Transcript Igneous rocks - HEDCen Science

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)
Igneous rocks
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
Igneous rocks
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
Igneous rocks
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)
Igneous rocks
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)
Igneous rocks
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)
Igneous rocks
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
For further study, go to the following slideshare page:
http://www.slideshare.net/croessler93/06-minerals-rocksforstudents
Igneous rocks
Island silicates
Single chain silicates
Framework silicates
Further study on these swf files:
http://web.visionlearning.com/silica_molecules.shtml.
Crystallization of magma
Igneous rocks
Papakolea Beach, Hawaii
Crystallization of magma
Igneous rocks
Olivine
• island silicate
• (Mg+2, Fe+2)2SiO4
Crystallization of magma
Igneous rocks
Crystallization of magma
Olivine
• used as gemstones (peridot- August birthstone)
• also found in meteors, star dust and comet tails
Igneous rocks
Augite
• single chain silicate
• (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6
Crystallization of magma
Igneous rocks
Crystallization of magma
Augite
• no industrial/economic importance
• used in reconstructing histories of rocks due to Ca
and temperature interaction
Igneous rocks
Cristobalite
• framework silicate
• SiO2
Crystallization of magma
Igneous rocks
Cristobalite
Crystallization of magma
• no industrial/economic importance
• used in scientific studies of crystal formation
• snowflake obsidian jewelry
Igneous rocks
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)
Igneous rocks
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
Igneous rocks
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
Igneous rocks
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
Igneous rocks
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
Igneous rocks
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
Igneous rocks
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
Igneous rocks
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
Igneous rocks
Naming igneous rocks
Granitic rocks
• primarily composed of potassium feldspar and
quartz
• felsic – rich in feldspar and silica (quartz)
• light in color
Igneous rocks
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
Igneous rocks
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
Igneous rocks
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
Igneous rocks
Naming igneous rocks
Igneous rocks
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
http://danny.oz.au/travel/iceland/vik.html
http://images.encarta.msn.com/xrefmedia/aencmed/targets/images/scp/T014530A.gif
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
http://www.chemistry.wustl.edu/~edudev/LabTutorials/Water/PublicWaterSupply/PublicWaterSupply.html
http://www.cofc.edu/~martine/111LectWeek1.htm
http://www.engineering.uiowa.edu/~cfd/gallery/images/flo12.jpg
http://www.env.duke.edu/eos/geo41/rks009.gif
http://www.mineraltown.com/galeria/volcanic_minerals/volcanic_rocks.jpg
http://www.pbs.org/wgbh/nova/volcanocity/images/anat-magma-l.jpg
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