UNIT 3. GEOLOGY

Text 1. Deep Earth

(by Dr. F.Borrero et al. from Earth Science : Geology, the Environment and the Universe)

The surface of the Earth is covered by a relatively thin, cold, hard crust. Beneath the oceans it is about 7 or 8 kilometres thick; in the continents, 30 to 60 kilometres thick. At its base lies the Mohorovičić discontinuity or Moho, a layer which reflects seismic waves, probably as a result of a change in composition to the dense rocks of the mantle beneath. The lithosphere, the complete slab of cold, hard material on the Earth’s surface, includes not only the crust but the top of the mantle as well. In total, the continental lithosphere may be 250 or even 300 kilometres thick. It thins under the oceans, as you approach the mid-ocean ridge system, down to little more than the 7-kilometre crust. The lithosphere is not a single rigid layer, however. It is split into a series of slabs called tectonic plates. They are our principal clue as to how the deep Earth works. To understand what’s going on, we must probe beneath the crust.

Only 30 kilometres away from us lies a place we can never visit. If the distance was horizontal it would just be an easy bus ride away, but the same distance beneath our feet is a place of almost unimaginable heat and pressure. No mine can tunnel that deep. A proposal in the 1960s to use ocean-drilling techniques from the oil exploration industry to drill right through the ocean crust into the mantle, the so-called Moho project, was ruled out on grounds of cost and difficulty. Attempts at deep drilling on land on Russia’s Kola Peninsula and in Germany had to be abandoned after about 11,000 metres. Not only was the rock difficult to drill, but the heat and pressure tended to soften the drill components and squeeze the hole shut again as soon as it was drilled.

There is one way in which we can sample the mantle directly: in the outpourings of deep-rooted volcanoes. Most of the magma that erupts from volcanoes comes from only partial melting of the source material, so basalt, for example, is not a complete sample of mantle rock. It does, however, carry isotopic clues to what lies beneath.

Violent volcanic eruptions do sometimes carry in their magma more direct samples of mantle rocks. These so-called xenoliths are samples of mantle rock that have not been melted, just carried along in the flow. They are typically dark, dense, greenish rocks such as peridotite, rich in the mineral olivine, a magnesium/iron silicate. Similar rock is sometimes found in the deep cores of mountain ranges which have been thrust up from great depths.

Recent analysis of seismic data from around the world has revealed a thin layer at the base of the mantle, the D-layer, up to 200 kilometres thick. It is not a continuous layer but seems more like a series of slabs, a bit like continents on the underside of the mantle. This could be regions where silicate rocks in the mantle are partly mixed with iron-rich material from the core. But another explanation is that this is where ancient ocean lithosphere comes to rest. After its descent through the mantle, the slab is still cold and dense so it spreads out at the base of the mantle and is slowly heated by the core until, perhaps a billion years later, it rises again in a mantle plume to form new ocean crust.

Clues to the deep interior of the Earth also come from measuring tiny variations in day length. Our spinning planet is gradually slowing down due to the pull of the moon on the tides and to the rising of land compressed by ice in the last Ice Age. But there are other even smaller variations of a few billionths of a second. Some may be due to atmospheric circulation blowing on mountain ranges like wind on a sail. But there is another component which seems to be caused by circulation in the outer core pushing on ridges in the base of the mantle like ocean currents pushing on the keel of a ship. So there may be ridges and valleys like upside-down mountain ranges on the base of the mantle. There seems to be a great depression in the core beneath the Philippines that is 10 kilometres deep, twice the depth of the Grand Canyon. Bulging up beneath the Gulf of Alaska is a high spot on the core; a liquid mountain taller than Everest. Maybe sinking cold material indents the core, while hotspots bulge up.

