Igneous rocks Igneous rocks – rocks formed by the crystallization of magma.

Igneous rocks

Igneous rocks – rocks formed by the crystallization of magma

Igneous rocksIgneous rocks

• 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?

Guide questions

Igneous rocks IntroductionIgneous rocks

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 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• 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


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


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)


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

Composition of magma • consists of the eight primary constituents of silicate minerals

• silicon• oxygen• aluminum• sodium

Crystallization of magma

• potassium• calcium• iron• magnesium

• volatiles (water vapor, CO2)

Igneous rocks

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


Igneous rocks

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


Igneous rocks

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 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


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:• 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• 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• 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• 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


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• from ejected molten rock which cools rapidly in the atmosphere• ions did not have time to form embryo crystals


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?


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

• 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


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


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


Andesitic rocks• intermediate rocks containing minerals found near the middle of Bowen’s series• primarily made of amphibole and the intermediate feldspars


Some andesitic rocks• Andesite• Diorite


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


Some basaltic rocks• Basalt• Gabbro


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

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.jpghttp://core.ecu.edu/geology/harper/igneous/display.cfm?ID=18http://danny.oz.au/travel/iceland/vik.htmlhttp://images.encarta.msn.com/xrefmedia/aencmed/targets/images/scp/T014530A.gifhttp://ist-socrates.berkeley.edu/~eps2/wisc/Lect4.htmlhttp://magma.nationalgeographic.com/ngm/exploration/images/WallpaperLarge04.jpghttp://volcanoes.usgs.gov/Imgs/Jpg/Monitoring/Deformation/magma.jpghttp://wps.prenhall.com/wps/media/objects/476/488316/ch11.htmlhttp://www.allmyeye.com/images/pcd/pcd0601/yosemite-sentinel-rock.3.htmlhttp://www.chemistry.wustl.edu/~edudev/LabTutorials/Water/PublicWaterSupply/PublicWaterSupply.htmlhttp://www.cofc.edu/~martine/111LectWeek1.htmhttp://www.engineering.uiowa.edu/~cfd/gallery/images/flo12.jpghttp://www.env.duke.edu/eos/geo41/rks009.gifhttp://www.mineraltown.com/galeria/volcanic_minerals/volcanic_rocks.jpghttp://www.pbs.org/wgbh/nova/volcanocity/images/anat-magma-l.jpghttp://www.chemistry.wustl.edu/~edudev/LabTutorials/Water/PublicWaterSupply/PublicWaterSupply.htmlhttp://www.solcomhouse.com/volcandang.gifhttp://www.uwm.edu/Course/422-100/Mineral_Rocks/igrox.index.html

Igneous rocks Igneous rocks – rocks formed by the crystallization of magma.

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