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Igneous Rocks
How do Igneous rocks form?
The answers are found in your:
Text book on page 112 or
Reference tables page 6
Write the answer in your notes. Then write what you
see on the slides for notes.
Intrusive Vs. Extrusive
<--Forms from-->
<-- Crystal size-->
<-- Alternate name-->
<-- Texture-->
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Felsic Vs Mafic
<-- Color -->
<-- Density -->
<-- Composition -->
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4 Factors that affect the formation
of Magma
1.
2.
3.
4.
Temperature
Pressure
Water content
Mineral composition
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1. Temperature
The temperature needs to be between 800 to 1200
degrees Celsius before they melt.
-Where does this heat come from in nature?
Geothermal gradient: The deeper into the Earth’s
crust you go, the higher the temperature gets.
Home
T H
e
m
L
p
Depth increasing -->
Back
2. Pressure
Like the geothermal gradient
Pressure increases with
depth due to the overlying
rocks
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P
r
e
s
s
H
L
Depth increasing -->
3. Water content
The amount of water will determine the melting point of
the rocks.
As the water content
increases the melting
point decreases.
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W
a
t
e
r
H
L
4. Mineral content
Different minerals have
different melting points
Mafic (rich with iron)
rocks have a higher
melting point than
Felsic (Al rich) rocks
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Partial melting
Because not all minerals have the same
melting points, not all parts of a rock melt at the
same time.
-Text book
page 102
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Fractional Crystallization
The process by which different minerals are
created at different temperatures.
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Bowen’s Reaction series
This illustrates the relationship between cooling
magma and mineral formation.
There are 2 main branches
1. Feldspars
2. Iron rich minerals
page 103 in text
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Igneous rock chart.
Found in text book page 107
Found in the ESRT’s page 6
The environment of
formation is found on the left
side of the chart. This
corresponds to the texture on
the right side. The deeper in
the ground a rock forms, the
slower it cools and the bigger
the crystal (grain size) is.
Intrusive --> coarse grained
Extrusive --> glassy and or fine grained
How do you use the chart?
-The further down on the chart you go, the rougher
the rock is
-The further to the right you go, the more dense, dark and mafic it is
-The further to the left you go, the less dense, lighter and felsic it is
-If you forget, look at the arrows underneath the main chart on pg 6
How can you determine mineral
composition?
1.
2.
3.
Pretend that the top igneous chart is made of
clear plastic.
Visualize that you are sliding the top chart
over the bottom one.
What are the three minerals that Rhyolite
(Middle left side) looks like it is made of?
Potassium Feldspar
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and
Quartz!
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Grain size
The faster the cooling rate the smaller the crystals.
The slower the cooling rate the larger the crystals.
Be able to show this by drawing a graph.
The bigger the crystal size the rougher the texture.
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Vocabulary
IGNEOUS: ROCKS THAT FORM FROM THE
SOLIDIFICATION OF MAGMA
INTRUSIVE: A ROCK THAT FORMS WITHIN EARTHS
CRUST COOLING SLOWLY and forms LARGER CRYSTALS
EXTRUSIVE: ROCKS THAT FORM ABOVE EARTHS
SURFACE, COOL QUICKLY and forms SMALLER CRYSTALS
FELSIC: HAS HIGH SILICA CONTENT, LIGHT IN COLOR,
LOW IN DENSITY AND HAS ALUMINUM and silicon IN ITS
COMPOSITION
MAFIC: LOW SILICA CONTENT, DARK IN COLOR, HIGH
IN DENSITY, HAS IRON AND MAGNESIUM
LAVA: MAGMA THAT IS ON EARTH’S SURFACE
MAGMA: MOLTEN ROCK BENEATH EARTHS SURFACE
TEXTURE: HOW IT FEELS BASED ON ITS GRAIN SIZE
PORPHYRY: A ROCK WITH 2 DIFFERENT TEXTURES DUE
TO TWO DIFFERENT COOLING RATES.
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Ch 5 Quiz
1. What is the color, composition and
density of a mafic rock?
2. What are the 4 Factors that affect the
formation of Magma?
3. Define Geothermal Gradient
4. What is the relationship between the
cooling rate and the grain (crystal) size?
List the minerals that are found in
Obsidian