We have no direct experience or samples of the Earth’s core. But we do know from seismic waves that the outer part of it is liquid and only the inner core is solid. We also know that the core has a much higher density than the mantle. The only material that is dense enough and sufficiently abundant in the solar system to make up the bulk of the core is iron. Although we do not have samples of the Earth’s core, we do have pieces of something that’s likely to be similar, in iron meteorites. Though not as common as stony meteorites, they are easier to spot. They are believed to come from large asteroids in which an iron core separated out before they were smashed by bombardment early in the history of the solar system. They are mostly made of iron metal but contain between 7% and 15% of nickel. Often, they have a structure of intergrown crystals of two alloys, one containing 5% nickel, the other about 40% nickel, in proportions that give the bulk composition.

An iron core must have formed in the Earth by gravitational separation from the silicate mantle when the new Earth was at least partially molten. As the layers separated, so-called siderophile elements such as nickel, sulphur, tungsten, platinum, and gold that are soluble in molten iron would have separated with them. Lithophile elements would have been held back by the silicate mantle. Radioactive elements such as uranium and hafnium are lithophile, whereas their decay products, or daughters, are isotopes of lead and tungsten so would have been separated out into the core at its formation. That consequently reset the radioactive clock in the mantle at the time the core formed.

The centre of the Earth is frozen. Frozen at least from the viewpoint of molten iron at the incredible pressures down there. As the planet cools, solid iron crystallizes out from the molten core.

Today, the inner core is about 2,440 kilometres across, 1,000 kilometres smaller than the Moon. But it is still growing. The iron is crystallizing at a rate of about 800 tonnes a second. That releases a considerable amount of latent heat, which passes through the liquid outer core, contributing to the churning of the fluid within it. As the iron or iron-nickel alloy crystallizes out, impurities within the melt, mostly dissolved silicates, separate out. This material is less dense than the molten outer core, so it rises through it in a steady rain of perhaps sand-like particles. It probably accumulates on the base of the mantle like a sort of upside-down sedimentation, collecting in upside-down valleys and depressions. There are seismic hints of a very low velocity layer at the base of the mantle that this upward sedimentation could explain. The sandy sediment would trap molten iron just as ocean sediment traps water. By holding iron within it, the layer provides material that can magnetically couple the magnetic field generated in the core with the solid mantle. If some of this material rises in super-plumes to contribute to flood basalts on the surface, it could explain the high concentrations of precious metals such as gold and platinum in such rocks.

Like the Earth as a whole, the inner core is rotating, but not exactly in the same way as the rest of the Earth. It is in fact rotating slightly faster than the remainder of the planet, gaining nearly one-tenth of a turn in the past 30 years. Understanding why the inner core is spinning so fast may give insight into what is going on in that strongly magnetic environment. It could be that currents in the outer core, analogous to the jet streams in the atmosphere, are putting a magnetic tug on the inner core.

So far, only about 4% of the total core has frozen. But, in 3 or 4 billion years’ time, the entire core will have solidified and we may lose our magnetic protection.

Vocabulary

asthenosphere – геол. астеносфера (земли)
bulge – выпучиваться, выпячиваться, выдаваться
bulk – масса, основная масса
convergent boundaries – сходящиеся границы
core – ядро (небесного тела)
crust – геол. земная кора, поверхностные отложения
D-layer – слой D
depression – геофиз. впадина, депрессия, углубление
divergent boundaries – расходящиеся границы
eruption – извержение (вулкана); выброс, прорыв (пламени, вод)
igneous – геол. изверженный, магматический, вулканический
impurity – примесь, загрязнение
indent – вдавливать, делать зазубрины
lead – свинец
lithosphere – геофиз. литосфера
magnesium silicate – силикат магния (белый пигмент)
mantle – мантия (часть внутренней структуры земного шара)
melt – расплав
Mohorovičić discontinuity (Moho) – слой Мохоровичича (Мохо) (назван так в честь сербского сейсмолога Мохоровичича)
oceanic trench – океаническая впадина
outpouring – выливание
olivine – минерал оливин, хризолит, перидот
peridotite – минерал перидотит
plume – тепломассопоток, плюм, струя, шлейф
rift – трещина, расселина, разлом
sediment – отстоявшийся слой, осадок
sedimentation – осаждение, оседание
seismic wave – сейсмическая волна
sideriphil – сидерофил (что-л., имеющее тенденцию к поглощению железа)
slab – плита
squeeze – нефт. закупоривающий материал
subduction – геол. субдукция, подвиг, пододвигание (одной тектонической плиты под другую)
sulphur – сера
tungsten – вольфрам
tug – рывок, тянущее усилие
xenolith – минерал ксенолит

     

Exercise 7. Translate into Russian/ Belarusian.

