VolcanoesVolcanoes

Source: Photograph copyright Paul Hickson, 1980Image by NPS

I. Volcanoes release magmaI. Volcanoes release magma

A.A. Molten rock is less dense than solid rock Molten rock is less dense than solid rock so it rises through fractures so it rises through fractures

Shield Volcano

B. Speed of magma depends on silica B. Speed of magma depends on silica content.content.

1. felsic magma1. felsic magma--

a. High silicaa. High silica

b. Thickb. Thick

c. Lightc. Light--coloredcoloredc. Lightc. Light--coloredcolored

d. Slowd. Slow--movingmoving

e. violent eruptionse. violent eruptions

2.2. Mafic magmaMafic magma

a. Low silicaa. Low silica

b. Thinb. Thin

c. darkc. dark

d. Fastd. Fast--movingmovingd. Fastd. Fast--movingmoving

e. Eruptions are not violente. Eruptions are not violent

C. Gas content of magma C. Gas content of magma determines violence of eruptionsdetermines violence of eruptions

1.1. The more gas, the more violentThe more gas, the more violent

2.2. COCO22, sulfur, H, sulfur, H22, H, H22O vaporO vapor

High gas Low Gas

D. Lava composition are a little D. Lava composition are a little different than their magma was. different than their magma was. Why?Why?

1.1. Gas is releasedGas is released

2.2. New material is addedNew material is added

E. E. TephraTephra--solid fragments of lava solid fragments of lava

Tephra is classified by sizeTephra is classified by size

1. 1. ashash-- < 2 mm in diameter< 2 mm in diameter

Ash

2. 2. LapilliLapilli-- 22--64 mm64 mm

3. 3. BlocksBlocks-- >64 mm>64 mm

4. 4. BombsBombs-- >64 mm, but are >64 mm, but are ejected as liquid & harden as ejected as liquid & harden as they fallthey fall

Pyroclastic flow- hot, fast, high-density mixture of ash, pumice, rock fragments and gas formed during explosive eruptions.

Image from: U.S. Geological Survey, photo by B. Yount

II. 3 Types of Volcanoes

A. Cinder Cones

B. Composite Volcanoes

C. Shield Volcanoes

A. Cinder ConesA. Cinder Cones1. Steep-sided

2. Gas-charged, felsic lava

3. Single vent

4. Bowl-shaped crater at the summit

5. Rarely more than 1000 ft tall5. Rarely more than 1000 ft tall

Sunset Crater in Arizona

http://www.geology.sdsu.edu/how_volcanoes_work/

Cinder ConesCinder Cones

Cinder cone

Cinder Cones

B. Composite or Stratovolcanoes

1. Steep-sided symmetrical cones

2. Felsic lava

3. Explosive

4. Make up about 60% of Earth’s volcanoes4. Make up about 60% of Earth’s volcanoes

5. Layered structure consisting of layers of lava & pyroclastic material

6. Large (1-10 km across)

Composite volcanoComposite volcano

Mt. St Helens

Composite Volcano

Mt Fuji in Japan

Source:http://www.geology.sdsu.edu/how_volcanoes_work/

C. Shield Volcanoes

1. Built of fluid basaltic lava flows

2. Lava pours out in all directions from a central summit vent, or group of vents

3. Gently sloping sides like a “warriors shield”3. Gently sloping sides like a “warriors shield”

4. Some of the largest volcanoes are shield volcanoes (3-4 mi wide, 1,500-2,000 ft tall)

5. Usually no tephra

Shield VolcanoShield Volcano

Layers of basalt lava build over time.

Contrasting the 3 types of Volcanoes

Composite Volcano

Cinder Cones

Shield Volcano

III. Causes of VolcanoesIII. Causes of VolcanoesA.A. Rift eruptionsRift eruptions--occur at fractures in the occur at fractures in the

crust on the ocean floorcrust on the ocean floor

1.1. Flow smoothlyFlow smoothly

2.2. Can form shield conesCan form shield cones

3.3. Rift eruption at spreading center form pillow Rift eruption at spreading center form pillow 3.3. Rift eruption at spreading center form pillow Rift eruption at spreading center form pillow lavaslavas

4.4. On land, lava can spread out evenly over On land, lava can spread out evenly over large areas to form large areas to form basalt plateausbasalt plateaus..

