Igneous Rocks Chapter 3 Igneous rocks form as molten rock cools and crystallizes (solidifies) from either a magma or a lava
Formation of Igneous Rocks Magma: forms from partially melted rocks INSIDE the Earth Parent material of igneous rocks Intrusive classification Plutonic rocks
Magma Completely or partially molten Earth material Most consist of three parts: Liquid portion (melt) Mobile ions from Earth s crust Solid portion Silicate minerals that have already solidified Gaseous phase (volatiles) Mostly: water vapor, carbon dioxide, sulfur dioxide Volatiles: materials that readily vaporize at surface pressures
Crystallization of Magma Cooling of magma results in the systematic arrangement of ions into orderly patterns of interlocking crystals The silicate minerals resulting from crystallization form in a predictable order
Interlocking Crystals
Formation of Igneous Rocks According to Bowen s Reaction Series During crystallization, the composition of the liquid portion of the magma continually changes Composition changes due to removal of elements by earlier-forming minerals The silica component of the melt becomes enriched as crystallization proceeds Minerals in the melt can chemically react and change
Partial Melting
How do magmas move toward the surface? By Assimilation, that is, melting the surrounding rocks. By Stoping: the magma forces its way into fractures and large blocks (inclusions) drop into the magma chamber. By Forceful Intrusion: Simply pushing up the surrounding rock.
Magmatic Differentiation
Fractional crystallization of a basaltic magma Silicate minerals form in order according to Bowen s Reaction Series While the magma is still intrusive mafic minerals solidify The magma exits and becomes extrusive The magma is enriched in felsic minerals Enriched in silica
Assimilation When a molten body moves up through "country rock" it assimilates rock.(melts and incorporates elements from the surrounding rock). This changes the magma composition. http://www.calstatela.edu/facu lty/acolvil/igneous.html
Magma Mixing At convergent boundaries Two magmas may over take each other and mix Oceanic (basaltic) magma rises through continental (granitic) magma mixing to form Andesite Ryolite
Classification of Igneous Rocks Igneous rocks are typically classified by texture and composition Texture: Size and arrangement of mineral grains Composition: Mineral constituents
Classification of Igneous Rocks Texture
Texture in igneous rocks describes the overall appearance of a rock based on the size, shape, and arrangement of interlocking mineral grains Factors that contribute to igneous textures include: Rate of cooling Amount of silica present Long chain-like structures; increased viscosity Amount of dissolved gasses Upper zone of lava flow; vesicles
Factors affecting crystal size Rate of cooling Slow rate promotes the growth of fewer but larger crystals Fast rate forms many small crystals Very fast rate forms glass
Igneous Textural Classifications
Cooling rate vs. Crystal formation
Aphanitic: Extrusive, Volcanic Aphanitic: Fine-grained Rapid rate of cooling of lava Microscopic crystals Aphanitic glassy Very rapid rate of cooling lava No crystals Apahnitic vesicular Very rapid rate of cooling Contains vesicles Holes from trapped gasses
Phaneritic: Intrusive, Plutonic Phaneritic: Coarse grained Slow cooling of magma Crystals can be identified without a microscope
Pegmatitic: Very coarse grained, plutonic Pegmatitic: very coarse grained Cooled very slowly deep in Earth s interior Batholiths Stocks Usually granitic
Porphyritic texture Minerals form at different temperatures rates Large crystals phenocrysts Are embedded in a matrix of smaller crystals groundmass
5. Pyroclastic texture Various fragments ejected during a violent volcanic eruption Textures often appear to be similar to sedimentary rocks
Igneous Texture: Pyroclastic Tuff Angular rock fragments embedded in a matrix of ash
Welded Tuff deposits: Yellowstone
Tuff deposits: Crater Lake
Classification of Igneous Rocks Composition
Igneous rocks are composed primarily of silicate minerals Light (or nonferromagnesian) silicates Felsic Quartz Muscovite mica Feldspars Dark (or ferromagnesian) silicates Mafic Olivine Pyroxene Amphibole Biotite mica
Composition Felsic Granitic Light color Low density Mostly Potassium Feldspar (Kspar) and Silica (Si) Nonferromagnesium minerals Little to no Mg and Fe Continental crust Intermediate Andesitic Intermediate between granite and basalt At least 25% dark silicate minerals Of the feldspars, contains mostly plagioclase Convergent boundaries Oceanic/continental collisions
Composition Mafic Basaltic Dark color Higher density Mostly ferromagnesium minerals Mg and Fe Ocean crust Convergent boundaries Oceanic/Oceanic collisions Volcanic Island arcs Ultra Mafic Rare composition that is high in magnesium and iron Composed entirely of ferromagnesian silicates Earth s mantle Extremely rare on Earth s surface
Identification of Igneous Rocks http://www.mpes.org/grade3/l abtypesofrocks.htm
Volcanoes and Other Igneous Activity Chapter 4
Extrusive Igneous Bodies
Volcano!
