Rocks
Chapter 4
P. Lobosco
The Rock Cycle
Chapter 4, Section 1
• Objectives:
• Distinguish between a rock and a mineral.
• Describe the rock cycle and some changes
that a rock could undergo.
Rock Forming Mineral Groups
Chemical Classification
Mineral
Elements
Silicates
Feldspar, Pyroxene,
Olivine, Mica, Quartz
Silicon, Oxygen
Carbonates
Calcite, Dolomite
Calcium Carbonate
Oxides
Hematite
Iron
Sulfates
Gypsum
Sulfur, Calcium
Halides
Halite
Sodium, Chlorine
Rock Cycle
• The continuous
changing of rocks
from one kind to
another over long
periods of time is
called the rock cycle.
The rock cycle has no
definite sequence. It
can follow many
different pathways.
Matter and the Rock Cycle
• Because granite is made of hard materials it is
resistant to nature’s forces. It can be slowly worn
down until bits of granite flake off and fall in
streams and are eventually reduced to sand. The
sand from granite, along with other sediments is
carried to the sea and is deposited on the floor.
The weight of layers piling on puts pressure on
lower layers and with calcite the granite becomes
part of a sedimentary rock. After many years,
under great pressure and temperature the
sedimentary rock will change to a metamorphic
rock, quartzite
James Hutton
• James Hutton, a Scottish physician and
naturalist, discovered in 1788 that rocks
undergo changes. He noticed that layers of
rocks had been altered since they formed.
• His findings still influence geologists today.
Igneous Rock
Chapter 4, Section 2
• Objectives:
• Recognize that magma and lava are the
materials that form igneous rocks.
• Contrast the formation of intrusive and
extrusive igneous rocks.
• Contrast basaltic and granitic igneous rocks.
Formation of Igneous Rocks
• When lava or magma cools and hardens, it
forms igneous rocks.
• Because magma is less dense than the
surrounding rock, it is forced upward to the
surface.
• When it reaches the surface, it is called
lava.
Intrusive Rocks
• Rocks that forms from magma below the
surface are intrusive igneous rocks.
• They are found at the surface only after the
layers of soil covering them are removed by
erosion.
Extrusive Igneous Rocks
• Extrusive Igneous
rocks are formed as
lava cools on the
surface of the earth.
• Since is exposed to
air and water, it cools
quickly. The cooling
rate keeps the grains
from growing large.
Igneous Rock Classification
• Igneous rocks are classified according to
their composition and texture.
• Composition refers to the minerals of which
rocks are formed.
• Texture means the shape, size, arrangement
and distribution of the minerals that make
up rocks.
• Both are evident in a rock’s appearance.
Igneous Rock Composition
• There are three basic igneous rock
compositions which produce three colors of
igneous rocks.
• Dark
• Intermediate
• Light
Igneous Texture
•
•
•
•
•
There are four textures of igneous rock:
Fine (Aphantic)
Coarse (Phaneritic)
Glassy
Vesicular (Full of holes)
Glassy Igneous
• Glassy igneous rocks
are shiny and look like
glass. They cool so
quickly, the crystals
do not have a chance
to grow.
• Obsidian is an
example of a glassy
texture igneous rock.
Fine-Grained Igneous
• Fine-grained rocks,
have crystals too small
to be seen without the
help of a microscope.
These extrusive rocks
grains cool quickly on
the surface of the
earth. This fine
grained rock is
rhyolite.
Coarse-Grained Igneous Rock
• Coarse-grained rocks,
such as granite, consist
of interlocking mineral
crystals, which are all
roughly the same size
and visible to the
unaided eye.
• Coarse grains grow as
rocks cool in the earth
(intrusive).
Vesicular Igneous Rocks
• Vesicular Igneous
rocks are formed when
lava that is full of gas
bubbles cools and
produces a rock with
open holes.
• Pumice and scoria are
examples.
