VOLCANOESChapter 10

Vent or fissure through which solids, liquids, & gases move to the surface

Vent – straw-like opening through which magma flows Lava moves radially (in all

directions) to produce conical land form

Fissure – cracks in ground through which lava moves to surface Lava travels in 2 directions to make flat

landform

Cross-section of a volcano

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Crater – forms when rocks break off at the end of eruption

Flank eruption – eruption from side of mountain (flank opening)

Caldera – when magma is released, a void is left in magma chamber Volcano sinks (collapsed structure) Example: Crater Lake, Oregan

Magma/lava – liquid solution of ions @ high temps (800-1100 °C) O, Fe, Si, Ca, Al, K, Mg, Na

Caldera

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Magma Composition

Magma CompositionSilica Viscosity Dissolve

d GassesPyroclastics

Landforms

BasalticLeast ~50% Least Least 1-

2% least

Shields, cinder cones, basaltic plateaus

Andesitic

b/w ~ 60% b/w b/w 3-4% b/w Composit

e conesRhyolitic Most

~70% Greatest Most 4-6% Most

Domes, pyroclastic flows

Viscosity

Resistance to flow The more viscous a

substance the slower the flow Explosive volcano is more

viscous than quiet volcano Temperature: the warmer the

lava, the less viscous it is As it cools it hardens into rock –

viscosity increases

Silica Content The more silica the more viscous

Dissolved Gases Gases trapped in mama provide force to

eject lava Mostly water vapor and carbon dioxide Basaltic magma (thin) allows for

bubbling, gases escape Highly viscous magma doesn’t allow gas

to escape, rather it keeps expanding until eruption occurs

Pyroclastic Materials Particles

produced in eruption

As gases expand, pulverized rock and lava are blown from vent

Volcanic Products Solids – formed from land material,

sharp & irregular shapes Formed from cooled lava, teardrop &

rounded shapes Ash - <2 mm Bombs - >64 mm

Bombs – rounded obsidian & basalt Blocks – blocky (from landforms) hummice

Gases – water vapor (steam), ground water, and any combination of Cl, F, H, oxides of C, and oxides of S (ex. HCl, H2SO4)

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Gas Flow from Volcano

Volcanic Plug Creates pressure as gas builds up Pressure can push out plug in one piece

or cause the sides to swell out and gas escapes from cracks in side of volcano.

Example: Shiprock, NM

Volcanic Landforms Vents:

Shield volcanoes Low viscosity

basaltic lava Slopes less than 5 Most grown from

ocean to form islands

Example: Hawaiian Islands & Iceland

Mauna Loa, Hawaii, is an excellent example of a shield

volcano

Cinder Cones Gas rich, basaltic magma Composed of mostly loost pyroclastic material Steep sided slopes (30-40) Usually product of single eruption Magma solidifies after eruption Can form on side of other volcanoes Examples American Southwest

Composite cones (Stratavolcano) Large nearly symmetrical

structure Composed of layers of lava

and pyroclastic material Gas rich magma with

andesitic composition Low slopes increase

toward peak Mostly located in the ring

of fir Huge amount of

pyroclastic material (most explosive)

May generate mudflows called lahars (snow and ice melt or heavy precipitation)

Mt. Hood, a beautifully symmetric stratavolcano

Lava Dome Lava is so sticky that it doesn’t flow Rhyolitic lava, lots of silica Volcanic plugs are typical May have multiple flows

East Butte, on the eastern Snake River Plain in Southern Idaho (USA), is a rhyolitic

volcanic dome

Fissures Lava Plateaus

Rock units are horizontal flows that stack up Makes large flat plains

Volcanoes & Climate Change As volcanoes erupt, they blast large clouds of gases, particles, water vapor, and aerosols into the

atmosphere. As a result, they affect the Earth's atmosphere and global climate. When large masses of gas from an eruption reach the stratosphere, it can produce a large,

widespread cooling effect. This is sometimes referred to as an “Volcanic Winter.” This “winter” occurs because volcanic ash and droplets of sulfuric acid obscure the sun, causing the temperatures to drop.

Here are some examples: In 1783, Benjamin Franklin blamed the unusually cool summer on volcanic dust coming from Iceland, where the

eruption of Laki volcano had released enormous amounts of sulfur dioxide. The eruption killed much of Iceland's livestock and lead to a catastrophic famine, killing a quarter of the

population. Temperatures in the northern hemisphere dropped by about 1 °C in the year following the Laki eruption.

Known as the “year without a summer,” the summer of 1816, unexpected climate changes left countries in the Northern Hemisphere suffering from devastating famine and epidemic outbreaks. These weather patterns were the result of the volcanic eruption of Mount Tambora in Sumbawa, Indonesia, on 10th April 1815.

It snowed in June in the United States and Europe: Crops failed, there was starvation, people lost their farms, and it touched off a wave of emigration that led to the settlement of what is now the American Midwest. In the meantime, hundreds of thousands more starved around the world. 

New England and Europe were hit exceptionally hard. Snowfalls and frost occurred in June, July and August and all but the hardiest grains were destroyed. Destruction of the corn crop forced farmers to slaughter their animals. Soup kitchens were opened to feed the hungry. Sea ice migrated across Atlantic shipping lanes, and alpine glaciers advanced down mountain slopes to exceptionally low elevations.

In 1991 explosion of Mount Pinatubo, another stratovolcano in the Philippines, cooled global temperatures for about 2–3 years, interrupting the trend of global warming which had been evident since about 1970.

Another possible effect of a volcanic eruption is the destruction of stratospheric ozone. Researchers suggest that ice particles containing sulfuric acid from volcanic emissions may

contribute to ozone loss. When chlorine compounds resulting from the breakup of chlorofluorocarbons (CFCs) in the stratosphere are present, the sulfate particles may serve to convert them into more active forms that may cause more rapid ozone depletion.