Rocks are the building blocks of the earth's crust. They may be
massive, as in granite ledges, or tiny. Soil, gravel, sand and clay
are rocks. THERE ARE THREE MAIN TYPES OF ROCKS.
1. IGNEOUS rocks are those formed at very high temperatures or from
molten materials. They come from magmas--molten mixtures of minerals,
often containing gases. They come from deep below the surface of the
earth. If they cool off while below the surface, they form intrusive
rocks, which may later be revealed by erosion. When magmas reach the
surface red hot, they form extrusive rocks, such as volcanic rocks.
Thus, granite is an igneous, intrusive rock; lava is an igneous,
extrusive rock. (Notice how the type of rock tells its past
history--if you know what to look for.)
2. SEDIMENTARY rocks are formed by the action of wind, water, or
organisms. They cover about three quarters of the Earth's surface.
Most are laid down--as sediments--on the bottom of rivers, lakes
and seas. Many have been moved by water, wind, waves, currents,
ice or gravity. The most common sedimentary rocks are sandstones,
limestones, conglomerates and shales. Oil is found in sedimentary
formations.
3. METAMORPHIC rocks are those that have been changed from what
they were at first into something else--by heat, pressure, or
chemical action. All kinds of rocks can be changed. The result
is a new crystalline structure, the formation of new minerals,
or a change in the rock's texture. Slate was once shale. Marble
came from limestone. Gneiss (pronounced "nice") is perhaps
reworked granite.
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Igneous rocks are formed at high temperatures or from molten
materials. They come from deep beneath the earth. They can be
intrusive or extrusive--depending on where they cooled off.
Sedimentary rocks are formed by the action of wind, water, or
organisms. They usually are laid down on the bottom of rivers,
lakes and seas. Most of the earth's surface is covered by these
rocks. Oil is found in sedimentary formations.
Metamorphic rocks have been changed from their original state
into something else. Heat, pressure, chemical action change the
crystalline structure, the texture, even form new minerals. All
kinds of rock can be changed.