Rock Forming Processes
Big Idea
Understanding natural processes
and Earth materials allows Earth
history to be discerned.
The Rock Cycle
The rock cycle allows us to view
different relationships between the
parts of the Earth system.
No beginning or an end.
 We begin with magma.
 Magma cools and forms crystals in a process
called crystallization. When cooling occurs,
depending on the conditions and time,
atoms are arranged forming geometric
shapes.
 The rocks formed from cooling magma are
called igneous rocks. Once igneous rocks are
formed, they can undergo various changes.
 Igneous rocks can be weathered and eroded
through sedimentation. When sediment
undergoes a natural process of cementation
called lithification, they become sedimentary
rock.
 Igneous rocks can also be buried,
undergoing metamorphism, or experiencing
extreme pressure and temperatures.
 Rocks that experience metamorphism
become distorted from their original forms.
 Any of the three types of rocks under the
right circumstances can become another
type of rock. They can even cycle between
the same types.
 For example, sedimentary rocks can
become sediment again and become part of
a different type of sedimentary rock.
 The rock cycle takes place at locations where there is
activity.
 1. Convergent and
 2. Divergent plate boundaries
 3. Rivers/Streams
 4. Deserts, etc.
 Convergent boundaries can create volcanoes and many
different types of igneous rocks as well as cause erosion,
and metamorphism as one plate moves beneath another.
 Divergent boundaries allow magma to rise to the surface
of the earth creating new igneous rocks.
Igneous Rocks
 Igneous rocks form from magma or lava,
depending on the location. There are two
types of igneous rocks based on where they
form.
 Intrusive (plutonic) igneous rocks form
inside the earth and “intrude” upon existing
rock structures. These rocks cool slowly
because the Earth around them acts as
insulation.
 Intrusive (plutonic) igneous rocks form
inside the earth and “intrude” upon existing
rock structures. These rocks cool slowly
because the Earth around them acts as
insulation.
Intrusive rock features:
Coarse or large grained texture
Visible crystal arrangement
Minerals are identifiable
Examples: Granite, Diorite,
Gabbro, Peridotite
 Extrusive (volcanic) rocks are igneous rocks
that form when lava solidifies or extrudes
onto the surface of the earth. These rocks
often cool quickly when they are exposed to
the air.
 Extrusive rock features:
 Fine or small grained texture
 No crystals apparent
 Voids (vesicles) left by escaping gasses
(Scoria)
 Examples: Basalt, Rhyolite, Andesite
 Some igneous rocks form when lava or
magma flows into large bodies of water or is
ejected into the air. The rapid cooling
caused by the cooler water/air
temperatures forms volcanic glass.
 Obsidian is an example of volcanic glass,
and so is pumice.
 Sometimes, a rock will cool underground
and then suddenly be erupted. This results
in an igneous rock with a porphyritic
texture, or large and small crystals mixed
together.
 One magma can produce many different
types of igneous rocks. Bowen’s Reaction
Series is a chart that shows the sequence of
mineral crystallization in magma.
All igneous rocks are classified by their
texture and composition.
 The four textures of
igneous rocks are:
 Coarse-grained
 Fine-grained
 Porphyritic
 Glassy
 The four different
compositions are:
 Granitic
 Andesitic
 Basaltic
 Ultramafic (*rare
on Earth’s surface)
Sedimentary Rocks
 Sedimentary rocks are made from the
sediments, or tiny pieces, of other rocks.
 Weathering of other rocks with the help of
gravity and erosional agents carry sediment to
new locations. The sediment becomes lithified,
or turned to rock by compaction and
cementation.
 The word sedimentary comes from the Latin
word sedimentum, meaning, “settling”.
 Over time, weathering and deposition of
sediment leads to a build up of materials.
The layers on the bottom become
compacted and together with mineral
matter form solid rock.
 Sedimentary rocks make up about 5%
(volume) of the Earth’s crust, but make
up 75% of the rocks that are exposed at
the surface.
 Sedimentary rocks are used to look
back at the history of the Earth and give
answers to ancient climates and life.
 Sedimentary rocks are very valuable to world
economies. For example, the following are all found
in sedimentary deposits:
 Coal
 Petroleum
 Natural gas
 Iron
 Aluminum
 Sand
 Gravel
 Classifying sedimentary rocks
 There two ways that sedimentary rock can form:
 They can originate from solid particles weathered
from other rocks. These are called detrital
sedimentary rocks.
