Magmatism. Igneous rocks

What is "Volcanism"? What is the correct spelling of this word. Concept and interpretation.

Volcanism volcanic?zm is a term that has two meanings. In a narrow sense, it refers to the processes of formation of volcanoes and the whole complex of phenomena of volcanic activity. In a broad sense, volcanism refers to all phenomena associated with the activity of magma both at depth and on the surface of the earth. The most representative consequence of volcanism on the surface of the earth are volcanoes, at depth - the formation of intrusions and the change of host rocks under the influence of high temperatures and pressures. The most general definition of volcanism is a set of phenomena associated with the formation and movement of magma in the depths of the Earth and its eruption on the land surface, the bottom of the seas and oceans in the form of lavas, pyroclastic material and volcanic gases. In the process of volcanic activity in the depths of the earth, magma chambers and channels are formed, the rocks around which can change both under the influence of high temperatures and as a result of the chemical effects of lavas. Volcanic cones, domes, plateaus, calderas, lava flows, pumice covers, geysers, hot springs, etc. arise on the earth's surface. Rocks that have erupted onto the surface as a result of volcanic activity are called volcanic. Rocks from magma at depth are igneous. Due to all forms of manifestation of volcanism, the volume of the rocks of the earth's crust increases by more than 5 km? in year. Volcanism releases a huge amount of gases into the atmosphere, which largely form the gaseous envelope of the Earth and participate in the formation of the hydrosphere. Volcanism is most intense in mid-ocean ridges, island arcs, rift valleys, and young orogens. Entire groups of minerals are associated with volcanism: gold, silver, copper, antimony, arsenic, sulfur, alunite, borates, precious stones, building materials. Volcanism is a powerful planetary process. Volcanoes, calderas, lava flows and fields are characteristic of the Moon, Mars, Mercury and Io, its satellite.

Volcanism- All changes in the earth's crust, taking place before our eyes and marking the past geological ... Encyclopedic Dictionary F.A. Brockhaus and I.A. Efron

Volcanism- (geological) set of phenomena associated with the movement of magma in the earth's crust and on it ... Great Soviet Encyclopedia

Volcanism- VOLCANISM, volcanism, pl. no, m. (geol.). The activity of the internal forces of the globe, leading to change ... Ushakov's Explanatory Dictionary

Volcanism- m. 1. The totality of phenomena associated with the movement of a molten liquid mass (magma) in the earth's crust ... Explanatory Dictionary of Efremova

Volcanism- a set of processes and phenomena associated with the movement of magma (together with gases and steam) in the upper ... Encyclopedia Collier

Volcanism- VOLCANISM - a set of phenomena caused by the penetration of magma from the depths of the Earth to its surface ...

VOLCANISM
a set of processes and phenomena associated with the movement of magma (together with gases and steam) in the upper mantle and the earth's crust, its outpouring in the form of lava or ejection to the surface during volcanic eruptions (see also VOLCANOES). Sometimes large volumes of magma cool and solidify before they reach the Earth's surface; in this case they form igneous intrusions.

MAGMATIC INTRUSIONS
The sizes and shapes of intrusive bodies can be judged when they are at least partially exposed by erosion. Most of the intrusions formed at considerable depths (hundreds and thousands of meters) and are under a thick layer of rocks, and only a few reached the surface in the process of formation. Relatively small intrusive bodies were completely exposed as a result of subsequent erosion. Theoretically, intrusive bodies come in any size and any shape, but usually they can be attributed to one of the varieties, characterized by a certain size and shape. Dikes are plate-shaped bodies of intrusive igneous rocks, clearly bounded by parallel walls, which penetrate the host rocks (or lie unconformably with them). Dikes range in diameter from several tens of centimeters to tens and hundreds of meters, however, as a rule, they do not exceed 6 m, and their length can reach several kilometers. Usually in the same area there are numerous dikes, similar in age and composition. One of the mechanisms of dike formation is the filling of cracks in host rocks with magmatic melt. The magma expands the cracks and partially melts and absorbs the surrounding rocks, forming and filling the chamber. Near contact with the wall rock, due to the relatively rapid cooling, dikes usually have a fine-grained texture. The host rock can be altered by the thermal action of the magma. Dikes are often more resistant to erosion than wall rocks and their outcrops form narrow ridges or walls. Sills are sheeted intrusions similar to dykes, but occur in conformity with (usually horizontal) layers of host rock. Sills are similar in thickness and length to dikes, with thicker sills occurring more frequently. The Palisade sill, in the area of the famous Hudson River bank opposite New York, was originally over 100 m thick and ca. 160 km. The thickness of the Wyn sill in the north of England exceeds 27 m. Laccoliths are lenticular intrusive bodies with convex or domed upper surfaces and relatively flat lower surfaces. Like the sills, they lie in conformity with the layers of the enclosing deposits. Laccoliths are formed from magma flowing either through dike-shaped supply channels from below or from sill, such as the well-known laccoliths in the Henry Mountains in Utah, which are several kilometers across. However, larger laccoliths are also found. Bismalites are a special variety of laccoliths - cylindrical intrusions, broken by cracks or faults, with an elevated central part. Lopolites are very large lenticular intrusive bodies, concave in the central part (saucer-shaped), occurring more or less according to the structures of the host rocks. One of the largest lopoliths (about 500 km across) was found in the Transvaal (South Africa). Another fairly large lopolith is located in the area of the Sudbury nickel deposit (Ontario, Canada). Batholiths are large irregularly shaped intrusive bodies expanding downwards, going to a considerable depth (as a rule, their soles are not exposed by erosion). The area of batholiths can reach several thousand square kilometers. They are often found in the central parts of the folded mountains, where their strike generally corresponds to that of the mountain system. However, usually batholiths cut through the main structures. The batholiths are composed of coarse-grained granites. The surface of the batholith can be very uneven with outgrowths, protrusions and processes. In addition, large prisms of parent rocks, which are called roof remnants, can be located in the upper part of the batholith. Like many other intrusive bodies, batholiths are surrounded by a zone (halo) of rocks altered (metamorphosed) as a result of the thermal action of magma. The size of the batholiths is so large that it is still not entirely clear how they are emplaced. It has been suggested that the formation of the batholith chamber occurs as a result of the collapse of large blocks of bedrock into molten magma, and then their absorption, melting and assimilation by magma (the so-called magmatic collapse hypothesis). A less common hypothesis is that the batholith granitic rocks are remelted and recrystallized wall rocks with a small addition of new igneous material (granitization hypothesis). Stocks - similar to batholiths, but are smaller. Conventionally, stocks are defined as batholithic intrusive bodies with an area of less than 100 km2. Some of them are domed protrusions on the surface of the batholith. Necks are cylindrical intrusive bodies that fill the vents of volcanoes, usually having a diameter of no more than 1.5 km. Volcanic necks are stronger than the host rocks, due to which, after the destruction of volcanic structures by erosion, they remain in the relief in the form of spiers or steep hills.
Other magmatic intrusions. There are a large number of varieties of small intrusive bodies that are rarer than those discussed above. Among them, phacoliths stand out - conformably occurring, biconvex, lenticular bodies, usually formed in the crests of anticlines or in the depressions (hinges) of synclines; apophyses - branches from larger intrusive bodies that have an irregular shape; conical dikes, or conical layers, arc-shaped dikes, gently plunging towards the center of the arc, presumably formed as a result of the filling of concentric cracks above magma chambers; ring dikes - vertical dikes, having a round or oval shape in plan and formed during the filling of ring faults that occur during the subsidence of the underlying igneous mass.

Collier Encyclopedia. - Open Society. 2000 .

