Rocks are classified as
Igneous rocks are formed by volcanic
activity. They are classified further by
their origin, texture, and mineral
Origins of Igneous Rocks
These rocks are formed
above the ground
when lava flows or
explodes from a
These rocks form
Texture of Igneous Rocks
Texture depends on the size and shape of the
crystals when the rock forms. This is due to the
cooling rate of the rocks.
Rapid cooling produces fine
grains and small crystals.
This is associated with extrusive
Slow cooling produces coarse
grains with large crystals. This
is associated with intrusive
• Obsidian is formed above ground and
cools quickly, but it forms no cyrstals and
has a glassy texture.
Mineral Composite of Igneous
Igneous rocks with low
amounts of silica
usually produce dark
Igneous rocks high in
silica usually appear
light in color.
Uses of Igneous Rocks
For thousands of years people have used
igneous rocks for building and tools.
Granite is still used today for
Obsidian has been used for tool
making by Native Americans.
Some surgeons use obsidian
instead of steel in their scalpels
today because they cut better
and leave a thinner scar.
Pumice is used for cleaning
and for removing calluses from
Sedimentary rocks are formed through a
series of processes: erosion,
deposition, compaction, and
Erosion is the wearing away of rock due to
wind, water or ice.
Deposition is when the sediments from
erosion settle out of the wind, water and
ice and are left or deposited in a different
Sediments build up over time and press
down on the layers below. This squeezes
the layers so that the particles or grains
Minerals that are dissolved in water start to
crystallize. These crystallized minerals
“glue” the solids or grains together.
Types of Sedimentary Rocks
• Geologist classify sedimentary rocks
according to the type of sediments they
have. There are three major groups of
sedimentary rocks: clastic, organic, and
Clastic rocks are your basic sedimentary rock.
They are formed from broken bits and pieces of
other rocks that were eroded and deposited a
long time ago. These rock pieces were
squeezed together. The particles can be the
size of sand or large and jagged.
Organic Sedimentary Rocks
Organic sedimentary rocks are made from
the remains of plants and animals. These
get deposited, compacted, and cemented
just like rock sediments. Coal and
limestone are two examples. Sometimes
limestone can have fossils in it.
When the minerals that are dissolved in a solution
crystallize, chemical rocks are formed. One type
of limestone can form this way when calcite is
dissolved in lakes and becomes crystallized.
Halite is another example of a chemical rock.
Metamorphic rock forms
when pressure deep
within the earth melts
and changes existing
rock. This is different
than intrusive igneous
rock, because you are
starting with rock, not
magma. The rocks melt
only a little. There are
two different types:
foliated and nonfoliated.
Foliated Metamorphic Rock
In foliated rock the grains are arranged in
parallel layers or bands. These may
appear as alternating layers of color or just
as flat layers that break or cleave.
Muscovite Mica Schist
Nonfoliated Metamorphic Rock
In nonfoliated rock the grains are random.
They do not split into layers.
All types of rock can change
Metamorphic rocks originally start out as different
types of rocks. Because of pressure deep within
the earth, they melt and change. Below are a
few metamorphic changes.
The Rock Cycle
The earth recycles. Igneous,
metamorphic rocks are
constantly changing to form
new rocks. Old rock is
eroded and deposited
somewhere else to become
sedimentary rock. Tectonic
plates subduct into the
mantle to melt and form
igneous rock. Intrusive
igneous rock and
sedimentary rock are buried
deep in the earth and are
subjected to high
temperature and pressure to
become metamorphic rock.
This never-ending cycle has
been going on for over 4
billion years. The earth is
continually breaking down
and rebuilding itself.