Eruptive Violence and Characteristics of Lava Lava = magma at Earth s surface Level of explosivity depends on gas content: primarily H 2 O but also CO 2, SO 2, H 2 S, HCl viscosity temperature high silica = most viscous high basalt = least viscous
Materials extruded during an eruption Or HOT HOT HOT
Lava flows pahoehoe aa
Lava tube
Lava tube
Volcanic Ash
Volcanic Ash Scanning electron microscopic pics
Tephra
Lava bombs
Volcanic Hazards Pyroclastic flows - Nuee Ardente Lahars
Pyroclastic flows: Nuee ardente Mt. ST. Helens August 7, 1980
Mt. St. Helens 1980
Monserrat March, 2002
Lahars Mt. St. Helens
Mt. St. Helens
RUIZ Volcano had not erupted in over a century when the first volcanic earthquakes were recorded in late 1984. Colombian and international scientists monitored the volcano for over a year, recognized the possibility that devastating lahars (volcanic mudflows) would be generated if an eruption occurred, and published an accurate hazards map that clearly showed vulnerable areas. Little was done to warn the threatened populations though, and when the expected eruption finally took place on the evening of November 13, no believable evacuation orders were given. Over 25,000 people were needlessly killed that night on the flanks of Ruiz, and the scenes of death and devastation I witnessed later that month will never be erased from my memory. This eruption had major impact on my conscience, and I realized the obligation of volcanologists to do more than "good science" -- we have moral obligations to make sure that public officials fully understand and act on our warnings and that they reach the threatened people in understandable format Columbia, 1985 John P. Jack Lockwood Consulting Volcanologist
Mt. Pinatubo
Types of volcanoes
Cinder Cones Formed of pyroclastics only Steep sided: ~30 degrees Relatively small Short duration of activity
Sunset Crater Flagstaff, Arizona
Composite Cone Volcanoes Alternating layers of pyroclasts and lava Slopes intermediate in steepness Intermittent eruptions over long time span Mostly Andesite Distribution Circum-Pacific belt ( Ring of Fire ) Mediterranean belt
Typical composite cone volcano
Cascade Range
Shield Volcanoes Low viscosity lava flows Gently sloping flanks between 2 and 10 degrees Calderas Large circular depressions at summit Due to collapse of a summit Tend to be very large example: Mona Loa Kileauea
Shield volcanoes
Mona Loa Location 19.425 N 155.292 W Elev. Above Sea Level 1,277 m 4,190 ft Area 1,430 km 2 552 mi 2 (13.7% of Hawai`i) Volume 25,000-35,000 km 3 6,000-8,500 mi 3
Galapagos Islands
Caldera process
Crater Lake, Washington
Kileauea Caldera
Santorini
Fissure Eruptions
Flood Basalts Very extensive flows of fluid basaltic lava From fissure eruptions Columnar jointing
Flood basalts Columbia River Plateau, Pacific NW Covers area of ~ 80,000 square miles (200,000 square km) 1 km thick 17 million years ago Deccan Traps, India 195,000 sq miles (500,000 sq km); 66 m years ago Ontong Java Plateau Floor of Pacific Ocean
Columbia River flood basalts
Columnar jointing Giants Causeway National Park, Northern Ireland
Devil s Tower Wyoming
Volcanic Domes Forms above a volcanic vent Viscous lava usually silica-rich (or cooler magma) Associated with violent eruptions
Lava domes
Monserrat
Mt. St. Helens
Volcanic Floods and Submarine Eruptions Submarine Eruptions basalts pillow lava
Pillow lava
