Earth Science
Rocks Notes
There are many different types of rocks on the earth. Rocks are
naturally occurring solid materials made of one or more minerals.
Most of these minerals are composed of various combinations of
eight elements. These elements are oxygen, silicon, aluminum, iron,
calcium, sodium, potassium, and magnesium. They are the most
abundant elements on the earth. Most rock forming minerals are
composed of these elements.
Scientist who study rocks and there mineral composition are called
petrologists. Petrologists have grouped rocks into three main types.
The general classification of rocks is based on the way rocks were
formed.
Types of Rocks
One type of rock is formed from magma. Remember that magma is
hot liquid rock that contains dissolved gases. Magma is formed deep
in the lower part of the earth’s crust and in the upper part of the
mantle. When magma cools and hardens, it forms igneous rocks.
Igneous rocks are sometimes referred to as “fire formed” rocks. In
fact, the word igneous comes from the latin word igni, meaning
“coming from fire”.
Another type of rock is formed from sediments. Here the sediments
have been pressed and cemented together to form sedimentary
rocks. The third type of rock is formed as a result of changes that
occur in existing rocks. Tremendous heat, great pressure, and
chemical reactions change existing rocks into new rocks. These new
rocks are called metamorphic rocks.
The Rock Cycle
The continuous changing of rocks from one type to another is called
the rock cycle. Some igneous and sedimentary rocks are buried deep
beneath the earth’s surface. Great pressure and tremendous heat
change these rocks into new rocks containing different minerals.
These are metamorphic rocks. When metamorphic rocks are
exposed at the earth’s surface, they, too will be broken down into
sediments.
Igneous Rocks
There are two types of igneous rocks. These types are based on
where magma cools and hardens to form the rocks. Igneous rocks
formed deep within the earth are called intrusive rocks. When magma
reaches the surface, it is called lava. Lava is the liquid rock that pours
out of volcanoes. Igneous rocks formed from lava at the earth’s
surface are called extrusive rocks. Igneous rocks form when the
minerals in magma crystallize, or harden. So, igneous rocks consist
of tightly interlocking crystals that have different size and chemical
composition. The size of the crystals in an igneous rock is called its
texture. Texture and chemical composition are the properties used to
classify igneous rocks. The texture of an igneous rock depends on
the time it takes the rock to harden. This time is called the rock’s
cooling rate. In general, the slower the cooling rate is, the larger the
crystals will be. So intrusive rocks, which cool and harden slowly
beneath the earth’s surface, have large crystals. Igneous rocks made
of large crystals are called coarse grained. Granite is an example of a
coarse-grained igneous rock. It is not uncommon for magma to cool
extremely slowly. Some trapped magma takes nearly 100 years to
cool only a few degrees. As a result, rocks may take thousands of
years to form! If the cooling rate is fast, small crystals will form. So
extrusive rocks, which cool quickly at the earth’s surface, have small
crystals. Igneous rocks made of small crystals are called fine grained.
Basalt is the most common fine-grained igneous rock. Sometimes
magma that reaches the surface cools so quickly that crystals do not
form. Obsidian, or volcanic glass, is a rock that cools quickly and
does not form crystals. The color of an igneous rock is often a good
clue to its composition.
Common Igneous Rocks
One of the most common coarse-grained igneous rocks is granite.
The continental crust of the earth is made of granite. Granite is
composed mostly of the minerals quartz and feldspar. These minerals
are light colored. So granite is light colored. But some granite has
dark-colored minerals in it, usually dark mica and hornblende, a type
of amphibole. The most common fine-grained igneous rock is basalt.
Basalt is made of very small mineral grains that cannot be seen
without a microscope. Basalt has the same mineral composition as
gabbro. So basalt is a dark-colored igneous rock. Basalt can be found
beneath all of the ocean floors. An igneous rock whose cooling rate
has changed and thus has two or more different-sized crystals is
called porphyry. Sometimes fine-grained igneous rocks form so
quickly that gas is trapped in them. Scoria and pumice are formed
this way.
Igneous Rock Formation
Magma does not always flow out onto the earth’s surface. Sometimes
it flows upward into cracks in rocks or spreads out between rock
layers beneath the earth’s surface. There it hardens, forming irregular
formation of intrusive rock. These formations, called intrusions,
harden into many different shapes depending on the magma’s
position when it cools. Intrusions are classified according to their size
and their relationship to the older rocks surrounding them. A batholith
is a huge, irregularly shaped intrusion that extends deep into the
earth’s crust. The floor of the arch is flat and is parallel to the
sedimentary rock layer it rests on. This domelike intrusion of igneous
rock is called a laccolith. Magma that pushes its way between layers
of sedimentary rocks but does not cause the overlying rocks to arch
forms a sill. A sill is a sheetlike mass of igneous rock. The Palisades
rock formation along the Hudson river bordering New York and New
Jersey is a good example of a sill. It is 80 kilometers long 305 meters
thick. A dike, is a narrow, flat formation of igneous rock formed in
vertical cracks in the existing rocks. The sheets often build up,
forming an extrusion called a lava plateau.
Sedimentary Rocks
Seventy-five percent of the rock’s on the earth’s surface are
sedimentary rocks. Mud, sand and gravel are some of the sediments
that make up these rocks. Sediments are moved by wind and water.
The faster wind and water move, the larger the particles they can
carry with them. Slow-moving wind and water can carry only small
grains. Over millions of years, mud, sand and gravel, bones and
shells settle to the sea floor as sediments. The sediments pile up in
layers many hundreds of meters thick. Lower layers are pressed
together more and more tightly under the weight of the layers above.