5.5. Mafic magmaMafic magma

B. Subduction boundary eruptionsB. Subduction boundary eruptions

1.1. Felsic magmaFelsic magma

2.2. Explosive w/tephraExplosive w/tephra

3.3. Form Form cinder conescinder cones-- steepsteep--sided volcanic sided volcanic mountainmountainmountainmountain

C. C. Hot SpotsHot Spots--areas of volcanic areas of volcanic activity in the middle of platesactivity in the middle of plates1.1. ooze mafic magma through shield conesooze mafic magma through shield cones

2.2. Caused by concentrations of heat from Caused by concentrations of heat from radioactive isotopes in the asthenosphereradioactive isotopes in the asthenosphere

3.3. They stay in the same place even though They stay in the same place even though 3.3. They stay in the same place even though They stay in the same place even though plates move above them.plates move above them.

IV. Examples of Famous VolcanoesIV. Examples of Famous Volcanoes

A.A. EldfellEldfell

1.1. Off the coast of IcelandOff the coast of Iceland

2.2. Erupted in 1973Erupted in 1973

3.3. Villagers pumped sea water on lavaVillagers pumped sea water on lava3.3. Villagers pumped sea water on lavaVillagers pumped sea water on lava

4.4. On the midOn the mid--Atlantic ridge, but it is above sea Atlantic ridge, but it is above sea level.level.

5.5. Rift eruptionRift eruption

http://volcano.und.nodak.edu/vwdocs/volc_images/europe_west_asia/heimaey/heimaey.html

http://volcano.und.nodak.edu/vwdocs/volc_images/europe_west_asia/heimaey/heimaey.html

B. Mount St. Helens in WashingtonB. Mount St. Helens in Washington

1.1. Cascade rangeCascade range

2.2. Erupted 1980Erupted 1980

3.3. Subduction boundary eruption Subduction boundary eruption

4.4. Juan de Fuca plate slides under the North Juan de Fuca plate slides under the North 4.4. Juan de Fuca plate slides under the North Juan de Fuca plate slides under the North American plateAmerican plate

Mt. St Helens

C. KilaueaC. Kilauea--HawaiiHawaii

1.1. Has erupted every year since 1952Has erupted every year since 1952

2.2. Hot spotHot spot

Kilauea

Low Gas

V. Plutonic ActivityV. Plutonic Activity

A.A. PlutonPluton (igneous intrusion(igneous intrusion))-- rock masses rock masses that form when magma cools inside other that form when magma cools inside other rocks.rocks.

B. Volcanoes UndergroundB. Volcanoes Underground1.1. DikesDikes –– sheets of igneous rock that cut sheets of igneous rock that cut

across the rock layers they intrudeacross the rock layers they intrude

2.2. SillsSills –– sheets of igneous rocks sheets of igneous rocks parallel to the layers they intrudeparallel to the layers they intrude

3. 3. LaccolithLaccolith –– domelike masses that form domelike masses that form when magma that intrudes between rock when magma that intrudes between rock layers is unable to flow easily. As a result, layers is unable to flow easily. As a result, you get a bulge. The rock layers above you get a bulge. The rock layers above push into a dome.push into a dome.

4. 4. BatholithBatholith –– largest igneous intrusions largest igneous intrusions that form at the cores of many that form at the cores of many mountain rangesmountain ranges

5. 5. Volcanic neck Volcanic neck –– a plug of hardened a plug of hardened magma left in the vent from which lava magma left in the vent from which lava flowed. Found when an extinct volcano is flowed. Found when an extinct volcano is almost completely eroded.almost completely eroded.

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