Rock Types and the Rock Cycle
Magma is the parent material for all rocks. From the time magma
cools and hardens at or near the surface of the earth, resulting
rock begins to change. In time, the rock formed from the original
magma is altered many times. Geologists study not only the crust
of the earth but also the forces and processes that act upon the
rocks of the crust. From this study, they have determined that
rocks form in three ways. From this, they have classified rocks
into three types based on the way the rocks are formed.
Igneous Rocks: develop directly from the cooling of
magma.
Sedimentary Rocks: develop from sediment that is pressed
and cemented together, and then harden.
Metamorphic Rocks: certain forces and processes (pressure,heat,
chemical reactions) can cause igneous or sedimentary rocks to
metamorphosize (change into) another type of rock.
****Any of the 3 rock types may change into another type. This series of changes is called the rock cycle.
IGNEOUS ROCKS:
2 general types:
Intrusive-formed by slow cooling of magma below the earth’s
surface
Extrusive-formed by rapid cooling of lava on the earth’s
surface
-Texture:
Intrusive igneous rocks have large grains (slow cooling) and a
coarse texture
Extrusive igneous rocks have fine grains (rapid cooling) are very
smooth
(IR’s will be the abbreviation for Igneous rocks)
Composition: Mineral composition of an igneous rock is determined
by the chemical composition.
Divided into 3 families based on their mineral composition:
1) IR’s (the granite family) formed from felsic magma are
high is silica. They are light colored because their main components
(orthoclase feldspar and quartz)
ex. coarse grained granite and fine grained rhyolite. Obsidian
is one.
2) IR’s (the basalt family) form from mafic magma. Their
main components are plagioclase feldspar and augite. These provide
this rock a dark color. They may also contain dark colored minerals
such as olivine, biotite, and hornblende.
ex. coarse grained gabbro and fine grained basalt.
3) IR’s (the diorite family) form from the minerals plagioclase
feldspar, hornblende, augite, and biotite. The have little or
no quartz.
They are a medium color.
ex. coarse grained diorite and fine grained andesite.
IR- Structures:
Intrusions-(underground structures
a. batholith-lg mass of IR that covers hundreds of square kilos.
b. stock- a mass of IR that covers an area less then 100 square
kilos
c. lacolith-a flat bottomed intrusion that pushes up layers to
form an arc.
d. sill- a sheet magma that flows between the layers of rock and
hardens. It lies parallel to the rock layers.
e. dike-magma that forces its way through rock layers by either
following vertical fractures or creating their own by melting.
Extrusions-
a. volcano
1. volcanic neck- the solidified central shaft of the volcano.
b. lava flows and lava plateaus
(SR’s will be the abbreviation for SEDIMENTARY ROCKS)
SEDIMENTARY ROCKS:
Cementation and compaction are the processes that form SR’s.
-cementation- water w/ dissolved minerals in it moves through
sediment. The minerals remain between the sediment and act like
a cement to hold the sediment together.
-compaction-extreme pressure forces the air and water out between
the fragments.
SR’s are classified by the type of sediment they are made
of.
1) Clastic SR’s- formed from deposits formed by agents of
erosion.
These are sub-divided by size:
-Large gravel-size SR’s that are rounded are conglomerates.
-If the fragments are angular with sharp corners its breccia.
-Sandstones: sand sized grains cemented together.
-Shale: clay sized particles cemented together.
2) Chemical SR’s- are made up of rock fragments that have
formed from minerals once dissolved in water.
ex. certain types of limestone, gypsum, halite (rock salt)
3) Organic SR’s-rock formed from the remains of living
things
ex. coal and a form of limestone called chalk.
SR Features:
1) stratification- or layering, produced when there is a change
in the kind of sediment being deposited.
2) ripple marks-formed by the action of wind or water on sand. When the sandstone is preserved as sandstone, so are the ripple marks
3) mud cracks- typically formed by the drying up of a river plain.
4) Fossils
5) Concretions and Geodes
METAMORPHIC ROCKS:
Metamorphism: is the changing of one rock type to another by heat,
pressure, and chemical processes. This process occurs for the
most part, deep within the earth where the right conditions exist.
IR’s, SR’s, and MR’s all can be changed by the process of Metamorphism.
Classification of MR’s:- based on their structure- either foliated or unfoliated. A Foliated structure means it has visible layers or bands. Unfoliated- does not.
Examples of Foliated MR’s-slate, schist, and gneiss.
shale-is metamorphisized into slate
Unfoliated MR’s lack layered crystals.
ex. Sandstone is changed into quartzite