The pressure on the lower sediments changes them into rocks. This
process is called compaction. Some sediments are joined together, or
cemented, by minerals dissolved in water. This process is called
cementation. The result of these processes is the formation of layers
of sedimentary rocks called strata. The major characteristic of all
sedimentary rocks is that they form layers. Sedimentary rocks are
usually formed in water. Most of the earth has been covered by water
at some time in the past. Seventy percent of the earth is covered by
water now. So sedimentary rocks are common all over the world.
Sedimentary rocks are often rich in fossils. Sedimentary rocks are
classified according to their composition and texture, or grain size.
Clastic Rock
One type of sedimentary rock is clastic rock. Clastic rocks are formed
from broken pieces, or fragments, of rocks. Clastic sedimentary rocks
are classified according to the size and the shape of the fragments in
them. Some clastic rocks are made of rounded pebbles and other
rocks of different sizes cemented together by clay, mud, and sand.
These rocks are called conglomerates. Conglomerates form where
rivers slow down enough to deposit large pieces of rock. The classic
rock called breccia is similar to conglomerate. But its fragments are
sharp and angular rather than rounded. The fragments have not been
carried far enough by water to have their edges rounded. Clastic
rocks made of small, sand-sized grains are called sandstones.
Sandstones are usually made of their mineral quartz. Sandstone is
very resistant to water and decay. It is used as a building stone.
Mudrocks are formed when clay particles are carried by winter or
wind. When the water or wind stops moving, the clay particles are
deposited. Clay particles are flat and can be easily pressed together.
Shale is an example of a mudrock. Most shale can be split into flat
pieces. Shale forms in quiet waters such as swamps and bogs. Shale
often contains mica flakes, which give the rock surface a smooth,
slippery. Another type of mudrock is Silverstone. Silverstone issimilar
to sandstone, but it has much smaller grains. Silverstone contains
large quantities of clay.
Organic Rocks
Another type of sedimentary rock is organic rock. Organic rocks are
formed either directly or indirectly from material that was once living.
Limestones are organic rocks. Limestones are made mostly calcium
carbonate, also known as calcite. Many animals, such as clams and
oysters, grow shells made of calcium carbonate. When the animals
die, their shells sink to the ocean floor. At some point, these shells
form limestones. Sometimes an entire large shell is found in a
limestone rock. But most of the time, the ocean breaks these shells
into smaller fragments. Sometimes many animals with calcium
carbonate shells live together. They cement their shells together and
over time form large structures called reefs. Corals build limestone
reefs off the coast of Florida and around many of the Caribbean and
Pacific islands. Oysters build limestone reefs along the Texas Gulf
Coast. Chalk is another organic sedimentary rock. It is a type of
limestone composed of small pieces of animal shells and crystals of
calcium carbonate that have been passed together. Unlike other
limestone, chalk is soft. Many large chalk deposits were formed
millions of years ago. Coal is another rock that is formed from the
remains of living things. It is made from plants such as ferns that lived
millions of years ago. When these plants died, their remains began to
build up in swamps. As more and more layers of decaying plants built
up, pressure changed the layers on the bottom into coal. The layers
above these may be only partly changed into coal. These layers are
called peat.
Chemical Rocks
A third type of sedimentary rock is chemical rock. Chemical rocks are
formed by chemical means that do not involve any living organisms.
Some chemical rocks are formed when water evaporates and leaves
behind mineral deposits. The rocks formed this way are called
evaporites. Rock salt and gypsum are evaporites. Large deposits of
rock salt and deposits of gypsum can be found in New York,
Michigan, and Kansas. Often thick layers of rock salt and gypsum are
deposited between layers of shale and limestone. Some limestone
rocks are formed directly from ocean water rather than living
organisms. Chemical changes in ocean water cause grains of
calcium carbonate to form. They begin as small grains and get larger
as thin layers of calcium carbonate are added from the ocean water.
So these limestones are chemical rocks rather than organic rocks.
Sedimentary Rock Structure
Sedimentary rocks have certain features that help to identify them.
Foe example, many sedimentary rocks have visible layers. Also,
sedimentary rocks often contain fossils. Ripple marks and mud
cracks are other common features of sedimentary rocks.
Metamorphic Rocks
Rocks that have been changed from an existing type of rock into a
new type are called metamorphic rocks. Metamorphic rocks can be
formed from igneous rocks, sedimentary rocks, and other
metamorphic rocks. Most metamorphic rocks are formed deep
beneath the earth’s surface.
The Metamorphic Process
The changing of one type of rock into another as a result of
tremendous heat, great pressure, and chemical reactions is called
metamorphism. Metamorphic rocks begin to form at depths of 12 to
16 kilometers beneath the earth’s surface and at temperatures of 100
degrees C to 200 degrees C. They continue to form at temperatures
up to 800 degrees C. The mineral crystals may change there size or
shape. Or they may separate into layers.
Types of Metamorphism
There are two basic types of metamorphism. The first type is called
contact metamorphism. Contact metamorphism occurs when rocks
are heated by contact with magma or lava. The second type of
metamorphism is called regional metamorphic rocks with a foliated
texture have mineral crystals arranged in parallel layers, or bands.
These rocks tend to break along these bands. A common finegrained metamorphic rock with a foliated texture is slate. Slate is
used today to make flagstones for paths. A common medium to
coarse grained metamorphic rock with a foliated texture is schist.
Mica is a mineral often found in schist. Schist’s can form from a
number of different rocks, including granite, basalt, and slate. A
common coarse grained metamorphic rock is gneiss. Gneiss is made
from the igneous rock granite and is usually the result of regional
metamorphism. Metamorphic rocks with an unfoliated texture do not
have bands of crystals and do not break in layers. Two metamorphic
rocks with unfoliated textures are quartzite and marble. Quartzite is
formed from sandstone, a sedimentary rock made mostly of quartz.
Marble is formed from limestone.