1. The surface of the Earth is covered by a relatively thin, cold, hard crust. Beneath the oceans it is about 7 or 8 kilometres thick; in the continents, 30 to 60 kilometres thick. At its base lies the Mohorovičić discontinuity or Moho, a layer which reflects seismic waves, probably as a result of a change in composition to the dense rocks of the mantle beneath.
2. The lithosphere, the complete slab of cold, hard material on the Earth’s surface, includes not only the crust but the top of the mantle as well. In total, the continental lithosphere may be 250 or even 300 kilometres thick. It thins under the oceans, as you approach the mid-ocean ridge system, down to little more than the 7-kilometre crust.
3. The lithosphere is not a single rigid layer, however. It is split into a series of slabs called tectonic plates. They are our principal clue as to how the deep Earth works. To understand what’s going on, we must probe beneath the crust.
4. No mine can tunnel that deep. Attempts at deep drilling on land on Russia’s Kola Peninsula and in Germany had to be abandoned after about 11,000 metres. Not only was the rock difficult to drill, but the heat and pressure tended to soften the drill components and squeeze the hole shut again as soon as it was drilled.
5. There is one way in which we can sample the mantle directly: in the outpourings of deep-rooted volcanoes.
6. Violent volcanic eruptions do sometimes carry in their magma more direct samples of mantle rocks. These so-called xenoliths are samples of mantle rock that have not been melted, just carried along in the flow. They are typically dark, dense, greenish rocks such as peridotite, rich in the mineral olivine, a magnesium/iron silicate.
7. Recent analysis of seismic data from around the world has revealed a thin layer at the base of the mantle, the D" layer, up to 200 kilometres thick. It is not a continuous layer but seems more like a series of slabs, a bit like continents on the underside of the mantle. This could be regions where silicate rocks in the mantle are partly mixed with iron-rich material from the core.
8. So there may be ridges and valleys like upside-down mountain ranges on the base of the mantle. There seems to be a great depression in the core beneath the Philippines that is 10 kilometres deep, twice the depth of the Grand Canyon. Bulging up beneath the Gulf of Alaska is a high spot on the core; a liquid mountain taller than Everest. Maybe sinking cold material indents the core, while hotspots bulge up.
9. We have no direct experience or samples of the Earth’s core. But we do know from seismic waves that the outer part of it is liquid and only the inner core is solid. We also know that the core has a much higher density than the mantle. The only material that is dense enough and sufficiently abundant in the solar system to make up the bulk of the core is iron. Although we do not have samples of the Earth’s core, we do have pieces of something that’s likely to be similar, in iron meteorites.
10. An iron core must have formed in the Earth by gravitational separation from the silicate mantle when the new Earth was at least partially molten. As the layers separated, so-called siderophile elements such as nickel, sulphur, tungsten, platinum, and gold that are soluble in molten iron would have separated with them. Lithophile elements would have been held back by the silicate mantle. Radioactive elements such as uranium and hafnium are lithophile, whereas their decay products, or daughters, are isotopes of lead and tungsten so would have been separated out into the core at its formation. That consequently reset the radioactive clock in the mantle at the time the core formed.
11. Continental crust is less dense, granite-type rock, while the oceanic crust is denser basaltic rock. Both types of crust behave as if they were floating on the mantle, which is denser than either type of crust.

Exercise 8. Translate into English.

