• There are 2 types of volcanismThere are 2 types of volcanism

IntrusiveIntrusive--------magma magma cools below the cools below the surface and makes plutons (igneous surface and makes plutons (igneous intrusions)intrusions)

Extrusive-Extrusive---liquid rock --liquid rock lava lava cools on the cools on the surface (volcanoes, lava plateaus)surface (volcanoes, lava plateaus)

Igneous IntrusionsIgneous Intrusions

Dikes•Small magma intrusions (no more than a few 100 m wide)•Cut across other rock layers•Force their way along lines of weakness such as faults.

Shiprock, NM ---volcanic plug Shiprock, NM ---volcanic plug and dikeand dike

DikeDike

Igneous IntrusionsIgneous Intrusions

Sills• Small magma intrusions (no

more than a few 100 m wide)• Run parallel to rock layers• Force their way along lines of

weakness such as bedding planes.

A sill in the Salt River A sill in the Salt River Canyon, Arizona Canyon, Arizona

COLUMNAR JOINTINGCOLUMNAR JOINTING• Form in sills and dikes that cool Form in sills and dikes that cool

slowly and form six-sided columns slowly and form six-sided columns

These pictures were taken on Oregon These pictures were taken on Oregon 19, just north of US 20 19, just north of US 20

99

Columnar

jointing in

basalt at

Devil’s

Postpile near

Mammoth

Lakes, CA

1010

Top of jointed basalt columns, Devil’s Postpile, CA

Igneous IntrusionsIgneous Intrusions

Laccoliths - • Lens shaped igneous intrusion• Have a “floor”---there are sedimentary rock layers

beneath the base

Batholiths -• larger • have no “floor”

A batholith exposed by erosionA batholith exposed by erosion

Enchanted Rock Enchanted Rock batholith; exfoliation domebatholith; exfoliation dome

Intrusive Igneous BodiesIntrusive Igneous Bodies

Locations of some of EarthLocations of some of Earth’’s s major volcanoesmajor volcanoes

• A volcano is active, or alive, when it erupts often.

• When a volcano is dormant, it has not erupted for a long time – but it might in the future.

• A volcano is extinct when it hasn’t erupted for at least 100,000 years.

Active, Dormant, or Extinct?Active, Dormant, or Extinct?

Types of EruptionsTypes of Eruptions

Depends on trapped gases and magma composition

•Violent and explosive•Quiet and flowing

• http://dsc.discovery.com/convergence/pompeii/videogallery/videogallery.html

Trapped GasesTrapped Gases•Water vapor and carbon

dioxide are trapped in magma•At low pressure, they escape

quietly when they reach the surface

•At high pressure, they escape violently when they reach the surface

Magma CompositionMagma Composition

•Two types:–Basaltic – less silica and very fluid; produces quiet eruptions

–Granitic – lots of silica, high water vapor content, and very thick; produces very violent eruptions

GLOSSARYGLOSSARY

Bomb – a lump of rock thrown out in an eruptionCrater – a deep hollow at the top of a volcanoLava – melted rock that flows down the volcanoMagma – melted rock inside the EarthMolten – melted, liquidVent – a crack on the side of a volcano where magma can

escape

Forms of VolcanoesForms of Volcanoes

•Three forms:–Shield–Cinder Cone–Composite

Shield VolcanoShield Volcano

•Formed by quiet eruptions•Basaltic lava builds up in flat layers

•Gently sloping sides•Ex: Hawaiian Islands (Mauna Loa)

Shield volcanoShield volcano

Mauna Kea

Shield volcanoHot Spot

Mauna Loa inbackground

Kilauea is behind MaunaLoa (erupting since 1983)

Kilauea

Pahoehoe (smooth, ropy) lava flowPahoehoe (smooth, ropy) lava flow

aa lava flow (slow, chunky)aa lava flow (slow, chunky)

• http://www.learner.org/interactives/volcanoes/movies/movies3.html

Cinder Cone VolcanoCinder Cone Volcano•Caused by explosive eruptions•Granitic lava thrown high into

the air•Lava cools into different sizes of

volcanic material called tephra•Steep-sided, loose slopes•Ex. – Paricutin (Mexico)

Cinder coneCinder cone

Composite/StratovolcanoComposite/Stratovolcano

•Alternating layers of ash and lava

•Quiet or violent•Basaltic or granitic•Steep or gentle slopes•Ex. – Mt. St. Helens

Composite volcano Composite volcano

Guagua Pichincha, EcuadorQuito in foregroundComposite volcanoes - explosive

Arenal Volcano – Costa RicaArenal Volcano – Costa Rica

Photo by E. L Crisp, 2007

Mt. St. Helens Mt. St. Helens –– a typical a typical composite volcanocomposite volcano

Mt. St. Helens following the Mt. St. Helens following the 1980 eruption 1980 eruption

http://www.youtube.com/watch?v=bgRnVhbfIKQ&feature=related

NueNueéé ardente (hot pyroclastic ardente (hot pyroclastic flow) flow) on Mt. St. Helenson Mt. St. Helens

Mt. Vesuvius – 79 a.d.Mt. Vesuvius – 79 a.d.

A A laharlahar (mud flow caused by melting (mud flow caused by melting snow) along the Toutle River near snow) along the Toutle River near

Mt. St. HelensMt. St. Helens

Composite VolcanoMt Rainier

Subduction zones

A size comparison of the three A size comparison of the three

types of volcanoestypes of volcanoes

Other volcanic landforms Other volcanic landforms Calderas

• Steep walled depression at the summit • Formed by collapse • Nearly circular • Size exceeds one kilometer in diameter

Fissure eruptions and lava plateaus • Fluid basaltic lava extruded from crustal

fractures called fissures • e.g., Columbia Plateau

Crater Lake, Oregon is a good Crater Lake, Oregon is a good example of a calderaexample of a caldera

Crater Lake in OregonCrater Lake in Oregon

Lava PlateausLava Plateaus• Magma seeps through

cracks on surface, spreads out and cools

• Process can go on for millions of years and cover very wide areas

The Columbia River basaltsThe Columbia River basalts

Other volcanic landforms Other volcanic landforms

Volcanic pipes and necks • Pipes are short conduits that connect a

magma chamber to the surface • Volcanic necks (e.g., Ship Rock, New

Mexico) are resistant vents left standing after erosion has removed the volcanic cone

Formation of a volcanic neckFormation of a volcanic neck

Shiprock (volcanic neck) New Shiprock (volcanic neck) New MexicoMexico

Devil’s Tower---WyomingDevil’s Tower---Wyoming

EyjafjallajokullEyjafjallajokull

• http://www.boston.com/bigpicture/2010/04/more_from_eyjafjallajokull.html