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Rocks
Igneous Rocks
 Sedimentary Rocks
 Metamorphic Rocks
 The Rock Cycle

When classifying a rock sample geologists
observe the rock’s color and texture and
determine its mineral composition.
Texture: the size, shape, and pattern of the
rock’s grain.
Color: the apparent color of the rock, on the
inside and the outside.
Mineral composition: The minerals that make
up the different parts of a rock.
Texture: Grain Size
Often, the grains in a
rock are large and easy
to see.
Such rocks are said to be
coarse-grained. In other
rocks, the grains are so
small that they can only
be seen with a
microscope.
These rocks are said to
be fine-grained.
Notice the difference in
texture between the finegrained slate and the
coarse-grained diorite to
the right.
Igneous Rocks
Igneous means ‘of fire’
 Igneous rocks are formed when magma or lava
cools and hardens into rocks

Intrusive Igneous Rocks
Intrusive or plutonic rocks form deep
within the Earth’s crust.
 They cool slowly so crystals can form.
 Sometimes they cool as a large mass of
rock called a batholith or laccolith. Over
time these rock formations can be pushed
up to the surface to form mountains.

Extrusive Igneous Rocks
Extrusive or volcanic rocks
form at or near the Earth’s
surface.
 The lava or magma cools
quickly so crystals don’t
have time to form.
 Sometimes gas bubbles are
trapped inside the cooling
lava making a rock like
pumice, or sometimes it
hardens like black glass
called obsidian.

Sedimentary Rocks
Sedimentary rocks form when there is a
build up of deposited material called
sediments.
 As new layers are added the ones on the
bottom become compacted together by
the heavy weight from above.
 Water is often involved in forming
sedimentary rocks, as sediments fall to the
bottom of seas, rivers and lakes.

Sedimentary Rocks

As sediments settle slowly into layers, the
smallest particles squeeze in between the
larger pebbles and rocks, cementing them
together.
Often
these rocks
are rich in fossils as
the remains of
plants and animals
get trapped in the
sediments over
time.
Three types of Sedimentary Rocks

Clastic Rocks: these rocks are made up of
rock fragments mixed with sand and clay
which cements them together. These rocks
are called conglomerates, sandstones,
shale, or siltstone depending on the size
of the fragments found in them.
Three Types of Sedimentary Rocks

Organic Rocks: these rocks contain within
them the remains of once living things.
Shells and fossils are a common
identifying features. Examples are
limestone and coal.
Three types of Sedimentary Rocks

Chemical rocks: these are formed when
water evaporates leaving behind mineral
deposits. This can happen to lakes or
seas, and within underground caves.
Examples are rock salt and gypsum.
Metamorphic Rocks
Metamorphic rocks form when existing
rocks are changed in some way into a new
type of rock.
 These rocks usually form deep within the
Earth’s crust at depths of more than
12km. Here they have a lot of pressure on
them, and temperatures can be 100 to
800 degrees Celcius.

Metamorphic Rocks
At these high pressures and temperatures
rocks can soften and change shape. They
are squeezed and flattened into more
compact shapes. Chemical reactions inside
the rocks can change the internal
structure and composition also.
 Metamorphic rocks are usually harder and
denser than other rocks.

Metamorphism
Contact metamorphism occurs in just a
small area as rocks are contacted by heat
from magma or pressure in a specific
area, such as near volcanic activity.
 Regional metamorphism occurs when
rocks are changed by the Earth’s internal
heat and pressure over a huge region.

Two Types of Metamorphic Rocks
Foliated rocks have easy to see layers or
bands of minerals and colors. They will
break on these lines. Examples are slate
and gneiss.
 Unfoliated rocks do not have any visible
layers or bands. Examples are marble and
quartzite.

Metamorphic Rocks
The Rock Cycle
These three types of rocks, igneous,
sedimentary and metamorphic are always
changing. Each type of rock can be
changed over time into one of the other
types.
 The processes involved in the rock cycle
are weathering, erosion, deposition,
heating, high pressure, cooling,
solidification, compaction and
cementation.
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