Assignments

1. Answer the questions.

1. What is rock to the geologist?

2. What is solid rock?

3. Is oil a rock?

4. When is the identification of rocks easy?

5. What can be used as clues in identification of minerals?

2. Find in the text the English equivalents for the Russian words and expressions. Reproduce the contexts in which they are used.

горная порода, твердый, подразумевать, почва, органический, мелкозернистый, текстура, естественный, дюны (ползучие пески), залежи, петрология

3. Find synonyms among the following words:

man-made, to look for, layer, to change, help, strange, artificial, bed, to search for, odd, stratum, aid, to alter, pure, to eliminate, dirty, to remove

4. Find antonyms among the following words:

alive, smooth, wet, possible, fine-grained, likely, thin, rough, course-grained, unlikely, thick, dead, dry, impos­sible, organic, ancient, inorganic, modern

5. Match the term on the left with its definition on the right.

Man-made	The hard, solid mineral matter that makes up much of the Earth’s crust.
Lava	Manufactured, created, or constructed by human beings.
Rock	A natural accumulation (as of iron ore, coal, or gas).
Coal	A science that deals with the origin, history, occurrence, structure, chemical composition, and classification of rocks.
Deposit	Molten rock that issues from a volcano.
Petrology	A black or brownish black solid rock that is widely used as a natural fuel.

6. Fill in the gaps with the words and expressions from the text.

1. The word _____ rules out the air and other gases, the oceans, rivers, lakes, and other ______.

2. The definition _______ that rocks are large masses of _______, solid material, big enough to form a distinct part of the earth's ______.

3. The rock which forms ______ is the basis for _____ on land.

4. Granite, made mainly of three minerals — ______, ______ and ______, is undoubtedly a rock.

5. Coal and oil ______ are the _______ of ancient plants.

6. ______ means that it is in no way related to life or ______ things.

7. _____, _____ and ______ are not minerals but they are rocks.

8. The study of rocks is ______.

9. The geologist looks for the geologic _______ in which the rock ______.

7. Work in teams and write down false statements based on the text (no fewer than 6 statements). Present them in class. Use the information from the text to refute (опровергнуть) the other team’s false statements.

8. Point out the main ideas of the text. Make a list of them. Sedimentary rocks

The other and perhaps most familiar group of rocks is made of debris, waste and rubble. The geologist speaks of most of them as fragmental or clastic rocks. Clastic rocks cannot be the oldest, for by their very definition they have been formed from other rocks.

Sedimentary rocks provide us with evidence that for at least a billion years — and perhaps more — conditions on the earth's surface were very much the same as they are to­day. An atmosphere contained the sun's heat. Rain, wind, running water, and the chemicals of the air changed the sur­face rocks. This process of weathering and erosion began as soon as the first rocks were formed, and it continues today. In older regions, snow changes into ice and ice forms glaciers, which move slowly over the land, wearing down the rocks and carrying fragments away.

Even raindrops help wear away the rocks. But the water that runs over the land or through the cracks in the rocks wears them out even more. The heat of the sun splits rocks open. Frosts of winter splinter them, too. Plant roots, grow­ing into cracks, swell and push the rocks apart. All these kinds of actions break the rocks into smaller and smaller fragments which are finally carried away by gravity, water and even by the wind. Wherever sand, mud, silt and dust are deposited, they be­come cemented together, forming sedimentary rock.

Sedimentary rocks form in different ways in many places. But the rocks are always the formation of particles or fragments from older rocks. Sometimes fragments are stones and boulders, sometimes pebbles, sometimes fine sand or finer clay. Chemical action in shallow seas and in hot springs may form deposits of even finer chemical sediments. Sedi­mentary rocks make up about 75 per cent of the exposed land surface of the earth. But they make up less than 5 per cent of the total volume of the earth's crust.

Sedimentary rocks are often classified according to the way they had been formed and to the size of the particles in them. Following this classification, we begin with fragments which were broken or worn off from older rocks. When such fragments are of pebbles size or even larger, then cementing they form a rock called conglomerate.

When the rock fragments are smaller, like grains of sand, the rock is called a sandstone. Sandstones may be fine or coarse, hard or soft. Usually they are composed of grains of sand or silica. Sometimes other minerals are present and then the sandstone is given a special name. Arkose, for example, is a sandstone formed under desert conditions. It con­tains grains of feldspar as well as grains of quartz. Sandstones form about 32 per cent of all sedimentary rock.

When the rock particles are still smaller in size, so small that they are difficult to see, the rock is a shale. Shales are rich in clay minerals, though they may contain large amounts of silica. Shales are usually fine-grained, light or dark in color, and they often break in a typical way. Shales are the most common of sedimentary rocks, forming about 46 per cent of all sedimentary rocks known.

Limestone is a sedimentary rock which does not easily fit in with the others. Limestone has many origins. In some cases it is a clastic rock formed of fragments of shells piled on beaches and cemented together by the lime itself. Coquina is coarse limestone made of shelf fragments. Of the sedimentary rocks 22 per cent are limestones.

When clastic fragments are first deposited, nothing holds them together. They may remain in this state for hundreds of thousands of years. Big piles of sand and gravel were deposited 25,000 or more years ago during the great ice age. In only a few places have these begun to cement together.

Rock fragments will cement together faster when water circulates through them, especially if the water contains dissolved chemicals. Lime is such a chemical. Iron rust is another. It gives sandstone and shale a yellowish or reddish color. Silica is a third element, and when sandstones are ce­mented by silica they are likely to be hard and tough.

The other group of sedimentary rocks includes those of chemical origin. In shallow seas, lakes, and caves, and under certain special conditions, lime is deposited chemically. Acid-bearing water trickling out through limestone dissolves some of it. The limestone is re-deposited as the water evaporates in cracks or caves. Such drippings form stalactites and stalagmites. Dolomite, a magnesium limestone, halite (or salt) and gypsum may also be deposited chemically. All they form beds that are large enough to be classified as sedimentary rock.

Finally, plants and animals produce a few sedimentary rocks. Coal deposits begin as peat, a soft organic material that is formed from decaying ferns and swamp plants. As this material is buried by layers of sand and debris, water in the peat is squeezed out and gases are liberated. The amount of carbon gradually increases as the material changes from peat to lignite, a brown coal, and from the brown coal to soft, black coal. Coral and similar animals form deposits of organic lime in reefs.

Sometimes sedimentary rocks give a clue to the kinds of life and conditions which existed millions of years ago when the rocks were formed. Fossils are the remains of life buried in the rock. In nearly every case fossils are formed in sedimentary rock, when plants and animals have been buried under debris. Tough parts of plants and hard parts of animals produce the best fossils. Shells, bones, teeth, leaves, wood and bark are often preserved. Fossils tell the history of the earth and the development of life. In the oldest rock, only the simplest kinds of plants and animals are found. In more recent rocks the plants and animals are different and more complex. All these features are preserved in sedimentary rocks and help to show how they were formed.