Volcanoes

Volcanoes

The first question you should ask about volcanoes is why do they erupt?

For the most part, volcanoes erupt, and lava is produced, because of gravity. Gravity? Yes, gravity!

Remember that the magma has a lower density than the surrounding rock so it will rise until the rock that is below it has a greater density. You can think of a bubble of air rising through water as an analogy. If, for example, the bubble is carbon dioxide which has a density greater than average air, then the bubble will rise until is leaves the water. Once free it will float above the water's surface, but won't dissipate (unless there's a wind). Magma is the same - it will rise until it reaches a level in the Earth's crust at which it is stable. If it makes it all the way to the surface of the Earth then it will erupt, sometimes violently, as lava. The explosiveness of the eruption is primarily controlled by the magma composition.

The nature of the eruption may change during the eruption owing to a number of factors:

change in magma chemistry
change in temperature
change in the vent geometry

In the long run, however, the rise and eruption of the magma is controlled by zones of weakness through which the magma must pass.

Types:

Cinder Cones
Composite, or Strato-volcanoes
Shield Volcanoes
Lava Domes
Lava Plateaus or Flood Basalts

As you can well imagine, the size and shape of the various types of volcanoes can vary a great deal. The following diagram, from the US Geological Survey, shows the relationship between Mt. Rainier, a composite volcano in the Cascades to Mauna Loa, on Hawaii. As a side-light, the tallest mountain in the world, as measured from base to peak is Mauna Loa.

Several factors determine the shape of the volcano. The most important factor is the composition of the magma

Cinder Cones:

simplest type of volcano

generally circular
formed from congealed lava blobs (agglomerate)
usually less than 1,000' tall
numerous, especially as parasitic cones
an excellent example is Lava Butte or the cone on Wizard Island

Composite, or Strato-volcanoes:

typically steep sided (~30-45°), circular

formed by alternating layers of ash and lava
may be up to 8,000' tall
examples include Mount Fuji, Shasta, St. Helens, Mount Rainier, and Mount Hood
have well developed conduit and plumbing systems
dikes act as supporting ribs for volcano

Shield Volcanoes:

built almost entirely from lava flows

typically have very gently dipping slopes (~5-10°)
large diameters (many miles)
can be exceptionally tall (Mauna Loa is >28,000' tall)
examples are the Hawaiian volcanoes

Lava Domes:

formed by small volume, viscous eruptions

grows by expansion from within
commonly form in the central crater or on the flank of composite volcanoes
examples include Mono domes and the dome in Mt. St. Helens

Lava Plateaus:

cover vast areas (thousands of square kilometers)
basaltic in composition
generally associated with spreading rifts or mantle hot-spots
excellent example would be the Columbia River Basalt