12ESS Volcanoes

Subduction Zone

When a dense oceanic plate slides under a less dense continental plate then sinks down into the mantle

Diverging Plate Boundary

When two tectonic plates stretch and move apart from one another

Magma

Molten rock beneath the Earth's surface

Lava

Molten rock that has reached the surface of the Earth

Magma Chamber

The underground body of molten rock that feeds a volcano

Vent

The opening through which molten rock and gas leave a volcano

Viscosity

Measure of fluid thickness. The higher this is the more it resists flowing

Silica content

Amount of silica inside of a magma this determines viscosity

Basalt Magma

Low silica high temperature magma that forms when the mantle melts. Can flow very far and doesn't trap many gas bubbles.

Andesite Magma

Intermediate silica mid temperature magma that forms when molten mantle mixes with minerals in continental crust. Can flow intermediate distances and can trap gas bubbles.

Rhyolite Magma

High silica low temperature magma that forms when continental crust melts. Does not flow very far at all, traps lots of gas and highly explosive.

Lava Flow

The spread of lava as it pours out of a volcano vent

Pyroclastic Flow

Very fast very hot avalanche of ash and gas that forms from a collapsed ash cloud. Made by Andesite and Rhyolite magmas.

Ash Cloud

Dense cloud of small rock fragments formed in explosive eruptions.

Lahar

An avalanche of volcanic water and mud down the slopes of a volcano, forms often on steep sided Stratovolcanoes.

Pumice

Very light coloured rock made of Rhyolite magma full of trapped gas bubbles, erupts from Caldera and Dome volcanoes.

Magma Formation

Requires decreased pressure, increased temperature, or water added to the mantle so it melts.

Magma Ponding

When molten mantle pools under continental crust, heat transfers up into the crust and melts it creating Rhyolite magma.

Granite

Low density silica rich rock that makes up continental crust

Shield Volcano

A low, flat, gently sloping volcano built from many flows of fluid, low-viscosity Basalt magma

Stratovolcano

A steep sided volcanic cone built up of alternate layers of lava and ash, formed from intermediate viscosity Andesite magma

Dome Volcano

A steep sided round shaped volcano made of layers of high viscosity Rhyolite, layers grow from the inside out

Caldera

A large crater caused by the violent explosion of a volcano that collapses into a depression and gets filled by water. Formed by explosive Rhyolite magma full of trapped gas bubbled.

Hot Spot

An area where magma from deep within the mantle melts through the crust above it, far away from plate boundaries.

Mantle Plume

A mass of hotter-than-normal mantle material that rises toward the surface, where it erupts through the crust forming a hot spot.

Monogenetic

Volcanoes that only erupt once then never again as the crust has moved past the mantle plume feeding the volcano with magma.

Tuff Ring

Broad flat crater with low rims and shallow slope formed by ash fall from a phreatomagmatic eruption

Phreatomagmatic stage

First step of a hot spot eruption where magma meets overlying cold water and contracts rapidly, forming a large ash cloud.

Scoria Cone

A steep, cone-shaped hill made by fire fountaining of gas rich Basalt magma. Lava bombs and scoria make up the steep cone.

Scoria

Basalt magma that contains trapped gas bubbles forming a "bubbly" rock when it cools

Tuff

Volcanic ash that has solidified into a rock, formed when basalt erupts explosively due to contacting cold water

Fire Fountaining Stage

Second stage of a hot spot eruption where frothy magma erupts through the Tuff ring, expanding gas bubbles drive the magma up into the sky

Lava Flow Stage

Third stage of a hot spot eruption where de-gassed basalt magma flows up and out the vent as lava flows