1. Астрономы изучают космос, получают информацию о планетах и звездах, несмотря на их огромную удаленность. При этом на самой Земле не меньше тайн, чем во Вселенной. И сегодня ученые не знают, что внутри нашей планеты. Наблюдая, как выливается лава при извержении вулкана, можно подумать, что внутри Земля тоже расплавленная. Но это не так.
2. Центральная часть земного шара называется ядром. Его радиус составляет около 3500 км. Ученые полагают, что внешняя часть ядра находится в расплавленно-жидком состоянии, а внутренняя – в твердом. Температура в нем достигает +5000°С. От ядра к поверхности Земли температура и давление постепенно снижаются.
3. Ядро Земли покрыто мантией. Ее толща составляет приблизительно 2 900 км. Мантию, как и ядро, никто никогда не видел. Но предполагают, что чем ближе к центру Земли, тем давление в ней выше, а температура – от нескольких сотен до -2500°С. Считают, что мантия твердая, но одновременно и раскаленная.
4. Поверх мантии наша планета покрыта корой. Это верхний твердый слой Земли. По сравнению с ядром и мантией земная кора очень тонкая. Ее толща составляет лишь 10–70 км. Но это та земная твердь, по которой мы ходим, текут реки, на ней построены города.
5. Земная кора образована различными веществами. Она состоит из минералов и горных пород (гранит, песок, глина, торф и др.). Минералы и горные породы различаются по цвету, твердости, строению, температуре плавления, растворимости в воде и другим свойствам. Многие из них человек широко использует, например, как топливо, в строительстве, для получения металлов.
6. Толщина земной коры (внешней оболочки) изменяется от нескольких километров (в океанических областях) до нескольких десятков километров (в горных районах материков). Сфера земной коры очень небольшая, на ее долю приходится всего около 0,5 % общей массы планеты.
7. На долю мантии приходится около 67 % общей массы планеты. Твердый слой верхней мантии, распространяющийся до различных глубин под океанами и континентами, совместно с земной корой называют литосферой –- самой жесткой оболочкой Земли. Под ней отмечен слой, где наблюдается некоторое уменьшение скорости распространения сейсмических волн, что говорит о своеобразном состоянии вещества. Этот слой, менее вязкий и более пластичный по отношению к выше- и нижележащим слоям, называют астеносферой. Считается, что вещество мантии находится в непрерывном движении.
8. Земное ядро разделяется на 2 отдельные области: жидкую (внешнее ядро) и твердую (внутреннее), переход между ними лежит на глубине 5156 км. По современным данным внешнее ядро представляет собой вращающиеся потоки расплавленного железа и никеля, хорошо проводящие электричество. Именно с ним связывают происхождение земного магнитного поля. Считается, что электрические токи, текущие в жидком ядре, создают глобальное магнитное поле.
9. Слой мантии, находящийся в соприкосновении с внешним ядром, испытывает его влияние, поскольку температуры в ядре выше, чем в мантии. Местами этот слой порождает огромные, направленные к поверхности Земли тепломассопотоки – плюмы.
10. Земная кора – внешняя твердая оболочка (кора) Земли, верхняя часть литосферы. Ниже коры находится мантия, которая отличается составом и физическими свойствами – она более плотная, содержит в основном тугоплавкие элементы. Разделяет кору и мантию граница Мохоровичича, или сокращенно Мохо, на которой происходит резкое увеличение скоростей сейсмических волн. С внешней стороны большая часть коры покрыта гидросферой, а меньшая находится под воздействием атмосферы. Земная кора схожа по структуре с корой большинства планет земной группы, за исключением Меркурия. Кроме того, кора схожего типа есть на Луне и многих спутниках планет-гигантов. При этом Земля уникальна тем, что обладает корой двух типов: континентальной и океанической. Масса земной коры оценивается в 2,8•10¹⁹ тонн (из них 21 % – океаническая кора и 79 % – континентальная). Кора составляет лишь 0,473 % общей массы Земли. В большинстве случаев кора состоит из базальтов. Для земной коры характерны постоянные движения: горизонтальные и колебательные.
11. Характерная черта эволюции Земли – дифференциация вещества, выражением которой служит оболочечное строение нашей планеты. Литосфера, гидросфера, атмосфера, биосфера образуют основные оболочки Земли, отличающиеся химическим составом, мощностью и состоянием вещества.