Synonyms:

See what "VOLCANISM" is in other dictionaries:

1) a geological doctrine that attributes the formation of the earth's crust and upheavals on the globe to the action of fire. 2) the same as plutonism. Dictionary of foreign words included in the Russian language. Chudinov A.N., 1910. VOLCANISM The system of geologists, ... ... Dictionary of foreign words of the Russian language

A set of processes and phenomena associated with the movement of magmas. masses and often accompanying gas-water products from the deep parts of the earth's crust to the surface. In a narrow sense, V. the totality of phenomena associated with the volcano. and accompanying her ... ... Geological Encyclopedia

The totality of phenomena caused by the penetration of magma from the depths of the Earth to its surface ... Big Encyclopedic Dictionary

The geological process caused by the activity of magma at the depth of the Earth's surface ... Geological terms

VOLCANISM, volcanic activity. The term is general for all aspects of the process: eruptions of molten and gaseous masses, the formation of mountains and craters, the occurrence of lava flows, geysers and hot springs ... Scientific and technical encyclopedic dictionary

VOLCANISM, volcanism, pl. no, husband. (geol.). The activity of the internal forces of the globe, leading to a change in the geological structure of the earth's crust and accompanied by volcanic eruptions, earthquakes. Explanatory Dictionary of Ushakov. D.N. Ushakov. 1935 ... Explanatory Dictionary of Ushakov

Exist., number of synonyms: 1 cryovolcanism (1) ASIS Synonym Dictionary. V.N. Trishin. 2013 ... Synonym dictionary

volcanism- a, m. volcanisme m. German A set of phenomena associated with the movement of a molten liquid mass (magma) in the earth's crust and its outpouring onto the surface of the Earth. BAS 2. Here .. for an area approximately equal to the entire area of Belgium ... ... Historical Dictionary of Gallicisms of the Russian Language

volcanism- An endogenous process associated with the movement of magmas and associated gas-water products from deep zones to the surface. [Glossary of geological terms and concepts. Tomsk State University] Topics geology, geophysics Generalizing ... ... Technical Translator's Handbook

volcanism- A set of processes and phenomena associated with the outpouring of magma on the surface of the Earth. Syn.: volcanic activity… Geography Dictionary

Volcanic eruption on Io ... Wikipedia

Books

Volcanism and sulfide mounds of paleooceanic margins. On the example of pyrite-bearing zones of the Urals and Siberia, Zaikov V.V. The monograph describes the volcanism and ore content of Paleozoic rifts of marginal seas, ensimatic island arcs, and interarc basins. On the example of the Urals of Siberia, it is shown that ...

VOLCANISM, a set of endogenous processes associated with the formation and movement of magma in the bowels of the Earth and its eruption on the land surface, the bottom of the seas and oceans. It is an integral part of magmatism. In the process of volcanism, magma chambers are formed in the depths of the earth, the rocks around which can change under the influence of high temperature and the chemical action of magma. When the magmatic melt reaches the Earth's surface, the most spectacular manifestation of volcanism is observed - a volcanic eruption, which consists in the outpouring or gushing of liquid lava (effusion), squeezing out viscous lava (extrusion), destruction of the volcanic structure by an explosion and ejection of solid products of volcanic activity (explosion). As a result of eruptions of different types and forces, volcanoes of various shapes and sizes are formed, volcanic rocks are formed. Volcanism is associated with phenomena that precede (harbingers), accompany and complete (post-volcanic phenomena) volcanic eruptions. Harbingers observed from several hours to several centuries before the eruption include some volcanic earthquakes, deformations of the earth's surface and volcanic structures, acoustic phenomena, changes in geophysical fields, composition and intensity of fumarolic gases (from active volcanoes), etc.

Phenomena observed during eruptions: volcanic explosions, associated shock waves, sharp jumps in atmospheric pressure, electrified eruptive (eruptive) clouds with Elmo fires, lightning, volcanic ashfalls and acid rains, the occurrence of lahars (mudstone flows), the formation of a tsunami - during falling into the water of huge volumes of landslide and explosive deposits. Volcanic phenomena also include a decrease in the level of solar radiation and temperature, the appearance of purple sunsets caused by clouding of the atmosphere with volcanic dust and aerosols during catastrophic explosive eruptions. After eruptions, post-volcanic phenomena are observed associated with the cooling of the magma chamber - outflows of volcanic gases (fumaroles) and thermal waters (thermal springs, geysers, etc.).

According to the place of manifestation, volcanism is distinguished terrestrial, underwater and subaerial (underwater-surface); according to the composition of the eruption products - sequentially differentiated basalt-andesite-rhyolite, contrast-differentiated basalt-rhyolitic (bimodal), alkaline, alkaline-ultrabasic, basic, acidic and other volcanism is most characteristic of the convergent boundaries of lithospheric plates, where in the process of their mutual interaction volcanic belts (island-arc and marginal-continental) are formed above the zone of subduction (subduction) of one plate under another or in the area of collision (collision) of their continental parts. Volcanism is also widely manifested at the divergent boundaries of lithospheric plates, confined to mid-ocean ridges, where, as the plates move apart in the course of underwater volcanic activity, a new formation of the oceanic crust occurs. Volcanism is also characteristic of the inner parts of lithospheric plates - structures of hot spots, continental rift systems, trap provinces of continents, and intraoceanic basalt plateaus.

Volcanism began in the early stages of the Earth's development and became one of the main factors in the formation of the lithosphere, hydrosphere and atmosphere. The development of all three shells due to volcanism continues: the volume of rocks in the lithosphere increases annually by more than 5-10 km 3, and an average of 50-100 million tons of volcanic gases per year enter the atmosphere, some of which is spent on the transformation of the hydrosphere. Many deposits of metallic (gold, silver, non-ferrous metals, arsenic, etc.) and non-metallic (sulfur, borates, natural building materials, etc.) minerals, as well as geothermal resources, are genetically associated with volcanism.

Manifestations of volcanism have been identified on all planets of the terrestrial group. On Mercury, Mars, and the Moon, volcanism has probably already ended (or almost ended), and intensively continues only on Venus. At the end of the 20th - beginning of the 21st century, volcanic forms and ongoing volcanic activity were discovered on the satellites of Jupiter and Saturn - Europa, Io, Callisto, Ganymede, Titan. On Europa and Io, a specific type of volcanism is noted - cryovolcanism (eruption of ice and gas).

Lit .: Melekestsev IV Volcanism and relief formation. M., 1980; Rast H. Volcanoes and vulcanism. M., 1982; Vlodavets V. I. Handbook of volcanology. M., 1984; Markhinin E.K. Volcanism. M., 1985.

T.I.FROLOV
Volcanic rocks are products of a deep process - volcanism. According to the definition of the famous volcanologist A. Jaggar, volcanism is a set of phenomena occurring in the earth's crust and under it, leading to a breakthrough of molten masses through the solid crust. Volcanism is associated with the flow of hot deep gases - fluids from the bowels of the Earth. Fluids contribute to the decompaction and local rise of deep matter, which, as a result of a decrease in pressure (decompression), begins to partially melt, forming deep diapirs - sources of magmatic melts. Depending on the intensity of heating, the formation of melts occurs at different levels of the mantle and the earth's crust, starting from depths of 300 - 400 km.

Volcanology is the science of volcanoes and their products (volcanic rocks), the causes of volcanism due to geodynamic, tectonic and physico-chemical processes occurring in the bowels of the Earth. In addition to the actual geological sciences: historical geology, geotectonics, petrography, mineralogy, lithology, geochemistry and geophysics, volcanology uses data from geography, geomorphology, physical chemistry, and partly astronomy, since volcanism is a planetary phenomenon. Being products of deep (endogenous) processes, volcanoes that form on the Earth's surface affect the environment, atmosphere and hydrosphere, and the formation of precipitation. Volcanology, as it were, focuses the problems linking the processes of internal and external energy of the Earth.

