INFORMATIVE READING- Rocks from fragments

Date: 2015-10-07; view: 491.

3.2.1 Read the text (Part 1, Part 2) and for statements 1-12, choose the best answer: A, B, C or D. Then explain the words in bold.

Part 1

Most sedimentary rocks form from particles eroded from the rocks on land. Their main ingredients are clasts (rock fragments) of quartz, feldspar and clay minerals. These fragments range in size from microscopic grains to boulders.

More than 90% of all sedimentary rock contains particles no bigger than a sand grain. Many geologists classify such particles by size in two main groups. The (fine-grained) lutites with grains less than 0.06 mm diameter produce mudstone, siltstone and shale. The (medium-grained) arenites or sandstones with grains of 0.06-2 mm give arkose, graywacke and orthoquartzite. Here are brief descriptions of six fine- and medium- grained rocks.

1. Mudstone- solid rock made of clay minerals of less than 0.004mm diameter.

2. Shale- mudstone, siltstone or similar fine-grained rock of silt and clay split easily along its bedding planes. Shale accounts for more than 80% of all sedimentary rock.

3. Siltstone-rock formed of particles 0.004-0.06mm in diameter.

4. Orthoquartzite- a “clean” or pure arenite mainly made of quartz after other substances have been removed. (Arenites account for more than 10% of all sedimentary rock)

5. Arkose- an arenite rich in feldspar derived from gneiss or granite.

6. Graywacke- a muddy, often grayish sandstone with mixed-size particles including quartz, clay minerals and others.