Exercise 9. Translate the text using PROMT (Déjà Vu, TRADOS). Analyze the translation and make corrections.

History of Earth’s Interior

(by Dr. F.Borrero et al. from Earth Science : Geology, the Environment and the Universe)

Many of the properties and characteristics of Earth, including the structure of its interior, can be explained from current theories of how it formed and evolved. Here is the theoretical summary of how Earth's interior was formed, discussed as if it were a fact. Keep in mind, however, that the following is all conjecture, even if it is conjecture based on facts.

In brief, Earth is considered to have formed about 4.6 billion years ago in a rotating disk of particles and grains that had condensed around a central protosun. The condensed rock, iron, and mineral grains were pulled together by gravity, growing eventually to a planet-sized mass. Not all the bits and pieces of matter in the original solar nebula were incorporated into the newly formed planets. They were soon being pulled by gravity to the newly born planets and their satellites. All sizes of these leftover bits and pieces of matter thus began bombarding the planets and their moons. Evidently, the bombardment was so intense that the heat generated by impact after impact increased the surface temperature to the melting point. Evidence visible on the Moon and other planets today indicates that the bombardment was substantial as well as lengthy, continuing for several hundred million years. Calculations of the heating resulting from this tremendous bombardment indicate that sufficient heat was liberated to melt the entire surface of Earth to a layer of glowing, molten lava. Thus, the early Earth had a surface of molten lava that eventually cooled and crystallized to solid igneous rocks as the bombardment gradually subsided, then stopped.

Then Earth began to undergo a second melting, this time from the inside. The interior slowly accumulated heat from the radioactive decay of uranium, thorium, and other radioactive isotopes. Heat conducts slowly through great thicknesses of rock and rock materials. After about 100 million years or so of accumulating heat, parts of the interior became hot enough to melt into pockets of magma. Iron and other metals were pulled from the magma toward the center of Earth, leaving less dense rocks toward the surface. The melting probably did not occur all at one time throughout the interior but rather in local pockets of magma. Each magma pocket became molten, cooled to a solid, and perhaps repeated the cycle numerous times. With each cyclic melting, the heavier abundant elements were pulled by gravity toward the center of Earth, and additional heat was generated by the release of gravitational energy. Today, Earth's interior still contains an outer core of molten material that is predominantly iron. The environment of the center of Earth today is extreme, with estimates of pressures up to 3.5 million atmospheres (3.5 million times the pressure of the atmosphere at the surface). Recent estimates of the temperatures at Earth’s core are about the same as the temperature of the surface of the Sun, about 6,000°C (11,000°F).

The melting and flowing of iron to Earth's center were the beginnings of differentiation, the separation of materials that gave Earth its present-day stratified or layered interior. The different crystallization temperatures of the basic minerals further differentiated the materials in Earth’s interior.

Exercise 10. Write a précis and an annotation of the texts given below in Russian/ Belarusian.

Earth’s Internal Structure

(by Dr. F.Borrero et al. from Earth Science : Geology, the Environment and the Universe)

The theoretical formation of Earth and the layered structure of its interior are supported by indirect evidence from measurements of vibrations in Earth, Earth's magnetic field, gravity, and heat flow. First, we will consider how vibrations tell us about Earth's interior.

If you have ever felt vibrations in Earth from a passing train, an explosion, or an earthquake, you know that Earth can vibrate. In fact, a large disturbance such as a nuclear explosion or really big earthquake can generate waves that pass through the entire Earth. A vibration that moves through any part of Earth is called a seismic wave. Geologists use seismic waves to learn about Earth's interior.

Seismic waves radiate outward from an earthquake, spreading in all directions through the solid Earth's interior as do sound waves from an explosion. There are basically three kinds of waves:

1. A longitudinal (compressional) wave called a P-wave. P-waves are the fastest and move through surface rocks and solid and liquid materials below the surface. The P stands for primary.

