Rocks
Rocks are always on the move through the rock
cycle!
Click on parts of this rock cycle diagram to find out more about
- sedimentary rocks
- igneous rocks
- metamorphic rocks
and how they change over
geologic time
All rock (except for meteorites!) that is on Earth today is made of the
same stuff as the rocks that dinosaurs and other ancient life forms walked,
crawled or swam over. While the stuff that rocks are made from stays the
same, the rocks do not. Over millions of years, rocks are recycled into
other rocks. Moving tectonic
plates help to destroy and form many types of rocks.
What Is a Sedimentary Rock?
Have you ever been to the beach and nestled your toes in the sand? Over thousands of years that sand might become part of a
sedimentary rock!
Sedimentary rocks make up about three-quarters of the rocks at the
Earth’s surface. They form at the surface in environments such as beaches,
rivers, the
ocean,
and anywhere that sand, mud, and other types of sediment collect.
Sedimentary rocks preserve a record of the environments that existed
when they formed. By looking at sedimentary rocks of different ages,
scientists can figure out how climate and environments have changed
through
Earth’s history. Fossils of ancient living things are preserved in sedimentary rocks too.
Many sedimentary rocks are made from the broken bits of other rocks. These are called
clastic sedimentary rocks.
The broken bits of rocks are called sediment. Sediment is the sand
you find at the beach, the mud in a lake bottom, the pebbles in a river,
and even the dust on furniture. The sediment may, in time, form a rock
if the little pieces become cemented together.
There are other types of sedimentary rocks whose particles do not come from broken rock fragments.
Chemical sedimentary rocks are made of
mineral crystals such as
halite and
gypsum formed by chemical processes. The sediment particles of
organic sedimentary rocks are the remains of living things such as
clamshells, plankton skeletons, dinosaur bones, and
plants.
Igneous Rocks
Igneous rocks form when molten rock cools and becomes solid. Molten rock is called
magma when it is below the Earth’s surface and
lava when it is above.
Igneous rocks are divided into two groups, based on where the rock forms.
Igneous rocks that form below the Earth’s surface are called
intrusive igneous rocks (or plutonic). They form when magma enters an underground chamber, cools very slowly, and forms rocks full of large crystals.
Igneous rocks that form above the Earth’s surface are called
extrusive igneous rocks. These rocks, also called volcanic rocks, form when lava cools quickly at or above the Earth’s surface.
Metamorphic Rocks
Have you have heard that caterpillars metamorphose into butterflies?
Well, rocks can metamorphose too! They don't grow wings like a
butterfly. But they do change! Rocks metamorphose when they are in a
place that is very hot and
pressure
is high. You can find such a place where Earth's tectonic plates are
coming together. There, the colliding plates squish rocks, and hot
pools of magma heat them deep underground.
Some rocks only
change a little, while others
change a lot.
When a rock is metamorphosed, its mineral crystals change. Usually,
the same chemical ingredients are used to form new crystals during
metamorphism. Sometimes new types of minerals grow that weren't in the
rock before.
Often, flat minerals like mica become lined up perpendicular (at a
right angle) to the direction of pressure. When minerals within a
metamorphic rock are organized this way, it is called foliation. Some
metamorphic rocks are
foliated and others are
non-foliated.
Any type of rock, can be metamorphosed. The rocks are changed either in small areas of
contact metamorphism or large areas of
regional metamorphism
Read more
http://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry13.html
