|Class and Origin
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.
originates deep in the crust, forming
very large crystals due to slow cooling.
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
commonly called volcanic glass. It cooled
so quickly that it has no crystals.
||PUMICE is very
light, full of holes, and floats in
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
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
||GYPSUM is the
result of the evaporation of water from
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
||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.
||ANTHRACITE, or hard
coal, was originally bituminous coal.
||MARBLE was once