2. A transverse (shear) wave called an S-wave. The S stands for secondary. S-waves are second fastest after the P-waves. S-waves do not travel through liquids

Using data from seismic waves, scientists were able to determine that the interior of Earth can be broken down into three zones. The crust is the outer layer of rock that forms a thin shell around Earth. Below the crust is the mantle, a much thicker shell than the crust. The mantle separates the crust from the center part, which is called the core.

THE CRUST

Seismic studies have found that Earth's crust is a thin skin that covers the entire Earth, existing below the oceans as well as making up the continents. According to seismic waves, there are differences between the crust making up the continents and the crust beneath the oceans. These differences are that (1) the oceanic crust is much thinner than the continental crust and (2) seismic waves move through the oceanic crust faster than they do through continental crust. The two types of crust vary because they are made up of different kinds of rock.

The boundary between the crust and the mantle is marked by a sharp increase in the velocity of seismic waves as they pass from the crust to the mantle. Today, this boundary is called the Mohorovičić discontinuity, or the "Moho" for short. The boundary is a zone where seismic P-waves increase in velocity because of changes in the composition of the materials. The increase occurs because the composition on both sides of the boundary is different. The mantle is richer in ferromagnesian minerals and poorer in silicon than the crust.

Studies of the Moho show that the crust varies in thickness around Earth's surface. It is thicker under the continents and thinner under the oceans.

The age of rock samples from Earth's continents has been compared with the age samples of rocks taken from the seafloor by oceanographic ships. This sampling has found the continental crust to be much older, with parts up to 3.8 billion years old. By comparison, the oldest oceanic crust is less than 200 million years old.

Comparative sampling also found that continental crust is a less dense, granite-type rock with a density of about 2.7 g/cm³. Oceanic crust, on the other hand, is made up of basaltic rock with a density of about 3.0 g/cm³. The less dense crust behaves as if it were floating on the mantle, much as less dense ice floats on water. There are exceptions, but in general, the thicker, less dense continental crust "floats" in the mantle above sea level, and the thin, dense oceanic crust "floats" in the mantle far below sea level.

THE MANTLE

The middle part of Earth's interior is called the mantle. The mantle is a thick shell between the core and the crust. This shell takes up about 80 percent of the total volume of Earth and accounts for about two-thirds of Earth's total mass. Information about the composition and nature of the mantle comes from (1) studies of seismological data, (2) studies of the nature of meteorites, and (3) studies of materials from the mantle that have been ejected to Earth's surface by volcanoes. The evidence from these separate sources all indicates that the mantle is composed of silicates, predominantly the ferromagnesian silicate olivine. Meteorites, as discussed in chapter 15, are basically either iron meteorites or stony meteorites. Most of the stony meteorites are silicates with a composition that would produce the chemical composition of olivine if they were melted and the heavier elements separated by gravity. This chemical composition also agrees closely with the composition of basalt, the most common volcanic rock found on the surface of Earth.

THE CORE

Information about the nature of the core, the center part of Earth, comes from studies of three sources of information: (1) seismo-logical data, (2) the nature of meteorites, and (3) geological data at the surface of Earth. Seismological data provide the primary evidence for the structure of the core of Earth. Seismic P-waves spread through Earth from a large earthquake. However, there are places between 103° and 142° of arc from the earthquake that do not receive P-waves. This region is called the P-wave shadow zone, since no P-waves are received here. The P-wave shadow zone is explained by P-waves being refracted by the core, leaving a shadow. The paths of P-waves can be accurately calculated, so the size and shape of Earth's core can also be accurately calculated.

Seismic S-waves leave a different pattern at seismic receiving stations around Earth. Recall that S-waves (sideways or transverse) can travel only through solid materials. An S-wave shadow zone also exists and is larger than the P-wave shadow zone. S-waves are not recorded in the entire region more than 103° away from the epicenter. The S-wave shadow zone seems to indicate that S-waves do not travel through the core at all. If this is true, it implies that the core of Earth is a liquid or at least acts as a liquid.

