Download Igneous Rock

Unit 3 - Rock Types
Igneous Rocks
Basic Rock Classifications
● Igneous
● Sedimentary
● Metamorphic
Igneous Rocks
An igneous rock is formed when magma or lava
cools and solidifies
Molten rock
in the Earth is magma
- Magma is buoyant, rises to surface, & sometimes breaks
through
Magma reaches Earth’s surface it is called lava
Igneous Rocks
Two types of igneous rocks
● Plutonic/Intrusive – cools underground
● Volcanic/Extrusive – cools above ground
Plutonic rocks
To see them, they must be uplifted to surface
And softer surrounding rock eroded away
Plutonic rocks
Volcanic necks
• Shallow intrusion formed when magma solidifies
in throat of volcano
Plutonic rocks
Dikes
• Tabular intrusive structure that cuts across any
layering in existing rock
Plutonic rocks
Sills
• Tabular intrusive structure that parallels layering
in existing rock
Plutonic rocks
Plutons
• Large, blob-shaped intrusive body formed of
coarse- grained igneous rock, commonly granitic
• Small plutons are called stocks,
large plutons are called batholiths
Geothermal Gradient
– the rate at which temperature increases with depth
in the Earth
*important for the
formation of
igneous rocks
Igneous Rock Formation
As magma cools, atoms arrange in an
orderly crystal structure
crystallization
Igneous Rock Classification
Igneous rocks are classified two ways
1. Texture
2. Mineral composition
Igneous Rock Classification - Texture
Igneous Rock Classification - Texture
5 Types of Textures
1. Phaneritic (coarse-grained)
2. Aphanitic (fine-grained)
3. Glassy (compact and frothy)
4. Vesicular
5. Porphyritic
Textures - Phaneritic (coarse-grained)
forms far below surface
Slow Cooling Large Crystals
Examples: Gabbro, diorite,
and granite
Textures - Aphanitic (fine-grained)
fast cooling magma/lava
forms at or near surface
can’t see individual
crystals (small)
Examples: Basalt, andesite, and rhyolite
Texture - Glassy (compact and frothy)
Very Rapid Cooling
No Crystal Structure
Example: Obsidian
Texture - Vesicular
Has many cavities at its surface and inside
Forms from gas bubbles in the magma
Example: Pumice
*Can be aphanitic vesicular or glassy vesicular rocks
Texture - Porphyritic
2 cooling stages: magma cooled slowly for
a while then erupted and cooled quickly.
minerals crystallize at different
temperatures and or rates
Different sized crystals
Example: Porphyritic Granite
Worksheet - Homework
Igneous Rock (Texture) Worksheet
Igneous Rock Classification
Igneous rocks are classified two ways
1. Texture
2. Mineral composition
Igneous Rock Classification - Chemical Composition
mainly silicate minerals
determined by composition of magma
from which it crystallized
magma mainly 8 elements:
Si, O, Al, Ca, Na, K, Mg, Fe
Bowen's Reaction Series
The geologist N.L. Bowen (Norman) found
that minerals tend to form in specific
sequences in igneous rocks
These sequences are assembled into
temperature gradient chart
Bowen’s Reaction Series
Igneous Mineral Composition Types
1. Mafic
2. Felsic
3. Intermediate
4. Ultramafic
Igneous Mineral Composition Types - Mafic
Have ~50% silica, by weight,
Contain dark colored minerals with iron,
magnesium and calcium
• Examples: Intrusive/extrusive - gabbro/basalt
Mafic Chemical Composition
Igneous Mineral Composition Types - Felsic
Have >65% silica, by weight,
Contain light-colored minerals that are abundant in
silica, aluminum, sodium and potassium
Examples: Intrusive/extrusive - granite/rhyolite
Felsic Chemical Composition
Igneous Mineral Composition Types - Intermediate
rocks have silica between (50-65%) those of mafic
and felsic rocks
Examples: Intrusive/extrusive - diorite/andesite
Intermediate Chemical Composition
Igneous Mineral Composition Types - Ultramafic
Have the least amount of <45% silica, by weight,
Are composed almost entirely of dark-colored
ferromagnesian (Fe and Mg) minerals
Example: peridotite (intrusive)
Special Note
Different minerals crystallize from magma at different
temperatures. (Bowen’s Reaction Series)
As minerals crystallize from magma the remaining composition
is forever changed.
Therefore, it is possible for more than one igneous rock to form
from the same magma depending on the cooling
environment.
Worksheet - Homework
Igneous Rock (Composition) Worksheet
Diamonds STSE - Kimberlite
What is a kimberlite?
kimberlite is a ultramafic, igneous rock which
occurs as small volcanic pipes, dykes, and sills.
Kimberlite commonly contains inclusions of
ultramafic rocks.
Kimberlite and Diamonds
they are formed deep within the mantle, at are
erupted rapidly and violently,
It is this depth of melting and generation
which makes kimberlites prone to hosting
diamond xenocrysts.
Kimberlite indicators
formed under high pressure and temperature within the mantle
Complete STSE
Complete Lab 4 Part 1