The general classification of all igneous rocks, including volcanic ones, is based on their chemical composition and, first of all, on the content and ratio of silica and alkalis in the rocks (Fig. 1). According to the content of silica, the most common oxide in igneous rocks, the latter are divided into four groups: ultrabasic (30 - 44% SiO2), basic (44 - 53%), medium (53 - 64%), acidic (64 - 78%). Another important feature of the classification is the alkalinity of rocks, which is estimated by the sum of the contents of Na2O + K2O. On this basis, rocks of normal alkalinity and alkaline are distinguished.

The most widely distributed among the volcanic rocks of the Earth are the main rocks - basalts, which are derivatives of the mantle substance and are found both in the oceans and on the continents. They can be compared with the "blood" of our planet, which appears in any violation of the earth's crust. Depending on the geological position, basalts differ in composition. Most of them belong to rocks of normal alkalinity. These are lime-rich low-alkaline (tholeiitic) and calc-alkaline basalts. Less common are alkaline basalts undersaturated with silica. Basalt magmas, when differentiated, give a series of rocks (tholeiitic, calc-alkaline and alkaline), united by origin from a single magma, retaining common features with parental basaltic magmas, up to extremely acidic ones. Among the intrusive rocks, granites are the most common. They belong to the group of silicic rocks, in the formation of which the substance of the earth's crust plays a significant role. The rocks of average composition, which are represented mainly by volcanic andesites, are less common and only in the mobile belts of the Earth. At the same time, the average composition of the earth's crust corresponds to andesites, and not to basalts or granites, corresponding to a mixture of these latter in a ratio of 2: 1.

HOW VOLCANISM EVOLVED IN THE HISTORY OF THE EARTH

The earliest processes of volcanism are synchronous with the formation of the Earth as a planet. In all likelihood, already at the stage of accretion (the concentration of planetary matter due to gas-dust nebulae and the collision of solid cosmic debris - planetosimals) its heating took place. The release of energy due to accretion and gravitational contraction turned out to be sufficient for its initial, partial or complete melting, with the subsequent differentiation of the Earth into shells. A little later, these sources of heating were joined by the release of heat by radioactive elements. The concentration of the iron-stony mass of the Earth, as well as on other planets of the solar system, was accompanied by the separation of a gaseous, predominantly hydrogen shell, which it subsequently lost during the period of maximum solar activity, in contrast to the large planets of the Jupiter group remote from the Sun. This is evidenced by the depletion of the modern earth's atmosphere in rare inert gases - neon and xenon compared to cosmic matter.

According to A.A. Marakushev, the differentiation of the iron-stony mass of the Earth, similar in composition to meteorites - chondrites and a hydrogen gas shell completely melted under high pressure, led to a high concentration of essentially hydrogen fluids (volatile components in the supercritical state) in the metallic (iron-nickel) core that began to separate. Thus, the Earth acquired a large fluid reserve in its bowels, which determined its subsequent, unique in its duration, in comparison with other planets, endogenous activity. As the Earth consolidated in the direction from its outer shells to the center, the internal fluid pressure increased and periodic degassing occurred, accompanied by the formation of magmatic melts that came to the surface when the frozen crust cracked. Thus, the earliest volcanism, which was characterized by an explosive, highly explosive nature, was associated with the beginning of the cooling of the Earth and was accompanied by the formation of the atmosphere. According to other ideas, the primary atmosphere, formed at the stage of accretion, was subsequently preserved, gradually evolving in its composition. One way or another, approximately 3.8 - 3.9 billion years ago, when the temperature on the Earth's surface and in the adjacent parts of the atmosphere dropped below the boiling point of water, the hydrosphere was formed. The presence of the atmosphere and hydrosphere made possible the further development of life on Earth. At first, the atmosphere was poor in oxygen until the simplest forms of life that produced it appeared, which happened about 3 billion years ago (Fig. 2).

The composition of the earliest volcanic rocks of the Earth, now completely reworked by subsequent processes, can be judged by comparing it with other terrestrial planets, in particular with our relatively well-studied satellite, the Moon. The Moon is a planet of more primitive development, which has used up its fluid reserves early and, as a result, has lost its endogenous activity. It is currently a "dead" planet. The absence of a metallic core in it indicates that the processes of its differentiation into shells stopped early, and a negligibly weak magnetic field indicates the complete solidification of its interior. At the same time, the presence of fluids in the early stages of the development of the Moon is evidenced by gas bubbles in lunar volcanic rocks, which consist mainly of hydrogen, which indicates their high reduction.

The most ancient, currently known rocks of the Moon, developed on the surface of the lunar crust on the so-called lunar continents, have an age of 4.4 - 4.6 billion years, which is close to the estimated age of the formation of the Earth. They are crystallized at shallow depths or on the surface, rich in high-calcium feldspar - anorthite - light-colored basic rocks, which are commonly called anorthosites. The rocks of the lunar continents were subjected to intense meteorite bombardment with the formation of fragments, partially melted down and mixed with meteorite matter. As a result, numerous impact craters coexisting with craters of volcanic origin were formed. It is assumed that the lower parts of the lunar crust are composed of rocks of a more basic, low-silica composition, close to stony meteorites, and anorthosites are directly underlain by anorthite gabbro (eucrites). On Earth, the association of anorthosites and eucrites is known in the so-called layered mafic intrusions and is the result of differentiation of basaltic magma. Since the physical and chemical laws that determine differentiation are the same throughout the Universe, it is logical to assume that on the Moon the most ancient crust of lunar meteorites was formed as a result of early melting and subsequent differentiation of the magmatic melt that formed the upper shell of the Moon in the form of the so-called "lunar ocean of magma". The differences in the processes of differentiation of lunar magmas from terrestrial ones lie in the fact that on the Moon it extremely rarely reaches the formation of high-silica felsic rocks.

Later, large depressions formed on the Moon, called lunar seas, filled with younger (3.2 - 4 billion years) basalts. On the whole, these basalts are close in composition to the basalts of the Earth. They are distinguished by a low content of alkalis, especially sodium, and the absence of iron oxides and minerals containing the OH hydroxyl group, which confirms the loss of volatile components by the melt and the reducing environment of volcanism. Feldspar-free rocks known on the Moon - pyroxenites and dunites, probably compose the lunar mantle, being either a remnant from the melting of basalt rocks (the so-called restite), or their heavy differentiate (cumulate). The early crust of Mars and Mercury is similar to the cratered crust of the lunar continents. On Mars, moreover, later basaltic volcanism is widely developed. There is also a basaltic crust on Venus, but data on this planet is still very limited.

The use of data from comparative planetology allows us to state that the formation of the early crust of the terrestrial planets occurred as a result of the crystallization of magmatic melts that underwent greater or lesser differentiation. The cracking of this frozen proto-crust with the formation of depressions was later accompanied by basaltic volcanism.

Unlike other planets, Earth did not have the earliest crust. More or less reliably, the history of the Earth's volcanism can be traced only from the Early Archean. The oldest known age dates belong to Archean gneisses (3.8 - 4 billion years) and grains of the mineral zircon (4.2 - 4.3 billion years) in metamorphosed quartzites. These dates are 0.5 billion years younger than the formation of the Earth. It can be assumed that all this time the Earth developed similarly to other planets of the terrestrial group. From about 4 billion years ago, a continental proto-crust was formed on the Earth, consisting of gneisses, predominantly of igneous origin, differing from granites in lower silica and potassium contents and called "gray gneisses" or the TTG association, after the name of the three main igneous rocks corresponding to the composition of these gneisses : tonalites, trondhjemites and granodiorites, subsequently subjected to intense metamorphism. However, "gray gneisses" hardly represented the primary crust of the Earth. It is also unknown how widespread they were. In contrast to the much less silicate rocks of the lunar continents (anorthosites), such large volumes of felsic rocks cannot be obtained by differentiation of basalts. The formation of "gray gneisses" of igneous origin is theoretically possible only during the remelting of rocks of basalt or komatite-basalt composition, which, due to their gravity, have sunk to the deep levels of the planet. Thus, we come to the conclusion about the basaltic composition of the crust, which is earlier than the "gray-gneiss" known to us. The presence of an early basaltic crust is confirmed by finds in the Archean "gray" gneisses of older metamorphosed mafic blocks. It is not known whether the parent magma of the basalts that formed the early crust of the Earth underwent differentiation to form lunar-like anorthosites, although this is theoretically quite possible. Intensive multi-stage differentiation of planetary matter, which led to the formation of acidic granitoid rocks, became possible due to the water regime established on Earth due to the large fluid reserve in its depths. Water promotes differentiation and is very important for the formation of acidic rocks.

