Rocks
Table of Contents
Classifying Rocks
Igneous Rocks
Sedimentary Rocks
Rocks from Reefs
Metamorphic Rocks
The Rock Cycle
Rocks - Classifying Rocks
Studying Rocks
• When studying a rock sample, geologists
observe the rock’s mineral composition,
color, and texture.
Rocks - Classifying Rocks
Mineral Composition and Color
• Rock – a solid mixture of minerals and
other materials.
• Rock-forming minerals – the common
minerals that make up most of the
rocks of Earth’s crust.
– Granite – tends to be a light-colored
rock that has high silica content
– Basalt – tends to be a dark-colored
rock that is low in silica
Rocks - Classifying Rocks
Texture
• Grains – particles of minerals or other
rocks that give a rock its texture.
• Texture – the look and feel of a rock’s
surface, determined by the size, shape,
and pattern of a rock’s grains.
Rocks - Classifying Rocks
Texture
• Grain size
– Coarse-grained – large easy to see grains
– Fine-grained – so small they can only be
seen under a microscope
Rocks - Classifying Rocks
Texture
• Grain Shape – varies greatly from sand grains to
large triangular grains
• Grain Pattern – can vary also from layers to
swirls to bands
Rocks - Classifying Rocks
How Rocks Form
• Geologists classify rocks into three major
groups:
– igneous rock, sedimentary rock, and
metamorphic rock.
Rocks - Classifying Rocks
How Rocks Form
• Igneous rock – type of rock that forms from
the melting then cooling of molten rock at or
below the surface.
• Sedimentary rock – type of rock that forms
when particles from other rocks or the
remains of plants and animals are pressed
and cemented together.
• Metamorphic rock – type of rock that forms
from an existing rock that is changed by heat
and pressure
Rocks - Igneous Rocks
Classifying Igneous Rocks
• Igneous rocks are classified according to
their origin, texture, and mineral composition.
Rocks
Classifying Igneous Rocks
• Extrusive igneous rock – forms from lava on
Earth’s surface.
• Intrusive igneous rock – forms when magma
hardens beneath Earth’s surface.
• The faster igneous rock cools, the finer the
grains
– Extrusive igneous rock tends to be a finergrained rock than intrusive igneous rock
Rocks - Igneous Rocks
Mineral Mixture
• Granite is a mixture of
light-colored minerals,
such as feldspar and
quartz, and dark-colored
minerals, including
hornblende and different
types of mica. But granite
can vary in mineral
composition. This affects
its color and texture.
Rocks - Igneous Rocks
Mineral Mixture
• Reading Graphs:
– What mineral is most
abundant in granite?
– Feldspar
Rocks - Igneous Rocks
Mineral Mixture
• Reading Graphs:
– About what
percentage of
granite is made up of
dark minerals?
– 10%
Rocks - Igneous Rocks
Mineral Mixture
• Calculating:
– If the amount of quartz
increases to 35 percent
and the amount of
dark-colored minerals
stays the same, what
percentage of the
granite will be made up
of feldspar?
– 100% - (35% + 10%)
= 55%
Rocks - Igneous Rocks
Mineral Mixture
• Predicting:
– How would the color of
the granite change if it
contained less feldspar
and more mica and
hornblende?
– The overall color
would be darker.
Rocks - Igneous Rocks
Links on Igneous Rocks
• Click the SciLinks button for links on
igneous rocks.
Rocks - Sedimentary Rocks
From Sediment to Rock
• Most sedimentary rocks are formed
through a series of processes: erosion,
deposition, compaction, and cementation.
Rocks - Sedimentary Rocks
From Sediment to Rock
• Sediment – small, solid pieces of
material that come from rocks or
organisms.
– Usually forms due to erosion
• Erosion – a destructive process
in which water or wind loosens
and carries away fragments of
rock.
• Deposition – process by which
sediment settles out of the water
or wind that is carrying it.
Rocks - Sedimentary Rocks
From Sediment to Rock
• Compaction – process by which
sediments are pressed together
under their own weight.
– Occurs over millions of years as
layers build
• Cementation – process by which
dissolved minerals crystallize and
glue particles of sediment together
into one mass.
– Takes place when sediment is in
the presence of water
Rocks - Sedimentary Rocks
Types of Sedimentary Rock
• There are three major groups of
sedimentary rocks:
– clastic rocks, organic rocks, and
chemical rocks.
