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Which are most hazardous, destructive or conservative boundaries?
From the Andes to the Himalayas, Plate boundaries are famed for the astounding formed
around them and have become geological and tourist hotspots. However constant
lithospheric movement plate boundaries pose an extreme hazard when in contact with
human populations, damaging economies and communities. Plates move in varying ways,
while at conservative boundaries parallel plates slide past each other; convergent (or
destructive) margins are characterised by one plate subducted under another. The
magnitude and frequency of volcanism and seismic activity at each margin discerns the
relative hazards they pose; yet the global distribution is also a critical factor in the dangers
carried.
One of the most hazardous features at plate boundaries are volcanoes where magma
extrudes to the surface. As conservative boundaries do not feature any crustal subduction
or creation- as destructive margins do- they are undoubtedly less hazardous in this sense.
Meanwhile at destructive margins where continental crust collides with oceanic, or two
oceanic plates meet, volcanoes are prevalent features. This is visible across the globe with
major volcanoes like Krakatoa and Mount Etna both created as denser crust-of 3g/cm2 – is
subducted under more buoyant continental crust, melting into the mantle. The less dense
magma now rises through the crust, emerging as a volcano. Volcanism at destructive
boundaries are some of the most perilous as highly viscous andesitic and rhyolite lava is
produced, catalysing huge and powerful eruptions. This was witnessed at Mount Etna in
1992 where pyroclastic flow reached the village of Zafferana, over 7km away. In addition the
strombolian and convex shape of destructive boundaries-like Vesuvius- holds the further
hazard of flank collapse. With steep sides avalanches similar to which engulfed Valle De
Bove after Etna’s 1928 eruption can destroy adjacent communication links, hindering
evacuation and rescue efforts. When mount Chaiten erupted in 2008, local industry took 5
years to return to 2007 levels, forcing 30% of the male population to migrate to surrounding
cities. Consequently destructive margin volcanoes potentially hold more danger that the
Icelandic shapes on constructive boundaries.
Hazards are also enhanced when populations are pushed out of their zone of
tolerance; two similar magnitude volcanoes may have differential impacts, depending on
the capacity to respond. The lack of frequency of volcanoes at destructive boundaries
means many authorities are not prepared for the impact of lava flows, Lahars and
pyroclastic flow. As the 19 fatalities during Soufriere hills eruption in Montserrat exhibits,
mitigation was not implemented and residents were not accustomed to dealing with such
hazards. In comparison to Hawaiian authorities, adapted to Mauna Loa’s continual eruption,
Montserrat’s government was not prepared for the eruption injecting a mere $30,000 was
in evacuation projects. In addition due to the 60% silica rich soil neighbouring volcanoes
agricultural industries have traditionally been attracted here. Today these areas are densely
populated, on Java over 120million live in the shadow of 30 volcanoes. This is an increasing
concern following mount Sinabung’s eruption earlier this month where over 40,000
residents need to be evacuated to comprehensively ensure survival. Furthermore the
Indonesian government cannot afford to divert Lava flows as Icelandic authorities did in the
1974 Heimaey eruption. Indeed geologists at Naples University warn that despite building
regulations introduced in 1995, should Vesuvius erupt again, only 5% of the city’s
population would be successfully evacuated. Both substantial populations and the
significant lethal features at Convergent margin volcanism enhance the social and economic
hazards posed. Regardless of the lack of volcanic activity at Conservative boundaries,
convergent margins independently pose immense threats.
Furthermore gases emitted from pressure release when magma rises to the surface can
extend the hazard posed from a local scale to global reverberations. As destructive margins
contain Rhyolite magma, volcanoes along this margin emit potent gases like Sulphur
Dioxide, CO2 and Carbon monoxide, as Krakatoa did in 1883- when injection of sulphur
dioxide into the Atmosphere caused temperatures to plummet by 1.2 o. However these
remain isolated events and debate remains over the actual impact of volcanically emitted
gases. Compared to 35 bn tons of anthropogenic CO2 produced per year, the 0.44 Gigatons
from volcanic movement is dwarfed. Consequently the long term atmospheric hazard posed
by volcanoes significantly less than human induced dangers; generating scepticism over the
potency of volcanic climate change.
