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Volcanoes

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Different Volcanic Settings

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Types of EruptionsEruptions will generally be of two types:

Quiet (Rift) eruptions

Explosive (Subduction) eruptions

Each will produce a different type of volcanic cone (shield or composite)

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RIFT (Quiet) ERUPTIONSGenerally at hot spots, spreading plates

Mantle comes directly to surface

Hot lava; low viscosity, flows easily, gases escape easily, mafic lava

Forms shield volcanoes, flood basalts

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SUBDUCTION (Explosive) ERUPTIONS

Found at subduction zones

Magma low temperature, high viscosity, does not flow easily, gases trapped, hard to predict explosions of felsic lava

Forms composite (strata) volcanoes, cinder cones, calderas, ash, lapilli, bombs, gas flows, very destructive

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Shield Volcanoes broad, gently sloping cones

constructed of solidified lava flows

lava spreads widely and thinly due to its low viscosity

produces a volcano in the shape of a flattened dome or ‘shield’

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Composite (Strata) Volcanoes constructed of alternating layers of pyroclastics (ash and rock particles) and rock solidified from lava flows.

built over long spans of time

eruption is intermittent, (a few years of intense activity separated by hundreds or thousands of years of inactivity)

most of the better known volcanoes of the world are composite volcanoes.

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Composite Volcanoes

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Shield and Composite Volcanoes

Figure 12.32

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Columbia River basalt flow(Ancient Shield)

Composite Volcano

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Some Interesting Volcano Facts

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Pyroclastic flows are superheated gas and particles (ash & dust or larger cinders, blocks or bombs) At Mt. St. Helens’s the pyroclastic flow extended up to 8 km with a temp of 700 C Mount Pelee, (Carribean island of Martinique) - 1902 eruption killed all but 2 of the more than 20,000 people in the town of St. Pierre

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AshfallAsh from eruptions that falls on surrounding ground

When ash is carried to high altitudes this can spread great distances

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MOUNT ST. HELEN’S

Before 1980 Eruption

May 18, 1980

After 1980 Eruption

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Mt. Pinatubo

Philippines

June 1991

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Volcanoes & Environmentbrilliant sunsets and sunrises due to fine ash and gases in the stratosphere, (cloud drifts around world)

Mt. Pinatubo - 22 million tons of sulfur dioxide (SO2) formed droplets of sulfuric acid

blocked some sunlight from reaching Earth, cooling temperatures worldwide by 0.5 C

An eruption the size of Mount Pinatubo could affect the weather for a few years.

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•August 26,1883, Eruption destroyed much of the island of Krakatau ("Krakatoa") in Indonesia,•One of the most catastrophic natural disasters in recorded history (heard more than 3000 miles away)•Fine ashes from the eruption were carried by upper level winds as far away as New York City. •Volcanic dust in the upper atmosphere affected incoming solar radiation and the earth's weather for several years.

ANAK KRAKATAU: (Child of Krakatoa)

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• series of large tsunami waves followed main explosion, some nearly 40 meters above sea level

• more than 36,000 people in the coastal towns and villages killed

• Tsunami waves were recorded or observed throughout the Indian Ocean, the Pacific Ocean, the American West Coast, South America, and even as far away as the English Channel.

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• April 1815 - Tambora Volcano, Indonesia the most powerful eruption in recorded history.

• Tambora's volcanic cloud lowered global temperatures by as much as 3 C

• Even a year after the eruption, most of the northern hemisphere experienced sharply cooler temperatures during the summer months.

• In parts of Europe and in North America, 1816 was known as "the year without a summer."

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Mt. St. Helen’s - Oct. 1, 2004

Mt. St. Helen’s - Oct. 5, 2004

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Mt. St. Helen’s

Oct. 12, 2004

Mt. St. Helen’s Today