 They can form from chemical sediment, or
precipitate out of liquids to reform solids. These
are called chemical sedimentary rocks.
Detrital sedimentary rocks
 Particle size is used to distinguish between detrital
rocks. There are four size categories that are used to
organize them.
 Conglomerate- large rounded pieces of rock dominate
 Breccia- large angular pieces of rock dominate
 Sandstone- sand size grains are present
 Shale- fine-grained sediment is present, also the most
abundant
 These size classifications also provide clues about
the environment in which the sediment was
deposited. The size reflects the amount of energy
present in the environment and the shape can
reflect the time of sediment transport.
 For example, glacier movement vs. stream vs.
wind vs. lakes
 Names can be mixed when more than one size of
sediment is present, i.e. sandy siltstone,
depending on which size predominates.
 Chemical sedimentary rocks
 Formed from material carried in solution,
or dissolved in liquids. Precipitation (not
rainfall) occurs by physical processes or by
organisms.
 Limestone is the most abundant chemical
sedimentary rock. It is composed of mostly
biochemical sediment, or stuff that was once
alive. (Coquina or chalk)
 Other chemical sedimentary rocks form from
solids precipitating out of solution. This can
occur because of water chemistry changes or
temperature changes.
 Travertine is an example of a chemical
sedimentary rock. It is often found in
caves, forming as groundwater drips or
trickles.
 Agate is a popular Michigan rock that is
a chemical sedimentary rock.
 Evaporation can also form chemical
sedimentary rocks. As the water leaves the
minerals are left behind. Halite and gypsum
are both common evaporates.
 Halite - cooking and seasoning
 Gypsum - drywall or sheet rock.
 Coal is a special type of sedimentary rock.
It forms when organic matter such as forest
or woods are buried for extended periods of
time.
 Swampy areas have the right conditions for
coal to form.
 Lithification of sediment
 Compaction and cementation are the two
processes that are needed to turn sediment into
rock.
 Compaction is the smooshing or crushing of
sediment beneath a buildup of more sediment.
 Cementation is the use of a natural cementing
agent such as calcite, silica or iron oxide to “glue”
or “stick pieces of sediment together.”
 The cementing agent can clearly be
distinguished because:
 Calcite- fizzes in diluted acid
 Silica- the hardest cementing agent
 Iron oxide- orange or red color present
Features of sedimentary rocks
 Strata, or beds- flat surfaces on which sediments are
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deposited and form
Fossils- traces or remains of prehistoric life,
environments, etc.
Ripple marks- form in areas that had gently lapping
waves
Mud cracks- mud or clay dries and shrinks, leaving gaps
Geodes- rocks that form from dissolved minerals getting
inside sedimentary rocks
Graded bedding-sorted layers of rocks with larger at
bottom
Metamorphic Rocks
 Metamorphism means, “to change form”.
Metamorphic rocks are existing rocks that
have been changed in size, shape or texture
due to three agents:
 Heat
 Pressure
 Chemically active fluids
 There are different levels of
metamorphism ranging from low-grade
(parent rock is identifiable) to highgrade (parent rock is obliterated).
 Metamorphic rocks do not melt!! If
they did, they would be igneous.
 Metamorphism usually occurs in one of two
settings:
 *Regional metamorphism – like mountain
building, or
 Contact metamorphism – when rocks are near
magma masses
 *Most metamorphic rocks form from regional
metamorphism.
 The three agents of metamorphism can work
together, but most often work in ones or twos. Heat
is the most important metamorphic agent.
 Temperature and Pressure increase with depth, so it
makes sense that rocks at great depths under go
intense metamorphism. In these situations rocks can
experience the great bending and rippling that are
often seen in metamorphic formations.
 Water is the mostly common chemically active fluid.
 The texture changes that occur with
metamorphism create the two types of
metamorphic rocks:
 Foliated metamorphic rocks
 Nonfoliated metamorphic rocks
 Foliated metamorphic rocks
 Rocks that undergo recrystallization often do so in
a preferred direction creating bands or stripes of
certain minerals or rocks.
 This usually occurs during regional
metamorphism.
 Foliated rocks form when minerals are brought
into alignment.
 Example: Shale becomes slate; any schists; gneiss
 Nonfoliated metamorphic rocks
 Do not exhibit banding or stripes. These rocks
are often formed from only one mineral.
 This usually occurs during contact
metamorphism.
 Example: limestone becomes marble; sandstone
becomes quartzite