| Class and Origin |
|
Common Types |
| Igneous These
rocks are formed from the hot, molten magma which
either comes to the surface as lava, or is cooled
and solidified within the crust. The rate of
cooling and the mineral content of the magma
determine the kind of igneous rock. Rocks with
visible mineral crystals result from slow
cooling. The faster the magma cools, the smaller
are the crystals.
|
 |
PEGMATITE
originates deep in the crust, forming very large
crystals due to slow cooling. |
 |
GRANITE
results from slow cooling and consists of
crystals of quartz, mica, feldspar, and
hornblende which are easily visible. |
 |
PORPHYRY is
formed from magma that cooled deep in the crust
first and then moved nearer the surface. Large
crystals are set in a background of very small
crystals. |
 |
BASALT is a
dark, dense rock. Crystals are microscopic in
size, which tells us that this rock once flowed
near or on the surface or the earth. |
 |
OBSIDIAN is
commonly called volcanic glass. It cooled so
quickly that it has no crystals. |
 |
PUMICE is
very light, full of holes, and floats in water. |
| Sedimentary Rocks
are broken up into small particles by weathering
and erosion. These materials, or sediments, are
carried by streams and deposited in the oceans in
layers. These layers are eventually buried by
more sediments and compressed into rock. Some
sedimentary rock is made of tiny particles that
settle out of sea water onto the ocean floor, and
some is made from the actual remains of plants
and animals.
|
 |
CONGLOMERATE
consists of pebbles and finer material. It
resembles concrete. |
 |
SANDSTONE is
made of sand particles, principally quartz. |
 |
SHALE, the
most common sedimentary rock, is made of visible
layers of silt and clay. |
 |
LIMESTONE
can be made up of tiny particles of calcium
carbonate that settled to the ocean floor, or it
can be entirely made of shells or coral. |
 |
PEAT is the
compressed remains of plants. Compressed further,
it forms lignite, then bituminous coal. |
 |
GYPSUM is
the result of the evaporation of water from
ancient seas. |
| Metamorphic Metamorphic
means change of form. When rocks are subjected to
sufficient heat, pressure, or movement (like
faulting), there is a "recooking" of
minerals within the rocks and the minerals are
rearranged, in some cases in distinct bands.
|
 |
GNEISS was
once granite or conglomerate. It has dark and
light minerals in wide bands. |
 |
SCHIST may
have originally been shale, slate, granite,
basalt, sandstone, or limestone. It is usually
rich in mica. |
 |
SLATE, once
shale, breaks easily into layers. |
 |
QUARTZITE
was originally sandstone. |
 |
ANTHRACITE,
or hard coal, was originally bituminous coal. |
 |
MARBLE was
once limestone. |
