A Rock is… A group of minerals
that have been “put together” in
several different ways.
Depending on how they are put
together, rocks are classified as:
1. Sedimentary
2. Igneous
3. Metamorphic
Sedimentary Rocks are formed
by several different processes.
1. Deposition, Compaction and Cementation
(Lithification) of Sediments.
2. Crystallization or Chemical Reaction of
Minerals in Solution in water.
Igneous Rocks are formed by the Crystallization,
by Cooling and Hardening, of molten Magma
(molten rock below ground) or Lava (molten rock
above ground).
Metamorphic rocks are formed by the
Recrystallization or Chemical Change of
minerals due to Heat and/or Pressure.
Let’s Focus on…
How are Igneous Rocks
Classified?
Igneous Rocks are classified based
upon several different qualities or
characteristics:
1. Mineral Composition
2. Location of Crystallization
3. Texture of Grain Size
1. Igneous rocks are identified
based upon Mineral
Composition
(high or low Silica content)
High Silica rocks are called Felsic
Igneous Rocks and are light in color.
Felsic Rocks and Minerals
Silica-Rich, or Felsic Rocks
Igneous rocks that have a Silicate
composition of 65% or more are
considered to be Felsic.
Low Silica rocks are called Mafic
Igneous Rocks, and are dark in color.
Mafic Rocks
Silica-Poor, or Mafic Rocks
Igneous rocks that have a Silicate
composition of 45-50% or less are
considered to be Mafic.
Igneous rocks that have a Silicate
composition of 50-65% are considered
to be Intermediate or Andesitic (named
after a rock made from a mixture of
Mafic magma and minerals from Felsic
“country rock”).
Igneous rock can be classified
by Silica and Oxide content
Pure Silica
2. Igneous Rocks are identified
based upon where they are
formed (above or below ground).
Igneous rocks formed underground
from Magma are called Plutonic or
Intrusive rocks.
Notice the large crystals
Igneous rocks formed above ground from lava
are called “Extrusive” or “Volcanic” rocks.
Felsite
Rhyolite
Obsidian
3. Igneous Rocks are identified
based upon Texture (size of
mineral crystals)
Large-Grained
Glassy
Texture of Grain Size is evidence
of rate of cooling; The faster
magma or lava cools, the smaller
the size of the crystals in the
solidified material will be.
Pumice
Obsidian
When Magma cools slowly
underground, large crystals have time to
form. Therefore, Igneous rocks with
large crystals are most likely Plutonic.
When Lava cools rapidly above
ground, crystals have a limited time
to form. Therefore, Igneous rocks
with small or no crystals are most
likely Volcanic.
Igneous Rock Textures
Obsidian-Glassy Felsic
Basalt- Fine-grained Mafic
Andesite- Fine-grained
Intermediate
Rhyolite- Fine-grained Felsic
Granite- Coarse-grained Felsic
Diorite- Coarse-grained
Intermediate
Gabbro- Coarse-grained Mafic
Pumice- Vesicular
Felsic
Vesicular Basalt- Mafic
Scoria-Vesicular Mafic
Basalt Porphyry- Mafic
Rhyolite Porphyry-Felsic
Andesite PorphyryIntermediate
How does Mineral Composition
of magma or lava affect an
Igneous Rock or the molten
material that it forms from?
High Silica content causes
Felsic Magma to be thick, or
viscous, and slow-flowing.
Felsic Dome
Felsic Dome
Low Silica content causes
Mafic Magma to be thin or
low-viscosity, and fast-flowing.
Flood Basalt Plateau
Igneous rock can be classified
based on where they were formed.
(location of Crystallization)
Intrusive Igneous Rock forms
structures such as…
Dikes
Dike: Sheet-like Igneous rock that cuts
across the structure of pre-existing rock.
Dike
Volcanic Neck
Volcanic Dike
This would form
a Dike.
Sills
Sills: Sheet-like Igneous rock that lies
parallel to the layering of pre-existing
rock.
Sill
Sill
Laccoliths
Laccoliths: A Laccolith is a vertically
developed sill.
Exposed/Eroded Laccoliths
Batholiths
Batholiths: A Batholith is a large Plutonic
mass of Igneous Rock (old Magma
“bubble”).
Halfdome- Yosemite, California
Enchanted Rock-Texas
Extrusive Igneous Rock
forms structures such as…
Felsic Lava Flows
Mafic Lava Flows
Pahoehoe Lava
Domes
Flood Basalt PlateausMafic Lava
Tephra or Ash Deposits
Volcanic Cones
Composite Cone (lava and
Tephra)- Felsic
Cinder Cone- Mafic Lava
Shield Cone-Mafic Lava