volcanism & magma volcanism and plate tectonics chain of extinct volcanoes as lithosphere moves...

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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 …

Dissolved gases form bubblesBubbles expand & burstMore gases = greater explosion

Popcorn & Soda pop

Volcanism & magmaVolcano = magma erupting through opening atearth’s surfaceLava = magma at earth’s surfaceChanges in composition

Some gases escapeNew material from rock melted by magma

Felsic lava = more (full of) Si; thick & stickyMafic = less Si; thin & (more) fluidGases escape mafic lava easily; felsic lava moreexplosive

Volcanism & magmaLava fragments

Tephra = solid framents produced duringexplosive eruptionsAsh - smallest particles (<2mm)Lapilli - larger pieces (2 - 64 mm)Blocks - large, solid piecesBombs - ejected as liquids & hardenPyroclastic flows - dense, superheatedcloud of gases & tephra

Kinds of Eruptions1) Rift Eruptions

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

Pyroclastic flowsThreatsDefinitionsHazards

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Volcanic Hazards

Causes lightening;large pieces crushthings; ash disruptscommunication &transportation,breathing problems,buries objects &collapses roofs

Rock fragments ejectedduring an eruption;includes ash, lapilli, blocks& bombs

Ashfall/tephra

Covers & burnseverything in its path;can melt snow/ice &cause lahars

Low-silica basalt lava; canmove 10-30 mph & covergreat distances

Lava flowsThreatsDefinitionsHazards

Volcanic Hazards

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

Plutonic features

laccolith

batholith

sill

volcanic neck

dike