Transcript Igneous rocks

Igneous rocks
Igneous rocks –
rocks formed by the
crystallization of
magma
Igneous rocks
Guide questions
• How are igneous rocks formed?
• How does magma differ from lava?
• What two criteria are used to classify igneous
rocks?
• How does the rate of cooling of magma influence
the crystal size of minerals in igneous rocks?
• How is the mineral makeup of an igneous rock
related to Bowen’s reaction series?
• In what ways are granitic rocks different from
basaltic rocks?
• How are economic deposits of gold, silver, and
many other metals formed?
Igneous rocks
Introduction
Magma
• molten rock found ~200km beneath the surface
• consists primarily of:
• elements found in silicate minerals
• gases (volatiles, water vapor) – confined
within the magma by surrounding rocks
• volatile = substance that occurs as gas at
Earth’s surface temperature and pressure
Igneous rocks
Introduction
Magma
• less dense than surrounding rock and so works
upward
• breaks through as lava in volcanic eruptions
• explosions are caused by escaping volatiles
• blockage of the vent, surface water seepage
into the magma chamber to produce steam can
aggravate explosions
• eruptions produce ejected rock fragments and
extensive lava flows
Igneous rocks
Introduction
Lava
• magma that has reached the surface
• extrusive or volcanic rocks – formed as lava
solidifies
• intrusive or plutonic rocks – formed as magma
crystallizes at depth
Igneous rocks
Magma
• hot fluid
• contains suspended
crystals and a gaseous
component
• the liquid portion
(melt) is composed of
ions that move about
freely (recall kinetic
molecular theory)
Crystallization of magma
Magma with crystals
(green bars) and gas
bubbles (white dots)
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Crystallization of magma
Crystallization of magma
• as magma cools, the ions lose kinetic energy
• the ions slow down and begin to form orderly
bonds
• the process is called crystallization
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Crystallization of magma
Disintegration of crystals
• ions in crystals form orderly bonds
• they vibrate in place
• increasing heat = stronger movements (higher
KE)
• ions acquire enough KE to slide past each other
• crystal disintegrates into a liquid whose ions
move about randomly
• the reverse process is crystallization
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Crystallization of magma
Igneous rocks
Crystallization of magma
Embryo crystals
• in crystallization, not all of the molten material
crystallizes at once
• small crystals form throughout (embryo crystals)
• ions are systematically added as the embryo
crystals grow
• the liquid is then transformed into a solid mass of
interlocking crystals
Igneous rocks
Crystallization of magma
Igneous rocks
Crystallization of magma
Rate of cooling
• slow cooling
• few and large embryo crystals
• rock is made of large crystals (mm to meters
in diameter)
• rapid cooling
• small and numerous embryo crystals
• rock is made of small, intergrown crystals
(often microscopic)
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Crystallization of magma
Formation of glass
• liquid magma is quenched instantly
• this leaves no time for the ions to form orderly
bonds
• rock consists of a solid mass of unordered ions
(glass)
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Crystallization of magma
Composition of magma
• consists of the eight primary constituents of
silicate minerals
• silicon
• potassium
• oxygen
• calcium
• aluminum
• iron
• sodium
• magnesium
• volatiles (water vapor, CO2)
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Crystallization of magma
Crystallization of magma
• silicon-oxygen tetrahedra form first
• the tetrahedra join with each other and with other
ions to form embryo crystals
• embryo grows as ions are added to the
crystalline network
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Crystallization of magma
Crystallization of magma
• certain minerals crystallize at higher
temperatures than others
• hence, magma often consist of solid crystals
surrounded by molten material
Igneous rocks
Crystallization of magma
Factors that influence the crystallization of magma
• rate of cooling
• mineral composition of magma
• volatile components
This results to physical and mineral differences
among igneous rocks.
Igneous rocks
Crystallization of magma
Igneous rocks are therefore classified according to
texture and mineral composition.
• texture
• size and arrangement of mineral grains
(crystals)
• reflects environment of crystallization
• mineral composition
• proportion of the eight constituents of silicate
minerals
• volatile components
Igneous rocks
Igneous rock textures
Igneous rock textures
• texture
• overall appearance of the rock
• based on size and arrangement of interlocking
crystals
• important field characteristic
• rock’s origin can be inferred
• when equipment is not available
Igneous rocks
Factors influencing texture
• rate of cooling of magma
• aphanitic
• phaneritic
• porphyritic
• glassy
• composition of magma
• basaltic magma (very fluid)
• granitic magma (more viscous)
• pyroclastic texture
Igneous rock textures
Igneous rocks
Igneous rock textures
Rate of cooling of magma may be:
• slow (thousands of years) – magma chambers
deep within the crust
• rapid (hours) – thin lava flows
• instantaneous (seconds) – small molten blobs
ejected into the air
Igneous rocks
Igneous rock textures
Aphanitic rocks – fine-grained texture
• forms at the surface or as small masses within
the upper crust
• crystal grains are indistinguishable by the
unaided eye
• mineral identification not possible
• often contains vesicles (openings produced by
expanding gas bubbles)
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Igneous rock textures
Classes of aphanitic rocks
• light aphanitic rock – primarily composed of lightcolored nonferromagnesian silicates
• intermediate aphanitic rock
• dark aphanitic rock – primarily composed of
dark-colored ferromagnesian silicates
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Igneous rock textures
Phaneritic rocks – coarse-grained texture
• from crystallization of large masses of magma
far below the surface
• mass of intergrown crystals
• equal in size
• large enough to be identified by the naked eye
• exposed only through erosion
Igneous rocks
Igneous rock textures
Porphyritic rocks – large crystals embedded in a
matrix of small crystals
• from eruption of magma containing large crystals
• phenocrysts – large crystals
• groundmass – matrix of smaller crystals
• porphyry – rock that has porphyritic texture
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Igneous rock textures
Glassy rocks – no crystal formation
• from ejected molten rock which cools rapidly in
the atmosphere
• ions did not have time to form embryo crystals
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Igneous rock textures
Igneous rocks
Igneous rock textures
Igneous rocks
Igneous rock textures
Igneous rocks
Igneous rock textures
Composition of magma
• basaltic magma (very fluid) – usually forms
crystalline rocks
• granitic magma (more viscous) – usually forms
glassy rocks
• pyroclastic texture – composed of rock
fragments
Igneous rocks
Igneous rock composition
Igneous rock composition
• ultimately determined by the chemical
composition of the original magma
• Question: Are there as many types of magma as
there are igneous rocks?
