Man's physical universe

xanabras

;

UNIT III

SECTION 7

GEOLOGICAL PROCESSES PRODUCE MANY DIFFERENT

KINDS OF ROCKS

Introduction.

For half a million years or so, early Eolithic man was content with

the improvisation of natural stones for use as hammers or knives.

no longer live in the Stone Age,^ but it is probable that more stone is

being used by modern man than ever before in man's history, because

of the refined tools which we have available for handling rock.

The solid portion of the earth is made up of three classes of rocks:

igneous, sedimentary, and metamorphic.

Igneous rocks were produced by high temperatures; sedimentary

rocks are those which were produced by deposition (by water or wind)

and metamorphic rocks are produced by changes in the above two

types of rocks.

Igneous Rock Was Molten at One Time.

Igneous rock was produced by high temperatures. If the earth

was molten at one time, all of this molten rock was the original rock.

Most of the igneous rock of today, however, undoubtedly represents

later forms of rock which were liquefied below the surface and returned

to the surface again by lava flows, volcanic activities, or isostatic

readjustments.

This molten rock, called "magma," generally contains superheated

steam and other gases, whose rapid expansion in a volcanic eruption

causes the magma to spray out into the air, where it cools quickly to

form volcanic dust. When the slower bodies of magma work their way

up through fissures in the rocks, they solidify to form layers of lava

on or beneath the surface.

Pumice is formed by the cooling of the froth produced in the magma

by the rapid expansion of dissolved gases as the result of the lowering

of pressure on reaching the surface. If certain lavas cool quickly,

volcanic glass or "obsidian" is formed, as has been previously mentioned.

Slower cooling produces fine-grained, igneous rocks. The grain

1 Except perhaps mentally and in some places politically.

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