Sockie Basics: Volcanoes

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Sockie Basics: Volcanoes

Post by Lurker on Thu Feb 10, 2011 7:12 am

Volcanoes originate from deep within the earth's mantle, at regions where the melting of solid rocks forms magma. Different mechnism allow for hot magma, rocks, steam, gas etc to rise to the surface. A volcano is basically a vent in the earth's crust through which these materials are expelled. The structure of volcanoes varies quite a bit, from cracks or raptures to domes or shields, and mountain-like structures with a crater in the summit.

The term "magma" is used for molten rock material below the earth's crust. When magma reaches the surface it's called "lava".

Volcanism can be found in the whole Solar System, the most earth-similar formations are on Mars and Venus.

The origin of the word "volcano" comes from the latin words "vulcano" (burning mountain), "Vulcanus" (roman god of fire). The name was first applied to Mount Etna in the 1610s.

Melting of rock is controlled by three physical parameters: its temperature, pressure, and composition. The most important geological mechanism ist the melting via decompression, as it creates the largest quantity of magma.

A phenomenon called „thermal convection“ is responsible for the movement of either rocks or magma.

There are two types of convection curcuits:
The first being either down- and upward currents which lead to a) subduction zones (convergent, downward), and b) mid-ocean ridges (divergent, upward) between lithospheric plates.

The chain of volcanoes along the Pacific Rim, often referred to as the Ring of Fire, is an example of subduction zone volcanism.
Iceland would be an example for the mid-ocean ridges.

The other types are mantle plumes or quasi-cylindrical currents, which rise above melting regions, also known as „hot spots“
Hawaii would be an example for that.

The Eruption:
There are three types of eruptions: the magmatic, the phreatomagmatic, and the phreatic eruption.

Magmatic Eruptions:
several subtypes: Plinian, Strombolian, Hawaiian, Volcanian, Pelean.

One basic form of an explosive magmatic eruption is the plinian eruption, which would be the most spectacular eruption as columns of gas and volcanic ash are thrown high into the stratosphere. .
Plinian eruptions often change into pyroclastic flow , which is a fast-moving current of extremely hot gas and rock, moving downhill. Such a flow can have very devestating effects, for e.g. Pompeji was destroyed by one.

Examples for plinian eruptions:
1645 Santorini in Greece
1883 Krakatoa in Indonesia
1980 Mount St. Helens in the US
2010 Eyjafjallajökul in Iceland

The Volcano explosivity index (VEI):
This index is a scale (from 0 to 8 ) for measuring the strength of eruptions. It's similar to the Richter Scale used for Earthquakes. Most eruptions fall into index 0-2.

Forms of Volcanoes:

Shield Volcanoes:
built mostly from fluid lava flows, characterized by gentle slopes
(e.g. Hawaii volcanic chain)

Strato Volcanoes:
a composite volcano, built from many layers (strata) of lava, plumice, ash etc.
these are tall conical mountains, with a steeper slope.
(e.g. Vesuv, Krakatoa, Mt. St. Helens, Stromboli, Fuji)

There are also a few other types like lava domes, crypto domes, cinder cones, submarine volcanoes, subglacial volcanoes, …

Links of Interest:

General Infos:

Volcano Hazards Program:

How Volcanoes Work:

FEMA: How can I protect myself from a volcanic eruption?

Interactive :

Webcams (USA):

Webcams (worldwide):


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