Where:

1. Almost all volcanoes are found @ Plate Boundaries.

Most of those volcanoes are found along the Pacific Rim – a subduction zone called “The Ring of Fire”

2. Hot spots

Volcanoes

Formation of Volcanoes

1. Magma forms: rock melts, forming liquid magma (melted rock + trapped gas)

2. Magma rises through the crust, erupting at the surface.

Magma rises b/c it is less dense than rock (it’s hotter & has gas in it).

3. Magma collects & melts more rock …Pressure builds as more gas is added.

4. Eruption: pressure gets too high.

Melting Rock to Make Magma

1. Heat Melts RockHeat from mantle; Heat from friction of grinding plates; heat

from magma that intrudes into crust from other locations

2. Decrease Pressure on Rock

Decompression Melting: When pressure is reduced rock can melt at lower temps. Pressure depends on depth. As hot rock rises; it melts because there is less pressure. * Rock deep in crust should melt; but is solid because of pressure)

3. Add Water: “Wet” rock melts at lower temps.Subduction drives water out of the subducting rock (metamorphic change). “Dewatering the slab” lowers the temperature of rock above.

Magma Composition and Characteristics

Granitic

Ocean Crust Melt (Mafic Magmas) Continental Crust

Melt(Felsic Magmas)

Structure of Volcanoes

Magma collects in magma chamber before eruption

Magma exits through a central vent or pipe to the crater at the summit.

Each eruption adds a layer to the volcano

Eruption dates can be determined by isotopic dating the rock layers

Structure of Volcanoes

Caldera : depression formed from collapsed volcanoes (usually extinct); usually fill up with water & b/cm lakes

Eruption Types: Quiet = low silica/ high water/ high temp. / Low viscosity/ low gas content

Eruption TypesExplosive Eruptions: high-silica / Low temps./ low water / high viscosity/ high gas content

“pyroclastic”

Mount St. Helens, WA

Pre 1980 eruption

Eruption Types

Mount St. Helens, WA

Post 1980 eruption

Today

Volcano Hazards1.Lava flows (burns/

fire)2.Ash (buries,

suffocates)3.Pyroclastic flows

(gas, ash, superheated rock fragments <bombs>)

4.Mudflows/Lahars5.Acidification of

water6.Climate

change/mass extinction

Least Hazard

Great Hazard

Lahar: Volcanic landslide/mudslide

Eruptions and Eqs that accompany them trigger landslides;Lava melts snow at the top of the volcano MudslideVery Hazardous wipe out villages/ fatalities

Volcano Types

1. Shield: Broad / Flat, Gently sloping cone, (b/c runny lava travels far before solidifying)

Quiet Eruptions: Liquid lava (low viscosity / high water / low silicates) fr. Single vent

Ex: Hawaiian Volcanoes

Volcano Types2. Strato or Composite VolcanoBuilds in layers of lava and ash & debris

Explosive “pyroclastic” eruptions (Hot gas, rock, and ash)High viscosity / high silicates /low water

Tall & Steep & Side Vents

Most Dangerous

Ex: Mt. St. HelensMt. Pinatubo

Volcano Types

3. Cinder Cones

Simple, small, steep sided

Made from blobs of lava & ash ejected from a single vent that fall back to the surface.

No repeated eruptions

Ex: California Volcanoes – Lassen Peak

Volcano Types4. Under Water Volcanoes

Seamount – an underwater mt. (volcano) that does not reach the surface

Volcanoes - Most are located near plate boundaries

“Ring of Fire” = Edge of Pacific Ocean stretching from Alaska to Japan to Indonesia, where most of the world’s volcanoes are located. (subduction zone)

Ring of Fire = Edge of the Pacific Plate. Most of the worlds volcanoes are found here due to SUBDUCTION of the pacific plate.

3 Ways Volcanoes Form

1. Subduction Zones (Convergent Plate Boundaries)(Ring of Fire)

2. Rifting & Sea Floor Spreading (Divergent Plate Boundaries)(African volcanoes & Iceland’s volcanoes)

3. Hot Spots – can be in the middle of a plate (Yellowstone, Hawaii) – a region of active magma under a plate. This active magma forces its way through weaknesses in the crust to form volcanoes.

Hot SpotsThe Hawaiian Islands were formed as seamounts grew over hot spots in the Pacific.

Hot Spots Around the World

Hot Spot Volcanoes

Hot Spot Volcanoes

Hot spot stays in the same place while the plate moves above.

Other Extrusive Igneous Features (not volcanoes)

Lava Plateau: Large amounts of easily flowing lava erupting fr. cluster of long, thin cracks in crust. Lava spreads out over enormous area before solidifying.

Ex.: Columbia Platueau in Pacific NW (1 km thick / 200,000 square mi)

Columbia Plateau in Idaho =

Lava Plateau

Basalt

Intrusive Igneous Features

Batholith: Large type of intrusive igneous rock mass that can form the core of a mountain range (Sierra Nevada’s in Calif.)

Sill: Magma squeezes through cracks in rock layers and hardens / Paralles to rock layers.

Dike: Igneous rock cuts across rock layers.Volcanic Neck: magma hardens in

volcanic neck.

Half Dome at Yosemite National Park (Sierra Nevada Mts) = Forms when batholith that was originally under surface reaches surface (rock on top of erodes away or it is pushed upwards)

Types of Volcanoeshttp://vulcan.wr.usgs.gov/Glossary/

VolcanoTypes/volcano_types.htmlhttp://vulcan.wr.usgs.gov/Photo/Pictograms/

volcano_types.html

Volcano World – All things volcanohttp://volcano.und.edu/

Smithsonian’s Weekly Volcano Reporthttp://www.volcano.si.edu/reports/usgs/