Half Dome from Yosemite Valley 1

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Dike, Arizona 3

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Ship Rock, New MexicoThe peak rises 520m above the surrounding plain

Wall-like dike(s) extend far out from the central peak

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Ship Rock, New MexicoThe peak rises 520m above the surrounding plain

Wall-like dike(s) extend far out from the central peak6

Ship Rock, New MexicoThe peak rises 520m above the surrounding plain

Wall-like dike(s) extend far out from the central peak

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ERUPTIONS

• Under high pressure, magma contains large amounts of dissolved gases such as water vapor and carbon dioxide.

– Pº, [Dissolved gases]

• As the magma slowly rises to the surface, the lower pressure cannot hold all those gases.

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ERUPTIONS

• The force of eruption will be greatly determined by the types of lava:

– Lava with higher silica content will tend to be cooler and thicker; it could even clog the vent, building up more pressure and causing the gases to boil out explosively.

[Si], Tº , Density Explosive

– Lava with lower silica content is hotter and thinner flowing more easily; the gases will be released quietly.

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Magma Source Gas SiO2 Viscosity Eruption Location of magma

Lava related rocks

Mafic1000-1200ºC

Upper mantle 1-2% 50% Low Quiet

Oceanic crust

(Hawaii)

Basalticdarker

Intermediate800-1000ºC

Oceanic crust 3-4% 60% Medium Medium

Continental margins at subduction

zone(Mt St-Helens)

Andesitic

Felsic650-800ºC

Continental crust 4-6% 70% High Explosive

Continental crust

(Yelllowstone)

Rhyoliticlighter

Material’s internal resistance to flow (viscosity) increases when Tº decreases, and when silica content increases. Higher the viscosity, the more difficult it is for gases to escape, thus explosive eruption 10

Volcanic material

• TEPHRA rock fragments thrown into the air during an eruption

– Classified according to size

• DUST: less than 0.25mm [texture like flour]

• ASH: > 0.25mm but smaller than 2mm [rice]– Small particles of ROCKS and GLASS

– While most of the dust and ash settle on land immediately surrounding the volcano, some maytravel around the world in the upper atmosphere.

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Ash-fall tephra deposit, 9cm thick, at the former US Clark Air Base, 25km east of Mt Pinatubo, Philippines [June 1991]

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13VOLCANIC ASH

Volcanic material• TEPHRA– Classified according to size

• LAPILLI [“little stones”]: >2mm but < 64mm

• BOMBS [up to size of a car / small building]– CINDERS are LIQUID blobs of lava which harden in the air,

forming round / streamline bombs[smaller, the size of golf balls ]

– BLOCKS are angular SOLID fragments ejected up to 10km into the air.

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HAZARDS

• LAHARS volcanic mudflows [mixture of mud, water and rocks]

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HAZARDS

• PYROCLASTIC FLOWS– Clouds of hot-poisonous gas/ash• Can reach up to 200km/hour• Temperature may exceed 700°C

– Most deadly P.F. occurred in 1902• 29,000 deaths by suffocation or burning

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Mayon Volcano, Philippines. Maximum height of the eruption column was 15 km above sea level

Rushing down

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HAZARDS

• LAVA FLOWS.

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Basaltic lava with low viscosity, clocked at 16km/hour [7mph]

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HAZARDS

• LAVA FLOWS– Their physical characteristics depend on:• composition• slope• duration of flow

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HAZARDS

Where do they occur?Convergent volcanism

Formation of two major volcanic belts:Mediterranean Belt involving the African, Arabian, and Eurasian

plates [e.g. Mt Vesuvius, Mount Etna]

Circum-Pacific Belt or Ring of Fire, located along the western coasts of North and South America, across the Aleutian Islands, and down the eastern coast of Asia[e.g. Cascade Range, and Mt Pinatubo in the Philippines]

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Ring of Fire and Mediterranean Belt

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Where do they occur?

Divergent volcanism

Rift volcanism occursmainly along ocean ridges [Mid-Atlantic Ridge],

but in few places, above sea level like in Iceland

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Iceland doted with volcanoes, part of the

Mid-Atlantic Ridge

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Where do they occur?

“In the middle of nowhere”Hot Spots

Pocket of very high-temperature mantle material rising towards the surface to form volcanoes

The magma moves vertically but not laterally, therefore a trail of progressively older volcanoes form as a plate moves over a hot spot.

Hawaiian Islands continue to rise above the ocean floor as the Pacific Plate moves slowly.

It provides important information regarding plate motions: rate and direction can be calculated.

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Kilauea

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Other famous “Hot Spot”Yellowstone - Supervolcano

• Beneath the North American Plate lies a huge hot spot [plate is moving southwest]

• It is currently under the Yellowstone National Park

• Within the last 2M years, 3 major eruptions occurred [largest eruption ever: 2400km3 tephra; over 6000 times > 1980 Mt St. Helens’ eruption]

• The park is a caldera 80km long by 45km wide

• Recent uplifts [surface bulges], earthquakes, and change in gases emitted and hot springs, geyser activities are signs of possible “near” future eruption

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GEYSERSuperheated water reaching the surface may form and eruptviolently to form a geyser:

I. Water is heated to extremely high T°II. Water expandsIII. Expansion forces some water out of the ground, releasing P°IV. Drop of P° causes the remaining water to boil rapidlyV. Steam P° forces the water and steam high into the airVI. Geyser erupts, water falls back and refills the underground chamber,

and the cycle repeatsVII. They are rare but can be found in Western US, Chile, Iceland, and

New Zealand

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Yellowstone National Park, WY

Old Faithful geyserwas named by the first official expedition to Yellowstone, the Washburn Expedition of 1870. Old Faithful erupts every 35 to

120 minutes for 1 1/2 to 5 minutes. Its maximum height

ranges from 90 to 184 feet.

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Yellowstone National Park / Grand Prismatic Spring -Geyser-

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Hot Springs form when groundwater heated by magma rises to the surface and collect in a pool.

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When will it erupt again?VEI Return period yrs Example

Level 5 10 St Helens, 1980

Level 6 100 Pinatubo, 1991

Level 7 1,000 Tambora, 1815 [Indonesia]

Level 8 50,000+ Yellowstonesupervolcano

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• Yellowstone itself has not erupted for 630,000 years, but is known to produce a Level 8 eruption roughly every 600,000 yrs

– Yellowstone is “overdue”.

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If Yellowstone would erupt again on this scale:….

• Sheets of magma and pyroclastic flows would cover the entire Yellowstone region, killing tens of 1,000s of people.

• An area about the size of Texas would be covered with several cm of ash [almost all of the US would experience enough ash-fall to cause a serious health hazard].

• 2,000M tons of SO2 would be ejected into the atmosphere.• The ensuing plume of ash and dust would spread worldwide,

producing a global cooling effect – T° might fall by 10 to 20°C due to the blanket of dust preventing sunlight

from reaching the earth’s surface– Photosynthesis would be inhibited, food webs would be severely disrupted,

and productivity of agricultural and natural biomes would be reduced– Widespread famines would be expected and could last for 4 to 5 years

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