TEXT 2 Occurrence of sedimentary rocks

Date: 2015-10-07; view: 557.

Read the text and fulfill the after reading exercises (R.P – 6.2)

Estimates of the volumes for the major rock types include in the order about 10% sedimentary strata, which occur as a nearly sphere relatively thin surficial covering of both continental and ocean floors. Sedimentary units are of great economic importance, for in them is gathered much of the world's readily extractable mineral ores, including coal, petroleum, natural gas, nuclear fuels.

Sediments are accumulations of materials that have been formed by surficial processes from preexisting rocks of any type distinguishing between clastic sediments or detritus, the products of mechanical accumulation of individual grains; and chemical sediments –materials that are precipitated from inorganic solutions. As sedimentary rocks result from the following processes:

1. weathering of source materials;

2. transportation (commonly in running water);

3. deposition by organic or inorganic means;

4. lithification, that is, compaction and cementation whereby the original unconsolidated sediment is converted into a more cohesive, less porous rock.