- Circling the Pacific Basin, on the bottom
of the sea bed, lie a dramatic series of volcanic arcs and oceanic trenches.
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- The zone - the 'Ring of Fire' - notorious
for frequent earthquakes and volcanic eruptions, coincides with the edges
of one of the world's main tectonic plates.
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- More than half of the world's active
volcanoes above sea level are part of the ring.
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- Intense geological activity
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- In the past 25 years, scientists developed
a theory called plate tectonics explaining the locations of volcanoes and
other large-scale geologic features.
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- According to tectonic theory, the surface
of the Earth is made up of a patchwork of massive rigid plates, about 80km
thick, which float in slow motion on top of the Earth's hot, pliable interior.
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- The plates change size and position over
time, moving at speeds of between 1cm and 10cm every year - about the speed
at which fingernails grow.
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- New sea bed is constantly being created
in the middles of the oceans - flowing out as hot lava, and rapidly cooling
on contact with cold deep sea water.
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- To make room for the continual addition
of new ocean crust, all the earth's plates move. And as they move, intense
geologic activity occurs at the plate edges.
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- At the edges, one of three things may
occur.
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- The plates can be moving away from each
other, leaving space for new ocean floor.
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- Some plates are moving towards each other,
causing one to submerge beneath the other.
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- Other boundaries slide past each other
without much disturbance.
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- Tectonics and earthquakes
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- Parts of the plate boundary that slide
past one another in opposite directions - such as the San Andreas Fault
- cause minor earthquakes.
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- The faults may also create cliffs or
scarps thousands of feet high on the ocean bed.
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- But where one oceanic plate collided
with and is forced deep into the Earth's interior, the subsumed plate encounters
high temperatures and pressures that partially melt solid rock.
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- Some of this newly-formed magma rises
to the Earth's surface and erupts, forming chains of violent volcanoes
- like the Ring of Fire.
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- These narrow plate-boundary sites, known
as subduction zones, are also associated with the formation of deep ocean
trenches and big earthquakes.
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- When there is an earthquake under the
sea, one side of the ocean floor suddenly drops downward, beneath the top
edge of the subducting plate.
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- The resulting vertical fault will generate
a tsunami - much as a wave machine in a swimming pool will generate one.
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- The movements of the plates usually allow
little warning for those at risk in coastal areas.
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- One warning of a tsunami is that there
is a rush of water away from the coastline - but this predictor may mean
the forthcoming seismic wave is only minutes away.
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- One week before Papua New Guinea's seismic
activity, a large quake was recorded to the west of Western Samoa, and
another took place in Vanuatu.
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- The frequency of Pacific quakes and seismic
activity is not coincidence.
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