What is a rock?
Different rocks have different characteristics. Some are
smooth, some are rough, some are striped, and some are
What are common rocks?
Most buildings and public monuments are made from
common rock. Rock used for building stone often contains
one or more common materials, called rock-forming
minerals. Two rock-forming minerals are quartz and calcite.
A rock is a mixture of rock-forming minerals and other
materials such as volcanic glass, organic material, or other natural materials.
The Rock Cycle
Scientists have created a model to show how rocks slowly
change over time. The rock cycle shows the processes that
create and change rocks. The three types of rocks shown in
the rock cycle are igneous, metamorphic, and sedimentary.
The rock cycle shows how rocks can change from one type
of rock to another.
How do rocks change?
The rock cycle in the figure above shows there are several
processes that change rocks. Weathering breaks down rocks
into tiny mineral grains, or sediments. Erosion moves the
sediments by wind or water. Layers of sediments pile up.
They are compacted, or packed down, by more layers of
sediment piling on top of them. Over time, the pressure of
compaction turns the sediment into sedimentary rock.
Heat and pressure deep inside Earth may change
sedimentary rock into metamorphic rock. The metamorphic
rock can then melt and later cool to form igneous rock. The
igneous rock may then be weathered into mineral grains.
The grains eventually form new sedimentary rock. Any rock
can change into any of the three major types of rock.
A rock can even change into another rock of the same type.
No matter what happens, the mineral material is never lost
or destroyed. It is conserved, or used in other forms.
Who discovered the rock cycle?
Scottish scientist James Hutton noticed that some rocks
have straight layers, while others are tilted. He saw that
some rocks are weathered, while others are not. Hutton
observed that rocks change constantly over time.
Before You Read
Think about an erupting volcano you may have seen on TV
or in the movies. On the lines below, describe what comes
out of an erupting volcano.
Formation of Igneous Rocks
When a volcano erupts, red-hot material may flow out of
it. The extremely hot material is melted rock, called magma.
Magma flows like a liquid. When magma flows near or onto
EarthÕs surface, it cools and hardens. Igneous rock forms
from the cooled and hardened magma.
How does magma become lava?
Inside Earth, the temperature and pressure in certain
places are just right to melt rocks. As a result, magma
forms. Magma can be found at depths ranging from near
EarthÕs surface to about 150 km below the surface. The
temperature of magmas range from about 650¡C to 1,200¡C.
Where does the heat come from that melts rock inside
Some heat comes from the decay of radioactive
elements in rocks. Some heat is left from when Earth was
formed. At first, Earth was very hot, molten material.
Magma is less dense than the solid rock around it.
Because it is less dense, it is forced up toward EarthÕs
surface. When magma reaches EarthÕs surface and flows
from volcanoes, it is called lava.
What are intrusive igneous rocks?
Magma is melted rock made up of common elements and
liquids. Magma cools as it rises toward EarthÕs surface. As
magma cools, the atoms and compounds in the liquid
rearrange themselves into new crystals called mineral grains.
As cooling continues, mineral grains grow together to form
rocks. Sometimes this process takes place beneath the
surface. Intrusive igneous rocks form from cooling magma
beneath EarthÕs surface, as shown in the figure below. It
takes a long time for magma beneath EarthÕs surface to cool.
Cooling is so slow, mineral grains grow quite large.
Intrusive igneous rock has large mineral grains.
Intrusive igneous rocks can be found on EarthÕs surface.
After many years, the layers of rock and soil that once
covered them are removed by erosion. Erosion occurs when
the rocks are pushed up by forces inside Earth.
What are extrusive igneous rocks?
Extrusive igneous rocks form as lava cools on the surface
of Earth, as shown in the figure above. When lava reaches
the surface, it is exposed to air and water, which cools it
quickly. The atoms in the liquid do not have time to
arrange into large crystals. Therefore, the mineral grains in
extrusive igneous rock are quite small.
What is volcanic glass?
Sometimes, lava that comes out of a volcano cools so
quickly that few or no mineral grains form. A rock that
forms from this quickly cooling lava is called volcanic glass.
Volcanic glass has few or no crystals because the atoms are
not arranged in an orderly pattern.
Obsidian is a volcanic glass that looks like shiny black
glass. Pumice and scoria are also volcanic glasses, but they
do not look like glass. They have lots of holes, or pores.
These materials form from a gooey liquid that contains
pockets of gases. Some of these gases escape and holes are
left where the rock formed around the gas pocket.
Classifying Igneous Rocks
Igneous rocks can be grouped as either intrusive or
extrusive depending on how they are formed. Igneous rocks
can also be grouped according to the type of magma they
come from. An igneous rock can form from basaltic,
andesitic, or granitic magma. The type of magma that cools
to form an igneous rock affects the properties of that rock.
Some of the chemical and physical properties of a rock are
its mineral composition, density, color, and melting
What are basaltic rocks?
Igneous rocks that are dense and dark-colored are
basaltic (buh SAWL tihk). They form from magma
containing a lot of iron and magnesium, but little silica,
which is made of silicon and oxygen. Basalt gets its dark
color from the iron and magnesium it contains. Basaltic
lava is fluid and flows freely.
