Notes on volcanoes
Definition of a volcano: an opening in the surface of the Earth through which magma erupts (as lava or
A volcano is underlain by a magma chamber, a reservoir that stores magma
before it rises as an eruption.
Not all volcanoes erupt lava; some erupt rock and magma fragments (pyroclasts); some erupt gases.
A common misconception is that magma exists everywhere beneath the surface, and seeps out wherever
the surface is cracked, producing a volcano.
This is not true.
Experiments have shown that if a significant (2%) of magma exists in the mantle, it will rises due to
buoyancy and will eventually reach the surface as a volcano.
All volcanoes are underlain by a source of
magma, and all significant sources of magma produce volcanoes.
Therefore the presence of a volcano
implies melting, and the ABSENCE of volcanoes implies NO MELTING.
To disprove the misconception
you need only examine the distribution of volcanoes on Earth:
Most volcanoes occur along the edges of continents, but
not all continental edges have volcanoes.
the eastern margin of North America does not have
Some volcanoes form island chains in the middle of the
sea, for example the Aleutian chain of Alaska;
There are some isolated volcanoes in the middle of
continents or oceans, such as the Hawaiian islands (in the
middle of the ocean), Yellowstone volcano (in the middle
of a continent), or volcanoes in eastern Africa (i.e., Mt.
There is a continuous string of submarine volcanoes in the middle of the oceans.
This distribution can be interpreted in terms of plate tectonics.
If you look at the distribution of the
world’s volcanoes on a plate-tectonic map you can see:
The volcanoes at the edges of continents occur above subduction zones.
without subduction zones do not have volcanoes;
Volcanic island chains in the middle of the sea occur above subduction zones as well.
The undersea volcanic chains correspond to the MORs.
Some of the volcanoes in the middle of continents are due to continental rifts (again, a plate
boundary) where a continent is being torn apart; for example, the east African volcanoes.
The volcanoes that occur in the middle of plates, such as Hawaii, are called hot spot volcanoes,
and are caused by anomalously warm mantle beneath them.
It is important to notice that most area on Earth do not have volcanoes!
Therefore volcanoes are a
relatively unusual feature on Earth, and it requires relatively unusual conditions to form them.
‘normal’ state of the Earth is without volcanoes.
In order to understand the origin of volcanoes need to understand the origin of magma.
Magma is molten
rock, and may also contain some crystals or gas bubbles.
Magma does not melt like ice, wax, plastic, or other substances you may be familiar with.