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Evaluate how plate tectonics theory helps our understanding of the distribution of seismic and volcanic
events
Throughout human history we have always wondered about the reasons for volcanic and seismic events, but it
wasn’t until geologist Alfred Wegener’s theory of continental drift that anyone seriously considered a
geological rather than theological cause. Wegener based his theory primarily on the continental jigsaw – an
observation of the way in which South America and Africa’s coastlines appeared to match one another, as if
they had once been joined together and then drifted apart. His theory was supported by the fossil evidence of
plants and animals existing at the same time on continents separated by vast oceans, and geological evidence
such as the location of coal deposits outside of tropical latitudes, or precise geological sequences and rare
deposits such as gold and diamonds in geographically separated locations. However it wasn’t until the 1950s
that advances in Science and Technology provided the solid proof required for his theory to be truly accepted,
by which time Wegener’s theory had been almost forgotten. Since then the true extent to which plate tectonic
theory can explain the distribution of volcanic and seismic events has become clear.
In 1954 the plotting of polar wandering paths provided a major piece of evidence for Wegener’s theory. In this,
geologists looked at the way iron rich particles were aligned in rocks in Europe. It soon became clear, as
European rocks of the same age were studied, that either the pole had moved, or the continents had. Once
rocks on other continents were studied, not only did it seem as though the pole was moving, but it also
seemed that there had been multiple poles at the same time. Since this can not be the case, the only
remaining theory was that the continents had moved. During the 1950s there was also discovery of mid ocean
ridges and of the faults associated with them. Palaeomagnetic banding found either side of the ridge in 1963,
suggested the sea floor was spreading. As magma welled up and solidified it aligning itself with the pole before
being split apart by the next divergence, with new magma welling up and aligning itself to the opposite pole if
a polar reversal had taken place. This created a symmetrical pattern of magnetic banding either side of the
ridge, with the oldest rocks found furthest away. The final piece of the puzzle was the explanation for why the
Earth wasn’t getting bigger if the plates were diverging; the plate tectonic model suggested a solution – that of
subduction of plates elsewhere, and the study of deep ocean trenches using sonar in the 1960s suggested
likely areas where this was occurring.
Today we view this ‘theory’ more as fact and use it to explain the location and behaviour of the Earth’s
volcanoes and earthquake zones. We know now that we inhabit a very thin layer of solidified rock with hot
molten rock underneath – the mantle. Movements in the mantle due to convection currents have caused this
thin layer of solid lithosphere to be broken up in piece called plates, and continued movements causes these
plates to move. Depending on the direction of movement, three main types of plate boundary result –
convergent, divergent and transform – and the differing interactions are what give rise to tectonic events,
which mostly occur in wide bands that mostly follow the boundaries of the Earth’s tectonic plates.
Convergent boundaries can be split into two: collision and subduction. Subduction zones, where oceanic crust,
along with layers of sedimentary deposits on the ocean floor and water, dives underneath another plate, are
associated with both severe earthquakes and violent volcanoes. As the oceanic crust and sea floor sediments
melt under the enormous heat, friction and pressure of the mantle, gases cause the resulting magma to rise
upwards through the crust towards the surface. In areas where there is oceanic-oceanic subduction this gives
rise to island arcs – a series of volcanic islands that form a little way beyond the plate boundary such as the
Caribbean, or older more developed islands arcs such as Japan or Indonesia. Volcanoes on these boundaries
are numerous due to the oceanic crust they erupt through being thinner, and this is the most obvious around
the ‘Pacific Ring of Fire’, or Indonesia which has 127 active volcanoes. Volcanoes tend to be violently explosive
due to the higher amounts of gases which build up between eruptions, with famous eruptions such as
Krakatoa, Tambora (both Indonesia), Soufriere Hills in Montserrat and Pinatubo being associated with these
boundaries.
Less common are the volcanoes on subduction zones where there is oceanic-continental convergence. Here
the crust is much thicker and magma often does not find a way through but instead intrudes into the
continental crust and solidifies slowly. There are exceptions however, and where magma does find a way
through, eruptions can be extremely powerful such as the famous eruption of Mount St Helens where the
energy released was equivalent to an estimated 20,000 Hiroshima bombs and blew 0.67 cubic miles of rock off
the top of the mountain. South America only has around 19 active volcanoes despite having a subduction
coastline that is over four times the length of Indonesia’s. The lower number of volcanoes here is not
surprising given the thickness of the South American plate at this point, but the ones that are there are
violently eruptive due to huge quantities of magma and gas being delivered to magma chambers by the rapidly
diverging East Pacific plate boundary driving the Nazca plate beneath the South American at around 9cm per
year. This makes South American volcanoes unpredictable with dramatic eruptions albeit mostly in areas
where populations are sparse, although there are exceptions such as the Nevado Del Ruiz eruption in
Columbia in 1985 which killed 23,000 people.
Convergent boundaries are also infamous for the magnitude of the earthquakes they create. As the oceanic
plate subducts, it drags the continental crust downwards along the subduction zone (raising fold mountains in
the continental crust in the process). When the pressure gets too much, rock will give way and in some cases
the continental plate can ‘flick’ upwards displacing millions of cubic meters of seawater along the subduction
zone and leading to a tsunami such as the Boxing Day tsunami in 2004, or the Japanese tsunami in 2011.
