Volcanoes

Volcano Definition

- A vent at which lava, pyroclastic debris (ash and fragments of previously solidified rock), and gas erupt.

- Eruption may build a mountain around the vent. (Mountain is also called a volcano).

- Anatomy of a volcano:

- magma chamber at depth

- a vent to the surface

Word bank: conduit, vent, crater, side vent, ash cloud, crust, magma

chamber, lava

Where do Volcanoes Occur? - Divergent volcanism:

- Mid-ocean ridge

- Creates new sea floor

- Rifts

- e.g., East African rift

Great African Rift Valley

• Bbb

Mid Ocean Ridge

It is thought that the Great Rift Valley will eventually become an ocean.

Where do Volcanoes Occur?

Convergent Boundaries

- the majority of major volcanoes

- e.g., the Andes, Japan, Aleutians (ring of fire)

Where do Volcanoes Occur?

- Hot spots

- e.g., Hawaii; Yellowstone Park

Nature of lava

• Characteristics of lava

depend on its temperature

and viscosity.

– viscosity is a measure of

the ability of a fluid to

flow • high viscosity means sticky

• slow, low viscosity means

watery and fast

• The higher the temperature

of a lava, the lower its

viscosity (i.e., the easier it

flows)

• The greater the gas

content, the lower the

viscosity. Frothy lava flows

more easily.

Types of Eruptions

• Lava dominated eruptions: – Characteristic of low viscosity eruptions.

– If lava has low viscosity, it fountains out easily. You will see lava fountains, lakes and rivers, that flow long distances.

• Explosive eruptions: – Sticky, very viscous lava clogs up the vent. Gas (CO2 and H2O) that come out

of solution and make bubbles. Gas pressure builds up and the volcano explodes.

– Result is a cloud of ash that blankets countryside or an ash flow that rushes down the volcano side at speeds up to 100 km/h, incinerating everything in its path.

• Explosions can also occur when water gets into magma chamber and turns to steam or when part of the mountain slips away and pressure is released.

Lava Vocab

• Tephra – materials that explode along with lava – Cooled chunks of lava

– Rocks, ash, parts of crust, etc.

• Pyroclastic flow – Rapidly moving clouds of tephra mixed with hot,

suffocating gases

– http://www.youtube.com/watch?v=WbkPo0Hws0M

Types of Volcanoes

• The fissure volcano: is a long crack in the earth's surface through which

magma erupts. These cracks may form as two tectonic plates pull apart.

• The shield volcano: is a broad, shallow volcanic cone, which arises

because the running lava, which is fluid and hot, cools slowly.

Types of Volcanoes

• The dome volcano: has a steep, convex slope from thick, fast-cooling lava.

• The ash-cinder volcano: throws out (in addition to lava) much ash into the

air. Through this the volcanic cone is built up from alternate layers of ash and

cinder.

Types of Volcanoes

• The composite volcano: are also built up from alternate layers of lava and

ash but, besides its main crater, it has many little craters on its slope.

• The caldera volcano: is an older volcano with a large crater which can be 62

miles (100km) wide. In this crater many little new craters are formed.

Intrusive Activity

• Plutons –

– Intrusive igneous rock bodies formed by cooled magma that forced its way into the crust and cooled before hitting the surface

• 4 types

– Batholiths and stocks

– Laccoliths

– Sills

– Dikes

Batholiths • Large, irregularly shaped

• Coarse-grained igneous rock

• Common in interior of many mountain chains

Laccoliths

• Lens-shaped

• Smaller than batholith

• Stone Mountain, GA

Sills

• Magma intrudes parallel to layers of rock

Dike

• Cuts across preexisting rocks

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