What are granitic rocks?
Granitic igneous rocks are light-colored and not as dense
as basalt. They form from thick, stiff magma that contains
lots of silica, but smaller amounts of iron or magnesium.
Stiff granitic magma can build up lots of gas pressure. This
pressure is released in violent volcanic eruptions.
What are andesitic rocks?
Andesitic igneous rocks have mineral compositions
between those of basalt and granite. Like granitic magma,
andesitic magma can produce violent volcanic eruptions.
Before You Read
Think about a time you packed a sandwich for lunch and
placed a can of juice or soda on top of it. Describe how the
sandwich looked at lunchtime and why it looked that way.
Formation of Metamorphic Rocks
Like a sandwich that has been flattened by a can of soda,
rocks can be affected by changes in pressure. Changes in
temperature also can affect rocks. Metamorphic rock is rock
that has been changed because of changes in temperature
and pressure, or the presence of hot, watery fluids. These
conditions may change the rockÕs form, the material it
contains, or both.
Metamorphic rock may form from sedimentary rocks,
igneous rocks, and even other metamorphic rocks.
How do heat and pressure change rock?
Rocks deep beneath EarthÕs surface are under great
pressure from the layers of rock above them. Temperature
also increases with depth. In some areas, the pressure and
the temperature are just right to melt rock. The melted rock
forms magma. Different types of metamorphic rock may
form from the magma.
In other places deep inside Earth where there is a lot of
liquid, rocks do not melt. Instead, some mineral grains
dissolve in the liquid and then form new crystals. Under
these conditions, minerals sometimes exchange atoms with
surrounding minerals and new minerals form.
How does shale change to gneiss?
Depending on the amount of pressure and the
temperature under Earth, one type of rock can change into
several different types of metamorphic rock. For example,
shale, a sedimentary rock, will change into slate, a
metamorphic rock. As the temperature and pressure on it
increase, the slate can change into phyllite, then into schist,
and finally into gneiss (NISE).
How do hot fluids change rock?
Hot fluids from magma flow through spaces in and
between underground rocks. The hot fluids are mostly
water, but they also contain dissolved elements and
compounds. These fluids can react with the rock they flow
through and change its composition. As shown in the figure
below, the hot fluid flows into the rock and chemically
changes it into a type of metamorphic rock.
Classifying Metamorphic Rocks
Metamorphic rocks form from igneous, sedimentary, or
other metamorphic rocks. Heat, pressure, and hot fluids
cause these rocks to change. The types of metamorphic
rocks that form can be classified based on their composition and texture.
What are foliated rocks?
As some metamorphic rocks form, their mineral grains
line up in parallel layers. Metamorphic rocks with a foliated
texture have parallel layers of mineral grains. For example,
slate is a metamorphic rock that forms from shale, a
sedimentary rock. When shale is exposed to heat and
pressure, it changes. Its mineral grains line up in parallel
layers to form slate, a foliated metamorphic rock.
Slate SlateÕs parallel layers of mineral grains are pressed so
tightly together that water cannot pass between them easily.
Slate also breaks into smooth, flat pieces. Because it sheds
water and splits smoothly, slate is often used for paving
stones and roofing tiles.
Gneiss Gneiss is a foliated rock that forms when granite
and other rocks are changed by heat and pressure. The
foliated texture of gneiss is easily seen in its light and dark
bands. As gneiss forms, the movement of atoms separates
the dark minerals in the rock from the light-colored
minerals in the rock.
What are nonfoliated rocks?
Some metamorphic rocks are formed without layers. In
these rocks, the mineral grains grow and rearrange, but do
not form layers. Nonfoliated rocks are metamorphic rocks
that form without a layered texture.
Sandstone Sandstone is a sedimentary rock made mostly of
quartz grains. When it is heated under a lot of pressure,
sandstone is changed into quartzite. Heat and pressure cause
the sandstoneÕs quartz grains to grow larger and lock
together like pieces of a jigsaw puzzle. The quartz grains in
quartzite are not in layers, so quartzite is a nonfoliated rock.
Marble Another nonfoliated metamorphic rock is marble.
Marble forms from limestone that is under great heat and
pressure. Limestone contains the mineral calcite. Heat and
pressure change the calcite into marble, which does not have
a layered texture. In fact, marbleÕs fine, smooth texture
makes it the perfect material for sculptures and buildings.
Before You Read
Imagine you are stacking slices of bread, one on top of the
other. Then you put a heavy book on top of the stack and
leave it there overnight. Describe how the slices of bread
might look the next day.
Formation of Sedimentary Rocks
Weathering breaks down rocks into sediment. Sediment is
the loose material, such as tiny pieces of rock, mineral
grains, and bits of shell, that are moved by wind, water, ice,
or gravity. Sediments come from already-existing rocks that
are weathered and eroded.
Sedimentary rock forms when sediments are pressed and
cemented together, or when minerals form from solutions.