Analysis of P-wave data suggests that the core has two parts: a liquid outer core and a solid inner core. Both the P-wave and S-wave data support this conclusion. Overall, the core makes up about 15 percent of Earth's total volume and about one-third of its mass.

Exercise 11. Write an annotation of the text given below in Russian/ Belarusian.

Earth’s Interior

(by Dr. F. Borrero et al. from Earth Science : Geology, the Environment and the Universe)

What is deep inside Earth? What you can see – the rocks, minerals, and soil on the surface – is a thin veneer. Nothing has ever been directly observed below this veneer, however. The deepest mine has penetrated to depths of about 3 km (about 2 mi), and the deepest oil wells may penetrate down to about 8 km (about 5 mi). But Earth has a radius of about 6,370 km (about 3,960 mi). How far have the mines and wells penetrated into Earth? By way of analogy, consider the radius of Earth to be the length of a football field, from one goal line to the other. The deep mine represents progress of 4.3 cm (1.7 in) from one goal line. The deep oil well represents progress of about 11.5 cm (about 4.5 in). It should be obvious that human efforts have only sampled materials directly beneath the surface. What is known about Earth's interior was learned indirectly, from measurements of earthquake waves, how heat moves through rocks, and Earth's magnetic field.

Indirect evidence suggests that Earth is divided into three main parts – the crust on the surface, a rocky mantle beneath the crust, and a metallic core. The crust and the uppermost mantle can be classified on a different basis, as a rigid layer made up of the crust and part of the upper mantle, and as a plastic, movable layer below in the upper mantle.

Understanding that Earth has a rigid upper layer on top of a plastic, movable layer is important in understanding the concepts of plate tectonics. Plate tectonics describes how the continents and the seafloor are moving on giant, rigid plates over the plastic layer below. This movement can be measured directly. In some places, the movement of a continent is about as fast as your fingernail grows, but movement does occur.

Understanding that Earth's surface is made up of moving plates is important in understanding a number of Earth phenomena. These include earthquakes, volcanoes, why deep-sea trenches exist where they do, and why mountains exist where they do. This chapter is the "whole Earth" chapter, describing all of Earth's interior and the theory of plate tectonics. A bit of indirect information about Earth's interior, a theory of plate tectonics, and the observation of a number of related Earth phenomena all fit together. You can use this concept to explain many things that happen on the surface of Earth.

Exercise 12. Write a précis and an annotation of the text below in English.

Внутреннее строение Земли

(www.geographya.ru)

Земля, так же, как и многие другие планеты, имеет слоистое внутреннее строение. Наша планета состоит из трех основных слоев. Внутренний слой – это ядро, наружный – земная кора, а между ними размещена мантия.

Ядро представляет собой центральную часть Земли и расположено на глубине 3000 – 6000 км. Радиус ядра составляет 3 500 км. По мнению ученых, ядро состоит из двух частей: внешней – вероятно, жидкой, и внутренней - твердой. Температура ядра составляет около 5 000 градусов. Современные представления о ядре нашей планеты получены в ходе длительных исследований и анализа полученных данных. Так, доказано, что в ядре планеты содержание железа достигает 35 %, что обусловливает его характерные сейсмические свойства. Внешняя часть ядра представлена вращающимися потоками никеля и железа, которые хорошо проводят электрический ток.

Происхождение магнитного поля Земли связано именно с этой частью ядра, так как глобальное магнитное поле создается электрическими токами, протекающими в жидком веществе внешнего ядра. Из-за очень высокой температуры внешнее ядро оказывает значительное влияние на соприкасающиеся с ним участки мантии. В некоторых местах возникают громадные тепломассопотоки (плюмы), направленные к поверхности Земли. Внутреннее ядро Земли твердое и также имеет высокую температуру. Ученые полагают, что такое состояние внутренней части ядра обеспечивается очень высоким давлением в центре Земли, достигающим 3 млн атмосфер. При увеличении расстояния от поверхности Земли повышается сжатие веществ, при этом многие из них переходят в металлическое состояние.