Submarine basaltic flow (with pillow lava)
Plutonic bodies Bodies that solidify underground Instrusive
lacolith neck fissure sill dike dike batholith
Concordant-tabular Contacts are parallel to bedding plane Sill Lava flows between bedding plans Flat and parallel Laccolith similar to sill but thicker in center; looks like a mushroom; causes beds to uplift
Sill
Palisades Sill
Discordant Volcanic neck Shallow intrusion Opening that brings magma to summit Dike: cuts across bedding planes Associated with fissure eruptions
Dike at Shiprock
Volcanic Neck
Shiprock, New Mexico
Massive Batholith: very large area (>36 miles 2 )(100km 2 ) Stock: similar to batholith but smaller (<36 mile 2 )
Granite Batholiths
Half-Dome Yosemite National Park
Plate Tectonics and Igneous Activity At spreading centers At subduction zones Intraplate activity
Locations of Earth s major volcanoes
The good, the bad and the ugly Growth of new islands and additions to old ones Hawaii Lohii hot spot volcano Surtsey Iceland mid ocean ridge volcano
Surtsey, Iceland
Surtsey is about 1.5 km in diameter and has an area of 2.8 square km
The volcano that makes up this island may be older but the island itself was born on 15 Nov, 1963. Eruptions actually started below sea level about a half of a year prior to this. They were detected and located by earthquake activity (seismic tremors) in the area. Earliest indications of an impending eruption at the surface were from water temperature increases of 7-9 C at the sea surface, about 45 days before the abovesea-level eruptions began. The 1963 eruption actually started at over 100 meters water depth. As the volcano neared the surface, the eruptions created large explosions. When it neared the surface it produced an eruption column composed moslty of volcanic ash. The first observations of an eruption column were by Ólafur Vestman. He observed the volcanic debris from a ship on the morning of Nov. 14, 1963. Eventually, the volcano grew above sea level, lava flows were erupted, and a cone formed. Basaltic lava flows first erupted April 4, 1964. All eruptions ceased in 1967. This pyroclastic to lava flow transition occured after the volcano had breached sea level and had built up a cone that was large enough to keep sea water out of its eruption crater. Today, the island is being eroded by the large waves of North Atlantic Ocean and will probably disappear unless it erupts anew. It is presently volcanically quiet. Drs. Mike Garcia and Ken Rubin Department of Geology and Geophysics University of Hawaii, Honolulu, HI 96822
Hawaii
Loihi the next Hawaiian island
Geothermal Energy: How it works
Geothermal energy The Geysers California The largest geothermal energy plant in the world 48.8% of Iceland s energy supply comes from geothermal energy
Geysers Yellowstone National Park Old Faithful Geothermal pool
Effect on Climate Krakatau, 1883 Addition of gasses (Co2 and water) into the atmosphere Ash into upper atmosphere Blocks sunlight Greenhouse effect Pelee, Soufriere and Santa Maria, 1902 Agung, 1963 El Chichon, 1982 Composite Composite: average of the 4 previous graphs Pinatubo, 1991
Montserrat Volcanic Catastrophes Soufriere Hills, July 18, 1995 Mt. St. Helens May 20, 1980 St. Pierre, Martinique Mount Pelé, 1902 Vesuvius, near Naples, Italy AD 79 Pompeii and Hurculaneum Santorini, Greece 1,650 BC Paricutin Mexico, 1943
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