|Class and Origin
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
||GRANITE results from slow
cooling and consists of crystals of quartz, mica,
feldspar, and hornblende which are easily
|Large and very small
||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.
|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, it has no
||PUMICE is very light, is
full of holes, and floats in water.
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. Eventually these layers are 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
|Coarse and fine particles
||CONGLOMERATE consists of
pebbles and finer material. It resembles
|Medium coarse particles
||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.
|Fragmental to compact
||PEAT is the compressed
remains of plants. Compressed further, it forms
lignite, then bituminous coal.
||GYPSUM results from
evaporation of water from ancient seas.
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
||SCHIST may have originally
been shale, slate, granite, basalt, sandstone, or
limestone. It is usually rich in mica.
|Very fine banding
||SLATE, once shale, breaks
easily into layers.
||QUARTZITE was originally
||ANTHRACITE, or hard coal,
was originally bituminous coal.
||MARBLE was once limestone.