3. Reading comprehension

3.1 Scanning

3.1.1 Rocks from living things

Answer all the questions on the basis of what is stated in the passage. For each question select the best answer A, B, C, D.

Organic sediments produce the rocks we know as coals and limestone.

Coals are rich in carbon derived from swampy vegetation. Coal type varies with the processes involved. Coal formation starts when plants die in wet acid conditions; instead of rotting completely the plants turn to the soft, fibrous substance peat. Later, overlying sediments drive out much moisture and squash the peat, converting it to lignite (soft brown coal). Even greater pressure gives bituminous coal – harder, blacker and with a higher carbon content. The final stage is anthracite – a hard, black, shiny coal with the highest carbon content in the series. Most of the world’s coalmines tap remains of low-lying forests drowned from time to time by an invading sea and buried under sediments.

Limestones are rich in calcium and magnesium carbonates. They make up about 8% of all sedimentary rock; only shale and sandstone are more plentiful. Organic limestones contain calcium carbonate extracted from seawater by plants and animals that used this compound for protective shells. These rocks include reef limestones built up from the stony skeletons of billions of coral polyps and algae inhabiting the beds of shallow seas. Chalk is a white, powdery, porous limestone comprising tiny shells of fossil microorganisms, drifting in the surface waters before they died and rained down on the bottom of the sea.

(David Lambert “The Field Guide to Geology” 1988, Cambridge University Press)

Fig. 30. Coquina

1. What rocks don’t produce organic sediments?

A. coal

B. limestone

C. sandstone

2. What are coals rich in?

A. oxygen

B. carbon

C. hydrogen

3. When do coal formations start?

A. weathering of rocks

B. chemically dissolved minerals

C. plants dying in wet acid conditions

4. What is peat?

A. soft, fibrous substance

B. soft, clay substance

C. soft, sticky substance

5. Lignite is

A. hard black coal

B. soft brown coal

C. debris

6. Which is not coal?

A. anthracite

B. lignite

C. carbon

7. Which is more plentiful?

A. limestone

B. shale and sandstone

C. chalk

8. What is coquina?

A. cemented mass of shelly debris

B. cemented mass of coral polyps

C. cemented mass of stony skeletons

9. Chalk is

A. mudstone

B. sandstone

C. limestone

10. What is the origin of limestone?

A. seawater reefs

B. seawater microorganisms

C. seawater debris

3.1.2 Rocks from chemicals

Read the following text and fulfill the after reading exercises.

Some sedimentary rocks and minerals consist of chemicals that had been once dissolved in water.

Certain limestones formed this way. Oolitic limestone consists of billions of oolites: tiny balls produced by calcium carbonate accumulating on particles rolled around by gentle currents in warm, shallow seas. Oolite forms like this today on the Bahama Banks. Dolomitic limestone (limestone mainly made of the mineral dolomite) occurs where certain brines chemically alter preexisting limestone or where dolomite deposits form in an evaporating sea.

Such so-called evaporites underlie one-quarter of the continents in beds up to 4,000 ft. (1220 m) thick. Evaporites form now where chemical deposits accumulate in evaporating desert takes and coastal salt flats. But certain old evaporites could have been precipitated from chemically oversaturated deep offshore waters of almost landlocked seas such as the Mediterranean.

Three main minerals tend to settle in a sequence. First comes calcium carbonate. Next is gypsum (a granular crystalline form of calcium sulfate combined with water. Then comes sodium chloride in the form of halite (rock salt.) This is a soft, low density rock and liable to flow. Pressure from overlying rock forces up huge plugs / domes of salt beneath the coast of Texas and Louisiana and in parts of Germany, Iran and Russia.

Besides the rocks and minerals just named, there are other chemical deposits. A few have or had important economic uses – particularly borax, chert and flint; certain iron-rich compounds, nitrates and phosphorites. But some of these are partly biological in origin; and scientists disagree about how certain forms of iron occurred.

(David Lambert “The Field Guide to Geology” 1988, Cambridge University Press)

Fig. 31. Rock salt

Fig. 32. A combination of gypsum (light colored) and anhydrite (darker bands)