Igneous rocks – rocks formed by the crystallization of magma

How are igneous rocks formed? How does magma differ from lava? 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?

molten rock found ~200km beneath the surface consists primarily of: 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

less dense than surrounding rock and so works upward 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

magma that has reached the surface 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

contains suspended crystals and a gaseous component Crystallization of magma 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) Magma with crystals (green bars) and gas bubbles (white dots)

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

Disintegration of crystals ions in crystals form orderly bonds 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

Crystallization of magma

small crystals form throughout (embryo crystals) 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

Crystallization of magma

few and large embryo crystals 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)

liquid magma is quenched instantly 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)

consists of the eight primary constituents of silicate minerals Crystallization of magma Composition of magma consists of the eight primary constituents of silicate minerals silicon oxygen aluminum sodium potassium calcium iron magnesium volatiles (water vapor, CO2)

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

Crystallization of magma certain minerals crystallize at higher temperatures than others hence, magma often consist of solid crystals surrounded by molten material

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.

size and arrangement of mineral grains (crystals) 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 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

Factors influencing texture rate of cooling of magma aphanitic Igneous rock textures 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

Rate of cooling of magma may be: 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

Aphanitic rocks – fine-grained texture 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)

Classes of aphanitic rocks Igneous rock textures Classes of aphanitic rocks light aphanitic rock – primarily composed of light-colored nonferromagnesian silicates intermediate aphanitic rock dark aphanitic rock – primarily composed of dark-colored ferromagnesian silicates

Phaneritic rocks – coarse-grained texture 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

from eruption of magma containing large crystals 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

Glassy rocks – no crystal formation 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 rock textures

Igneous rock textures

Igneous rock textures

basaltic magma (very fluid) – usually forms crystalline 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 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?

Bowen’s Reaction Series N.L. Bowen (1900 to 1925) Key points 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

Bowen’s Reaction Series discontinuous reaction series 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 rock composition

assimilation – magma melts surrounding bedrock 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

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

primarily composed of potassium feldspar and quartz Naming igneous rocks Granitic rocks primarily composed of potassium feldspar and quartz felsic – rich in feldspar and silica (quartz) light in color

Some granitic rocks Granite Rhyolite Obsidian Pumice Naming igneous rocks Some granitic rocks Granite Rhyolite Obsidian Pumice

Naming igneous rocks

primarily made of amphibole and the intermediate feldspars 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

Naming igneous rocks Some andesitic rocks Andesite Diorite

primarily composed of calcium feldspar, pyroxene, and olivine 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

Naming igneous rocks Some basaltic rocks Basalt Gabbro

form from fragments ejected during volcanic eruptions Naming igneous rocks Pyroclastic rocks form from fragments ejected during volcanic eruptions tuff (welded tuff) volcanic breccia these names do not denote mineral composition

Naming igneous rocks

Mineral resources & igneous processes 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

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.solcomhouse.com/volcandang.gif http://www.uwm.edu/Course/422-100/Mineral_Rocks/igrox.index.html