A) fill in the gaps.

1. Wegener thought the continents… freely over the mantle.

2. The ocean floors are actually extremely… .

3. Plate tectonics sees the continents as passengers on a number of… … … .

4. These rigid rock plates continually … … the face of the earth.

5. The crust the earth is a completely … structure.

B) note down the terms used by the lecturer.

C) note down the thickness of the asthenosphere.

Part C. After listening Activities

Task 1. Discuss in pairs.

1. What is the asthenosphere?

2. What is the lithosphere distinguished from the asthenosphere in?

Task 2. Summarize the information about continental drift (in writing).

Revision

Ex. 1. Fill in the text with the appropriate word from the box.

Relief form of the earth

The irregularities of the earth’s surface, or its (1) _____ , are divided into major and minor groups. The former are the (2) _____ ; the minor groups consist of (3) _____ as opposed to (4) _____ valleys and basins on the (5) _____ . The (6) _____ height of all the lands above sea level is about 727 m; the (7) _____ depth of the sea – about 3,940 m. The highest (8) _____ of the land, Mt. Everest in the Himalayas, is 8,800 m, the lowest known point in the ocean, in the Pacific, is 9,394 m deep. This makes the greatest (9) _____ in relief – 18,194 m.

The features of the land are (10) _____ into plains, plateaus and mountains. It is of interest that the continents have a tendency to (11) _____ interior basins with mountain (12) _____ as coastal rims, while the ocean (13) _____ , on the contrary, often have deeps near the (14) _____ and submarine ridges or up-swells of the (15) _____ in mid ocean. Often the highest ranges on the (16) _____ of a continent border the important deeps in the ocean (17) _____ . It does not mean, however, that mountains are found only at the continental (18) _____ , since they may extend in a wide zone far into the (19) _____ .

Ex. 2. Translate the text into Russian (in writing).

Earthquake waves

An earthquake, the most destructive of natural phenomena, consists of rapid vibratory motions of rock near the earth's surface. A single shock usually lasts no more than a few seconds, though severe quakes may last for as much as three min; even in such brief times the damage done may be immense. Therapidity of the vibrations rather than the actual displacements involved is responsible for the damage. Man-made structures are shaken to pieces if they are too rigid to follow the back-and-forth motions of the underlying rock, and landslides are common. Widespread fires frequently follow earthquakes in inhabited regions since broken water mains hinder their control. But there is one useful feature of earthquakes: by studying the waves they send оut, it is possible to infer a surprising amount about the nature of the earth’s interior.

Earthquakes

Earthquakes occur without warning. Usually the first shock is the most severe, with disturbances of lessening intensity following at intervals for days or months afterward. A major earthquake may be felt over many thousands of square kilometers, but its destructiveness is limited to a much smaller area.

The great majority of earthquakes are caused by the sudden displacement of crustal blocks along faults. A fault is the scar left by a fracture which occurred when the stresses developed within the crust became too great for the rock to support. Additional stress may accumulate to the point where further slippage takes place, and this slippage in turn sends out the shock waves that are characteristic of an earthquake. The event responsible for an earthquake typically involves an area within the crust some tens of kilometers across located within a few kilometers of the surface, but in a fair number of cases depths of up to several hundred kilome­ters below the crust have been established. The place where an earthquake originates is called its focus, and its epicenter is the point on the earth's surface directly above the focus.

Sensitive instruments called seismographs have been devised which re­spond to the vibrations of even distant earthquakes. Seismographs of different types are needed to respond to vertical and horizontal movements. A vertical seismograph and two horizontal ones, one for the north-south direction and the other for the east-west direction, are needed at each observatory. Several hundred seismological stations are in operation around the world, and the data they obtain are routinely compared and correlated. It is possible to establish from such data where the focus of a given earthquake is located and something about how much energy it has released.