Divergent boundary volcanoes are common, although most do not appear on distribution maps owing to the
fact that most of them are submarine, found along the 60,000km of mid ocean ridges than run through the
Atlantic, Pacific and Indian oceans. On the few places where these underwater volcanoes have grown big
enough to break the surface – most notably in Iceland – eruptions tend to be much gentler than subduction
zones leading to wide shield volcanoes which tend to erupt basic lava and ash rather than the acidic lava and
pyroclastic flows of subduction volcanoes. More unpredictable are the volcanoes found on diverging margins
earlier in their evolution, such as those found along the African Rift Valley. The reasons for volcanoes here are
clear in plate tectonic theory as continental crust thins and weakens as convection currents drag the plate
apart. Perhaps due also to the position of hot spots, volcanoes here have basic lava and present a rare threat –
that of being overwhelmed by lava, such as the city of Goma which lay in the path of the Nyiragongo eruption
in 2002. Earthquakes along these boundaries are frequent but rarely as devastating as convergent boundaries
where the forces involved in plate-on-plate collision are much greater than that of diverging boundaries. Most,
like most divergent boundary volcanoes, occur deep under the ocean with no impact on human populations.
The exception to this is Iceland, but earthquakes here rarely rate above 6 on the Richter scale.
The other form of convergent boundary creates much more powerful earthquakes – continental-continental
convergence. The collision of the Indo-Australian plate with the Eurasian plate has been ongoing since around
70 million years ago with the Himalayas growing since around 50 million years ago when continental plate met
continental plate. They are still doing so today at a rate of 5mm a year as the Indo-Australian plate continues
to move northwards at around 67mm a year. Huge forces build up within the rocks and are occasionally
released in devastating earthquakes along fault lines radiating off from this collision boundary, such as the
Sichuan earthquake in China in 2008 on a known fault line which killed 68,000 people, or the earthquake in
Iran in 2003 which occurred well away from the plate boundary in an area with no history of earthquakes and
killed 23,000. Earthquakes in most instances are anticipated based on our knowledge of plate tectonic theory
and known fault lines with histories of seismic activity, but the margins of error are wide with multiple faults,
many of them predating written records and some distance away from earthquake zones and plate
boundaries, any of which could be the one to give way. Plate tectonic theory can help explain these events but
not predict their location with any accuracy, although with each event our knowledge of the location of these
fault lines becomes more complete.
The same is also true of the last form of plate boundary – transform. Most can be found in conjunction with
mid ocean ridges as plates pull apart at different rates along the boundary, causing a shearing motion between
different sections, albeit deep beneath the ocean. A more famous land based example is that of the San
Andreas fault – a visible crease in the Earth’s crust as the Pacific plate drags alongside the North American
plate at a faster rate, leading to regular shallow focus earthquakes which threaten the cities of San Francisco
and Los Angeles. Perhaps if we had formulated the theory of plate tectonics earlier these cities would never
have been located here, as it is we have to develop our understanding of the area’s fault lines and plate
movements and develop ways to deal with the effects. It is a conservative margin too that runs through the
island of Haiti which ‘unzips’ every few hundred years with devastating consequences for the settlements
there. But with intervals between large earthquakes being so long, it does little to dissuade development, and
that development all too often does not take potential seismic events into consideration, such was the case in
2010 when over 250,000 people died, most in poorly built buildings that collapsed. Thus plate tectonic theory
can aid our understanding of events, although this does not always translate into action that minimises effects.
Of course not all events occur on the plate boundaries themselves – in the case of volcanoes the Hawaiian
islands are famously in the centre of the Pacific plate, but even here the theory of plate tectonics can explain
their location. Hot spots are where plumes of mantle rise up and force magma through parts of the Earth’s
crust regardless of plate boundaries or, in some cases, can start the process of divergence. In Hawaii the hot
spot has created an island chain as the Pacific plate moved over the hotspot. The oldest island (and the
smallest due to erosion) is now furthest from the current active zone on the main island, and its volcano is long
extinct, whereas the current active caldera – Kilauea – spews out a constant stream of basic lava. Although we
know far less about hot spots, this appears to offer a logical explanation for the volcanic anomalies found away
from plate boundaries.
Plate tectonic theory has come a long way since Wegener’s original theory. We now have the ability to
measure the movement of plates and find sections where plates have been locked together. Our knowledge of
the causes of volcanic eruptions and of the source of the gases erupted is being applied to predict the location
and magnitude of future eruptions. Of course there are still instances when we are caught unawares,
particularly with earthquakes which more regularly occur away from the plate boundaries. There are also
instances of earthquakes being caused by human activity rather than by plate tectonics alone (e.g. dam
building or mining) but these are the exceptions rather than the rule. Plate tectonic theory is undoubtedly
more fact than theory and as our knowledge about volcanoes and earthquakes grows we are better able to
apply this theory to the world and comment on the distribution and even predict the location of volcanic and
seismic events.