1 Volcanism and Plate Tectonics Earth Science Ch. 14 Volcanism & magma What is magma? molten rock underground asthenosphere & plate boundaries - two locations where rock can melt requires proper balance of heat/pressure water content affects (reduces) “melting point” Felsic magma - full of silica; thick & slow; light- colored Mafic magma - Mg/Fe; more fluid; dark-colored Volcanism & magma Gases in magma Most important: water vapor(H2O), carbon dioxide (CO2), sulfur (S) Amount of dissolved gas affects kind of eruption Less pressure near surface … Dissolved gases form bubbles Bubbles expand & burst More gases = greater explosion Popcorn & Soda pop Volcanism & magma Volcano = magma erupting through opening at earth’s surface Lava = magma at earth’s surface Changes in composition Some gases escape New material from rock melted by magma Felsic lava = more (full of) Si; thick & sticky Mafic = less Si; thin & (more) fluid Gases escape mafic lava easily; felsic lava more explosive Volcanism & magma Lava fragments Tephra = solid framents produced during explosive eruptions Ash - smallest particles (<2mm) Lapilli - larger pieces (2 - 64 mm) Blocks - large, solid pieces Bombs - ejected as liquids & harden Pyroclastic flows - dense, superheated cloud of gases & tephra Kinds of Eruptions 1) Rift Eruptions Occur at long, narrow fractures (fissures) in crust Smooth basaltic flows with few gases Eruptions in ocean at spreading centers produce pillow lavas mid-Atlantic Ridge & East Pacific Rise Eruptions on land form basalt plateau Columnar jointing - 6-sided columns form as basalt cools East African Rift system; Columbia Plateau
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Volcanism & magma Volcanism and Plate Tectonics chain of extinct volcanoes as lithosphere moves over the hot spot Examples: Hawaii & Yellowstone Types of Volcanoes Famous Eruptions
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Volcanism and PlateTectonics
Earth ScienceCh. 14
Volcanism & magmaWhat is magma?
molten rock undergroundasthenosphere & plate boundaries - two locationswhere rock can meltrequires proper balance of heat/pressurewater content affects (reduces) “melting point”Felsic magma - full of silica; thick & slow; light-coloredMafic magma - Mg/Fe; more fluid; dark-colored
Volcanism & magmaGases in magma
Most important: water vapor(H2O), carbon dioxide(CO2), sulfur (S)
Amount of dissolved gas affects kind oferuptionLess pressure near surface …
Occur at long, narrow fractures (fissures) in crustSmooth basaltic flows with few gasesEruptions in ocean at spreading centers
produce pillow lavas mid-Atlantic Ridge & East Pacific Rise
Eruptions on land form basalt plateau Columnar jointing - 6-sided columns form as basalt cools East African Rift system; Columbia Plateau
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Kinds of Eruptions2) Subduction boundary eruptions
Magma usually thick with large amounts of gasExplosive eruptions of gas & tephraForms cinder cones or stratovolcanoesMost active volcanoes at subduction boundaries
Volcanic island chains: Indonesia (most active),Philippines, Japan, Aleutian
Young mountain ranges: Cascades, Central Am.; Andesin South Am.
