"ROCKS" see How A Story Ends

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   . Story Or Knowledge Of a Simple Rock.

Rocks are hard, natural masses of sol id

matter that make up the earth' s crust. With a

few exceptions (such as coal) , rocks are

composed of one or more mineral s.

Geologists classify rocks as either igneous,

sedimentary, or metamorphic, depending on

how they were formed.

Igneous rocks are formed from magma,

the molten matter deep within the earth.

There are two types of igneous rock. If

magma rises toward the surface, it slowly

cools and sometimes solidifies underground.

The result is i ntrusive igneous rock. If

magma reaches the earth 's surface, it

emerges from volcanoes or fissures (cracks)

as lava. Lava cools rapidly aboveground,

solidifying into extrusive i gneous rock.

Intrusive igneous rock. such as granite, can

be identified by its large, clearly visible

mineral grains (crystals). Because extrusive

rock solidifies more quickly than intrusive

igneous rock, it is characterized by tiny

crystals. Basalt, with its fine texture, high

density, and dark color, is the most common

extrusive igneous rock, lying beneath the

vast ocean floor. Pumice, another common

extrusive igneous rock used in some

abrasives, acquires its rough porous texture

from the explosive release of gas that often

accompanies volcanic eruptions.

Virtually all sedimentary rocks are

formed when particles, known as sediments,

acc umul ate in s t rata (layers). Most

sediments are created when rocks of any

kind are broken down by erosion or

weathering. When these particles cement or

compact together· and harden, they form

sedimentary rock. Shale, the most common

sedimentary rock, is formed from mud and

clay ; sandstone, as its name suggests, is

formed from sand. Some sediments,

however, are created from animal or plant

remains that have decayed or decomposed

in water. Most limestone, for example, is

formed from the minerals of decomposed

shells or skeletons of marine organisms,

while coal is formed from plants that have

decayed in swamps . Sedimentary rock

usually forms under water. It can frequently

be identified by characteristic layers or by

parti cles of differen t sizes, and often

contains fossils.

Metamorphic rocks are formed when

rocks of any type are changed by long

periods of intense heat or pressure within

the e arth . This process, known as

metamorphism, alters the texture, structure,

and mineral composition of the existing

rock, usually making it rougher and more

dense. Metamorphic rock can sometimes be

identified by its distorted structure, or by

wavy bands. When the sedimentary rock

l imestone undergoes me tamorphism, it

becomes marble. Shale, another sedimentary

rock, becomes slate under metamorphism,

while the igneous rock granite becomes

gneiss.

As hard as they are, rocks do not last

forever. Rocks above ground are contin-

uously exposed to weathering and erosion.

Over thousands or even millions of years,

they are broken down and worn away to

sediments, which can l ater form new

sedimentary rocks. Rocks below ground can

also change . Any rock s ubj ected to

sufficient heat and pressure undergoes

metamorphism and forms new metamorphic

rock. And if the heat is great enough, any

rock can be melted back into magma and

later form new igneous rock. Thus, any type

of rock can be transformed into one of the

others. This dynamic, never-ending process

of rock formation is known as the rock

cycle.