Thus, during the earliest (Katarchean) and Archean time, mainly as a result of magmatism processes, which were joined by sedimentation after the formation of the hydrosphere, the earth's crust was formed. It began to be intensively processed by the products of active degassing of the early Earth with the addition of silica and alkalis. Degassing was due to the formation of the solid inner core of the Earth. It caused the processes of metamorphism up to melting with a general acidification of the composition of the crust. So, already in the Archean, the Earth had all the hard shells inherent in it - the crust, the mantle and the core.

The growing differences in the degree of permeability of the crust and upper mantle, which were due to differences in their thermal and geodynamic regimes, led to the heterogeneity of the composition of the crust and to the formation of its different types. In areas of compression, where degassing and rising to the surface of the emerging melts was difficult, the latter experienced intense differentiation, and the previously formed basic volcanic rocks, being compacted, descended to a depth and were remelted. A protocontinental two-layer crust was formed, which had a contrasting composition: its upper part was composed mainly of acid volcanic and intrusive rocks, processed by metamorphic processes into gneisses and granulites, the lower part was composed of basic rocks, basalts, komatites and gabbroids. Such a crust was characteristic of protocontinents. The proto-oceanic crust, which had a predominantly basaltic composition, formed in the extension areas. Along the breaks in the protocontinental crust and in the zones of its junction with the protooceanic, the first mobile belts of the Earth (protogeosynclines) were formed, which were distinguished by increased endogenous activity. Even then, they had a complex structure and consisted of less mobile uplifted zones that underwent intense high-temperature metamorphism, and zones of intense extension and subsidence. The latter were called greenstone belts, since the rocks composing them acquired a green color as a result of low-temperature metamorphism processes. The stretching setting of the early stages of the formation of mobile belts was replaced by a prevailing compression setting during evolution, which led to the appearance of felsic rocks and the first rocks of the calc-alkaline series with andesites (see Fig. 1). The mobile belts, which had completed their development, attached themselves to the areas of development of the continental crust and increased its area. According to modern concepts, from 60 to 85% of the modern continental crust was formed in the Archaean, and its thickness was close to modern, that is, it was about 35 - 40 km.

At the turn of the Archean and Proterozoic (2700 - 2500 million years) a new stage began in the development of volcanism on Earth. Melting processes became possible in the thick crust formed by that time, and more acidic rocks appeared. Their composition has changed significantly, primarily due to an increase in the content of silica and potassium. Real potassium granites, which were smelted from the bark, were widely used. Intense differentiation of mantle basalt melts under the influence of fluids in mobile belts, accompanied by interaction with the crustal material, led to an increase in the volume of andesites (see Fig. 1). Thus, in addition to mantle volcanism, crustal and mixed mantle-crustal volcanism became increasingly important. At the same time, due to the weakening of the processes of degassing of the Earth and the heat flow associated with them, such high degrees of melting in the mantle, which could lead to the formation of ultrabasic komatite melts (see Fig. 1), turned out to be impossible, and if they did occur, then rarely rose to the surface due to their high density compared to the earth's crust. They underwent differentiation in intermediate chambers and their derivatives, less dense basalts, fell to the surface. The processes of high-temperature metamorphism and granitization also became less intense, which acquired not an areal, but a local character. In all likelihood, two types of the earth's crust were finally formed at that time (Fig. 3), corresponding to continents and oceans. However, the time of formation of the oceans has not yet been finally determined.

In the subsequent stage of the development of the Earth, which began 570 million years ago and is called the Phanerozoic, those trends that appeared in the Proterozoic were further developed. Volcanism is becoming more and more diverse, acquiring clear distinctions in oceanic and continental segments. In extension zones in the oceans (mid-ocean rift ridges), tholeiitic basalts erupt, and in analogous extension zones on the continents (continental rifts), they are joined by and often dominated by alkaline volcanic rocks. The mobile belts of the Earth, called geosynclinal, are magmatically active for tens and hundreds of millions of years, starting from early tholeiite-basalt volcanism, which together with ultrabasic intrusive rocks form ophiolite associations under extensional conditions. Later, as extension changes into compression, they give way to contrasting basalt-rhyolitic and calc-alkaline andesitic volcanism, which flourished in the Phanerozoic. After folding, the formation of granites and orogeny (growth of mountains), volcanism in the mobile belts becomes alkaline. Such volcanism usually ends their endogenous activity.

The evolution of volcanism in the Phanerozoic mobile belts repeats that in the development of the Earth: from homogeneous basalt and contrasting basalt-rhyolitic associations that prevailed in the Archaean, to continuous in silicic acidity with large volumes of andesites, and, finally, to alkaline associations, which are practically absent in the Archaean. This evolution, both in individual belts and on the Earth as a whole, reflects a general decrease in permeability and an increase in the rigidity of the earth's crust, which determines a higher degree of differentiation of mantle magmatic melts and their interaction with the material of the earth's crust, a deepening of the level of magma formation and a decrease in the degree of melting. The above is connected with the change in the internal parameters of the planet, in particular with the general decrease in the global heat flux from its interior, which is estimated to be 3–4 times less than in the early stages of the Earth's development. Correspondingly, local upward flows of fluids resulting from periodic degassing of the subsoil also decrease. It is they that cause the heating of individual areas (movable belts, rifts, etc.) and their magmatic activity. These flows are formed in connection with the accumulation of light components at the crystallization front of the outer liquid core in separate protrusions-traps that float up, forming convective jets.

Endogenous activity is periodic. It caused the presence of large pulsations of the Earth with alternating predominance of basic and ultrabasic magmatism, fixing extension, and calc-alkaline volcanism, granite formation and metamorphism, fixing the predominance of compression. This periodicity determines the presence of magmatic and tectonic cycles, which, as it were, are superimposed on the irreversible development of the Earth.

WHERE DO VOLCANO EVENTS OCCUR IN CENOSIOIC?

The geological structures where volcanic rocks are formed in the youngest, Cenozoic, stage of the Earth's development, which began 67 million years ago, are located both within the oceanic and continental segments of the Earth. The former include mid-ocean ridges and numerous volcanoes on the ocean floor, the largest of which form oceanic islands (Iceland, Hawaii, etc.). All of them are characterized by an environment of high permeability of the earth's crust (Fig. 4). On the continents, in a similar setting, volcanoes erupt, associated with large extension zones - continental rifts (East African, Baikal, etc.). In conditions of predominant compression, volcanism occurs in mountain structures, which are currently active intracontinental mobile belts (Caucasus, Carpathians, etc.). The mobile belts on the margins of the continents (the so-called active margins) are peculiar. They are developed mainly along the periphery of the Pacific Ocean, and in its western margin, as in the ancient mobile belts, they combine zones of predominant compression - island arcs (Kurilo-Kamchatka, Tonga, Aleutian, etc.) and zones of intense extension - rear marginal seas (Japanese, Philippine, Coral, etc.). In the mobile belts of the eastern margin of the Pacific Ocean, the extension is less significant. On the edge of the American continent there are mountain ranges (Andes, Cordillera), which are analogues of island arcs, in the rear of which there are continental depressions - analogues of marginal seas, where the stretching situation prevails. Under conditions of high permeability, as always in the history of the Earth, mantle melts erupt, and in oceanic structures they have predominantly normal alkalinity, while in continental structures they have increased and high. In settings of predominant compression on the continental crust, in addition to mantle rocks, rocks of mixed mantle-crustal (andesites) and crustal (some felsic volcanics and granites) origin are widespread (Fig. 5).

