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 dont 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, Ive 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 Im 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 agoif 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 notes
- Ice ages
The Ice Age column on the geological timeline above
gives only a rough impression of when ice ages occured.
Knowledge on ice ages is steadily increasing. In recent times and
back to 800,000 years ago, ice ages have been occuring roughly every
100,000 years. Before 800,000 years ago, ice ages were on an approximately
40,000 year cycle.
There is a lot of variation within these cycles, most of the causal
factors are at peresent speculative.
For information and background on what is known as the Little Ice
Age, NASA
provides a page with many annotated illustrations.
“[...] from the 1400s to the 1700s [there was a localised]
"Little Ice Age" in several regions including North America
and Europe.”
Smart
animation showing the localisation of global heating and cooling
events.
- Because the Richter scale
is logarithmic, an increase of 0.3 is equivalent to a doubling of
the strength of an earthquake.
- Eons, eras, periods
and epochs
- The names of the geological timespans, like the
classifications used for categorising life-forms, change
as those studying the topic learn more, make further discoveries,
or try to be more precise. And the discussions over the names
contuinue. We at abelard.org have attempted
to provide the least ambiguous namings. However, there is a
fair degree of naming confusion, not least because older names
for the same, or for slightly different, time periods are still
being used alongside the newer names.
Etymologies:
Prefixes and suffixes:
- -zoic: from
zoon [Greek], meaning: life or animal
- paleo-: from
palaios [Greek], meaning: ancient, or from palai,
Greek, meaning long ago
- meso-: from
mesos [Greek], meaning: middle
- neo-: from neos [Greek], meaning:
new
Eons:
- Phanerozoic: visible or evident life
(phaneros [Greek], meaning: visible or evident, + -zoic)
- Proterozoic: the eon before the Phanerozoic
eon
(proteros [Greek] meaning: earlier or former + -zoic)
Eras:
- Cenozoic: new
life (kainos [Greek], meaning: new or recent + -zoic)
- Mesozoic: middle life/animals
- Paleozoic: ancient life
- Archaean: from archaea [Greek],
meaning: ancient ones
- Hadean: After Hades, Greek for hell (from
the intense heat during part of this period).
Periods:
- Tertiary and Quaternary:
“The name Tertiary was first applied about the middle
of the 18th cent. to a layer of deposits, largely unconsolidated
sediments, geologically younger than, and overlying, certain
other deposits then known as Primary and Secondary. Later (c.1830)
a fourth division, the Quaternary, was added.
Although these divisions of the earth’s crust seemed adequate
for the region to which the designations were originally applied
(parts of the Alps and plains of Italy), when the same system
was later extended to other parts of Europe and to America it
proved to be inapplicable. It was realized that one scheme of
classification could not be applied universally.
The names Primary and Secondary were generally abandoned; Tertiary
and Quaternary were, and still are, used, but other geologic
literature substitutes other names, including the Palaeogene
and Neogene.”
[Quoted from The
Columbia Electronic Encyclopedia]
- Cretaceous: from cretaceus, Latin,
chalky.
There were widespread deposits of chalk rocks at this period.
- Jurrasic: from the Jura Mountains, which
run along the border of France and Switzerland.
- Triassic: from trias [Latin], meaning
triad.
This period is so named for the three distinct layers of rock
laid down during this period: continental redbeds, then marine
limestone and thirdly evaporites.
- Permian: named after the Perm region in Russia
where extensive areas of rock formed in this period are found.
- Carboniferous: Producing carbon or coal .
Extensive swampy forests in this period later formed coal deposits.
- Devonian: named after the marine formation
of Devonshire dtaing from this period.
- Silurian: from Silures [Latin],
an ancient people of southwest Wales, where the rocks were first
identified.
- Ordovician: from Ordovices [Celtic],
an ancient Celtic tribe of Wales.
- Cambrian: from medieval Latin name for Wales
- Cambria.
Epochs:
- Holocene: entirely recent time (from holos
[Greek], meaning: whole, entire + kainos,
recent).
- Pleistocene: most recent time (from pleistos
[Greek], meaning: most + kainos, recent).
- Pliocene: more recent time (from pleion
[Greek], meaning: more + kainos, recent).
- Miocene: less recent time (from meion
[Greek], meaning: less + kainos, recent).
- Oligocene: a little recent time (from oligos
[Greek], meaning: little, few + kainos, recent).
- Eocene: dawn of recent time (from eos
[Greek], meaning: dawn + kainos, recent).
- Palocene: oldest recent time (from paleo-
+ kainos, recent).
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