Igneous Variety
• Where and how magma cools determines
the size of mineral crystals. The longer it
takes magma to cool, the larger are the
crystals that form. Glassy and fine-grained
rocks form from lava that erupts from
volcanoes and hardens on the Earth’s
surface. Coarse-grained rocks form from
molten rock that cools and hardens within
the Earth.
Igneous Rock Identification
Metamorphic Rocks
Chapter 4, Section 3
• Objectives:
• Describe the conditions in Earth that cause
metamorphic rocks to form.
• Classify metamorphic rocks as either
foliated or nonfoliated.
Metamorphic Rocks
• When already existing rocks are buried
deep within the Earth, tremendous heat,
great pressure and chemical reactions may
cause them to change into different rocks
with different textures and structures. The
changing of one type rock into another as a
result of heat, pressure and /or chemical
reactions is called metamorphism.
Metamorphic Rocks
• Metamorphic rocks are formed when
chemical reactions from hot fluids,
tremendous heat and great pressure change
existing rocks into new kinds of rocks.
These new rocks have chemical and
physical properties usually quite different
from the original rocks.
Metamorphic Rocks
• The amount of heat, pressure and chemical
reactions varies during metamorphism.
Thus the degree of metamorphism also
varies. The characteristics of the original
rock also affect the degree of
metamorphism. Many metamorphic rocks
can be produced from more than one kind
of rock.
Metamorphic Classification
• Like igneous and sedimentary rocks,
metamorphic rocks can be classified
according to texture and composition.
• There are two types of texture: foliated and
nonfoliated.
Foliated Rocks
• In the first group, the mineral crystals are
arranged in parallel layers, or bands. The
word foliated comes from the Latin word
for leaf. It describes the layers in such
metamorphic rocks, which are thin and flat.
Most metamorphic rocks are foliated, like
schist, slate and gneiss.
Unfoliated Rocks
• In the second, smaller group of
metamorphic rocks, the rocks are not
banded and do not break into layers. These
rocks are said to be unfoliated. Marble and
quartzite are examples of unfoliated rocks.
Metamorphic Identification
Sedimentary Rocks
Chapter 4, Section 4
• Objectives:
• Explain how sedimentary rocks form from
sediments.
• Classify sedimentary rocks as detrital,
chemical or organic in origin.
• Summarize the rock cycle.
Detrital
• The rocks that form by cementation or
compaction are known as detrital or clastic
sedimentary rocks.
• They are further subdivided by size of the
particles.
Chemical Sedimentary
• Chemical sedimentary rocks form from
solution
• An example would be rock salt or gypsum.
Organic Sedimentary
• These sedimentary rocks form from the
remains of once living organisms or the
processes of living organisms.
• Coal is an organic rock formed from plants.
• Coquina or chalk from from sea creatures.
Weathering
• Rocks are broken down over by chemical
weathering or physical weathering.
• Chemical weathering, like acid rain, alters
the structure of a rock by adding or
removing an element.
• Mechanical weathering. Like frost wedging,
just breaks the rock into smaller pieces.
Weathering
• Quartz is resistant to weathering because it
is so hard. Sand on the beach is often little
pieces of quartz.
• Sedimentary rocks are often compacted and
cemented together. This process of making
a rock is known as lithification.
Cementation
• Cementation occurs when water carrying
minerals runs over the sediments and
deposits minerals that act like glue sticking
them together.
Compaction
• Compaction is the pressing together of
particles over time due to layers being
deposited again and again. The weight and
pressure pushes the small pieces together.
Erosion
• Erosion occurs when the small pieces are
carried away.
• The agents of erosion are running water,
waves, and wind.
Extrusive Rocks
• Rocks formed from lava are called extrusive
rocks. Because lava is brought to the
surface by volcanoes, extrusive rocks are
also known as volcanic rocks. Basalt and
obsidian are two kinds of extrusive rocks
that are quite solid. Pumice, another
extrusive rock, is filled with bubbles.
