An important geological process, the movement of molten
rock, or magma, through the lithosphere of the Earth to its surface,
where it is eventually extruded through volcanoes and upon which it is
eventually deposited as lava or volcanic ash. The process
also interests geomoprhologists
because of the landforms created by the material after it cools, and geographers because of the soils and environments thus created.
Volcanism occurs when heat gains the upper hand in its perpetual tug-of-war with
gravity in materials that are normally solid.
As such, it has been observed on bodies other than the Earth: the Moon,
Mars, Io, Triton. Of course, the former two bodies cooled past
the point of active volcanism long ago, and while the latter two are still
active, their volcanoes are driven by quite different materials from those
on Earth, and the underlying processes are not well understood. Because
of this, this writeup will be limited to Earthly volcanism.
Volcano terms are dealt with quite well in the volcano node but we
will touch on some in this discussion of the process.
Although the residual heat of the Earth's formation would have radiated
away early in the 4.5 billion years since it happened, the slow decay of
radioactive isotopes deep within the Earth (as well as exothermic chemical
reactions at the boundary between the Earth's core and mantle)
keeps the inside hot. The difference in temperature between the Earth's
surface and its interior leads to convection currents within the mantle
to dissipate the heat.
These convection currents are the primary driving force of volcanism,
although a secondary effect of plate tectonics, which we will discuss
later, drives much of it.
When convection currents reach the top of the mantle, they dissipate
heat into the lithosphere and then spread out laterally. This process
of advection drives plate tectonics in the lithosphere, but the dissipated
heat can also have a direct effect at the surface: It can melt through
the lithosphere directly above the top of the convection column.
This magma, lighter than lighter than the surrounding solid or semisolid
rock, migrates upwards through the lithosphere until it reaches the surface,
in one of two forms.
- divergent plate boundaries
Most heat reaches the surface at tectonic spreading centers, and the Mid-Ocean Ridge can e considered one immense volcano, of which the volcanoes of Iceland are minor volcanic vents. Divergent plate boundaries often begin their lives as rift valleys in the middle of contenents, such as the East African Rift Valley system.
- hot spots
- Some convection columns have their effects confined to (more or less) point
locations. At the top of these mantle plumes are "Hot spots", such as the one that created the Hawaiian Islands, the Azores and Canary Islands, or the sleeping giant underneath Yellowstone National Park. As a tectonic plate migrates over a hot spot, the spot burns a line of volcanoes through the plate. The Hawaiian legend of an angry Mother Ocean chasing Pele from island to island is a marvelous depiction of this process.
- subduction zones
- The end of an oceanic plate's life usually comes when it is subducted beneath another plate. This can be a piece of continental crust, or even another oceanic plate, since plates subside as they age. Regardless of the force that pushes a plate deep into the lithosphere, the plate, as well as the sediment deposited on top of it, melts as it is driven into the mantle. Plumes of magma, or diapirs, migrate through the overriding plate, and eventually break through to form a line of volcanoes a short distance rom the plate boundary. Subduction volcanoes form most of the "Ring of Fire" surrounding the Pacific Ocean: the Andes, the Cascade Range, the Aleutian Islands, Kamchatka, the Kuril Islands, and Japan, the Marianas Islands, the Phillippines, the southern margin of Indonesia, and Melanesia. The volcanoes of Italy and the Mediterranean Sea are also subductionvolcanoes, as are the volcanoes of the Lesser Antilles.
Although the causes of volcanism operate continuously, their effects are
mostly episodic. Because large and small volcanic events have been
a constant throughout the Earth's history, they can be considered a "continuum"
of sorts if you take an extremely long view. But on a human scale,
volcanic eruptions can happen suddenly, and without warning, after a long
period of calm.
Some magma cools permanently before it has a chance to reach the surface,
forming intrusive igneous formations that are only exposed by erosion
millions or billions of years later. Many intrusive formations are
associated with nearby volcanoes:
Of course, most people associate the term "volcano" with the extrusive
effects of the volcanic process on the Earth's surface.
Molten rock flowing out onto the Earth's surface is forced to cool much
rapidly than magma left in a batholith, and its structure is changed
enough that a new term, lava, is used for it. Lava can take several
forms, depending upon its chemical composition. It can fail to form
crystals, forming glasses like obsidian, it can form tiny crystals,
and a crust of crumbly stones called a'a. Lava may even polymerize
into the ropy pahoehoe.
The form of an individual volcano depends on the composition of the
magma, as well as the country rock it has to melt through. Some
volcanoes have an easy time of it, and lava streams out onto the surface,
making a mudpie-like shield volcano. Others build up piles of
debris around the original vent, giving rise to the familiar stratovocanoes
and smaller cinder cones.
Stratovolcanoes and cinder cones are dangerous for another reason:
They are poorly consolidated, creating the potential for huge landslides
even when the volcano is not erupting. When the volcano is near water
(as many are), such a landslide can cause a tsunami that may have global
The gases dissolved in magma cause the most dramatic and far-reaching
effects. When a magma with dissolved gas reaches the surface, the
sudden decrease in pressure can cause an effect much like opening a soda
or beer bottle after shaking it: A violent explosion, atomizing some
material into clouds of volcanic ash, tossing large rocks ("bombs")
for hundreds of meters, or destabilizing the side of the volcano, causing
devastating pyroclastic flows. Sometimes, the entire mountain
collapses as the full contents of the magma chamber are thrown into the
air, resulting in a volcanic pit or caldera.
But volcanism's effects are felt far beyond the vicinity of individual
volcanoes. On a regional level, lava flows can cover vast areas,
such as the Deccan Traps of India, or the entire eastern two-thirds
of Oregon, caused by the Yellowstone supervolcano. Ash clouds
can fill Earth's atmosphere and affect global climate, as was evidenced
by the 1991 eruption of Mt. Pinatubo in The Philippines.
When pressures have built during a long period of inactivity, volcanic
explosions can be quite violent. Pinatubo and Krakatoa are tiny compared
to the eruption of Thera which plunged the Eastern Mediterranean into
a dark age 3,500 years ago, and Thera was just a popgun compared to the
supervolcano eruptions of Toba 74,000 years ago and Yellowstone 600,000
years ago. Volcanic events are responsible for some of the mass
extinctions that have occurred from time to time during Earth's history.
Don't call your local representative to request a law banning volcanoes
just yet. They are an important reason why life exists on Earth today,
because the gases released during individual volcanic disasters make up
the Earth's atmosphere, and gases from new disasters replenish it.
More information about Yellowstone can be found at the Yellowstone
Volcano Observatory site,