Volcanism and Techtonics.

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 volcanism is
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 Mid-Oceanic
Ridge winds its way across many ocean floors – including, predominantly, the Atlantic –
and emerges in islands such as Tristan da Cunha and Iceland.

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 eruptions.

Figure Seven: The Oceanic crust is subducted beneath the Continental crust. This results
in an arc of active volcanoes, highly explosive and potentially catastrophic. Examples
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 the asthenosphere
(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
of Kick-'em'Jenny..

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 the
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
Mid-Atlantic Ridge.

Figure Nine: An eruption on the island of Hawaii; this is Kilauea, which has been in
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, which
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 gaining great
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.