• Evidence: a volcano often produces different
lava flows and pyroclastic material
• Hypothesis: Could a single magma type produce
rocks of varying mineral content?
Igneous rocks
Igneous rock composition
Bowen’s Reaction Series
• N.L. Bowen (1900 to 1925)
• Key points
• minerals with higher melting points crystallize
first
• during crystallization, the composition of the
melt (liquid portion of magma) continually
changes
• minerals react with melt to produce next set of
minerals (hence, reaction series)
• minerals that form in the same temperature
regime are found in the same igneous rock
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Igneous rock composition
Bowen’s Reaction Series
• discontinuous reaction series
• each mineral has a different crystalline
structure
• does not normally run to completion (all
reactants are not consumed)
• continuous reaction series
• minerals become progressively rich in certain
ions (does not change crystalline structure)
• crystals typically have cores and outer zones
with varying ionic content
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Igneous rock composition
Igneous rocks
Igneous rock composition
Magmatic differentiation – the process of developing
more than one rock type from a common magma
(parent magma)
• crystal settling – dense minerals settle at the
bottom of the magma chamber
• assimilation – magma melts surrounding
bedrock
• magma mixing – happens as two rising magma
bodies merge and mix by convection
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Naming igneous rocks
Naming igneous rocks
• Three main types
• basaltic
• andesitic
• granitic
• Classification closely corresponds to Bowen’s
reaction series
• Gradations among the types exist
• Silica (SiO2) content plays an important role in
classification
• Different textures result to different rocks
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Naming igneous rocks
Granitic rocks
• primarily composed of potassium feldspar and
quartz
• felsic – rich in feldspar and silica (quartz)
• light in color
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Some granitic rocks
• Granite
• Rhyolite
• Obsidian
• Pumice
Naming igneous rocks
Igneous rocks
Naming igneous rocks
Igneous rocks
Naming igneous rocks
Andesitic rocks
• intermediate rocks containing minerals found
near the middle of Bowen’s series
• primarily made of amphibole and the
intermediate feldspars
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Some andesitic rocks
• Andesite
• Diorite
Naming igneous rocks
Igneous rocks
Naming igneous rocks
Basaltic rocks
• primarily composed of calcium feldspar,
pyroxene, and olivine
• these minerals are high in iron, magnesium or
calcium, and low in silicon
• mafic – rich in magnesium and ferric (iron)
• dark colored and slightly denser than most other
igneous rocks
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Some basaltic rocks
• Basalt
• Gabbro
Naming igneous rocks
Igneous rocks
Naming igneous rocks
Pyroclastic rocks
• form from fragments ejected during volcanic
eruptions
• tuff (welded tuff)
• volcanic breccia
• these names do not denote mineral composition
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Naming igneous rocks
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Mineral resources and igneous processes
Mineral resources & igneous processes
• some of the most important accumulations of
metals (gold, silver, copper, mercury, lead,
platinum, nickel) are produced by igneous
processes
• such processes are
• magmatic differentiation
• hydrothermal solutions
• vein deposits
• disseminated deposits
• surface deposits
Igneous rocks
References
http://community.weber.edu/museum/Philliteweb.jpg
http://core.ecu.edu/geology/harper/igneous/display.cfm?ID=18
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http://ist-socrates.berkeley.edu/~eps2/wisc/Lect4.html
http://magma.nationalgeographic.com/ngm/exploration/images/WallpaperLarge04.jpg
http://volcanoes.usgs.gov/Imgs/Jpg/Monitoring/Deformation/magma.jpg
http://wps.prenhall.com/wps/media/objects/476/488316/ch11.html
http://www.allmyeye.com/images/pcd/pcd0601/yosemite-sentinel-rock.3.html
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http://www.chemistry.wustl.edu/~edudev/LabTutorials/Water/PublicWaterSupply/PublicWaterSupply.html
http://www.solcomhouse.com/volcandang.gif
http://www.uwm.edu/Course/422-100/Mineral_Rocks/igrox.index.html