Rocks are all around us. They make up the backbones of hills and
mountains and the foundations of plains and valleys. Beneath the
soil you walk on and the deep layers of soft mud that cover the
ocean basins is a basement of hard rock.
What are rocks made of?
Rocks are made up mostly of crystals of different kinds of
minerals, or broken pieces of crystals, or broken pieces of
rocks. Some rocks are made of the shells of once-living animals,
or of compressed pieces of plants.
We can learn something about the way a rock formed from by looking carefully at the evidence preserved inside. What a rock is made of, the shapes of the grains or crystals within the rock, and how the grains or crystals fit together all provide valuable clues to help us unlock the rock's history hidden within.
Where do rocks come from?
Rocks are divided into three basic types, igneous,
metamorphic , depending upon how they were formed. Plate tectonics
provides an explanation for how rocks are recycled from igneous
to sedimentary to metamorphic and back to igneous again.
Igneous rocks (from the Greek word for fire) form from when hot,
molten rock (magma) crystallizes and solidifies. The melt originates
deep within the Earth near active plate boundaries or hot spots,
then rises toward the surface. Igneous rocks are divided into two
groups, intrusive or extrusive,
depending upon where the molten rock solidifies.
Extrusive , or volcanic, igneous rock
is produced when magma exits and cools outside of, or very
near the Earth's surface. These are the rocks that form at erupting
volcanoes and oozing fissures. The magma, called lava when molten
rock erupts on the surface, cools and solidifies almost instantly
when it is exposed to the relatively cool temperature of the atmosphere.
Quick cooling means that mineral crystals don't have much time to
grow, so these rocks have a very fine-grained or even glassy texture.
Hot gas bubbles are often trapped in the quenched lava, forming
a bubbly, vesicular texture. Pumice, obsidian, and basalt are all
extrusive igneous rocks.
The cinder cone above and the close up at right are made of basalt.
Intrusive, or plutonic igneous rock forms
when magma is trapped deep inside the Earth. Great globs
of molten rock rise toward the surface. Some of the magma may feed
volcanoes on the Earth's surface, but most remains trapped below,
where it cools very slowly over many thousands or millions of years
until it solidifies. Slow cooling means the individual
mineral grains have a very long time to grow, so they grow to
a relatively large size. Intrusive rocks have a coarse grained texture.
The image at right shows
granite, an intrusive igneous rock.