Earthquake severity is usually expressed on the Richter scale, which isbased upon the maximum amplitude of an earthquake's vibration. Each step of 1 on this scale represents a change in vibrational amplitude of a factor of 10 and a change in energy release of a factor of about 30; thus an earthquake of magnitude 5 produces vibrations 10 times larger than one of magnitude 4 and evolves 30 times more energy. An earthquake of magnitude 0 is barely capable of being detected, and the energy released, if it could be concentrated, is just about sufficient to blow up a tree stump. An inhabited area will suffer some damage if a magnitude 4.5 quake occurs nearby, and one of magnitude 6 or more may lead to significant destruction. The energy associated with amagnitude 6 earthquake is equivalent to that of a medium-size nuclear bomb, though its effects are different because the earthquake energy is spread out over a much wider area. The energy released in a magnitude 8.6 earthquake, the greatest that have occurred to date, is about double the energy content of the coal and oil produced each year in the entire world; the Alaska earth­quake of 1964 was of nearly this magnitude.

Of the million or so earthquakes per year strong enough to be experienced as such (that is, of magnitude 2.5 or more), only a small proportion liberate enough energy to do serious damage to man-made structures. About 15 really violent earthquakes (magnitude 7 or more) occur each year on the average, and only 9 of magnitude 8.4 to 8.6 have occurred since 1899. Regions in which severe earthquakes are comparatively frequent include the mountain chains that fringe the Pacific and a broad belt extending from the Mediterranean basin across southern Asia to China. Major earthquakes have occurred sporadically elsewhere, but by far the greatest number has been concentrated in these zones. In or near the earthquake belts lie most of the world’s active volcanoes – which is no coincidence.

Erosion

All the processes by which rocks are worn down and by which the debris is carried away are included in the general term erosion. The underlying cause of erosion is gravity. Such agents of erosion as running water and glaciers derive their destructive abilities from gravity, and gravity is responsible for the transport of removed material to lower and lower elevations. The leveling of landscape by erosion is often referred to as gradation.

Weathering

We have all seen the rough, pitted surfaces of old stone monuments and buildings and the progressive obliteration of their markings. This sort of disintegration, brought about by rainwater and atmospheric gases, is called weathering.

Weathering is in part a chemical process, in part a mechanical process. It participates in an important way by preparing rock material for easy removal by the more active erosional agents. Among the agents whose work is most obvious are streams, glaciers, wind, and waves. Less apparent is the erosional work of groundwater, water in crevices and channels beneath the surface. All these agents are capable of cutting slowly into solid, unweathered rock, but their work is greatly speeded by the disintegration of rocks into the softer material of the weathered layer.

Some of the minerals in igneous and metamorphic rocks are especially susceptible to chemical weathering, since they were formed under conditions very different from those at the earth’s surface and minerals stable under the former conditions are not necessarily stable under the latter. Most sedimentary rocks, on the other hand, consist of rock debris that has already undergone chemical weathering, and so are relatively resistant to further attack; the chief exception is limestone.

Ferromagnesian minerals are readily attacked by atmospheric oxygen with the help of carbonic acid (formed by the solution of carbon dioxide in water) and of organic acids from decaying vegetation. Their iron content changes to rustlike compounds whose red and brown colors are familiar as stains on the surface of rocks containing these minerals. Feldspars and other silicates containing aluminum are in large part altered to clay minerals. Among common sedimentary rocks limestone is most readily attacked by chemical weathering because of the solubility of calcite in carbonic acid. Exposures of this rock can often be identified simply from the pitted surfaces and enlarged cracks that solution produces. Quartz and white mica are extremely resistant to chemical attack and usually remain as lose grains when the rest of a rock is thoroughly decayed. Rocks consisting wholly of silica, like chert and most quartzites, are practically immune to chemical weathering.

Mechanical weathering is often aided by chemical attack; not only is the structure of a rock weakened by the decomposition of its minerals, but fragments are actively wedged apart because the chemical changes in a mineral grain usually result in an increased volume. The most effective process of mechanical disintegration that does not involve chemical action is the freezing of water in crevices, since water expands when it turns into ice and considerable forces can be developed in this way. Just as water freezing in an automobile engine on a cold night may split the block, so water freezing in tiny cracks is an effective wedge for disrupting rocks. Plant roots aid in rock disintegration by growing and enlarging themselves in cracks.

Weathering processes clothe the naked rock of the earth’s crust with a layer of debris made up largely of clay mixed with rock and mineral fragments. The upper part of the weathered layer, in which rock debris is mixed with decaying vegetable matter, is the soil. From a human point of view the formation of soil is by all odds the most important result of weathering.