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A rock is a naturally occurring solid
mixture of one or more minerals, or
organic matter
Rocks are classified by how they
are formed, their composition, and
Rocks change over time through
the rock cycle
Igneous rock begins as
 Magma can form:
▪ When rock is heated
▪ When pressure is released
▪ When rock changes
 Magma is a mixture of
many minerals
Coarse-grained: takes longer to cool,
giving mineral crystals more time to
 Fine-grained: cools quickly with little to
no crystals
Big crystals cool slow
Small crystals cool fast
Intrusive Igneous Rocks:
magma pushes into
surrounding rock below the
Earth’s surface
Extrusive Rocks: forms when
magma erupts onto the Earth’s
surface (lava), cools quickly
with very small or no crystals
Obsidian is a dark-colored volcanic glass that forms from the very rapid
cooling of molten rock material. It cools so rapidly that crystals do not form.
Is this rock Felsic or
Is it fine-grained or
Is this rock Intrusive or
Mafic, fine grained, extrusive
 Means to change shape
 Changes with heat and
pressure, but remains solid
 Usually takes place deep
in the Earth
Foliated - contain aligned grains of flat
minerals (lines)
Gneiss is foliated
metamorphic rock
that has a banded
appearance and is
made up of granular
mineral grains.
It typically contains
abundant quartz or
feldspar minerals.
Non-Foliated – mineral grains are not
arranged in plains or bands
Marble is a nonfoliated metamorphic
rock that is produced
from the
metamorphism of
It is composed
primarily of calcium
Determine if the following rock samples are
foliated or non-foliated:
Sedimentary rock is formed by erosion
Sediments are moved from
one place to another
Sediments are deposited in
layers, with the older ones
on the bottom
The layers become compacted
and cemented together
Sedimentary Rocks are formed at or near the
Earth’s surface
No heat and pressure involved
Fossils found in these rocks
 Strata – layers of rock
 Stratification – the process in
which sedimentary rocks are
arranged in layers
Clastic – made of fragments of rock
cemented together with calcite or quartz
Breccia is a term most often
used for clastic sedimentary
rocks that are composed of
large angular fragments
(over two millimeters in
The spaces between the
large angular fragments can
be filled with a matrix of
smaller particles or a mineral
cement that binds the rock
Chemical sedimentary – minerals crystallize
out of solution (water) to become rock
Limestone is a sedimentary rock
composed primarily of calcium
carbonate (CaCO3) in the form of
the mineral calcite. It most
commonly forms in clear, warm,
shallow marine waters.
It is usually an organic
sedimentary rock that forms
from the accumulation of shell,
coral, algal and fecal debris.
Organic sedimentary – remains of plants and
Coal is an organic
sedimentary rock that
forms from the
accumulation and
preservation of plant
materials, usually in a
swamp environment.
Coal is a combustible rock
and along with oil and
natural gas it is one of the
three most important
fossil fuels.
 The continuous changing of rocks
from one kind to another over long
periods of time is called the rock
 The rock cycle has no definite
sequence. It can follow many
different pathways.
 Igneous:
Melting to Magma/Volcanic Activity
 Sedimentary:
Erosion – Weathering-Deposition Cementation – Compaction
 Metamorphic:
Heat and Pressure
Fossils are found in Sedimentary rocks
 They help provide information of life forms
that lived thousands of years ago.
Petrified – Minerals have filled in where an
organisms once was
Molds – A hollow area that an organism has
Cast – A copy of the shape of the organism
(opposite of a mold)
Carbon Film – a thin coating of carbon that is
left behind when gases have been release by
the decaying fossil (like a leaf print on a rock)
Trace – An imprint that an organism left
behind (like a footprint)
Fossils in which minerals replace all
or part of an organism.
 How does this happen?
 Water rich in dissolved minerals
seeped into spaces, evaporated,
leaving the hardened minerals
 Example – petrified wood
Most common type of fossil.
Both copy the shape of the
 A mold is a hollow area of sediment
in the shape of the organism.
 A cast is a copy of the shape of an
Carbon film is an extremely thin
coating of carbon
 How does this happen?
 All organisms are made of carbon.
When they are buried, the
materials that make up the
organism evaporates. These gases
escape leaving carbon behind.
Trace fossils provide evidence of the
activities of ancient organisms.
 Examples
▪ A footprint provide clues about the size
and behavior, the speed, how many legs it
walked on, lived alone or with others.
▪ A trail or burrow can give clues about the
size and shape of the organism, where it
lived, and how it obtained food.
 Relative Age – It’s age when
compared to others ( I am older than
 Absolute age – It’s actual or real age
 We use radioactive dating to find
 Unconformity – a gap in the the
geological record (like from erosion)
Sometimes magma pushes, or intrudes, into cracks in
existing rocks. When the melted rock cools and solidifies,
the resulting feature is called an igneous intrusion.
An igneous intrusion
is always younger
than the rock it cuts
This image shows metamorphic rock in
Death Valley, California, cut by a darker
igneous intrusion.