Post on 13-Feb-2017
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Earth Materials:
Intro to rocks & Igneous rocks
• Three main categories of rocks:
• Igneous
• Sedimentary
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• Sedimentary
• Metamorphic
• The most common minerals in rocks
(a.k.a. the “rock-forming” minerals)
• Igneous Rocks – description and classification
The three major categories of rocks Fig 3.1 Understanding Earth
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The three major categories of rocks Fig 3.1 Understanding Earth
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Composed of crystals and/or glass
May have layers if volcanic
Classified using composition and texture
Most are composed of fragments (e.g. sand); Some may be crystalline.
Always layered/stratified at some scale
Classified using composition and texture
Always crystalline
Some have aligned crystals (foliation), some do not
Classified using composition and texture
Relative abundance of rock types in the earth’s crust
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Fig 3.4 Understanding Earth
The “Rock-forming” minerals
The 8 most abundant mineral in the earth’s crust and the most common minerals in rocks are often called the “rock-forming”
minerals. All 8 can be found in igneous rocks, and all 8 are silicates.
Quartz (1)
Feldspars: Alkali feldspar (2)
Pl i l f ld
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Plagioclase feldspar (3)
Micas: Muscovite mica (4)
Biotite mica (5)
Pyroxene (6)
Amphibole (7)
Olivine (8)
Understanding Earth6
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Igneous rocks
• General categories based on where they form• Intrusive (plutonic)
• Extrusive (volcanic)
• Types of Intrusions
• Description and classification
• Texture
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Texture• crystal size(s)
• bubbles
• fragments
• Composition (minerals and color)
• Felsic
• Intermediate
• Mafic
• Origin of magma
Formation of igneous rocks: intrusive and extrusive rocks
Magma vs. Lava
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g
Magma is molten rock underground. Intrusive or Plutonic
Lava is molten rock which has reached the surface. Extrusive or VolcanicFig 3.2 Understanding Earth
Extrusive rocks:Volcanoes and Lava flows
Fig 4.8 Understanding Earth
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October 1915 eruption of Mt. Lassen (photo by Chester Mullen)
Fig 4.8 Understanding Earth
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Fig 4.2 Understanding Earth
Veins of granite cutting through metamorphosed sedimentary rock
Dike
Intrusive rocks:veins & dikes
Intrusive rocks: plutons & batholiths 11
Part of the Sierra-Nevada batholith in Yosemite National Park
A huge mass of intrusive rocks composed of many individual large intrusions (plutons)
Classification of igneous rocks by texture and composition
Igneous textures: crystal size
all large crystals
d d b l ti l l li d d d
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- produced by relatively slow cooling deep underground
-found in intrusive igneous rocks(also known as plutonic igneous rocks)
phaneritic texture - large crystals
pegmatitic texture - extremely large crystals
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Classification of igneous rocks by texture and composition
Igneous textures: crystal size
at least partly small crystals and/or glass
- produced by relatively fast cooling, usually at or near the earth’s surface
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- typical of extrusive (volcanic) rocks
- sometimes in intrusive rocks (e.g. shallow dikes)
glassy texture - no crystals
aphanitic texture - very small crystals
porphyritic texture - mixed sizes of crystals
Glassy texture: Obsidian, pumice, and volcanic ash
Fig 4.3 Understanding Earth
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Igneous texturesGeology 2nd ed Chernicoff
fine = aphanitic mixed = porphyritic15
Geology 2 ed. - Chernicoff
coarse = phaneritic extremely coarse = pegmatitic
Classification of igneous rocks by texture and composition
Igneous textures: other textures
bubbles: vesicular texture
Gas bubbles trapped in an igneous rock are called vesicles.
Examples: vesicular basalt and pumice
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Examples: vesicular basalt and pumice
broken fragments: pyroclastic texture
If there is enough gas, expanding bubbles may cause lava to explosively break into fragments.
Examples: volcanic ash and pumice
Pyroclastic deposits may have some of the same features as sedimentary rocks.
Vesicular textures:basalt with vesicles (left) and pumice (below)Understanding Earth
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Pyroclastic (fragmental) textures
Volcanic ash
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Tuff (this one contains both ash and pumice)
Pumice
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Classification of igneous rocks by texture and composition
Composition
observed as variations in:
• mineral abundance (quartz, feldspar, etc.),
• elemental abundance (silicon, oxygen, iron, etc.),
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• color (dark, medium, light)
Ultramafic i.e. peridotite
Mafic i.e. basalt and gabbro
Intermediate i.e. andesite and diorite
Felsic i.e. rhyolite and granite
Classification of igneous rocks: texture and composition
Mafic rocks: basalt (fine) and
gabbro (coarse)
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Fig 4.5 Understanding Earth
Extrusive/Volcanic(glassy, aphanitic, or porphyritic)
Intrusive (coarse or porphyritic)
Felsic rocks: rhyolite (fine) and
granite (coarse)
Ultramafic rocks
Understanding Earth
Classification of igneous rocks: texture and composition
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Example: Peridotite
Composition
Felsic Intermediate Mafic Ultramafic
Extrusive (glassy, aphanitic, porphyritic, Rhyolite Dacite Andesite Basalt Komatiite
(rare)re
Igneous rock classification22
fragmental, vesicular, etc.)
(rare)
Te
xtu
r
Intrusive (phaneritic, pegmatitic)
Granite Granodiorite Diorite Gabbro Peridotite
more silica; less iron & magnesium
less silica; more iron & magnesium
lighter darker
Igneous rockcompositions
More silica (SiO2), sodium, potassium
More iron, magnesium, calcium
Lighter color Darker color
g 4.
6 U
nder
stan
ding
Ear
th
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mod
ifie
d fr
om F
ig
Rhyolite or Granite
Dacite or Granodiorite
Andesite or Diorite
Basalt or Gabbro
Komatiite or Peridotite
How are magmas formed?
by partial melting of crust or mantle rocks
Three ways to melt rocks in the Earth’s mantle and crust:
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• increase temperature
• decrease pressure on already hot rock (decompression)
• add fluids (water) to already hot rock
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Magma formation at plate boundaries and hot spots
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Fig 4.8 Understanding Earth
Magma formation by addition of fluids at subduction zones
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Modified from Fig 4.19 Understanding Earth
Where and how are magmas formed?
Mid-ocean ridges – decompression
mafic (basaltic) magma
Continental rifts and other areas of extension – decompression
mostly mafic (basaltic) magma
S bd ti lti d t dditi f fl id
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Subduction zones – melting due to addition of fluids
mafic, intermediate, and felsic magmas - often water-rich
Mantle plumes (hot spots) – decompression melting
ocean basins: mafic magma
continents: mostly mafic magma, some intermediate and felsic magmas
Note: secondary melts may occur due to addition of heat from primary magma