• Clastic rock – sedimentary rock that
forms when rock fragments are
squeezed together under high pressure.
– Can range in size from microscopic
clay particles to large boulders
– Ex: shale, sandstone, conglomerate,
and breccia
Rocks - Sedimentary Rocks
Types of Sedimentary Rock
• Organic rock – sedimentary rock
that forms from remains of
organisms deposited in thick layers.
Peat
Lignite
– Ex: Coal and Limestone
– Coal forms from the remains of swamp
plants buried in water.
– Limestone forms in the ocean, where Bituminous
many living things, such as coral,
clams, and oysters, have hard shells
made of calcite that collect on the
Anthracite
ocean floor.
Rocks - Sedimentary Rocks
Types of Sedimentary Rock
• Chemical rock –
sedimentary rock that
forms when minerals
crystallize from a
solution. (Reacts to
HCl)
– Ex: Limestone forms
from calcite deposits
dissolved in lakes,
rivers, streams.
Rocks - Sedimentary Rocks
Uses of Sedimentary Rock
• Sedimentary rock
has been used for
thousands of years in
building structures,
tools, and statues.
– Ex: The White House
in Washington D.C.
is constructed of
sandstone.
Rocks - Sedimentary Rocks
Links on Sedimentary Rocks
• Click the SciLinks button for links on
sedimentary rocks.
Rocks - Rocks From Reefs
Coral Reefs
• Coral reef – a structure of calcite skeletons
built up by coral animals in warm, shallow
ocean water.
– When coral animals die, their skeletons remain.
More corals build on top of them, gradually
forming a coral reef.
– Only form in ocean water between 30°N and
30°S latitude
Rocks
Coral Reefs(not in notes)
• Coral reef is really organic(from living
things) limestone.
• Deposits of organic limestone help
geologists figure out what the environment
was like long ago.
• If they find a fossil of coral reef, they would
know that location used to contain warm,
shallow ocean water.
Rocks - Rocks From Reefs
More on Coral Landforms
• Click the PHSchool.com button for an
activity
about coral landforms.
Rocks - Metamorphic Rocks
Metamorphic Rock
• Metamorphic rock –
forms when heat and
pressure beneath Earth’s
surface changes the
appearance, texture,
crystal structure, and
mineral content of either
igneous, sedimentary, or
other metamorphic rock.
Rocks - Metamorphic Rocks
Types of Metamorphic Rocks
• Geologists classify
metamorphic rocks according
to the arrangement of the
Shale
grains that make up the
rocks.
– Foliated rocks – metamorphic Slate
rocks that have grains arranged
in parallel layers or bands.
Gneiss
– Ex: Slate – metamorphic shale
(denser and more compact
than shale)
Rocks - Metamorphic Rocks
Types of Metamorphic Rocks
• Nonfoliated rocks – metamorphic rocks
that have randomly arranged metamorphic
grains that do not split into layers
– Ex: Quartzite – metamorphic sandstone
Marble – metamorphic limestone
Rocks - Metamorphic Rocks
Links on Metamorphic Rocks
• Click the SciLinks button for links on
metamorphic rocks.
Rocks - The Rock Cycle
A Cycle of Many Pathways
• Forces deep
inside Earth and
at the surface
produce a slow
cycle that builds,
destroys, and
changes the
rocks in the crust.
Rocks - The Rock Cycle
• Rock cycle –
processes on
the surface
and inside
Earth that
slowly
change rocks
from one kind
to another.
Rocks - The Rock Cycle
The Rock Cycle and Plate Tectonics
• Plate movements start the rock cycle by
helping to form magma, the source of
igneous rocks.
• Plate movements also cause faulting,
folding, and other motions of the crust that
help to form sedimentary and
metamorphic rocks.
Rocks - The Rock Cycle
Rock Cycle Activity
• Click the Active Art button to open a
browser window and access Active Art
about the rock cycle.
Rocks - The Rock Cycle
Rock Cycle
Magma
Metamorphic
Sedimentary
Igneous
Rocks
Graphic Organizer
Rocks
can be
Igneous
Sedimentary
Metamorphic
include
include
include
Intrusive
Extrusive
Clastic
Organic
Chemical
Foliated
Nonfoliated