Classifying
Rocks
Figure 3 Texture helps geologists classify
rocks. Forming Operational Definitions –
Looking at the rocks below, describe the
characteristics of a rock that help you define
what a rock’s “grain” is.
Figure 3: Sample Answer
• Texture is described with the terms
based on:
• Grain size
– Fine-grained
– Coarse-grained
• Grain shape
– Smooth
– Jagged
• Grain pattern
– banded
– nonbanded
What characteristics are
used to identify rocks?
• When studying a rock sample,
geologists observe the rock’s
– Color
– Texture
– Determine its mineral composition
What are the three major
groups of rocks?
• Igneous rock
– Forms from the cooling of molten rock – either
magma below the surface or lava at the surface
• Sedimentary rock
– Forms when particles of other rocks or the
remains of plants and animals are pressed and
cemented together
– Forms below the surface
• Metamorphic rock
– Forms when an existing rock is changed by heat,
pressure, or chemical reactions
– Most metamorphic rock forms deep
underground
What tests do geologists
use to identify minerals in
rocks?
• Scratch Test (Mohs)
• Use acid to determine whether the
rock contains the mineral calcite
• Magnet test
Igneous Rock-0rigin
• Igneous rocks form
when molten rock
cools and solidifies.
Molten rock is
called lava when it
is above the Earth’s
surface and magma
when it is below.
Intrusive Igneous Rocks
• Igneous rocks that form
below the Earth’s surface
are called intrusive
igneous rocks .
• They form when magma
enters a pocket or
chamber underground
that is relatively cool and
solidifies into large
crystals as it cools very
slowly.
Intrusive Igneous Rock
EXAMPLES:
1. Granite
2. Gabbro
3. Diorite
Intrusive Igneous Rock:
• inside the volcano
• cools slowly
• large crystal
• coarse grained
Extrusive Igneous Rocks
• Extrusive igneous rocks, or
volcanic, form when magma
makes its way to Earth's
surface. The molten rock
erupts or flows above the
surface as lava, and then
cools forming rock.
• Most extrusive (volcanic)
rocks have small crystals.
Examples include basalt,
rhyolite, and andesite.
Glassy Igneous Rocks
Glassy Igneous Rocks cool so rapidly,
that atoms don’t have enough time to
get together, bond and form crystals.
To cool this quickly the rocks MUST
be extrusive.
•
•
•
•
Pumice (left)
Scoria (bottom left)
Obsidian (bottom)
Note gasses in the lava can cause fine holes as
seen in the pumice and scoria.
Extrusive rocks:
• Outside the volcano
• Cools quickly
• Small crystal
• Fine grained
Rock Cycle Questions
1. What are the 3 types of rocks?
2. Why is the set of natural processes by which
rocks change into other types of rocks called a
cycle?
3. If a rock began as a metamorphic rock, what
could happen to it?
4. Draw a diagram showing how an igneous rock
could change into a metamorphic rock and how
the metamorphic rock could change into a
sedimentary rock.
Sedimentary Rocks
Rocks made of bits & pieces of other
rocks. Sedimentary rock forms
when sediments-rocks made of bits
and pieces of other rocks are pressed
and cemented together, or when
minerals form from solutions.
How Sedimentary Rocks Form:
• When rocks, plants, and loose material get
pressed into layers over time.
• This process includes:
• Erosion-running water, wind, or ice loosen and
carry fragments of rock.
• Deposition-is the process by which sediment
settles out of the water or wind carrying it.
• Compaction-particles squeezed together under
great pressure.
• Cementation- particles are glued together.
Types of sedimentary rocks?
• There are 3 main types of sedimentary
rocks.
• 1.Rocks from sediments
• 2.Rocks made from remains of plants and
animals. (contains fossils)
• 3. Rocks made from minerals that are
dissolved in solutions
What is a metamorphic rock?
• The term "metamorphic" means
"to change form."
• Any rock (igneous, sedimentary,
or metamorphic) can become a
metamorphic rock. If rocks are
buried deep in the Earth at high
temperatures and pressures, they
form new minerals and textures
all without melting. If melting
occurs, magma is formed, starting
the rock cycle all over again.
Metamorphic Rock Types
• There are two types of
metamorphic rocks.
• Each is classified
according to its
composition and texture.
– Foliated
– Nonfoliated.
Foliated Metamorphic
Rocks
• FOLIATED metamorphic rocks
are those in which the minerals
have been flattened and pushed
down into parallel layers. The
bands in foliated metamorphic
rock look like pages in a book.
• Examples of foliated rocks are
slate, shale, and gneiss.
Non-foliated
• NON-FOLIATED metamorphic
rocks do not display layers. Rather,
they are massive structures with no
obvious banding. The mineral grains
grow and rearrange, but they don’t
form layers.
• A good example of non-foliated rock
is quartzite, the smooth-textured,
metamorphosed form of the mineral
quartz.
• A coarse-textured non-foliated rock
is marble.
• Anthracite, or hard coal, is a nonfoliated rock that forms when
intense pressure drives gases out of
soft coal, causing it to harden.
The Rock Cycle
Igneous
Sedimentary
Metamorphic
The Rock Cycle
Rock – a naturally formed solid that is usually
made up of one or more types of minerals
Rock Cycle – the set of natural processes that
form, change, break down, and reform
rocks.
3 Rock Types Review
Igneous Rock – forms when molten rock cools
and becomes solid. This can happen within
earth or on the surface
Sedimentary Rock – forms when pieces of
older rocks, plants, and other materials get
pressed and cemented together
Metamorphic Rocks – forms when heat and
pressure causes older rocks to change into new
types of rocks. These are formed deep under
the earth.
Describing the Rock Cycle
A rock’s cycle begins by becoming one of
the three types of rocks.
Describing the Rock Cycle
• If it begins as igneous, it can be changed with
heat and pressure to become a metamorphic
rock or it can reach the surface, be broken
down and turn into a sedimentary rock.
• If the rock begins as sedimentary, then it can
be changed by heat and pressure into a
metamorphic rock or it can be broken down
into sediment.
Describe the Rock Cycle
• If the rock begins as a metamorphic rock
then it can be turned into magma which
eventually becomes an igneous rock or it
can be brought up to the surface of earth
and broken down into sediment to become a
sedimentary rock.
Rock Cycle Vocabulary
• Erosion – the movement of rock pieces from
one place to another by wind, water, or ice
• Weathering – the breaking down of rocks by
wind, water, ice, or plant roots
• Heat – high temperatures
• Pressure – caused by layering or weight
• Melting – occurs due to magma
• Deposition – when rocks are moved to a new
location due to erosion
Rock Cycle