Yet volcanism is not the sole measure of hazard posed by a plate margin, seismicity also
holds major dangers. Here conservative plate margins rival the threat posed at destructive
margins. Pressure release from the friction of two parallel plates sliding past each other can
instigate large tremors like in Haiti in 2004 and Turkey 1999, with both quakes reached 7 on
the Richter scale. This happens often around the immensely active San-Andreas fault line
where the pacific and North American plates migrate north at 6cm pa and 1cm pa
respectively. The epicentres of these quakes are relatively shallow- only 10km deep in Haiti
2004- creating more powerful surface waves. In comparison to deeper epicentres at
destructive margins these waves pose a far greater hazard to human life. Bolivia’s 1994
quake exposes this, despite reaching 6 on the Richter scale; only 1000 homes were damaged
in comparison to the 1999 Anatolian fault quake where over 50,000 Turkish residences were
damaged. Lower epicentres (or Benioff zones) at destructive boundaries are caused by the
compression of crust being subducted deep into the lithosphere, as seen along Japan’s coast
where the Eurasian plate is subducted at 40O. Despite variations in tremor waves
converging margins have equally damaging potential, and the hazardous nature of
destructive boundaries cannot be overlooked. Indeed the biggest earthquake ever recorded
– in Chile 1960- occurred where the Nazca plate meets the South American, reaching a
staggering 9.5 in magnitude. Conclusively, though quakes at these margins differ in
magnitude and formation, they pose equally fatal hazards to surrounding communities.
Despite varying magnitudes produced by the physics at each boundary, it could be argued
that it the prime determinant for the scale of hazard posed is in fact the location of the
Earthquake. A seismic tremor in a highly populated area will undoubtedly cause more social
and economic damage than those in isolated regions. Consequently destructive boundaries
could be considered far more hazardous, as my diagram shows, subduction zones straddle
densely populated regions including New Zealand and Indonesia, contiguous to Japan’s
coastline.
By comparison over 70% of Conservative boundaries are located in the oceans, across the
Atlantic and south pacific. Thus, while the movement between the Pacific plate and the
Honushu in 2011 killed 15,000 and destroyed over 120,000 buildings, the same magnitude
quake at a transform margin in the Mid-Atlantic earlier this month went largely unobserved.
In addition lower economic development can also exacerbate the hazard posed by
earthquake events. Many LEDC countries cannot afford to construct quake resistant
buildings, with over 80% of casualties stemming from building collapse; fatalities are likely
to be far higher. Compared to conservative boundaries like the San- Andreas fault in
California where GDP is $1.8 trillion, destructive boundaries are largely near less developed
regions like the Philippines and Indonesia with GDPs half this figure at $500bn and $830bn
respectively. The dramatic disparity between the Loama Prieta quake in 1989 and in the
Philippines in 2013 highlights the relative hazard to human life. With over 60% of housing
constructed illegally and only the capital Manila boasting steel re-enforcements for
buildings, there were over 5,000 estimated deaths, a stark contrast to San- Francisco where
only 63 died. Here California state council invested over $2bn in pendulums and steel cores
making the city far more resistant to seismic hazard. Yet it could be argued that the risk of
collapse in MEDCs is a greater economic risk. Hosting 30% of west coast trade, the Pacific
Exchange base and 2,800 international technology companies, destruction in San-Francisco
would have global repercussions, unlike the strike on Tacloban (Philippines 2013) where
global stock markets did not move. By comparing Haiti and San-Francisco’s quake the effect
of development is palpable. Despite both occurring on conservative margins at similar
magnitudes, the 316,000 Haitian deaths in contrast to San-Francisco’s mere 63 exposes how
expert infrastructure and resources to combat secondary diseases can dramatically reduce
seismic hazard, irrespective of plate boundary movement. The frequent isolated or
economically developed locations of many Conservative boundaries, suggests that
Earthquake activity is significantly less hazardous to human activity than their convergent
counterparts.
In addition due to rapid improvements in seismic prediction technology, the hazard posed
by conservative boundaries may begin to reduce in the future. This is crucial as unlike
volcanic events, Earthquakes are sudden impact hazard. With improved forewarning
systems evacuation procedures can be instigated; pivotal to reducing human fatalities.
Improvement has been seen along the Anatolian fault, where in 1999 the quake was in fact
successfully predicted using radon gas sensors. Doubly the San-Andrea’s fault has 5 seismic
research centres along the line, where by tracking plate movement researchers at USGS
have produced ‘Deterministic and Scenario Ground-Motion Maps’; the slower onset
enabling emergency response units to conduct training exercises based on realistic
earthquake situations. Although this potentially reduces the relative hazard at conservative
boundaries in comparison to continued ambiguity at destructive margins, effectively
coordinated evacuation is still demanded. Judging by Turkey’s 1999 failure to respond
leaving 17,000 dead, this continues to demand improvement.
Undoubtedly lack of volcanic activity as Conservative margins means Converging boundaries
are the obvious volcanic risk. Yet Seismicity is an equally potent hazard and the difficulty in
mitigating and responding to Earthquake events makes them fatally dangerous to those
affected. However as both margin formations create equally large quakes; their relative
locations determine the extent and significance of the hazard posed. Despite catastrophic
Conservative boundary quakes like Haiti, due to the predominantly less developed and more
densely populated locations along destructive margins, these unquestionably carry greater
long and short term, social and economic hazards.