Rocks have different chemical and physical properties that make them useful in our everyday lives. We use rocks for construction, fuel, art, and other purposes. Rocks are classified into three main groups based on their method of formation: igneous, sedimentary, and metamorphic. Igneous rocks form from the cooling and hardening of molten material from within the Earth. Their texture, mineral composition, and other features provide clues to how quickly or slowly they cooled.

1. ROCKS:
You probably don’t realize it,
but we depend on many rocks in
our everyday lives. Rocks have different
chemical and physical properties that make
them quite useful to us. Besides providing a
nice hard place for us to stand on Earth, we use
rocks for the construction of our homes, for fuel,
and for the creation of statues and art. Go home
today and look around your house. You may be
surprised by what rocks you find.

2. ROCKS:
A ROCK is composed of one or more
minerals. Rocks composed of only one
mineral are said to be
MONOMINERALLIC.
This is a photo of a slice of Dunite, a
monominerallic igneous rock, under a
microscope. Almost all of the
grains in this rock are olivine. Note
the high order interference colors
of olivine and the minor secondary
calcite which occurs as veinlets
through the sample.

3. ROCKS:
Most rocks are
POLYMINERALLIC,
that is, made of
many different
minerals. There are
over 2,500 different
minerals. However,
there are 10-15 minerals known as the rock-
forming minerals, which compose about 90%
of all the rocks in Earth’s crust.

4. METHOD OF FORMATION:
Rocks are classified by their
METHOD OF FORMATION, into
three groups:
1) IGNEOUS
2) SEDIMENTARY
3) METAMORPHIC
Let’s take a more detailed at each.

5. IGNEOUS ROCKS:
Igneous rocks form from the MELTING and
SOLIDIFICATION (hardening) of molten
material from within the Earth.

6. IGNEOUS ROCKS:
Molten material within the Earth is called
MAGMA. When this molten material
reaches Earth’s surface it’s called LAVA.
LAVA:
ON OR NEAR
EARTH’S
SURFACE
MAGMA:
INSIDE OF
EARTH

7. IGNEOUS ROCKS:
Igneous rocks are solid, hard, and compact and
are composed of randomly arranged
INTERGROWNMINERALCRYSTALS.
GRANITE IS AN IGNEOUS MAYBE YOU CAN SEE
ROCK WITH COARSE THEM BETTER CLOSE UP?
GRAINED CRYSTALS.

8. TEXTURE:
Igneous rocks are best identified by their
texture, environment of formation, and
mineral composition
Texture describes a rock’s “grain
appearance” which is determined by the
size of the crystal and the arrangement
of the crystals.
There are FOUR textures:
glassy, fine, coarse, and very coarse.

9. TEXTURE:
1) GLASSY: A glassy texture forms
when a mineral cools so quickly that no
crystals are visible. Glassy textures
occur when there is a rapid drop in
temperature and
pressure. This condition
OBSIDIAN
exists when magma
reaches Earth’s surface
through a volcano.

10. SCHEME FOR IGNEOUS ROCK IDENTIFICATION
IN YOUR ESRT:

11. TEXTURE:
There are TWO types of glassy texture:
PUMICE
a) VESICULAR: A vesicular
texture exhibits gas
pockets and the rock
appears holey. Ex: Pumice
b) NON-VESICULAR:
OBSIDIAN
A non-vesicular texture
looks like glass. Ex: Obsidian

12. SCHEME FOR IGNEOUS ROCK IDENTIFICATION
IN YOUR ESRT:

13. TEXTURE:
2) FINE: A fine texture
RHYOLITE
forms when a mineral
cools quickly at or near
Earth’s surface. The
grain size is less than
1mm. Ex: Rhyolite
BASALT SCORIA
ANDESITE

14. SCHEME FOR IGNEOUS ROCK IDENTIFICATION
IN YOUR ESRT:

15. TEXTURE:
3) COARSE: A fine texture
GRANITE
forms when a mineral cools
SLOWLY WITHIN the
Earth under high pressure.
The grain size ranges from
1-10 mm. Ex: Granite
PERIDOITE DIORITE GABBRO

16. SCHEME FOR IGNEOUS ROCK IDENTIFICATION
IN YOUR ESRT:

17. TEXTURE:
4) VERY COARSE: A course PEGMATITE
texture forms when a mineral
cools very slowly within the
Earth, producing grain sizes
10 mm or larger. Ex: Pegmatite
A CLOSER LOOK!!!

18. SCHEME FOR IGNEOUS ROCK IDENTIFICATION
IN YOUR ESRT:

19. TEXTURE:
TEXTURE IS DETERMINED BY THE
LENGTH OF COOLING!
Rocks that cool FAST have SMALLER crystals.
Rocks that cool SLOW have LARGER crystals.
SMALL LARGE
CRYSTALS CRYSTALS
COOLED COOLED
FAST SLOWLY

20. RATE & DEPTH OF COOLING:
SUPER IMPORTANT FACT!
The grain size of igneous rocks depends on the RATE
and DEPTH at which magma cools.
ROCK NAME GRAIN SIZE A ENVIRONMENT
The diagram Basalt 0.1 mm Surface
B
shows the
relationship Diabase 1 mm Near
between grain size C Surface
and rate/depth of
cooling. Granite 10 mm Deep
Slow Moderate Fast
COOLING RATE

21. RATE & DEPTH OF COOLING:
CRYSTAL SIZE
CRYSTAL SIZE
CRYSTAL SIZE
COOLING RATE COOLINGTIME DEPTH
THESE ARE GRAPHING RELATIONSHIPS THAT YOU SHOULD KNOW.