If we take into account the features of the modern stage of the Earth's development, which include the high intensity of the process of ocean formation and the widespread development of rift zones on the continents, it becomes clear that in the Cenozoic stage of development, extension predominates and, as a result, the mantle, mainly basalt volcanism associated with it, is widespread. , especially intense in the oceans.

HOW VOLCANISM IS TRANSFORMING THE EARTH'S CRUST

Even at the beginning of the last century, it was noticed that rocks form regularly repeating associations, called geological formations, more closely related to geological structures than individual rocks. Rows of formations that replace each other in time are called temporary, and those that replace each other in space are called lateral formation rows. Together, they make it possible to decipher the main stages in the development of geological structures and are important indicators in the restoration of geological settings of the past. Volcanic formations, including volcanic rocks, products of their washing and redeposition, and often sedimentary rocks, are more convenient to use for these purposes than intrusive ones, since they are members of layered sections, which makes it possible to accurately determine the time of their formation.

There are two types of series of volcanogenic formations. The first, called homodromous, begins with basic rocks - basalts, giving way to formations with gradually increasing volumes of medium and acidic rocks. The second series is antidromic, beginning with formations of predominantly felsic composition with an increase in the role of basic volcanism towards the end of the series. The first, therefore, is associated with mantle volcanism and high permeability of the crust, and only as the permeability decreases and the crust is heated by deep heat, the latter begins to participate in magma formation. The antidromic series is characteristic of geological structures with thick, poorly permeable continental crust, when direct penetration of mantle melts to the surface is difficult. They interact with the material of the earth's crust the more intensely, the more it warms up. Basalt formations appear only later, when the crust cracks under the pressure of mantle magmas.

Homodromic series of volcanic formations are characteristic of the oceans and geosynclinal mobile belts and reflect, respectively, the formation of the oceanic and continental crust. Antidromic series are characteristic of structures that are laid down on the continental crust heated after the previous cycle of magmatism. Typical examples are marginal seas and continental rifts that appear immediately after orogeny (epiorogenic rifts). From the beginning of magmatic cycles, mantle-crustal and crustal rocks of intermediate and acidic composition appear in them, giving way to basic ones as the continental crust is destroyed (destruction). If this process goes far enough, as, for example, in marginal seas, then the continental crust is replaced by an oceanic one as a result of a complex set of processes, including extension.

The processes of transformation of the crust in long-term developing mobile belts of the geosynclinal type, which are very heterogeneous in their structures, are the most diverse and multidirectional. They contain structures with both an extension regime and a compression regime, and the type of crustal transformation depends on the predominance of certain processes. However, as a rule, the processes of formation of a new continental crust dominate, which attaches to the previously formed one, increasing its area. But this does not always happen, since, despite the vast areas occupied by mobile belts of different ages, the vast majority of the continental crust is of Archean age. Consequently, the destruction of the already formed continental crust also took place within the mobile belts. This is also evidenced by the cutting of the structures of the margins of the continents by the oceanic crust.

Volcanism reflects the evolution of the Earth during its geological history. The irreversibility of the development of the Earth is expressed in the disappearance or sharp decrease in the volume of some types of rocks (for example, comatites) along with the appearance or increase in the volume of others (for example, alkaline rocks). The general trend of evolution indicates a gradual attenuation of the deep (endogenous) activity of the Earth and an increase in the processes of processing of the continental crust during magma formation.

Volcanism is an indicator of the geodynamic conditions of extension and prevailing compression that exist on Earth. Typomorphic for the former is mantle volcanism, for the latter, mantle-crustal and crustal.

Volcanism reflects the presence of cyclicity against the background of the general irreversible development of the Earth. Cyclicity determines the repeatability of formation series in one separately taken and in different time, but the same type of geological structures.

The evolution of volcanism in the geostructures of the Earth is an indicator of the formation of the earth's crust and its destruction (destruction). These two processes continuously transform the earth's crust, carrying out the exchange of matter between the solid shells of the earth - the crust and mantle.

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Tatyana Ivanovna Frolova - Professor of the Department of Petrology, Faculty of Geology, Lomonosov Moscow State University M.V. Lomonosov, Honored Professor of Moscow State University, full member of the Academy of Natural Sciences (RANS) and the International Academy of Sciences of Higher Education; specialist in the field of volcanism of the mobile belts of the Earth - ancient (Urals) and modern (West Pacific active margin); author of monographs: "Geosynclinal volcanism" (1977), "Origin of volcanic series of island arcs" (1987), "Magmatism and transformation of the Earth's crust of active margins" (1989), etc.

Magmatism is a set of processes and phenomena associated with the activity of magma. Magma is a fiery-liquid natural usually silicate melt enriched in volatile components (H 2 O, CO 2 , CO, H 2 S, etc.). Low-silicate and non-silicate magmas are rare. Crystallization of magma leads to the formation of igneous (igneous) rocks.

The formation of magmatic melts occurs as a result of the melting of local areas of the mantle or the earth's crust. Most of the melting centers are located at relatively shallow depths in the range from 15 to 250 km.

There are several reasons for melting. The first reason is associated with the rapid rise of hot plastic deep matter from the region of high to the region of lower pressures. A decrease in pressure (in the absence of a significant change in temperature) leads to the onset of melting. The second reason is related to the increase in temperature (in the absence of a change in pressure). The reason for the heating of rocks is usually the intrusion of hot magmas and the fluid flow that accompanies them. The third reason is associated with the dehydration of minerals in the deep zones of the earth's crust. Water, released during the decomposition of minerals, sharply (by tens - hundreds of degrees) reduces the temperature of the beginning of the melting of rocks. Thus, melting begins due to the appearance of free water in the system.

The three considered mechanisms of melt generation are often combined: 1) the rise of asthenospheric matter into the area of low pressure leads to the beginning of its melting - 2) the formed magma intrudes into the lithospheric mantle and lower crust, leading to partial melting of the rocks that make them up - 3) the rise of melts into less deep zones of the crust, where hydroxyl-containing minerals (micas, amphiboles) are present, leads, in turn, to the melting of rocks during the release of water.

Speaking about the mechanisms of the nucleation of melts, it should be noted that in most cases, not complete, but only partial melting of the substrate (rocks undergoing melting) occurs. The emerging melting center is a solid rock penetrated by capillaries filled with melt. The further evolution of the chamber is associated either with the squeezing out of this melt, or with an increase in its volume, leading to the formation of a "magmatic porridge" - magma saturated with refractory crystals. Upon reaching 30-40 volume% of the melt, this mixture acquires the properties of a liquid and is squeezed out into the region of lower pressures.

The mobility of magma is determined by its viscosity, which depends on the chemical composition and temperature. The lowest viscosity is possessed by deep mantle magmas, which have a high temperature (up to 1600-1800 0 C at the time of generation) and contain little silica (SiO 2). The highest viscosity is inherent in magmas that have arisen due to the melting of the material of the upper continental crust during the dehydration of minerals: they are formed at a temperature of 700-600 0 C and are maximally saturated with silica.

