Igneous Rocks and Processes - ausearthed.com.au
Transcript of Igneous Rocks and Processes - ausearthed.com.au
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Igneous Rocks and Processes
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Formation
• Molten rock cools and solidifies
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Formation
• Molten rock cools and solidifies
― Intrusive (plutonic)
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Formation
• Molten rock cools and solidifies
― Intrusive (plutonic)
― Extrusive (volcanic)
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Formation
• Molten rock cools and solidifies
― Intrusive (plutonic)
― Extrusive (volcanic)
• Interlocking crystals
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Classification
• Minerals
― Size = rate of cooling
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Classification
• Minerals
― Size = rate of cooling
― Composition = amount of silica present/ chemistry, gas in magma
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Terminology
• Aphanitic – fine grained crystals
• Phaneritic – coarse grained crystals
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Terminology
• Aphanitic – fine grained crystals
• Phaneritic – coarse grained crystals
• Porphyritic – large crystals in a background of smaller crystals (groundmass)
• Glassy – very rapid cooling
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Terminology
• Aphanitic – fine grained crystals
• Phaneritic – coarse grained crystals
• Porphyritic – large crystals in a background of smaller crystals (groundmass)
• Glassy – very rapid cooling
• Pyroclastic – rock fragments
• Pegmatic – very coarse grained
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Bowen’s Reaction Series
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Common Igneous Rocks
Mafic Intermediate Felsic
Aphanitic Basalt Andesite Rhyolite
Phaneritic Gabbro Diorite Granite
Dominant
Minerals
Pyroxene
Calcium-rich
plagioclase feldspar
Amphibole
Sodium & calcium-
rich plagioclase
feldspar
Quartz
Orthoclase feldspar
Sodium-rich
plagioclase feldspar
Accessory
minerals
Amphibole
Olivine
Pyroxene
Biotite
Muscovite
Biotite
Amphibole
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Other igneous rocks to know
• Komatiite
• Pumice
• Tuff
• Obsidian
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Earth resources
• Intrusive ore deposits – formed during the cooling of magma
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Earth resources
• Intrusive ore deposits – formed during the cooling of magma
• Exhalative ore deposits – formed from volcanic materials which are extruded or ‘exhaled’ onto the Earth’s surface through hydrothermal vents
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Intrusive ore deposits
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Fractional crystallisation
• As magma cools, some minerals form crystals before others
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Differentiation – Pegmatite deposits
• As magma crystallises out minerals like pyroxene and calcium rich plagioclase, it becomes progressively more felsic
• Late stage magmas can be fluid rich and contain rare elements like lithium, tin and tantalum
• E.g. Greenbushes tin-tantalum-lithium deposit
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Gravitational settling
• Early formed, heavier minerals sink to the base of the magma chamber
• E.g. chromium at Coobina, near Newman
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Immiscible liquid separation
• Like oil and water, some minerals don’t readily mix
• Heavy sulfur-rich liquid (high in Ni & Cu) separates from silicate-rich liquid and sinks. E.g. Kambalda
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Lamproite/Kimberlite Pipes
• Diamond-bearing igneous rocks (fine-grained ultramafic)
• ‘Punch up’ from depth of ~600km
• Usually vertical, pipe-like bodies
• Rise rapidly and cool rapidly
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Mantle hot spots
• Hot spots are thought to be abnormally hot sections of the mantle
• The material is thought to originate at the core-mantle boundary
(USGS 1999, public domain)
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Exhalative ore deposits
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Black smokers
• Hot vents spewing clouds of black sulfur-bearing compounds
• Rich in copper, iron, zinc, etc.
• Supports a unique ecosystem
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Volcanogenic Massive Sulfide (VMS) DepositOr volcanic hosted massive sulfide (VHMS)
• Hot, saline, metal-rich fluids are exhaled from hydrothermal seafloor vents or fumaroles
• Zinc, copper, lead, silver, gold, etc.
• E.g. The DeGrussa copper-gold deposit, 150 km NE of Meekarhara, hosted in what may have been a back arc basin
Image courtesy of
Sandfire Resources NL
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Hot spring deposits
• Hydrothermal solutions reach the surface
• Superheated water overflows and precipitates out very fine silica (sinter)
• May contain lead, zinc, mercury, etc.
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Volcanic fumarole deposits
• Hot volcanic gases condense and precipitate out hematite, sulfur and gypsum minerals
• Bacteria may also precipitate sulfur
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Igneous intrusions
• Massive or tabular
• Discordant or concordant
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References
•USGS, 1999, “Hotspots”: Mantle thermal plumes, accessed 15 May 2012 from http://pubs.usgs.gov/gip/dynamic/hotspots.html
Unless otherwise stated all information and
graphics are from:
• Tompkins, DE (Ed) 2011, Epxloring Earth and Environmental Science Stages 1, 2 and 3, Earth Science Western Australia
• ESWA photo/ graphic library
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