Промежуточный слой – мантия – покрывает ядро. Мантия занимает около 80 % объема нашей планеты, это самая большая часть Земли. Мантия расположена кверху от ядра, но не достигает поверхности Земли, снаружи она соприкасается с земной корой. В основном вещество мантии находится в твердом состоянии, кроме верхнего вязкого слоя толщиной примерно 80 км. Это астеносфера, что в переводе с греческого языка означает слабый шар. По мнению ученых, вещество мантии непрерывно движется. При увеличении расстояния от земной коры в сторону ядра происходит переход вещества мантии в более плотное состояние.

Снаружи мантию покрывает земная кора – внешняя прочная оболочка. Ее толщина варьирует от нескольких километров под океанами до нескольких десятков километров в горных массивах. На долю земной коры приходится всего 0,5 % общей массы нашей планеты. Часть земной коры, простирающаяся до глубин, доступных для геологического изучения, образует недра Земли, которые требуют особой охраны и разумного использования. В состав коры входят оксиды кремния, железа, алюминия, щелочных металлов. Континентальная земная кора делится на три слоя: осадочный, гранитный и базальтовый. Океаническая земная кора состоит из осадочного и базальтового слоев.

Литосферу Земли формирует земная кора вместе с верхним слоем мантии. Литосфера слагается из тектонических литосферных плит, которые как будто «скользят» по астеносфере со скоростью от 20 до 75 мм в год. Двигающиеся друг относительно друга литосферные плиты различны по величине, а кинематику передвижения определяет тектоника плит.

Согласно современной теории литосферных плит вся литосфера узкими и активными зонами – глубинными разломами – разделена на отдельные блоки, перемещающиеся в пластичном слое верхней мантии относительно друг друга со скоростью 2–3 см в год. Эти блоки называются литосферными плитами.

Особенность литосферных плит – их жесткость и способность при отсутствии внешних воздействий длительное время сохранять неизменными форму и строение.

Литосферные плиты подвижны. Их перемещение по поверхности астеносферы происходит под влиянием конвективных течений в мантии. Отдельные литосферные плиты могут расходиться, сближаться или скользить друг относительно друга. В первом случае между плитами возникают зоны растяжения с трещинами вдоль их границ, во втором – зоны сжатия, сопровождаемые надвиганием одной плиты на другую (надвигание – обдукция; поддвигание – субдукция), в третьем – сдвиговые зоны – разломы, вдоль которых происходит скольжение соседних плит.

В местах схождения континентальных плит происходит их столкновение, образуются горные пояса. Так возникла, например, на границе Евразийской и Индо-Австралийской плиты горная система Гималаи.

В результате столкновения континентальной и океанической литосферных плит образуются глубоководные желоба и островные дуги.

Для осевых зон срединно-океанических хребтов характерны рифты (от англ. rift – расщелина, трещина, разлом) – крупная линейная тектоническая структура земной коры протяженностью в сотни, тысячи, шириной в десятки, а иногда и сотни километров, образовавшаяся главным образом при горизонтальном растяжении коры. Очень крупные рифты называются рифтовыми поясами, зонами или системами.

Так как литосферная плита представляет собой единую пластину, то каждый ее разлом – это источник сейсмической активности и вулканизма. Эти источники сосредоточены в пределах сравнительно узких зон, вдоль которых происходят взаимные перемещения и трения смежных плит. Эти зоны получили название сейсмических поясов. Рифы, срединно-океанические хребты и глубоководные желоба являются подвижными областями Земли и располагаются на границах литосферных плит. Это свидетельствует о том, что процесс формирования земной коры в этих зонах в настоящее время происходит очень интенсивно.

Химический состав Земли схож с составом других планет земной группы, например Венеры или Марса. В целом преобладают такие элементы, как железо, кислород, кремний, магний, никель. Содержание легких элементов невелико. Средняя плотность вещества Земли 5,5 г/см³.