The melt squeezed out of the intergranular pores is filtered upward at a rate of several centimeters to several meters per year. If significant volumes of magma are introduced along cracks and faults, the rate of their rise is much higher. According to calculations, the rate of rise of some ultrabasic magmas (the outpouring on the surface of which led to the formation of rare effusive ultrabasic rocks - komatiites) reached 1-10 m/s.

Patterns of magma evolution and formation of igneous rocks

The composition and features of rocks formed from magma are determined by a combination of the following factors: the initial composition of magma, the processes of its evolution, and the conditions of crystallization. All igneous rocks are divided into 6 orders according to silicic acidity:

Magmatic melts come from the mantle or are formed as a result of the melting of rocks in the earth's crust. As is known, the chemical composition of the mantle and crust are different, which primarily determines the differences in the composition of magmas. Magmas arising from the melting of mantle rocks, like these rocks themselves, are enriched in basic oxides - FeO, MgO, CaO, therefore, such magmas have an ultrabasic and basic composition. During their crystallization, ultrabasic and basic igneous rocks are formed, respectively. Magmas arising from the melting of crustal rocks depleted in basic oxides but sharply enriched in silica (a typical acidic oxide) have an acidic composition; during their crystallization, acidic rocks are formed.

However, primary magmas in the course of evolution often undergo significant compositional changes associated with the processes of crystallization differentiation, segregation, and hybridism, which gives rise to a variety of igneous rocks.

crystallization differentiation. As is known, according to the Bowen series, not all minerals crystallize simultaneously - olivines and pyroxenes are the first to separate from the melt. Having a density greater than the residual melt, if the viscosity of the magma is not too high, they settle to the bottom of the magma chamber, which prevents their further reaction with the melt. In this case, the residual melt will differ in chemical composition from the original one (because some of the elements have become part of the minerals) and will be enriched in volatile components (they are not part of the minerals of early crystallization). Consequently, the minerals of early crystallization in this case form one rock, and the remaining magma will form other, different in composition, rocks. Processes of crystallization differentiation are typical for basic melts; The precipitation of femic minerals leads to layering in the magma chamber: its lower part acquires an ultramafic composition, while its upper part acquires a basic one. Under favorable conditions, differentiation can lead to the release of a small volume of felsic melt from the primary mafic magma (which was studied on the example of the frozen lava lakes of Alae in the Hawaiian Islands and volcanoes in Iceland).

Segregation is a process of separation of magma with a decrease in temperature into two immiscible melts with different chemical compositions (in the most general form, the course of this process can be represented as the process of separating water and oil from their mixture). Accordingly, rocks of different composition will crystallize from the separated magmas.

hybridism ("hybrida" - a mixture) is the process of mixing of magmas of different composition or assimilation of host rocks by magma. Interacting with host rocks of different composition, capturing and processing their fragments, the igneous melt is enriched with new components. The process of melting or complete assimilation of foreign material by magma is denoted by the term assimilation ("assimillato" - assimilation). For example, the interaction of mafic magmas with felsic wall rocks produces hybrid rocks of intermediate composition. Or, conversely, intrusion of silicic magmas into rocks rich in basic oxides can also lead to the formation of intermediate rocks.

It should also be taken into account that during the evolution of the melt, the above processes can be combined.

Furthermore, different rocks can form from the same chemical composition of magma. This is due to different conditions of magma crystallization and, above all, to depth.

According to the conditions of depth of formation (or on the basis of facies), igneous rocks are divided into intrusive, or deep, and effusive, or erupted, rocks. intrusive rocks are formed during the crystallization of magmatic melt at a depth in rock strata; Depending on the depth of formation, they are divided into two facies: 1) abyssal rocks formed at a considerable depth (several km), and 2) hypabyssal, which were formed at a relatively shallow depth (about 1-3 km). effusive rocks are formed as a result of the solidification of lava poured onto the surface or bottom of the oceans.

Thus, the following main facies are distinguished: abyssal, hypabyssal and effusive. In addition to the three named facies, there are also subvolcanic and vein breeds. The first of them are formed in near-surface conditions (up to a few hundred meters) and have a close resemblance to effusive rocks; the latter are close to hypabyssal. Effusive rocks are often accompanied by pyroclastic formations consisting of fragments of effusives, their minerals and volcanic glass.

Drawing - facies

Significant differences in the nature of the manifestation of magmatic processes in deep and surface conditions make it necessary to distinguish between intrusive and effusive processes.

Intrusive magmatism

Intrusive processes are associated with the formation and movement of magma below the Earth's surface. The magmatic melts formed in the depths of the Earth have a density lower than that of the surrounding solid rocks and, being mobile, penetrate into the overlying horizons. The process of magma intrusion is called intrusion (from "intrusio" - implementation). If magma solidifies before reaching the surface (among host rocks), then intrusive bodies are formed. In relation to the host rocks, intrusions are divided into consonants(concordant) and dissenters(discordant). The former lie in accordance with the host rocks, without crossing the boundaries of their layers; the latter have secant contacts. According to the shape, a number of varieties of intrusive bodies are distinguished.

Consonant forms of intrusives include sill, lopolith, laccolith, and other less common ones. Silla are conformable sheet-like intrusive bodies formed under the conditions of stretching of the earth's crust. Their thickness ranges from tens of cm to hundreds of meters. The intrusion of a large number of sills into the layered stratum forms something like a layer cake. At the same time, as a result of erosion, strong igneous rocks in the relief form “steps” ( English "sill" - threshold). Such multilevel sills composed of mafic rocks are widespread on the Siberian platform (as part of the Tunguska syneclise), on Hindustan (Dean), and other platforms. lopolites- These are large consonant intrusive saucer-shaped bodies. The thickness of the lopoliths reaches hundreds of meters, and the diameter is tens of kilometers. The largest is the Bushveld in South Africa. Formed under conditions of tectonic extension and subsidence. Laccoliths- a consonant intrusive body of a mushroom-like shape. The roof of the laccolith has a convex arched shape, the sole is usually horizontal. The Henry Mountains intrusions in North America are a classic example. They are formed under conditions of significant pressure of intruding magma on layered host rocks. They are shallow intrusions, since in deep horizons the pressure of magma cannot overcome the pressure of powerful strata of overlying rocks.

The most common unconformities include dikes, veins, stocks, and batholiths. Dike- a discontinuous intrusive body of a plate-like shape. They form in hypabyssal and subvolcanic conditions when magma is emplaced along faults and fissures. As a result of exogenous processes, the enclosing sedimentary dikes are destroyed faster than the dikes occurring in them, due to which, in the relief, the latter resemble destroyed walls ( name from English "dike", "dyke" - a barrier, a wall of stone). veins called small secant bodies of irregular shape. Stock (from him. "Stock" - stick, trunk) is an unconformity intrusive columnar body. The largest intrusions are batholiths, they include intrusive bodies with an area of more than 200 km 2 and a thickness of several km. Batholiths are composed of acidic abyssal rocks formed during the melting of the earth's crust in areas of mountain building. It is noteworthy that the granitoids that make up batholiths are formed both as a result of the melting of primary sedimentary "sialic" rocks (S-granites), and during the melting of primary magmatic, including basic "femic" rocks (I-granites). This is facilitated by the preliminary processing of the original rocks (substratum) by deep fluids, which introduce alkalis and silica into them. Magmas formed as a result of large-scale melting can crystallize at the place of their formation, creating autochthonous intrusions, or intrude into host rocks - allochthonous intrusions.

All large deep intrusive bodies (batholiths, stocks, lopolites, etc.) are often combined under the general term plutons. Their smaller branches are called apophyses.