Kinds of Eruptions3) Hot spots = eruptions in middle of lithosphericplates (not at boundaries)
Lava usually similar to rift eruptionsMay form shield cones with broad, gentle slopesCause is uncertain - likely some concentration ofheat from radioactive sources in theasthenosphereForms chain of extinct volcanoes as lithospheremoves over the hot spotExamples: Hawaii & Yellowstone
Types of Volcanoes Famous EruptionsEldfell
Small island near IcelandErupted for 5 months in 1973Rift eruption from a fissure along mid-Atlantic Ridge (spreading center)Lava flow & tephra eruptionTephra burned & buried homes; lavathreatened to block harbor, so villagerssprayed with water
Famous EruptionsMt. St. Helens
WA State - one of 15 Cascade volcanoesMajor eruption in 1980Subduction boundary eruption of a stratovolcano,with dome building in crater, caused by Juan deFuca Plate plunging under N. Am. PlateLittle lava, large gas & tephra eruption; includedEQ, landslide, pyroclastic explosion & mudflowsTrees blown down 25 km away; 20 km high ashcloud with large fallout; massive landslide;mudflows threatened towns downstream (57people died & 200 homes destroyed)
Famous EruptionsKilauea
Island of HawaiiHas erupted at least once/yr since 1952Hot spot eruption forming shield volcano &caldera; magma rising from depth > 50 kmEruptions produce lava flowsUnique lava lakes form in cone; lava flowsthreaten communities & covered highways
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Famous EruptionsVesuvius
(Italy) one of Mediterranean volcanoesErupted in AD 79 and repeatedly sinceSubduction boundary eruption [of a compositevolcano] as African Plate subducts beneathEurasian PlateVery explosive eruptions caused by thick magma& abundant gases produced heavy ashfallPompeii & two other Roman cities buried andpreserved by ash
Famous EruptionsKrakatau
Volcanic island in Indonesian chainFamous eruption of Aug. 27, 1883Subduction boundary eruption of a volcanic island[stratovolcano] caused by Indian Plate subductingunder Eurasian PlateLabeled the “most violent eruption of historictimes”More than 1/2 the island destroyed; ash cloud 30km high; giant sea wave drowned 36,000;affected sunrise/sunset for 2 yrs
Famous EruptionsCrater Lake
Oregon StateErupted approx. 7000 yrs agoSubduction boundary eruption [of stratovolcano -Mt. Mazama] caused by Juan de Fuca plungingbeneath N. Am. PlateViolent [catastrophic] eruptionAshfall up to 15 m thick >50 km from volcano;caldera (crater) formed after cone collapsed; snowmelt & rainfall have formed deepest lake in U.S.
Famous EruptionsMt. Pinatubo
PhilippinesErupted June 1991 after 635 yrs inactivitySubduction boundary eruption [of stratovolcano]caused by Pacific Plate plunging beneathPhilippine PlateBiggest volcanic eruption of the 20th century(1900’s)20 million tons of SO2 & ash erupted into upperatmosphere; ash cloud encircled earth causingvivid sunsets; potential effects on climate & ozonehole
Volcanic Hazards
http://volcanoes.usgs.gov/hazards/
Volcanic Hazards
Moves & buriesobjects in its path;may result in hilly(hummocky)landscape
Rapid downslopemovement of rock, dirt,snow & ice; eruption, EQ,or heavy rainfall causeshydrothermally alteredrocks to breakaway
landslides
One of the mostdeadly hazards; ripsout trees & houses;entombs everything inmud
Mixture of water, rock, dirt& mud; can travel >50 mi;20-40 mph
Lahars/mudflows
Knocks down & burnseverything in its way
Avalanche of hot ash, rockfragments, gases; 1500ºF;100-150 mph
Large waves candestroy shorelinecommunities -washing awaybuildings & drowningpeople
water displaced by seismicevent (EQ, landslides,eruptions); creates massivewaves
tsunamis
Can result in acid rain;high CO2 concen-trations can causesuffocation; mayaffect ozone &temperatures
Includes water vapor, CO2,SO2, H2S, H2
Volcanic gasesThreatsDefinitionsHazards
Plutonic ActivityPlutons & volcanism
Much more magma is present beneath thesurface than erupts at volcanoesThese igneous intrusions (“plutons”) = rockmasses that form when magma coolsinside other rocks
Plutonic ActivityDikes, sills, laccoliths, necks
Dike - igneous intrusion cutting across rock layers,forms when magma intrudes into vertical cracksSill - sheet of intrusive igneous rock that runsparallel to the existing rock layersLaccoliths - dome-shaped intrusive masses thatpush overlying rock layers upwardVolcanic neck - hardened magma left in the ventof extinct volcano
Plutonic ActivityBatholiths & stocks
Batholith - largest of all igneous intrusions;forms the core of many mountain ranges,such as Coast Range of British ColumbiaRocks of batholiths are usually made ofgranite or granodioriteStock - small batholith (less than 100 sq.km.) exposed at the surface