Intrusive Rocks
• Igneous rocks formed deep within the Earth are
called intrusive. They form when magma forces
its way upward into preexisting rocks and then
hardens. Intrusive rocks include granite and
pegmatite. Intrusive rocks are also known as
plutonic rocks. A mass of intrusive rocks are
known as a pluton. Plutons may produce
landforms by pushing up layers of rock above
them, such as domes.
Sedimentary Rocks
• Igneous rocks are the most common rocks
on Earth but most of them exist below the
surface. 75% of the rocks exposed at the
surface are sedimentary.
Classification of Sedimentary
Rocks
• The most widely used classification system
for sedimentary rocks places them into three
main categories according to the origin of
the materials from which they are made.
These three categories are: clastic or
detrital rocks, organic rocks and chemical
rocks.
Clastic Rocks
• Sedimentary rocks that
are made of the
fragments of
previously existing
rocks are known as
clastic rocks. Clastic
rocks classified
according to the size
and shape of the
fragments in them:
conglomerate,
sandstone and shale.
Conglomerates
• Some clastic rocks are made of rounded
pebbles cemented together by clay, mud or
sand. If over a third of the rock is made of
pebbles, the rock is called a conglomerate.
The pebbles in conglomerates are smooth
and rounded because they have been worn
down by the action of water. They are also
called puddingstones. If the pieces of the
rock are angular then it is called breccia.
Sandstones
• Clastic rocks made of small, and-sized
grains are called sandstones. At least half
the particles in a clastic rock must be sand
sized in order for it to be considered a
sandstone. Sandstones are very common
rocks. They are formed from the sand on
beaches, in riverbeds and in sand dunes. In
a sandstone, the grains are cemented
together by minerals such as quartz, calcite
and hematite, that harden.
Shale
• Many geologists use term shale to describe
all the clastic rocks that are made of
particles smaller than sand. Shale forms
from small particles of mud and clay that
settle to the bottom of quiet bodies of water
such as swamps. Most shale can be split
into flat pieces.
Organic Rocks
• Organic rocks come from
organisms. Limestone are
often but not always
organic rocks. Deposits of
limestone may be formed
from the shells of
creatures when they die.
Creatures may also
cement their shells
together and over time
form reefs. Coal is also
made from the remains of
living things. It is made
from plants that lived
millions of years ago.
Coal and Coquina
Chemical Sedimentary Rocks
• Water precipitates two ways to form
chemical sedimentary rocks
• 1. by the process of evaporation
• 2. through the life processes of marine
organism
Biochemical Sedimentary Rocks
• Coral is an example of
a chemical
sedimentary rock that
was formed through
the life processes of a
coral polyp.
Evaporites
• Halite and gypsum are
examples of
evaporites.
Chemical Rocks
• Some sedimentary rocks
are formed when a sea or
lake dries up, leaving
large amounts of minerals
that were dissolved in
water. Examples of
chemical rocks formed
this way include rock salt
and gypsum. Some
limestone rocks are
formed by inorganic
processes in caves. As
water evaporates, a thin
deposit of limestone is left
behind.
Limestone- the most abundant
sedimentary rock
• Calcium carbonate is
carried in solution in
ocean water. When the
calcium carbonates comes
out of solution as calcite,
the crystals grow together
and lime stone forms.
Limestone is 50% calcite.
Much is found in Kansas
which was underwater.
Rock Salt
When water that is rich in
salt evaporates, it deposits
the mineral halite. Halite
forms rock salt. Rock salt
deposits can be as thick as
400 meters. Companies
mine these salts. It is used
in making glass, paper,
soap and dairy products.
The halite is processed for
table salt.
Inorganic Chemical Sedimentary
Rocks
• Travertine is an
inorganic chemical
sedimentary rock that
is found in caves.
Silicates
• Chert, jasper and flint
all form from silica
that is left behind
when water
evaporates.
Sedimentary Rock Classification