Forms of occurrence of intrusive bodies

When interacting with the host rocks (“frame”), magma has a thermal and chemical effect on them. The zone of change in the near-contact part of the host rocks is being drilled exocontact. The thickness of such zones can vary from a few cm to tens of km, depending on the nature of the host rocks and the saturation of the magma with fluids. The intensity of changes can also vary significantly: from dehydration and slight compaction of rocks to complete replacement of the original composition by new mineral parageneses. On the other hand, the magma itself changes its composition. This occurs most intensively in the marginal parts of the intrusion. The zone of altered igneous rocks in the marginal part of the intrusion is called endocontact zone. Endocontact zones (facies) are characterized not only by changes in the chemical (and, as a consequence, mineral) composition of rocks, but also by differences in structural and textural features, sometimes saturation xenoliths(captured by magma inclusions) of host rocks. When studying and mapping territories within which several intrusive bodies are combined, the correct identification of phases and facies is of great importance. Each implementation phase are igneous bodies formed by the intrusion of one portion of magma. Bodies belonging to different penetration phases are separated by secant contacts. The diversity of facies can be associated not only with the presence of several phases, but also with the formation of endocontact zones. For endocontact facies, the presence of gradual transitions between rocks is characteristic (due to the decrease in the influence of host rocks with distance from the contact), rather than sharp boundaries.

Volcanic processes

Melts and gases released in the bowels of the planet can reach the surface, leading to volcanic eruption- the process of incandescent or hot solid, liquid and gaseous volcanic products entering the surface. The outlet openings through which volcanic products enter the surface of the planet are called volcanoes (Vulcan is the god of fire in Roman mythology.). Depending on the shape of the outlet, volcanoes are divided into fissure and central. Fissure volcanoes, or linear type have an outlet in the form of an extended crack (fault). The eruption occurs either along the entire crack, or in its individual sections. Such volcanoes are confined to zones of separation of lithospheric plates, where, as a result of stretching of the lithosphere, deep faults are formed, along which basaltic melts are introduced. Active stretch zones are the areas of mid-ocean ridges. The volcanic islands of Iceland, which represent the exit of the Mid-Atlantic Ridge above the ocean surface, are one of the most volcanically active parts of the planet; typical fissure volcanoes are located here.

At volcanoes central type the eruption occurs through the supply pipe-like channel - mouth- passing from the volcanic chamber to the surface. The upper part of the vent that opens to the surface is called crater. Secondary outlet channels can branch off from the main vent along the fissures, giving rise to lateral craters. Volcanic products coming from the crater form volcanic structures. Often, the term "volcano" is understood as a hill with a crater on top, formed by the products of the eruption. The shape of volcanic structures depends on the nature of the eruptions. With calm outpourings of liquid basaltic lavas, flat shield volcanoes. In case of eruption of more viscous lavas and (or) ejections of solid products, volcanic cones are formed. The formation of a volcanic structure can occur as a result of a single eruption (such volcanoes are called monogenic), or as a result of multiple eruptions (volcanoes polygenic). Polygenic volcanoes built from alternating lava flows and loose volcanic material are called stratovolcanoes.

Another important criterion for classifying volcanoes is their level of activity. According to this criterion, volcanoes are divided into:

current- erupting or emitting hot gases and waters in the last 3500 years (historical period);
potentially active- Holocene volcanoes that erupted 3500-13500 years ago;
conditionally extinct volcanoes that did not show activity in the Holocene, but retained their external forms (younger than 100 thousand years old);
extinct- Volcanoes, significantly reworked by erosion, dilapidated, not active during the last 100 thousand years.

Schematic representations of the central (top) and shield (bottom) volcanoes (after Rast, 1982)

The products of volcanic eruptions are divided into liquid, solid and gaseous.

solid eruptions represented pyroclastic rocks (from the Greek "ryg" - fire and "klao" - I break, I break) - clastic rocks formed as a result of the accumulation of material ejected during volcanic eruptions. Divided into endoclastitis, formed during the spattering and solidification of lava, and exoclastites formed as a result of crushing of pre-coclastic rocks formed earlier. According to the size of the debris, they are divided into volcanic bombs, lapilli, volcanic sand and volcanic dust. Volcanic sand and volcanic dust are combined under the term volcanic ash.

Volcanic bombs are the largest among pyroclastic formations, their size can reach several meters in diameter. Formed from fragments of lava ejected from the crater. Depending on the viscosity, lavas have different shapes and surface sculptures. Spindle-shaped, drop-shaped, ribbon-shaped and ink-shaped bombs are formed during ejections of liquid (mainly basaltic) lavas. The spindly shape is due to the rapid rotation of low-viscosity lava during flight. The ink-shaped form occurs when ejections of liquid lava to a small height, not having time to harden, when they hit the ground, they are flattened. Tape bombs are formed by squeezing lava through narrow cracks, they are found in the form of fragments of tapes. Specific forms are formed during the flowing of basalt lavas. Thin streams of liquid lava are blown by the wind and harden into threads, such forms are called "Pele's hair" ( Pele - the goddess, according to legend, lives in one of the lava lakes in the Hawaiian Islands). Bombs formed by viscous lavas are characterized by polygonal outlines. Some bombs become covered in a chilled, hardened crust during flight, which is torn apart by gases released from the interior. Their surface takes the form of a "bread crust". Volcanic bombs can also be composed of exoclastic material, especially in explosions that destroy volcanic structures.

Lapilli (from lat. "lapillus" - pebble) are represented by rounded or angular volcanic ejecta, consisting of pieces of fresh lava frozen in flight, old lavas, and rocks alien to the volcano. The size of fragments corresponding to lapilli ranges from 2 to 50 mm.

The smallest pyroclastic material is volcanic ash. Most of the volcanic emissions are deposited near the volcano. As an illustration of this, it is enough to recall the cities of Herculaneum, Pompeii and Stabia covered with ashes during the eruption of Vesuvius in 79. During strong eruptions, volcanic dust can be thrown into the stratosphere and, in suspension, move in air currents for thousands of kilometers.

Originally loose volcanic products (called "tephra") are subsequently compacted and cemented, turning into volcanic tuffs. If fragments of pyroclastic rocks (bombs and lapilli) are cemented by lava, then lava breccias. Specific, deserving special consideration, formations are ignimbrites (from lat. "ignis" - fire and "imber" - downpour). Ignimbrites are rocks composed of sintered acidic pyroclastic material. Their formation is associated with the emergence scorching clouds(or ash flows) - streams of hot gas, lava drops and solid volcanic emissions resulting from intense pulsed gas release during an eruption.

Liquid products of eruptions are lavas. Lava (from ital. "lava" - I flood) is a liquid or viscous molten mass that comes to the surface during volcanic eruptions. Lava differs from magma by a low content of volatile components, which is associated with degassing of magma as it moves towards the surface. The nature of the flow of lava to the surface is determined by the intensity of gas release and the viscosity of the lava. There are three lava flow mechanisms - effusion, extrusion and explosion - and, accordingly, three main types of eruptions. Effusive eruptions are calm outpourings of lava from a volcano. Extrusion- type of eruption accompanied by extrusion viscous lava. Extrusive eruptions can be accompanied by explosive outgassing, leading to the formation of scorching clouds. explosive eruptions- These are eruptions of an explosive nature, due to the rapid release of gases.

Facies of volcanogenic rocks(Field geology, 1989)
1-dykes, 2-sills, laccoliths, 3-explosive subfacies, 4-lava flows (effusive subfacies), 5-domes and obelisks (extrusive subfacies), 6-vent facies, 7-hypabyssal intrusion

Lavas, like their intrusive counterparts, are primarily classified into ultrabasic, basic, intermediate, and felsic. Ultrabasic lavas in the Phanerozoic are very rare, although in the Precambrian (under conditions of a more intense influx of endogenous heat) they were much more widespread. Basic - basaltic - lavas are usually liquid, which is associated with a low content of silica and a high temperature at the exit to the surface (about 1000-1100 0 С and more). Due to their liquid state, they easily give off gases, which determines the effusive nature of eruptions, and the ability to spill over long distances in the form of streams, and in areas with a poorly dissected topography form extensive covers. The structural features of the surface of lava flows make it possible to distinguish two types among them, which are given Hawaiian names. The first type is called pahoehoe(or rope lavas) and forms on the surface of rapidly flowing lavas. The flowing lava is covered with a crust, which, under conditions of active movement, does not have time to acquire a significant thickness and quickly wrinkles in a wave-like manner. These "waves" with the further movement of the lava get off and look like ropes laid side by side.

