• There 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)

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

COLUMNAR JOINTING

• Form 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

Enchanted Rock

batholith; exfoliation dome

Intrusive Igneous Bodies

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 eruptedfor 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

• http://dsc.discovery.com/convergence/pom

peii/videogallery/videogallery.html

Trapped Gases

• Water vapor and carbon dioxideare 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

Bomb – a lump of rock thrown out in an eruption

Crater – a deep hollow at the top of a volcano

Lava – melted rock that flows down the volcano

Magma – melted rock inside the Earth

Molten – melted, liquid

Vent – 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

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)

• http://www.learner.org/interactives/volcano

es/movies/movies3.html

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

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 following the

1980 eruption

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

Nueé 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

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

Eyjafjallajokull

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

more_from_eyjafjallajokull.html