Download The earths crust is separated into several tectonic plates

Compare and contrast two plate margins you have studied
to evaluate which is the most hazardous. (40).
The earths crust is separated into several tectonic plates. Heating from the Earths core
creates convection currents in the molten mantle this has the effect of causing the
magma to rise when heated and sink when cooling. The movement of the magma also
causes the plates to move. Where two of these plates meet is referred to as a plate
margin. There are many types of plate margins each exhibiting different characteristics.
This essay will attempt to compare and contrast constructive plate margins where the
crust is diverging and destructive plate margins where the crust is colliding and attempt
to evaluate the level of hazards associated with each in an attempt to ascertain which is
the most hazardous.
First there are the constructive/divergence plate boundaries these are created when the
rising magma reaches the top of the asphenosphere (mantle) and reaches the bottom of
the lithosphere crust it will diverge having the effect of pushing the plates apart. When
this happens less viscous basalts lava rises to the surface to create new crust. When
these oceanic crusts move apart they can create an oceanic ridge such as the Mid
Atlantic Ridge which runs down the length of the Atlantic Ocean. See diagram 1
As shown in diagram 2 the North Americas plate and Eurasian plate are being pulled
apart and magma rises from the mantle to create the Mid Atlantic Ridge. Where two
continental plates move apart they can lead to the crust fracturing which can cause a
section to drop down to create rift valleys. The hazards associated with these margins
appear to be quite minor in terms of intensity. Due to the low pressure created by the
crust moving apart allowing the rising magna to be basalts and so less viscous therefore
volcanic activity is less explosive. More liquid lava can rise from a Oceanic Ridge to form
Submarine volcanoes due to low pressure zones. If eruptions persist they may develop
until they reach the surface forming islands. Examples of this are Iceland and Isle of
Surtsey formed in 1963 to the South of Iceland. Also due to the gentler nature of
eruptions there are no great seismic events and any earthquakes are gentle, therefore
no tsunamis are created and as these tend to form at sea there are no people around so
these don’t present a hazard to them. However should constructive boundaries form
fissure volcanoes on land these can be more hazardous. These form when cracks in the
crust appear due to the diverging plates allowing lava to spill out over large area. Again
these eruptions are gentle so there are no great explosions and pyroclastic flows,
however due to less viscous nature of the lava it can spread out over a large area and so
has the potential to cause great damage. This can be seen in Heimay the main fishing
port on 23 January 1973when lava began pouring out of a fissure 2km in length and
activity lasted for 6 months. Many homes nearby had been burnt to the ground and
others had been buried under 5m of ash and the harbour entrance had been blocked.
However all the inhabitants were evacuated safely due to the predictable nature of
these eruptions. Another example is when the Grimsvoten volcano beneath the
Valnajoku ice field caused rapid melting of the ice in October 1996 creating massive
floods (Jokathlamps) with a peak flow of 45000 …………… This flood destroyed bridges and
10km of ring road that encircles the island. However again due to the predictable nature
of the eruption dykes were strengthened and people evacuated therefore there was no
loss of life or damage to settlements. Therefore due to the gentle nature of constructive
plate margin characteristics and the relatively few people in their areas they don’t
represent a substantial hazard and even in populated areas their predictable nature
allows people to mitigate against their effects.
Secondly there are the destructive (convergence) plate margins. These occur where two
plates collide with each other due to cooling magna converging and sinking into the
mantle. There are three types of destructive boundaries. First there is the Oceanic
continental crust convergence. When an oceanic collides with a continental crust the
denser oceanic crust subducteded beneath the less dense continental crust where it is
destroyed by the magma below. This can cause the continental crust to fold upward to
create Fold Mountains such as the Andes created by subduction of the Nazca plate
beneath the South American plate. Also deep ocean trenches like the Peruvian trench
are formed at the subduction zone. Where the crust is destroyed friction builds up this
has the effect of heating the magma to an even greater degree which can cause it to
rise through rocks to create intrusive features like batholiths locoliths sills and dykes.
However the magma can reach the surface to form extremely violent eruptions (see
diagram). Secondly when two oceanic crusts meet the dense one is subducted beneath
the less dense one. Again ocean trenches are formed at the subduction zones like the
Marianus trench in the Pacific. Again like oceanic and continental convergence friction
from subduction can form volcanoes. These form islands that erupt above the ocean and
go on to form island arcs as the crust move such as The Philippines. Finally there is
continental/continental plate divergence where two continental plates collide. This
time subduction does not occur instead the two plates push against each other forcing
the crust to fold upwards creating fold mountains. An example of this is when the Indian
subcontinent is propelled northwards by the Indo Australian plate until it collided with
the Eurasian plate some 40 million years ago, closing the Tethys sea and creating the
Himalayas.