About 75 percent of the rocks you see on EarthÕs surface
are sedimentary rocks.
What do sedimentary rocks look like?
Sedimentary rocks often form as layers, like a stack of
papers. The older layers are at the bottom because they
were deposited first. The newer layers are at the top because
they were deposited later. If sedimentary rock is not disturbed,
the layers will remain in place, with the oldest at the bottom
and youngest at the top.
Sometimes, though, forces within Earth overturn layers of
sedimentary rock. Then, the oldest layers are no longer on
the bottom. The order of the layers is changed.
Classifying Sedimentary Rocks
Sedimentary rocks can be made of just about any material
in nature. Sediments come from weathered and eroded
sedimentary, metamorphic, and igneous rock. Sediments
also can come from the remains of some organisms. The
composition of a sedimentary rock depends on what types
of sediments formed it.
Sedimentary rocks are classified by what they are made of.
They are also classified by the way in which they formed.
Sedimentary rocks are classified as detrital, chemical, or
Detrital Sedimentary Rocks
The word detrital (dih TRI tul) comes from the Latin
word detritus, which means Òto wear away.Ó Detrital
sedimentary rocks are made from the broken pieces of other
rocks. The tiny pieces are compacted and cemented together
to form solid sedimentary rock.
What are weathering and erosion?
Weathering is the process in which air, water, or ice breaks
down rocks into smaller and smaller pieces. The movement
of weathered material is called erosion.
What is compaction?
Erosion moves sediments to a new place, where they are
deposited in a thin layer. Over time, layer upon layer of
sediment builds up. The weight of the top layers pushes
down on the lower layers. Downward pressure causes small
sediments to stick together and form solid rock. The process
in which layers of sediments are pressed together to form rock
is called compaction. The figure below shows how rock pieces
are compacted to form sedimentary rock.
Compaction of Sediments
What is cementation?
Large sediments, like sand grains and pebbles, cannot
form rock from pressure alone. These large sediments form
rock only if something helps them stick together.
As water moves through rock and soil, it picks up materials
released by the weathering of minerals. The resulting solution
of water and dissolved minerals moves through open spaces
between larger sediments. The solution acts as a kind of glue
that holds the large sediments together. Cementation is the
process in which sediments are held together by dissolved
minerals produced when water moves through rock. Minerals
such as quartz, calcite, and hematite make the best cement
for holding large sediments together.
What are the shapes and sizes of sediments?
Detrital rocks have a grainy texture, like grains of sugar.
They are named according to the shapes and sizes of the
sediments that form them. For example, conglomerate and
breccia (BRECH uh) are detrital rocks that form from large
sediments. If the sediments are rounded, the rock is called
conglomerate. If the sediments have sharp angles, the rock
is called breccia. The farther sediments are carried by wind,
water, or ice, the more rounded they become.
What materials are found in sedimentary rocks?
Conglomerate and breccia are formed from gravel-sized
sediments that are cemented together by quartz or calcite.
These sediments may come from the minerals quartz or
feldspar, or may contain chunks of other rocks, such as
gneiss, granite, or limestone.
Sandstone forms from small sediments. The sand-sized
sediments in sandstone can come from almost any mineral,
though they usually come from quartz and feldspar. Shale
is a detrital sedimentary rock that is made from the smallest
Chemical Sedimentary Rocks
When water evaporates from a salt solution, salt grains
remain. In a similar way, when the water in a lake evaporates,
its minerals remain. The remaining mineral deposits form
sediments which, in turn, form rocks. Chemical sedimentary
rocks form when dissolved minerals come out of solution and
form sediments that become rocks.
How does limestone form?
Calcium carbonate is found dissolved in ocean water.
Calcium carbonate comes out of solution as the mineral
calcite. Calcite forms crystals, which bond to form limestone,
usually on the bottom of lakes and shallow seas. Long ago,
the central United States was covered with a shallow sea.
Over time, the water evaporated. As a result, much of the
central United States has limestone bedrock.
How does rock salt form?
Some bodies of water contain a lot of dissolved salts. When
the water evaporates, it deposits the mineral halite, or rock
salt. Rock salt is mined. It is used in manufacturing glass,
paper, and soap. It is also made into table salt.
Organic Sedimentary Rocks
Rocks made of materials that were once living things are
called organic sedimentary rocks. One of the most common
organic sedimentary rocks is fossil-rich limestone. It is made
of the remains of once-living ocean organisms. Ocean animals,
such as clams and snails, make their shells out of calcium carbonate,
which eventually becomes calcite. When the animals die, their
shells pile up and become cemented together to form fossil-rich limestone.
What are other organic sedimentary rocks?
Chalk Chalk is an organic sedimentary rock that is made
up of extremely tiny bits of animal shells. When you write
with chalk, you are crushing and smearing the calcite shell
remains of once-living ocean animals.
Coal Coal is a useful organic sedimentary rock that forms
when pieces of dead plants are buried under other sediments in
swamps. The plant material is chemically changed. The resulting
sediments are compacted to form coal. Today, coal is a fuel used
in power plants to make electricity.