
- •Contents
- •Text 1
- •Text 2
- •Text 3
- •Violent – інтенсивний
- •Text 4
- •Text 5
- •Text 6
- •Text 7
- •Text 8
- •Velocity – швидкість
- •In the course of development there will be some surprises, and there will modify the earlier decisions.
- •Text 9
- •Text 10
- •Text 11
- •Text 12
- •Volatile – леткий, непостійний
- •Text 13
- •Text 14
- •Text 15
- •Text 16
- •Test yourself
- •Текст 17
- •References
Text 4
extinct – вимерлий, поганий
ultimate – основний, критичний
break – тріщина, прогин
crumble – кришитися, обсипатися
boulder – валун; галька, голиш; брила
clastic – кластичний
precipitate – осаджувати (-ся), відмулювати (-ся)
abundant – рясний, багатий; поширений
conceivable – можливий, імовірний
After lavas, the rocks with the most obvious origin are sedimentary rocks. The commonest of these are compacted and cemented accumulations of gravel, sand, mud (silt and clay), or chemical precipitates – often containing shells, bones, or vegetal remains – that were deposited by water, wind, or ice.
Because the accumulation of sediment is almost never a steady process, most sedimentary rocks are stratified. Individual layers may be tens of feet thick, as in the Alaskan coal, or less than an inch thick. A rock composed almost entirely of extinct marine shells is a sample of the more unusual compositions found among sedimentary rocks.
The ultimate resting place of most sediment is on the bottoms of the world’s oceans, seas, and bays. Lesser quantities accumulate in river valleys, desert basins, and along the bases of steep mountains. When such sediments become so compacted and cemented that they break rather than crumble when struck with a hammer, or produce pebbles and boulders that remain coherent instead of disintegrating when moved, they are sedimentary rocks. Conditions intermediate between loose sediment and hard sedimentary are common; the coherence often varies greatly within a single rock mass.
Marine sedimentary rocks are far more common and widespread on land today than all other kinds of sedimentary rocks combined. This is one of those simple facts that fairly cry out for explanation and that lie at the heart of man’s continuing effort to understand more fully the changing geography of the geologic past.
All sediments may be divided into three groups. Clastic (or mechanical or detrital: accumulations of flakes and grains and pieces of weathered rock, such as silt, sand, and gravel. Chemical: natural precipitated, such as rock salt and gypsum. Organic: accumulations of organic remains, such as coal or shells. Clastic sediments are about three times as abundant as chemical and organic.
Mud (composed of silt and clay) is by far the most abundant sediment; it is the chief ingredient of about half the total known volume of sedimentary rocks. Sand is second in abundance, accounting for more than half of the rest. Coarse materials such as rubble and gravel are minor contributors, though they are important in many localities. Altogether, these silecious clastic (Greek: klastos, broken) sediments account for about 80 per cent of all sedimentary rocks. The rest are composed of a variety of sediments, chiefly chemical precipitates or of organic origin. These are nonclastic; most of their ingredients at the site of accumulation instead of being transported as the fragments from other sources.
Organic and chemical sediments vary greatly in composition. Coal is an altered and condensed derivative of vegetation preserved in swamps. Rock salt and gypsum are formed from chemical precipitates produced when shallow lagoons or lakes become natural evaporation pans. Limestone, by far the most abundant nonclastic sedimentary rock on the continents consists mainly of calcium carbonate. Many limestones are partly clastic in origin.
Nearly all limestones are marine and most contain well-preserved fossils. Many also contain considerable amounts of calcium magnesium carbonate; if this predominates the rock is technically dolomite.
Limestone had been much used as a building stone and is the chief source of lime as well as the main ingredient of commercial Portland cement.
There is, of course, no reason why clastic and nonclastic sediments may not be deposited more or less simultaneously in the same place. Limestones are frequently interbedded with shale or contain admixed clay, silt, and sand. Rocks representing almost every conceivable mixture of these four materials can be found in nature and are referred to as silty limestone, calcareous shale, and so on.