Rocks

The solid part of the earth's crust is rock. In general it
is covered by a layer of soil in which plants or trees may
grow. Even the soil is made up of tiny bits of rocks
usually mixed with organic materials from plants and
animals. Rock can also be found beneath the oceans and
under the polar icecaps. 

Rocks and minerals are used in many ways. Builders use
granite, marble, and other rocks in constriction work.
Cement made from limestone and other rocks binds crushed
stone into strong, long-lasting concrete for buildings,
dams, and highways. Metals such as aluminum, iron, lead,
and tin come from rocks that we call ores. Ores also supply
such radioactive elements as radium and uranium. 

Nonmetallic minerals such as borax and graphite can also be
found in rocks. All gems except amber, coral, and pearl,
come from rocks. Diamonds mined in Africa and Arkansas come
from a rock called peridotite. Emeralds are found in black
limestone in Colombia. 

The history of the earth can be traced by studying rocks.
Fossils, or imprints of plants and animals found in rocks,
help scientists learn about the kind of life that existed
millions of years ago. 

The three main kinds of rocks are: igneous rocks,
sedimentary rocks, and metamorphic rocks. Igneous rocks
form when molten or melted rock material called magma,
cools and solidifies. 

There are two types of igneous rocks: extrusive rocks and
intrusive rocks. Extrusive rocks form when magma is
extruded, or forced out, onto the surface of the earth.
This may occur when a volcano erupts great masses of hot
lava, cinders, and ash. Lava is the name for magma that
reaches the surface. Intrusive rocks form from magma that
does not rise all the way to the surface of the earth. It
may push up the surface rock in the shape of a huge
blister. Beneath the surface, the molten rock cools and
hardens slowly. 

Sedimentary rock consists of materials that once were part
of older rocks or of plants and animals. These rocks were
deposited millions of years ago as layers of loose
material. As time passed, the loose materials changed into
solid rocks. 

Metamorphic rock is rock that has changed its appearance
and sometimes its mineral composition. These changes may be
caused by hot magma, pressure and heat from
mountain-building movements in the earth, or the chemical
action of liquids and gases.