introduction
“Floating on the surface of a cauldron” is not quite the
way we usually view our ship sailing in space, but this is the reality.
Regularly, the cauldron sends up cubic kilometres of hell fire as a reminder
not to take our precarious home too casually, or a couple of the floating
islands heave like a slumbering giants turning in a dream, as with the
Sumatran earthquake.
The Earth’s surface is made up of a number of enormous rock plates
(islands) that move over the convection currents, caused by heat from
radioactive decay, in the molten rock nearer the Earth’s centre.
These plates can be as big, or bigger, than a continent or an ocean.
tectonic movements
As the tectonic plates move, they can
- diverge, creating rift valleys
- converge, occuring between a continental and an oceanic plate
- converge, occuring between two oceanic plates
- converge, occuring between two continental plates
diverge, creating rift valleys
On land, a clear example is the rift valley in Eastern Africa; while
the Atlantic Ocean is the result of two tectonics: tectonic plates moving
and being pushed apart by molten larva from the Earth’s core.
The molten rock also enlargens the plates.
converge, occuring between a continental and an oceanic
plate
This is what has occured with Sumatran quake. The denser oceanic plate
is subducted (slides under) beneath the lighter continental plate, lubricated
by the sea. As the oceanic plate subducts, it heats up and generates
volcanic activity along the margin.
converge, occuring between two oceanic plates
Here, also one plate subducts under another under the ocean, the lower
plate melting with the resulting magma possibly pushing up to make a
line of volcanic islands along the length of the subduction.
converge, occuring between two continental plates
This occurs when there is no sea or ocean to lubricate the movement
between the two plates, as is the case between the Indian and the Asian
plates. The Indian plate was subducting under the Asian plate, but instead
both plates were forced upwards to form the Himalayas.
the Sumatran
earthquake
The tectonic plates in the area of Sumatra, where the earthquake hit
on 26 December 2004, are moving at about the speed that your fingernails
grow, say five to ten centimetres per year.
This gradual movement builds up tension over decades (or even centuries)
until, explosively, the plates readjust – that readjustment is an
earthquake. It is these slow-moving adjustments that, over millions of
years, change the whole map of the planet: countries move, continents
move, mountains grow, rift valleys widen and split into new land masses.
Volcanoes form along the meeting of the tectonic plates, hence the long
strings of volcanic activity and associated earthquakes around the planet.
“Erupting volcanoes are among the most destructive forces in
Mother Nature's arsenal. But where many people live on or near the flanks
of such mountains, the real disaster often doesn't start until the eruption
has subsided and the world has stopped paying attention. It is then
that rain-swollen rivers emanating from volcanic peaks can send massive
lahars - large waves of mud made up of water, ash and volcanic rock
- careening down the mountainsides, often burying everything in their
paths, even entire towns and villages. Such lahars can occur for years
after an eruption, depending on how much debris the volcano deposits
and how much rain falls, until the sediment has either been cleaned
off the mountain or has stabilized so that it doesn't erode easily.”
—
“In one of the streams we're studying, nothing can live. If a
big storm hits, the whole riverbed moves," Gran said. That means
that more than 13 years after the eruption, some of the rivers studied
have not recovered to the point of having stable channels, which are
necessary for a return of aquatic species and a general ecological recovery.”
—
“Mount Pinatubo's eruption [1991], the second largest recorded
in the 20th century, deposited nearly 1.5 cubic miles of volcanic ash
and rock on its flanks, about 10 times more than Mount St. Helens in
Washington state deposited in its eruptions in 1980.”
The eruption of Katmai, Alsaka in 1912 was the largest volcanic eruption
of the 20th century.
analysis
of the tsunami generating Sumatran quake continues
“Seismologists initially used seismic waves with periods of about
300 seconds to set the magnitude of the Sumatran earthquake at 9.0 -
making it the fifth most powerful event on record.”
—
“ [Then they examined] seismograms taken from 7 stations around
the world in the week or so following the earthquake. They looked for
the longest-period waves possible - those lasting about 3200 seconds
(53 minutes). "We found [...] that there was three times more energy
out there than at the 300-second period [...]" The new work reclassifies
the earthquake on the logarithmic Richter scale at magnitude
9.3 - second only to the 9.5-magnitude quake recorded in Chile in
1960. ”
—
“The Burma plate rebounded upwards by about 10 metres at the quake's
epicentre - setting the deadly tsunami waves in motion. And the process
continued along the border between the two plates, causing the earth
to rupture along the fault line - running from south to north. But seismologists
are not sure exactly where the rip stopped.”
once
upon a time i used to wander on this neat solid ball of mud
Now as I was young and easy under the apple boughs
About the lilting house and happy as the grass was green.
 Dylan
Thomas, Fern Hill, 1946
Now us human monkeys are beginning to wake up and look around—
Global warming, new ice ages, AIDS and ebola, great starvations and
collapsed civilisations....
Then there are wandering asteroids set to wipe out dinosaurs, or us. That
is, of course, if we don’t contrive to blow ourselves up first, or
manage to ruin the land and water sufficiently that it will no longer
feed us.
And by the way, I’ve been told that we are blithely sitting on volcanos
fit to darken the sun and moon and leave us struggling to breathe; let
alone being able to continue to live our profligate lives, while waiting
for the oil to run out in a few years.
I open the door and the flies swarm in,
Shut the door and I'm sweating again;
And in the process I cracked my shin,
Just one darn thing after another.
[From
Life
gits te-jus don’t it, 1948]
So now folks, we have the ‘supervolcano’, where the earth opens
up and gobbles us all down, well almost. The last one was apparently 74,000
years ago, so the wiseacres tell me. Not very long, considering that our
written history only goes back about 10,000 years, and I’m told sommat
like us has been around half a million to a couple of million years. So
these things seem to come around every other Tuesday, whereas the last
serious asteroid was around 60 million years ago—if I am to believe
‘them’.
From
a longish TV interview:
“ROBERT CHRISTIANSEN: Quite amazingly we realised that there was
a cycle of caldera-forming eruptions, these huge volcanic eruptions
[occur] about every 600,000 years.
“NARRATOR: Yellowstone was on a 600,000 year cycle and the last
eruption was just 600,000 years ago. Yet there was no evidence of volcanic
activity now. The volcano seemed extinct. That reassuring thought was
about to change.”
end note
- Because the Richter scale is logarithmic, an increase
of 0.3 is equivalent to a doubling of the strength of an earthquake.
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