Download Igneous Rocks - My Illinois State

Earth Materials continued
• Mineral
•
•
•
•
•
Naturally Occurring
Inorganic
Solid
Chemical Composition
Crystalline Shape
Rock
• a coherent, naturally occurring solid, consisting of
an aggregate of one or more minerals, or a mass of
natural glass or organic matter.
Basic Rock Classifications
•Igneous
•Sedimentary
•Metamorphic
Rock Cycle
And
Sedimentary
Rocks
Metamorphic
Rocks
Erosion/ Weathering
Heat and Pressure
Magma
Sediment
Igneous
Rocks
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Define - Freeze
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Igneous Rocks
Igneous Rocks
-a rock that forms when hot molten rock
(magma or lava) cools and freezes solid
Two types of igneous rocks
• Plutonic – cools underground
• Volcanic – cools above ground
Factors controlling Melting
1. Temperature:
Melt at [800oC and 1200oC]
Source of heat [radioactive decay]
Geothermal Gradient – the rate at which
temperature increases with depth in the Earth
30OC/km
Norman Bowen
Bowen’s Reaction Series
Illustrates the order in which minerals crystalize from a magma
Liquid
Solid
Iron and Magnesium rich
Dark colored minerals
Al, Ca, and Sodium rich
Light colored minerals
Factors controlling Melting
2. Pressure:
• as pressure increases, melting temperature increases
• as pressure decreases, melting temperature
decreases
Lowering pressure causes melting in divergent margins.
Relationship between P, T, and Melt
Less Pressure
More Pressure
Divergent Plate Boundary
Factors controlling Melting
3. Water content:
As water content increases, melting temperature decreases
Water increases melting
at convergent plate boundaries
Where do igneous rocks occur?
Types of Molten Material
1. Magma – molten material below the
Earth's surface
2. Lava – molten material above the
Earth's surface
Magma Composition Changes
•Partial melting
•Assimilation
•Magma Mixing
•Fractional Crystallization
Partial
Melting
some
minerals
melt at a
lower
temperature
than others.
Magma Viscosity
Viscosity – resistance to flow
honey – high viscosity
water – low viscosity
Viscosity increases as silica content
increases.
Magma Chemistry
Molten material can consist of liquid rock,
mineral grains and gases (H2O, CO2, SO2).
Silicon and oxygen (SiO2) make up the
majority of magma
45% SiO2 - "low" silica content
75% SiO2 - "high" silica content
Endmember Magma Chemistry
Mafic – low SiO2 / high iron, magnesium
Silicic – high SiO2 / high aluminum, calcium, sodium
Igneous Rock Types
Intrusive (plutonic) rock – cools and
solidifies below the Earth's surface.
Extrusive (volcanic) rock – cools and
solidifies above the Earth's surface.
Textures of Igneous Rocks
Texture – overall appearance, related to size,
shape, and arrangement of minerals.
Texture is related to cooling history
of an igneous rock, not its
chemistry.
Textures of Igneous Rocks
1. Aphanitic (fine grained)
Earth’s surface
Lava
2. Phaneritic (course grained) Magma
Aphanitic Texture – Rapid Cooling
Small Crystals
Phaneritic Texture – Slow Cooling Large
Crystals
Silicic Chemical Composition
Phaneritic Texture –
Slow Cooling
Large Crystals
Granite
Aphanitic Texture –
Rapid Cooling
Small Crystals
Rhyolite
Intermediate Chemical Composition
Phaneritic Texture –
Slow Cooling
Large Crystals
Diorite
Aphanitic Texture –
Rapid Cooling
Small Crystals
Andesite
Mafic Chemical Composition
Phaneritic Texture –
Slow Cooling
Large Crystals
Gabbro
Aphanitic Texture –
Rapid Cooling
Small Crystals
Basalt
PHANERITIC texture
It means that the size of all grains in the rock are large
enough to be distinguished with the unaided eye
APHANITIC texture
It means that not all grains in the rock are large enough to
be distinguished with the unaided eye. Most of the rock is
background mass.
Pegmatites
Ingersoll Mine
Ingersoll mine, Pennington Co., South
Dakota, United States
One of the many adits at the old Ingersoll
pegmatite.
Private and Very dangerous. No Trespass.
Glassy Texture - Very Rapid Cooling
- No Crystal Structure
Pyroclastic Texture
Ash Fall
Ash Flow
Plutonic Structures
Plutonic Structures
Image courtesy of
geoinfo.nmt.edu