Destructive margins have the potential to be more hazardous than the constructive
margins. First the volcanoes created by oceanic/continental and oceanic collisions are
much more violent and explosive due to adesitic nature of the lava its viscosity.
Therefore the lava does not flow far. This may seem less hazardous as it will destroy
smaller area, however it may block up the main shaft leading to a huge explosive
eruption of ash and debris as the pressure inside the volcano the blockage is blown
away. The large amounts of ash can create severe problems. First it can flow down the
side of the mountain as pyroclaustic flows at speeds of 200km/h at temperatures of over
1000’C, destroying everything in their path and covering large areas. An example of this
is Soufriere Hills volcano on the Caribbean Island of Montserrat between 1995 and 1998
where flows destroyed much of the farmland and led to the evacuation of half of its
11000 population and the abandonment of the southern half of the island. Another
hazard is the ash plume itself as this can rain down on the surrounding area. This will
destroy crops and could lead to famine, also upon entering the lava and ash can become
like cement and can lead to breathing problems and other health problems. Also the ash
cloud can reach high into the stratosphere where it can be carried by the jet stream
over the world. Droplets of particles like sulphuric acid can reflect and absorb insulation
lowering the temperature, this could lead to harsher winters throughout the world. For
example the eruption of Mount Pinatubo in the Philippines in 1991 caused a significant
cooling of global climate the following year. Also due to the particles entering the
stratosphere this becomes a global hazard, not just a local one. Finally the melting of
snow and ice fields around the summit can lead to Lahars mixtures of water rock sand
and mud that flows down valley, leading away from a volcano. It is initially devastating
and can destroy buildings and bridges in the short term. However in the long term it can
pollute water supplies leading to the spread of diseases like cholera and TB. These are
also extremely hazardous as they follow where settlements and populations are
concentrated. (See diagram 3) Another hazard associated with all the destructive
boundaries are violent earthquakes. The subduction of oceanic crust can cause great
friction to build up. When this is released it can lead to powerful earthquakes which can
lead to great damage and loss of life, as buildings are destroyed trapping people inside
them. Also if an earthquake happens out at sea an oceanic/oceanic boundary, it can
create huge tsunamis such as Indonesia in 2004 where the Australian plate was
subducted beneath the Eurasian plate, a earthquake occurred measuring 9.1 on the
Richter scale, triggering a tsunami, responsible for the greatest loss of life in a tectonic
event since records began with 229866 people lost as well as widespread damage. The
wave also destroyed 60% of Shrilanka’s fishing fleet and coastal industrial infrastructure
adversely affecting the economy. Also diseases spread due to the contamination of the
drinking water. Therefore these more violent characteristics have the potential to be a
great hazard, however due to the unpredictable nature of these hazards, people cannot
be prepared for them and therefore people cannot mitigate effectively against them
leading to greater impact.
Therefore to conclude constructive boundaries are where crust s forced apart and new
lava rises to create new crust whereas destructive margins are where a denser oceanic
crust is subducted beneath a less dense continental/oceanic plate or where two
continental plates meet creating fold mountains. It does appear that destructive margins
have the potential to be more hazardous as constructive margins produce weak
earthquakes and gentle eruptions that produce less viscous lavas and their predictable
nature allows people to prepare against them. However destructive boundaries produce
a greater number of hazards which are more violent such as explosive volcanoes,
powerful earthquakes and massive tsunamis. However the nature of the hazard is
dependent on the effect on the human population. If an extremely powerful volcano
were to erupt in an unpopulated area it would be less of a hazard than a gentle volcano
erupting in a densely populated area similarly the ability of the country to mitigate the
risk also affects the level of hazard, for example a powerful earthquake hitting an MEDL
like America would be less of a hazard than an earthquake hitting a LEPL like Haiti as
the MEDL would be more able to deal with the effects than an LEPL. Therefore to
conclude as destructive margins produce more violent characteristics than constructive
margins and the fact that as most constructive margins are in the oceans and do not
affect many people whereas many people live near destructive boundaries, also as many
of these are LEDCs they will be more unable to cope with the impacts. Therefore
destructive boundaries are more hazardous than constructive.