Unit 9-1: The Rock Cycle
Part 1
The rock cycle begins as
magma cools and hardens
into rock. This type of rock is
known as Igneous Rock.
Extrusive Igneous Rock:
Forms at surface when
a volcano erupts.
So there’s basically two
types of igneous rock,
depending on where the
magma cooled!
Intrusive Igneous Rock:
Forms when magma cools
below the surface of the Earth.
Two types of lava/magma
Mafic Magma: -Low silica content, but high calcium,
iron and magnesium.
-Hotter and more fluid than Felsic.
Felsic
Magma:
-High
-Forms
darker
minerals
andsilica
rockscontent.
when it cools.
and slowAugite,
flowing.
-Ex.-Thick
Horneblende,
Basalt.
-Forms makes
light colored
minerals
and rocks.
-Mostly
up extrusive
igneous
rocks.
-Ex. Quartz, Orthoclase Feldspar, Granite.
-Mostly makes up intrusive igneous rocks.
Igneous
can betelling
glassy-smooth
So rocks
what you’re
me is thatin
texture,
or they
can be
rough and
a rock’s
physical
properties
are
coarseon
in its
texture.
dependent
composition,
AND how/where it cooled?
The texture of the rocks depends on the
size, shape, and arrangement of its grains
or crystals (from the minerals that make
up the rock.)
Another factor is the percentage
of dissolved gas in the magma.Some igneous rocks, like obsidian,
Gas keeps the magma hotter forcontain grains that are too small to
longer.
be seen by the naked eye.
Another factor is the order in The size of grains is dependent on
which the crystals form. This is
how fast the magma that formed the
because different rocks
rock cooled. The faster the
melt/solidify at different
magma/lava cools, the smaller the
temperatures.
crystals.
-Some igneous rocks have two
Now this
rock has
fine mineral
distinct
textures.
grains
and large crystals
mineral grains.
-When
are
So
how do we
where
surrounded
by figure
a fine out
grained
or
it formed?
glass rock (called
a groundmass),
the rock is a porphyry.
-A porphyry forms in two stages.
-First, it begins to form deep in the earth, where it cools slowly.
-This allows the large crystals to form.
-Then the magma slowly rises to the surface.
-Once it breaks to the surface, it cools very quickly.
-This forms the groundmass of very small crystals.
-Now there are three families of igneous rocks, based on
their composition. Each family has members based on
the grain size.
-The families are the granite family, the gabbro family, and
the diorite family.
-There is a fourth igneous rock type called granodiorite.
Now, let’s take a look at these individually.
The Granite Family
-Made of high-silica magmas.
-Made mainly of orthoclase
feldspar and quartz.
-There can also be plagioclase
feldspar, mica, and
horneblende.
-Rocks in this family are
usually light in color.
Based on grain size:
-Granite is coarse-grained.
-Rhyolite is fine-grained.
-Obsidian and Pumice are glassy.
-All three have the same chemical structure.
The Gabbro Family
-Rocks in this family have
low silica content.
-Made mainly of plagioclase
feldspar and augite.
-Can also contain olivine,
horneblende, and biotite.
-All minerals in this family
(except feldspar) contain
high levels of iron.
-These rocks are usually
dark in color.
-In this family, gabbro is coarse-grained.
-Basalt is fine-grained.
-Basalt glass is glassy.
-Diabase is somewhere between basalt and gabbro in texture.
The Diorite Family
-Rocks in the diorite family have
a texture that is somewhere in
between the granite and gabbro
family.
-They are made of plagioclase
feldspar, horneblende, augite
and biotite.
-They are a mix of dark and
light colors.
-Diorite is the coarse-grained rock in this family.
-Andesite is the fine-grained rock in this family.
-Obsidian without quartz in it is the glassy rock in this family.
Granodiorite
-Between the coarse textured rocks in
the granite and diorite family is
granodiorite.
-It shares in compositions somewhere
in between granite and diorite.
Three other coarse grained rocks are of interest as well:
-Pyroxenite is almost entirely pyroxene.
-Dunite is almost all olivine.
-Periodotite is a mixture of olivine and pyroxene.
I think that if rocks formed at
different locations and have
different compositions, then
they should have different
properties.
Hey, now that we’ve looked at
how igneous rocks form and their
classifications, can we look at
some of the properties of the
individual rocks themselves?
-Granite is made from quartz, orthoclase feldspar, and at
least one other mineral, such as mica or horneblende.
-The quartz looks like gray or smoky glass.
-The feldspar is usually white, gray, or pink, and is
noticeable by its flat fracture planes.
-The mica can be easily broken away, while the
horneblende cannot.
-Granite can come in many different
colors.
-Granite is the most common
continental igneous rock.
-It can be found in the Rockies, the
Adirondacks, the Black Hills, etc.
-Granite is an intrusive rock, so it is
only exposed to the surface after a
great amount of erosion.
-Obsidian is formed by very
rapid cooling of lava.
-It is dark brown to black in
color, and is glassy in texture.
-Obsidian is a very hard rock,
but also very brittle.
-When broken, it has very sharp
edges.
-Pumice is made by very rapid
cooling of lava, when dissolved
gasses cannot escape.
-It looks like a sponge, but this
rock is glass, like obsidian.
-Because of the trapped gas,
pumice is light enough to float.
-Basalt is the igneous rock of
the ocean floor.
-This is the type of rock that
forms from surface lava flows
that cool moderately slowly.
-Fine grained, colors range
from black to dark green.
-Diabase is similar to basalt.
-Made from the same material
as basalt, but with larger
grains.
-Also known as dolerite or trap
rock.