Video illustrating the formation of a rope surface

The second type, called aa-lava, is characteristic of more viscous basaltic (or other composition) lavas. Due to the slower flow, the crust becomes thicker and breaks into angular fragments; the surface of aa lavas is an accumulation of acute-angled fragments with spike-like or needle-like protrusions.

Formation of AA lavas (Kilauea volcano)

As the silica content increases, the lavas become more viscous and solidify at a lower temperature. If basaltic lavas remain mobile at temperatures of the order of 600-700 0 C, then andesitic (middle) lavas solidify already at 750 0 C or more. Usually the most viscous are felsic dacitic and liparitic lavas. Increased viscosity makes it difficult to separate gases, which can lead to explosive eruptions. If the viscosity of the lava is high and the pressure of the gases is relatively low, extrusion occurs. The structure of lava flows is also different. For viscous medium and acidic melts, the formation of blocky lavas is characteristic. blocky lavas outwardly close to aa-lavas and differ from them in the absence of spiky and needle-shaped protrusions, as well as in the fact that the blocks on the surface have a more regular shape and a smooth surface. The movement of lava flows, the surface of which is covered with blocky lavas, leads to the formation of lava breccia horizons.

When liquid basalt lava is poured into water, the surface of the flows rapidly solidifies, which leads to the formation of peculiar “pipes” inside which the melt continues to move. Squeezing out from the edge of such a “pipe” into the water, a portion of lava acquires a drop-like shape. Since the cooling is uneven and the inner part continues to remain in a molten state for some time, the lava “drops” are flattened under the action of gravity and the weight of the following portions of lava. Heaps of such lavas are called pillow lavas or pillow lavas (from English. "pillow" - pillow).

Gaseous products of eruptions represented by water vapor, carbon dioxide, hydrogen, nitrogen, argon, sulfur oxides and other compounds (HCl, CH 4 , H 3 BO 3 , HF, etc.). The temperature of volcanic gases varies from a few tens of degrees to a thousand or more degrees. In general, high-temperature exhalations (HCl, CO 2 , O 2 , H 2 S, etc.) are associated with magma degassing, low-temperature ones (N 2 , CO 2 , H 2 , SO 2) are formed both by juvenile fluids and due to atmospheric gases and groundwater seeping into the volcano.

With the rapid release of gases from magma or the transformation of groundwater into steam, gas eruptions. During eruptions of this kind, there is a continuous or rhythmic release of gas from the vent, no emissions or very small amounts of ash. Powerful eruptions of gas and steam pierce a channel in the rocks, from which rock fragments are ejected, forming a shaft that borders the crater. Gas eruptions also occur through the vents of existing polygenic volcanoes (an example is the gas eruption of Vesuvius in 1906).

Types of volcanic eruptions

Depending on the nature of the eruptions, several types are distinguished among them. The basis of such a classification was laid by the French geologist Lacroix back in 1908. He identified 4 types, to which the author assigned the names of volcanoes: 1) Hawaiian, 2) Strombolian, 3) Vulcan and 4) Peleian. The proposed classification cannot include all known eruption mechanisms (subsequently it was supplemented by new types - Icelandic, etc.), but, despite this, it has not lost its relevance today.

Hawaiian-type eruptions characterized by a calm effusive outpouring of very hot liquid basaltic magma under conditions of low gas pressure. Lava under pressure is thrown into the air in the form of lava fountains, from several tens to several hundred meters high (during the eruption of Kilauea in 1959, they reached a height of 450 m). The eruption usually occurs from fissure vents, especially in the early stages. It is accompanied by a small number of weak explosions that splatter lava. Liquid wisps of lava that fall at the base of the fountain in the form of spatter and blot-shaped bombs form spatter cones. Lava fountains, stretching along the crack, sometimes for several kilometers, form a shaft consisting of frozen lava splashes. Liquid lava drops can form Pele's hair. Hawaiian-type eruptions sometimes lead to the formation of lava lakes.
Examples are the eruptions of the volcanoes Kilauea, Hapemaumau in the Hawaiian Islands, Niragongo and Erta Ale in East Africa.

Very close to the described Hawaiian type Icelandic type; similarities are noted both in the nature of the eruptions and in the composition of the lavas. The difference lies in the following. During eruptions of the Hawaiian type, lava forms large dome-shaped massifs (shield volcanoes), and during eruptions of the Icelandic type, lava flows form flat sheets. The outpouring comes from cracks. In 1783, the famous eruption from the Laki fissure about 25 km long occurred in Iceland, as a result of which basalts created a plateau with an area of 600 km2. After the eruption, the fissure channel is filled with hardened lava, and a new fissure is formed next to it during the next eruption. As a result of the layering of many hundreds of covers over fissures that change their position in space, extended lava plateaus (extensive ancient basalt plateaus of Siberia, India, Brazil and other regions of the planet) are formed.

Strombolian type eruptions. The name comes from the volcano Stromboli, located in the Tyrrhenian Sea off the coast of Italy. They are characterized by rhythmic (with interruptions from 1 to 10-12 min) ejections relative to liquid lava. Fragments of lava form volcanic bombs (pear-shaped, twisted, less often spindle-shaped, often flattened when falling) and lapilli; material of ashy dimension is almost absent. Ejections alternate with lava outpourings (compared to eruptions of Hawaiian-type volcanoes, the flows are shorter and thicker, which is associated with a higher viscosity of lavas). Another typical feature is the duration and continuity of development: the Stromboli volcano has been erupting since the 5th century BC. BC.

Volcanic eruptions. The name comes from the island of Vulcano in the group of Aeolian Islands off the coast of Italy. Associated with the eruption of viscous, usually andesitic or dacitic lava with a high content of gases from volcanoes of the central type. The viscous lava solidifies quickly, forming a plug that clogs the crater. The pressure of the gases released from the lava periodically “knocks out” the cork with an explosion. At the same time, a black cloud of pyroclastic material with bombs of the "breadcrust" type is thrown upwards, rounded, ellipsoidal and twisted bombs are practically absent. Sometimes explosions are accompanied by outpourings of lava in the form of short and powerful streams. Then the plug is formed again, and the cycle repeats.
Eruptions are separated by periods of complete rest. Eruptions of the Vulcan type are characteristic of the Avachinsky and Karymsky volcanoes in Kamchatka. The eruptions of Vesuvius are also close to this type.

Peleian-type eruptions. The name comes from the volcano Mont Pelee on the island of Martinique in the Caribbean. Occur when very viscous lava enters volcanoes of the central type, which brings it closer to the eruption of the Vulcan type. The lava solidifies in the vent and forms a powerful plug, which is squeezed out in the form of a monolithic obelisk (extrusion occurs). On the Mont Pele volcano, the obelisk has a height of 375 m and a diameter of 100 m. Hot volcanic gases accumulating in the vent sometimes escape through a frozen cork, leading to the formation of scorching clouds. The scorching cloud that arose during the eruption of Mont Pele on May 8, 1902 had a temperature of about 800 ° C and, moving down the slope of the volcano at a speed of 150 m / s, it destroyed the city of Saint-Pierre with 26,000 inhabitants.
A similar type of eruption was often observed near volcanoes on the island of Java, in particular near the Merapi volcano, and also in Kamchatka near the Bezymyanny volcano.