Chapter 12

Volcanoes

Volcanoes

• An opening in the Earth that erupts gases, ash, and lava

• Volcanic mountains form when layers of lava, ash, and other materials build up around these openings

• Earth has more than 600 active volcanoes

The most active volcanoes…

• Kilauea, in Hawaii, is the world’s most active volcano and has been erupting for decades

The most active volcanoes…

• Hekla, in Iceland, is another very active volcano, but there are many active volcanoes in Iceland

• In 2010, the eruption of an Icelandic volcano caused the cancellation of 1000s of flights

• http://www.youtube.com/watch?v=oZeYJF-yIcE

Effects of Eruptions

• Lava flows destroy everything in their path

• Pyroclastic flow=when ash and debris rush down the side of a volcano

How Do Volcanoes Form?

Where Do Volcanoes Occur?

• Divergent plate boundaries—where plates are moving apart

• Convergent plate boundaries—where plates are moving together

Where Do Volcanoes Occur?

• Hot Spots—unusually hot areas between the mantle and the core; rock melts here and is forced up toward the crust.

• The Hawaiian Islands sit on top of a hot spot under the Pacific plate. The islands are all in a line b/c the Pacific plate is moving over a stationary hotspot.

The Ring of Fire

Volcanic Eruptions

• Two factors control the type of eruption

– Amount of water vapor and other gases trapped in the magma

– Amount of silica present in the magma

Volcanic Eruptions

• Trapped gases:– Gases such as water vapor and carbon

dioxide are trapped in magma by the pressure of the surrounding magma and rock

– As magma nears the surface, the pressure lessens, which allows gas to escape from the magma

Volcanic Eruptions

• Water vapor– The magma at some convergent plate

boundaries contains a lot of water vapor

– This is b/c at some convergent plate boundaries, oceanic plate material and water slide under other plate material

– Water becomes water vapor at high heat

Volcanic Eruptions

• Quiet eruptions– Magma that is relatively low in silica is called

basaltic magma– It is fluid and produces quiet, nonexplosive

eruptions– It pours from volcano vents and runs down

the sides of a volcano– This fluid lava is called pahoehoe

Volcanic Eruptions• Pahoehoe lava

– Forms a ropelike structure when it cools

• Aa lava– Same lava as pahoehoe, but flows at a lower

temperature, it is stiff and slower moving

Volcanic Eruptions

• Pillow lava– Lava that flows underwater and forms shapes

like tubes, balloons, and pillows

Volcanic Eruptions• Explosive eruptions

– Silica-rich, or granitic, magma produces explosive eruptions

– This magma is thick and gets trapped in vents causing pressure to build up

Forms of Volcanoes• Shield volcano

– Formed from quiet eruptions of basaltic lava that spread out on flat layers

– Forms a broad volcano with gently sloping sides

– Hawaiian Islands

Forms of Volcanoes• Cinder Cone Volcano

– Explosive eruptions throw lava and rock high into air

– Bits of rock and solidified lava dropped from the air are called tephra• Tephra varies in size from ash to cinders to

large rocks called bombs and blocks• When tephra falls to the ground, it forms a

steep-sided, loosely packed cinder-cone volcano

• Cinder Cone Volcanoes

• Paricutin– On February 20, 1943, a Mexican farmer

noticed a hole in his cornfield that had been there as long as he could remember, was giving off smoke

– Throughout the night, hot glowing cinders were thrown high into the air

– In just a few days, a cinder cone several hundred meters high covered his cornfield

Forms of Volcanoes

• Composite Volcano– Eruptions can vary between quiet and

explosive– An explosive period can release ash forming

a tephra layer– Then it can switch to a quieter period

releasing lava over the tephra– This cycle can be repeated over and over

again

• Composite Volcanoes

Mt. St. Helens• http://www.youtube.com/watch?v=FnDT_6

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• http://www.youtube.com/watch?v=-H_HZVY1tT4&safety_mode=true&persist_safety_mode=1&safe=active

• http://www.youtube.com/watch?v=pGImksoOwtU&safety_mode=true&persist_safety_mode=1&safe=active

• http://www.youtube.com/watch?v=BaRAGzjSkzo&safety_mode=true&persist_safety_mode=1&safe=active

• http://www.youtube.com/watch?v=Ifohb8_2WhE&safety_mode=true&persist_safety_mode=1&safe=active

• http://www.youtube.com/watch?v=55xikeGO6oQ&safety_mode=true&persist_safety_mode=1&safe=active