According to the theory of plate tectonics, the Earth’s crust is divided
into many fractured plates (Figure four) that float on the mantle.
The mantle is a hotbed of unrest, with swirling convection currents
resulting in plate movement.
Figure Four: The plates of the Earth.
It is on the boundaries of these plates that volcanism
tends to occur. Firstly, a band of
volcanism stretches across the borders of the Pacific; from South America
and the Andes it sweeps upwards, through North America’s Cascades range
and into Alaska and the Aleutian Islands, over into Russia’s Kamchatka
peninsula. Then, it continues south through Japan,
Indonesia and the Philippines, finally terminating in New Zealand. Explosive
also seen at other parts of the globe, for example in the Caribbean
West Indies. Volcanoes
are prevalent in Africa’s Rift Valley, and in Italy. Meanwhile, the
Ridge winds its way across many ocean floors – including, predominantly,
the Atlantic –
and emerges in islands such as Tristan da Cunha and Iceland.
The mid-oceanic ridges (Figure five) rise 3,000 meters from
the ocean floor; the Mid-
Atlantic Ridge (Figure six) is more than 60,000 km in length,
surpassing the Himalayas
in size. The mapping of the seafloor also revealed that these
huge underwater mountains
are linked with a network of transform faults that can be more
than 2,000 meters deep.
Figure Five: Topographic rendition of the Mid-Oceanic Ridge
at latitude 9 degrees North, courtesy of the University of Rhode
Island.This is a small part of the East Pacific Rise;
“warm” colours are above the sea floor, “cool” colours are below
Figure Six: The path of the Mid-Atlantic Ridge.
The Mid-Oceanic Ridges are entirely volcanic; here, plates are
moving apart (‘diverging’),
and magma is rising to fill the gaps. Other prominent examples
include the Juan de Fuca
Ridge, offshore of North America; also the Indo-Antarctic Ridge.
These lavas are fluid and produce vast lava flows (as seen
in Icelandic volcanism).
Other divergent marginscan be found at Rift Valleys, such as
the African Rift Valley, wherecountries aretorn apart by the
movement of the plates.
But it is at other plate boundaries that more explosive volcanoes
exist.For, in some places
– such as at the boundaries of the Pacific – one plate is being forced
beneath another, or ‘subducted’.As it falls, it melts, and a plume
of magma rises through the crust.The lavas
on these volcanoes are thick and viscous, and often jam in the vents;
as pressure builds,
it can only be released in a devastating explosion (Figure six)
Figure Six: The 1980 eruption of Mount
St. Helens, releasing a massive blast of ash
and gas known as a pyroclastic flow.
When an oceanic-continental convergence occurs (Figure seven)
the denser oceanic plate
will most commonly subduct beneath the less dense continental
plate creating a deep ocean trench. About 100km deep into
the mantle, the melting ocean crust forms a magma.
Some of this is pushed to the surface, resulting in volcanic
Figure Seven: The Oceanic crust is subducted beneath the Continental
crust. This results
in an arc of active volcanoes, highly explosive and potentially
of this type of volcano include El Chichon, Mexico, and Mount
St. Helens, USA.
Note that the lithosphere is another term for the crust, while
(sometimes spelled aesthenosphere) is the mantle.
But this was not the case for the volcano Mt.
Pelee, which erupted spectacularly
in 1902. In the case of Pelee, the volcano resulted from an
oceanic plate sinking
underneath another oceanic plate (Figure eight). The result?
An arc of volcanic islands
- the entire West Indies were created by volcanic activity,
and in fact eruptions have
been seen at Mt. Pelee, Martinique; Chances Peak, Montserrat;
Soufriere, St. Vincent;
and Soufriere, Guadeloupe,
as well as the submarine volcano that bears the uniqe name
Figure Eight: The Oceanic plate sinks
underneath another Oceanic plate, and the rising magma creates
a chain of explosive volcanoes as an island arc.
Occasionally, however, plumes rise within the Earth’s mantle
and produce volcanism at seemingly random points.These are
known as hotspots, and the hotspots may emerge
within continents or oceans, producing major volcanic landforms.An
ideal example of
an oceanic hotspot is the island chain of Hawai’i
(Figure nine), while a continental
hotspot is Yellowstone, USA.Occasionally hotspots may be juxtaposed
on other margins;
this is likely the case for Mount
Etna, Italy, and is certainly a regular occurrence on
Mid-Oceanic Ridge.Axial Volcano, on the Juan de Fuca Ridge,
is the product of a
hotspot; Iceland is a land created by a hotspot’s positioning
itself on the
Figure Nine: An eruption on the island of Hawaii; this is
Kilauea, which has
constant lava effusion since 1983.
The Hawai’ian hotspot is the most famous example; here, the
plate has slid over the semi-stationary plume, creating an
island chain (Figure ten).Until recently, it was believed
the hotspot had remained stationary; but there is a chink
in the pattern of island formation.
The original theory explaining this – that the direction of
plate movement changed –
has been cast aside, since there is no geological evidence
for this on the boundaries
of the Pacific plate.Instead, it seems that the hotspots do
indeed shift slightly.
Figure ten: Volcanism and tectonics.The dark arrows indicate movement
of magma or
convection cells. On the diagram can be seen several examples of volcanism,
occurs at points A to D. At point A, the Mid-Oceanic Ridge is seen;
note the abrupt
offset of the transform faults.At point B, an island arc system is
created ala the West Indies
or Japan. point C, a continental arc exists, such as the Cascades.And
point D illustrates
the hotspot process, although other examples of hotspot island chains
can be seen
on the diagram.
It must be noted, however, that the theory of plate tectonics is currently
criticism.It is being argued that it does not account for local conditions,
where often countless theories need to be built upon one another to
allow the concept to work.Furthermore, many scientists now doubt convection
currents would be capable of driving the plates in motion.
about the author:
Tom Bacon, age 19. Student at Batley Grammar School,
He has an active imagination, and is an enthusiastic researcher.
Organizing a lot of things at school.
There, he became fascinated with geography in general and volcanoes
particular, collecting and collating a mass of data and material that
he absolutely loves
to improve on.
He is almost obsessive about the collection and perfection of knowledge
(especially about volcanoes) and is considering teaching as a life-occupation.
If you asks how he finds time for all this, just don't ask That's
his big problem....
to other articles by Tom:
- Mount Pelee