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Chapter 3 Igneous rocks
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2) contains a mixture of one or more minerals
Definition of a rock -> different from that of a mineral. A rock: 1) is solid 2) contains a mixture of one or more minerals 3) occurs naturally as part of our planet
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minerals rock mineral minerals collection of one or more A collection
So far we have: minerals rock mineral collection of one or more minerals A collection of one or more types of atoms
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Molten rock Some definitions inside the Earth is magma
when magma reaches Earth’s surface(outside) it is called lava
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Figure 4.1
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An igneous rock is formed when magma or
lava cools and solidifies
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Igneous rocks that form at the surface(lava)
Volcanic (extrusive) Igneous Igneous rocks that form at the surface(lava) Plutonic (intrusive) Igneous Igneous rocks that form deep inside the earth (magma)
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Plutonic rocks To see them, they must be uplifted to surface
And softer surrounding rock eroded away Magma body
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Plutonic rocks For us to ever see them, they must be uplifted to the surface And softer surrounding rock eroded away
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Plutonic igneous rock example: El Capitan Yosemite Nat. Park,
California
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As magma cools, atoms arrange in an
orderly crystal structure crystallization
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Note: different crystal sizes
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Coarse-grained igneous rock
Close up of crystals Note the variety of crystal sizes
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Texture a. size b. shape c. arrangement the - of interlocking crystals
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cooling rate crystal size Slow cooling larger crystals Fast cooling
Coarse texture Fast cooling small or no crystals Fine texture
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Coarse-grained Fine-grained Large crystals in matrix Glassy
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fast cooling magma/lava forms at or near surface
sometimes holes present can’t see individual crystals forms far below surface slow cooling intergrown crystals Aphanitic Fine-grained Phaneritic Coarse-grained very rapid cooling ions unable to unite in orderly crystalline structure magma cooled slowly for a while then erupted. minerals crystallize at different temperatures and or rates Porphyritic Glassy Large crystals in matrix
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Porphyritic Texture
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Aphanitic Texture
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Texture Rock types Coarse grained Peridotite, Gabbro, Diorite, Granite Fine grained Komatiite, basalt, andesite, rhyolite Glassy Obsidian, Pumice, Scoria Vesicular Pumice, Scoria Prophyritic Andesitic porphyry Phenocrysts –large very crystals.
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mainly silicate minerals
determined by composition of magma from which it crystallized magma mainly 8 elements: Si, O, Al, Ca, Na, K, Mg, Fe Most abundant constituents
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Figure 4.7
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DARK silicates(simatic) LIGHT silicates(sialic)
Two major silicate mineral groups: DARK silicates(simatic) LIGHT silicates(sialic) Silicate group Rich in Poor in Examples DARK (Simatic) Fe and/or Mg Si olivine pyroxene amphibole biotite LIGHT (Sialic) Si, K, Na, Ca Fe and/or Mg quartz muscovite feldspars
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Bowen’s Reaction Series
In a magma body: crystals form as magma cools heavy crystals sink to bottom the composition of the magma changes variety of igneous rocks from the same source
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Bowen’s reaction series
Figure 4.23
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Igneous rocks are classified from
A) texture B) mineral composition depends on: how fast/slow magma cools depends on: chemical makeup of parent magma
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mafic felsic high in Mg, Fe. Dark, dense
Igneous rocks types mafic felsic magnesium + ferrum high in Mg, Fe. Dark, dense feldspar + silica (quartz) high in Si. Lighter, less dense
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zooming in rhyolite volcanic f e l s i c granite plutonic
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obsidian volcanic f e l s i c volcanic pumice
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zooming in andesite volcanic i n t e r m e d i a t e diorite plutonic
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zooming in basalt volcanic m a f i c gabbro plutonic
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volcanic m a f i c scoria
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magnesium + ferrum feldspar + silicate magma recipes
3 main types magnesium + ferrum high in Mg, Fe. Dark, dense “mafic” basaltic andesitic granitic feldspar + silicate high in Si, lighter, less dense “felsic”
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magma recipes basaltic andesitic granitic 3 main types “mafic”
magnesium + ferrum high in Mg, Fe. Dark, dense Most common volcanic Igneous rock. “mafic” basaltic andesitic granitic Most common plutonic Igneous rock “felsic” “felsic” feldspar + silicate high in Si, lighter, less dense
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magma recipes basaltic andesitic granitic 3 main types Partial
melt of: “mafic” basaltic andesitic granitic mantle rock: peridotite Mantle peridotites + felsic crustal rocks Si-rich continental crust rocks “felsic” “felsic”
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Kimberlite What is a kimberlite?
kimberlite is a potassic, ultramafic, igneous rock which occurs as small volcanic pipes. Kimberlite commonly contains inclusions of upper mantle ultramafic rocks. olivine, illmenite, pyrope, diopside,, enstatite and chromite.
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Kimberlites are formed deep within the mantle, at between 150 and 450 kilometres depth, from exotic mantle compositions, and are erupted rapidly and violently. It is this depth of melting and generation which makes kimberlites prone to hosting diamond.
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Most kimberlites are confined to the ancient cratons
Most kimberlites are confined to the ancient cratons. The ages of most kimberlites are Late Mesozoic Era (Jurassic-Cretaceous).
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Diamonds in the kimberlite
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Kimberlite Indicator Minerals
Cr-pyrope (purple colour,), eclogitic garnet (orange-red), Cr-diopside (pale to emerald green), Mg-ilmenite (black, conchoidal fracture), chromite (reddish-black), and olivine (pale yellow-green) are the most commonly used kimberlite indicator minerals
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