There are 2 types of volcanismThere are 2 types of volcanism Intrusive----magma cools below the surface and makes plutons (igneous intrusions) Extrusive---liquid rock lava cools on the surface (volcanoes, lava plateaus) Extrusive---liquid rock lava cools on the surface (volcanoes, lava plateaus)

Igneous 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 and dike

Dike

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

COLUMNAR JOINTING Form in sills and dikes that cool slowly and form six-sided columnsForm in sills and dikes that cool slowly and form six-sided columns These pictures were taken on Oregon 19, just north of US 20

9 Columnar jointing in basalt at Devil’s Postpile near Mammoth Lakes, CA

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

Igneous 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 erosion A batholith exposed by erosion

Enchanted Rock batholith; exfoliation dome

Intrusive Igneous Bodies

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Locations of some of Earth ’ s major volcanoes Locations of some of Earth ’ s major 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?

Types of Eruptions Depends on trapped gases and magma composition Violent and explosive Quiet and flowing

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Trapped 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 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

GLOSSARY BombBomb – a lump of rock thrown out in an eruption CraterCrater – a deep hollow at the top of a volcano LavaLava – melted rock that flows down the volcano MagmaMagma – melted rock inside the Earth MoltenMolten – melted, liquid VentVent – a crack on the side of a volcano where magma can escape

Forms of Volcanoes Three forms: –Shield –Cinder Cone –Composite

Shield Volcano Formed by quiet eruptions Basaltic lava builds up in flat layers Gently sloping sides Ex: Hawaiian Islands (Mauna Loa)

Shield volcano Shield volcano

Mauna Kea Shield volcano Hot Spot Mauna Loa in background Kilauea is behind Mauna Loa (erupting since 1983)

Kilauea

Pahoehoe (smooth, ropy) lava flow

aa lava flow (slow, chunky)

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Cinder 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 cone

Composite/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, Ecuador Quito in foreground Composite volcanoes - explosive

Arenal Volcano – Costa Rica Photo by E. L Crisp, 2007

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

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

Nue é ardente (hot pyroclastic flow) on Mt. St. Helens Nue é ardente (hot pyroclastic flow) on Mt. St. Helens

Mt. Vesuvius – 79 a.d.

A lahar (mud flow caused by melting snow) along the Toutle River near Mt. St. Helens

Composite Volcano Mt Rainier Subduction zones

A size comparison of the three types of volcanoes A size comparison of the three types of volcanoes

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 example of a caldera

Crater Lake in Oregon

Lava 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 basalts

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 neck

Shiprock (volcanic neck) New Mexico

Devil’s Tower---Wyoming

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