Geology - Igneous Rocks - Quick Guides

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Study GuideGeology–Igneous Rocks1. Extrusive Rock TypesExtrusive rocksform whenlava erupts onto Earth’s surfaceand cools quickly. The type of rockthat forms depends mainly on thechemical makeup of the magma, especially how muchsilicaitcontains. Silica strongly affects a lava’s color, thickness (viscosity), and mineral content.1.1 Basalt, Andesite, and RhyoliteThese three rocks are the most commonextrusive igneous rocks, and they differ mainly in silicacontent and mineral makeup.Basalt•Usually thefirst lava to erupt•Contains manyferromagnesian minerals(rich in iron and magnesium)

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Study Guide•Has about25–50% silica•Dark green, gray, or black•Very common on the ocean floorAndesite•Hasmore silicathan basalt•Contains moreplagioclase feldsparand fewer ferromagnesian minerals•Light greenish-gray in color•Common in volcanic arcsRhyolite•Themost silica-richextrusive rock•Contains65% or more silica, mostly in feldspar and quartz•Very few ferromagnesian minerals•Light-colored: tan, pink, or creamDacite•Composition fallsbetweenandesite and rhyolite•Slightly less potassium feldspar and quartz than rhyolite•Slightly more ferromagnesian minerals•Light grayish-green•Often hard to tell apart from rhyolite in the field

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Study Guide1.2 Mafic, Intermediate, and Felsic Extrusive RocksGeologists often group extrusive rocks using broader chemical terms:Mafic rocks•About50% silica•High iniron, magnesium, and calcium•Dark-colored•Example:basaltFelsic rocks•High insilica, potassium, sodium, and aluminum•Low in iron, magnesium, and calcium•Light-colored•Examples:rhyolite and dacite•Most viscousmagmas due to high silicaIntermediate rocks•Chemical composition between mafic and felsic•Example:andesite1.3 Ultramafic Extrusive RocksA much rarer group of extrusive rocks is theultramaficcategory.

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Study Guide•Made almost entirely offerromagnesian minerals•Containless than 45% silica•No feldspar or quartz•Thought to originate deep in themantle•Havevery low viscosity, so they floweasilyA common example iskomatiite, which is made mostly ofolivine and pyroxene, with smalleramounts of feldspar.2. Rock TexturesA rock’stexturegives important clues about how it formed. Texture refers to thesize, shape, andarrangementof mineral crystals or rock fragments within the rock. By carefully examining texture,geologists can often tell whether a rock formed from lava, explosive eruptions, or other processes.2.1 Grain Size and Cooling RateMostextrusive rockscool quickly at Earth’s surface, so they are usuallyfine-grained. This meanstheir mineral grains aresmaller than 1 millimeterand are often too small to see without amicroscope.Lava flows often show a pattern related to cooling:•Theouter edgeof a lava flow cools very fast, forming achilled marginwith an extremelyfine-grained (aphanitic) texture.•Moving toward thecenterof the flow, cooling is slower, so grain size gradually increases.•Thick lava flows may developmedium-to coarse-grained centers.

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Study Guide2.2 PorphyriƟc and Glassy TexturesAporphyritic texturecontainslarge crystals, calledphenocrysts, surrounded by a much finer-grained background called thegroundmass.•The large crystals formedearlier, while the magma was still underground.•The remaining magma then erupted and cooled quickly, forming the fine-grained matrix.Obsidian, also known as volcanic glass, forms when lava coolsso rapidlythat crystals do not havetime to grow. It is:•Very fine-grained•Hard and glassy•Composed mainly of silica2.3 Lava Flow Surface TexturesBasaltic lava flows can develop distinctive surfacetextures:•Pahoehoe(pa-hoy-hoy):○Has a smooth,ropy surface○Forms when the outer surface cools quickly while molten lava continues flowingunderneath•Aa(ah-ah):○Has a rough,jagged, rubbly surface○Forms when a partially solidified crustbreaks apart as the lava keeps moving forward2.4 Vesicular Rocks and Gas BubblesMagmas deep underground often containdissolved gasesbecause of high pressure. When magmaerupts at the surface, pressure drops suddenly, allowing gas to escape.

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Study GuideThis createsvesicles, which are holes or cavities left behind by gas bubbles.•Pumice○Extremely vesicular○So full of gas bubbles that it canfloat on water•Scoria○A vesicular basalt○Dark-colored, rough, and full of holes○Contains more gas spacethan solid rock2.5 PyroclasƟc and Fragmental TexturesExplosive volcanic eruptions ejectpyroclastic material, which includes fragments of lava, ash, andcrystals. When this material becomes compacted and hardened, it forms rocks withfragmentaltextures.Pyroclastic material is classified by size:•Dust and ash–very fine-grained•Cinders–about pea-sized•Lapilli–about walnut-sized•Bombs and blocks–can be1 meter across or larger•Blocksare solid fragments of hardened lava.•Bombsare blobs of molten lava that harden while flying through the air.Small crystals—often feldspars—that formed before eruption may also be mixed into these deposits.•Tuff–made of fine-grained pyroclastic material (for example, ash tuff or crystal tuff)

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Study Guide•Welded tuff–ash and glass fragments were still hot enough tofuse together•Tuff breccia (agglomerate)–contains large volcanic bombs or blocks2.6 Special Extrusive TexturesSome extrusive rocksshow textures linked to where and how they cooled:•Columnar jointing○Found inflood basalts○Cooling and contraction form vertical, parallel columns—oftensix-sided•Pillow structures○Form insubmarine lava flows○Rounded blobs of lava harden instantly in cold seawater○Help geologists identify rocks formed on theocean floorand determine which waywas “up” in the original flow

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