Igneous Rocks: Terminology

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Igneous Rocks Terminology

• Igneous rocks are formed as a result of cooling
  and crystallization from a magma
• Magma is molten rock, rich
• in silica (SiO2), which contains
• dissolved volatiles such as gasses
• Lava is magma extruded on or
• very near the Earths surface

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Igneous Rocks

• 2 Types
• Extrusive (Volcanic Rock)
• Cool above the Earths surface (fast cooling)
• Magma which has reached the surface is known as
  lava
• The heat from the lava goes quickly and the lava
  solidifies
• Example basalt, rhyolites, pumice, obsidian
  (smooth volcanic rock)
• Intrusive (Plutonic Rock)
• Cool beneath the Earths surface (slow cooling)
• Example granite

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Type of Igneous Rock Formed

• Depends on three factors
• Chemical composition of the magma
• Example The amount of silica (SiO 2 ) found in
  magma can vary from 75 to less than 45 .
• Temperature of solidification
• The temperature of cooling determines which types
  of minerals are found dominating the rock's
  composition
• Low temperature rocks are rich in minerals
  composed of silicon, potassium, and aluminum.
• High temperature igneous rocks are dominated by
  minerals with higher quantities of calcium,
  sodium, iron, and magnesium
• Rate of cooling which influences the
  crystallization process
• Igneous rocks that form through a gradual cooling
  process tend to have large crystals. Relatively
  fast cooling of magma produces small crystals.
• Volcanic magma that cools very quickly on the
  Earth's surface can produce obsidian glass which
  contains no crystalline structures.

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Classifying Igneous Rocks

• Classified based on
• A. Texture
• The size shape and arrangement of constituent
  minerals
• Related to the cooling history
• Fast cooling crystals do not grow to any great
  size
• Slow cooling crystals grow large
• B. Composition
• The percentage of particular minerals contained
  in the rock
• Certain minerals are light in color others dark
• Reflects the source of magma from which the rock
  was produced

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Igneous Rocks Cooling History

• Length of time for cooling determines the texture
  and degree of crystallization
• Extrusive
• Fast cooling
• Fine grained
• Example Basalt
• Glassy texture
• Example Obsidian (like glass)
• Example Pumice (holes)
• Lava that cools very quickly can lock in gases
  as it solidifies
• Result Rock with many holes
• Think Pumice stone

ie. Basalt
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Igneous Rocks Cooling History

• Intrusive
• Slow cooling
• Coarse grained and larger crystals
• Example Granite
• Example Diorite
• Example Gabbro

ie. Granite
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Extrusive Rock Bodies Volcanoes

• Mafic (basaltic) eruptions
• Character
• fissures or fractures
• mafic magma
• fluid
• not explosive

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Extrusive Rock Bodies Volcanoes

• Mafic (basaltic) eruptions
• Structures

Flood basalt
Shield volcano
Lava tube
Tephra
Caldera
Pillow lava
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Extrusive Rock Bodies Volcanoes

• Felsic or Silicic (granitic) eruptions
• Character
• typical at convergent plate boundaries
• extrude cool magma
• viscous so does not flow freely
• highly explosive

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Extrusive Rock Bodies Volcanoes

• Felsic or Silicic (granitic) eruptions
• Structures

Crater
Tephra
Pyroclastic flow
Composite cone
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Intrusive Rock Bodies Plutons

• Character
• cools below the surface of the earth
• Structures formed by Igneous Rocks
• Pluton
• Batholith
• Stock
• Neck
• Dike
• Laccolith
• Sill
• Pipe
• Xenoliths

Pluton
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• When magma cools below the surface of the earth
  it forms intrusive igneous rocks.  These rocks
  produce the following structures
• Pluton An intrusive rock body of any size.
• Batholith A pluton greater than 100 km2.
• Stock A pluton less than 100 km2.
• Dike Tabular intrusive rock which cuts
  discordantly (vertically) across strata.
• Laccolith A concordant (horizontal) intrusion
  that generally has a horizontal floor but which
  arches up the strata above it lens or spoon
  shaped.
• Sill A concordant intrusion that is injected
  between layers no upward arch.
• Pipe Volcanic neck conduit through which magma
  travels from chamber upwards.
• Xenoliths Inclusions of surrounding rock in
  magma.

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Intrusive Rock Bodies Plutons
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Volcanic Neck and Dike
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Aerial Photo of Shiprock, New Mexico (Volcanic
Neck)
From http//www.ngdc.noaa.gov/seg/hazard
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Volcanic Neckand Dike
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Edinburgh Sill
From http//www.ngdc.noaa.gov/seg/hazard
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Sill - Yellowstone National Park
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Sierra Nevada Batholith
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Stock Chisos Mountains Texas
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Laccolith Black Hills, Wyoming
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Patagonia Pluton - Arizona
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Xenoliths Petersburg, Virginia

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Pipe Organ Pipes National Park, Australia
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Igneous Rock Metallic Mineral Deposits

• Rare minerals often found in Igneous rocks
• Place found
• Often found in intrusive (plutonic rock)
• Think mining where is the good stuff found
• Copper, gold