22. RATE & DEPTH OF COOLING:
CRYSTAL SIZE
CRYSTAL SIZE
CRYSTAL SIZE
COOLING RATE COOLINGTIME DEPTH
THESE ARE GRAPHING RELATIONSHIPS THAT YOU SHOULD KNOW.

23. ENVIRONMENT OF
FORMATION:
By studying a rock’s texture,
you can infer a rock’s
environment of formation.
There are TWO different
environments of formation.
1) EXTRUSIVEorVOLCANIC: This is when an
igneous rock hardens NEAR the Earth’s
surface or ON the Earth’s surface to form a
rock with small or no crystals. These rocks
are found by volcanoes.

24. SCHEME FOR IGNEOUS ROCK IDENTIFICATION
IN YOUR ESRT:

25. ENVIRONMENT OF
FORMATION:
2) INTRUSIVEorPLUTONIC: This is when
an igneous rock hardens within the Earth
to form large crystals. When magma
doesn’t erupt it can flow underground.
Underground magma flows are called
INTRUSIONS because they can move into
and through fractures (cracks) in rock. If
an intrusion hardens, it produces a large
mass of igneous rock called a PLUTON.

26. SCHEME FOR IGNEOUS ROCK IDENTIFICATION
IN YOUR ESRT:

27. CHEMICAL COMPOSITION:
A rock’s chemical composition refers to a rock’s
mineral make-up. Igneous rocks are generally
described as having either a FELSIC composition
or a MAFIC composition.
1) FELSIC COMPOSITION: Rocks with a felsic
composition tend to contain a high percentage of
the minerals potassium feldspar and quartz. They
are RICH IN ALUMINUM (Al) and low in both iron
(Fe) and magnesium (Mg). These rocks are usually
LIGHTER IN COLOR and LOWER IN DENSITY
than mafic rocks. Felsic rocks make up a large
portion of continental crust. The granite family of
rocks is felsic.

28. SCHEME FOR IGNEOUS ROCK IDENTIFICATION
IN YOUR ESRT:
NOTE:
Even though
OBSIDIAN
is dark in
color, it is
considered
FELSIC, be
cause it is
low in
density.
This is the
one
exception
to the rule!

29. CHEMICAL COMPOSITION:
2) MAFIC COMPOSITION: Rocks with a mafic
composition tend to contain a high percentage
of the minerals plagioclase feldspar and
pyroxene. They are RICH IN IRON (Fe) and
MAGNESIUM (Mg) and low in ALUMINUM
(Al). These rocks are usually DARKER IN
COLOR and HIGHER IN DENSITY than felsic
rocks. Mafic rocks are most common in the
ocean basins. The Gabbro family of rocks is
mafic.

30. SCHEME FOR IGNEOUS ROCK IDENTIFICATION
IN YOUR ESRT:

31. MINERAL COMPOSITION:
WHAT IS MINERAL
COMPOSITION?
Mineral composition refers
to the different minerals
that a rock is made up of
and the percentages of
these minerals.
Granite is mostly made up of
POTASSIUM FELDSPAR
(pink/gray), BIOTITE MICA
(black), and QUARTZ
(clear/white).

32. MINERAL COMPOSITION:

33. SCHEME FOR IGNEOUS ROCK IDENTIFICATION
IN YOUR ESRT:

34. FINDING THE MINERAL COMPOSITION USING
YOUR ESRT:
You can determine the
possible mineral
composition of specific
igneous rocks by looking
at your ESRT. Simply
find a rock name on the
chart, then look at the
minerals found in the
column under that rock
name. Your rock name
can contain varying
amounts of those
materials.

35. FINDING THE PERCENTAGE OF MINERALS IN A
IGNEOUS ROCK USING YOUR ESRT:
You can determine the
possible mineral
composition of specific
igneous rocks by looking
at your ESRT. Simply
find a rock name on the
chart, then look at the
minerals found in the
column under that rock
name. Your rock name
can contain varying
amounts of those
materials.

36. FINDING THE PERCENTAGE OF MINERALS IN A
IGNEOUS ROCK USING YOUR ESRT:
You can determine the
possible mineral
composition of specific
igneous rocks by looking
at your ESRT. Simply
find a rock name on the
chart, then look at the
minerals found in the
column under that rock
name. Your rock name
can contain varying
amounts of those
materials.

38. REVIEW OF IGNEOUS ROCKS:
INTRUSIVE EXTRUSIVE
( PLUTONIC) (VOLCANIC)
INSIDE EARTH ENVIRONMENT ON/NEAR EARTH’S
OF FORMATION SURFACE
SLOW SPEED OF FAST
COOLING
LARGE SIZE OF SMALL
CRYSTAL
COARSE, VERY COARSE NAME OF FINE, GLASSY
TEXTURE
GRANITE, DIORITE, GABB ROCK EXAMPLES OBSIDIAN, SCORIA, PU
RO, PERIDOITE, DUNITE, MICE, RHYOLITE, AND
PEGMATITE ESITE, BASALT

39. REVIEW OF IGNEOUS ROCKS:
FELSIC MAFIC
Aluminum (Al) MINERAL Iron (Fe)
Silicon (Si) COMPOSITION Magnesium (Mg)
Light COLOR Dark
Low DENSITY High
TEMPERATURE AT
Low WHICH MINERALS High
IN ROCK
CRYSTALLIZE
Granite, Pegmatite, Rhyolite ROCK EXAMPLES
Scoria, Basalt, Gabbro,
, Pumice, Obsidian Peridoite, Dunite