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Earth Materials:
Intro to rocks & Igneous rocks
The three major categories of rocks
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Fig 3.1 Understanding Earth
• Three main categories of rocks:
• Igneous
• 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
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
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Fig 3.1 Understanding Earth
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Relative abundance of rock types in the earth’s crust
Always crystalline
Some have aligned
crystals (foliation),
some do not
Classified using
composition and
texture
Fig 3.4 Understanding Earth
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The “Rock-forming” minerals
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Understanding Earth
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:
Micas:
Alkali feldspar
Pl i l feldspar
Plagioclase
f ld
Muscovite mica
Biotite mica
(2)
(3)
(4)
(5)
Pyroxene
(6)
Amphibole
(7)
Olivine
(8)
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Igneous rocks
Formation of
igneous rocks:
intrusive and
extrusive rocks
• General categories based on where they form
• Intrusive
• Extrusive
(plutonic)
(volcanic)
• Types of Intrusions
• Description and classification
• Texture
Magma
g vs. Lava
• crystal size(s)
• bubbles
• fragments
Magma is molten rock
underground.
Intrusive or Plutonic
• Composition (minerals and color)
• Felsic
• Intermediate
• Mafic
• Origin of magma
Fig 3.2 Understanding Earth
Lava is molten rock
which has reached the
surface.
Extrusive or Volcanic
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Extrusive rocks:
Volcanoes and Lava flows
Some major types of igneous intrusions (plutons)
Fig 4.8 Understanding Earth
Vein
- very thin, tabular intrusion
- the smallest type of intrusion
Sill
- intrudes between rock layers, tabular shape
Dike
- cuts across p
pre-existing
g rock layers,
y , tabular shape
p
- the main way in which magmas reach the surface
Stock
October 1915
eruption of Mt.
Lassen (photo by
Chester Mullen)
- cuts across rock layers, irregular shape
Batholith - like a stock, but much larger (area of at least 100 km2)
- the largest type of igneous body
- usually comprised of numerous individual intrusions
Fig 4.8 Understanding Earth
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Veins of granite cutting through
metamorphosed sedimentary rock
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Fig 4.2 Understanding Earth
Sill
Intrusive rocks:
Sill and dike
Dike
Fig 4.15 & 4.16
Understanding Earth
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Part of the Sierra-Nevada batholith in Yosemite N. P.
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A huge mass of intrusive rocks composed of many individual intrusions (plutons)
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Classification of igneous rocks by
texture and composition
Igneous textures: crystal size
all large crystals
- produced
d d bby relatively
l ti l slow
l cooling
li deep
d
underground
d
d
-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
Glassy texture: Obsidian, pumice, and volcanic ash
Fig 4.3 Understanding Earth
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
- 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
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fine = aphanitic
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mixed = porphyritic
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
Igneous textures
Geology 2nd ed.
ed - Chernicoff
broken fragments:
coarse = phaneritic
extremely coarse = pegmatitic
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.
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Pyroclastic (fragmental)
textures
Vesicular textures:
basalt with vesicles (left)
and pumice (below)
Understanding Earth
Volcanic ash
Pumice
Tuff (this one contains both ash and pumice)
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Classification of igneous rocks by
texture and composition
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observed as variations in:
• mineral abundance (quartz, feldspar, etc.),
• elemental abundance (silicon, oxygen, iron, etc.),
• color (dark, medium, light)
i.e. peridotite
Mafic
i.e. basalt and gabbro
Intermediate
i.e. andesite and diorite
Felsic
i.e. rhyolite and granite
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Mafic
rocks:
basalt
(fine)
and
gabbro
(coarse)
Composition
Ultramafic
Classification of igneous rocks: texture and composition
Felsic
rocks:
rhyolite
(fine)
and
granite
(coarse)
Fig 4.5 Understanding Earth
Extrusive/Volcanic
Intrusive
(glassy, aphanitic, or porphyritic)
(coarse or porphyritic)
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Igneous rock classification
Classification of igneous rocks: texture and composition
Composition
Felsic
Understanding Earth
Intermediate
Mafic
Ultramafic
Andesite
Basalt
Komatiite
Diorite
Gabbro
Peridotite
Ultramafic rocks
Example: Peridotite
Texturre
Extrusive
(glassy,
aphanitic,
porphyritic,
fragmental,
vesicular,
etc.)
Rhyolite
Dacite
(rare)
Intrusive
(phaneritic,
pegmatitic)
Granite Granodiorite
lighter
more silica;
less iron &
magnesium
darker
less silica;
more iron &
magnesium
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More silica (SiO2),
sodium, potassium
Igneous rock
compositions
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Darker color
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How are magmas formed?
modified from Fig
g 4.6 Understanding Earth
Lighter color
More iron, magnesium,
calcium
by partial melting of crust or mantle rocks
Three ways to melt rocks in the Earth’s mantle and crust:
• increase temperature
• decrease pressure on already hot rock (decompression)
• add fluids (water) to already hot rock
Rhyolite or
Granite
Dacite or
Granodiorite
Andesite or
Diorite
Basalt or
Gabbro
Komatiite or
Peridotite
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Magma formation at plate boundaries and hot spots
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Magma formation by addition of fluids at subduction zones
Fig 4.8 Understanding Earth
Modified from Fig 4.19 Understanding Earth
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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 zones – melting
Subduction
lti due
d to
t addition
dditi off fluids
fl id
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
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