tectonics: tectonic plates - floating on the surface of a cauldron | briefing document

tectonic plates –
floating on the surface of a cauldron

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This page helpful? Share it! Tectonics: tectonic plates – floating on the surface of a cauldron discusses tectonic plates, including their relationship to tsunamis. This is a sub-document to tsunamis: tsunamis travel fast but not at infinite speed. tsunamis: tsunamis travel fast but not at infinite speed vulcania tectonics: tectonic plates – floating on the surface of a cauldron futuroscope anthropogenic global warming earthquakes

a geological timeline
tectonic movements
sumatran earthquake
after a volcano blows
once upon a time i used to wander on this neat solid ball of mud


“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. These movements take place over, what to a human, is enormous periods of time. As usual, humans work to organise these incredible time periods in a manner to help people make sense of them.

The geological timeline below is not to scale On human classification systems Note that eons are sub-divided into eras, which are sub-divided into periods, which are sub-divided into epochs.
Eons, eras, periods and epochs - origins of these classifications, and their etymologies time before present
(millions of years) geological eon era period epoch data ice ages
(millions of years before present) 0.11 to present Cenozoic
(age of mammals) Quaternary and Recent Holocene

400 to present: life on the land

1.8 to 0.11 Pleistocene 6 to 1.8 Tertiary Pliocene 26 to 6 Miocene 38 to 26 Oligocene 40 to 25 55 to 38 Eocene 65 to 55 Palocene 135 to 65 Mesozoic
(age of reptiles - dinosaurs) Cretaceous 205 to 135 Jurassic 250 to 205 Triassic 290 to 250 Paleozoic
(ancient life forms) Permian 350 to 260 355 to 290 Carboniferous 410 to 355 Devonian

600 to present: 15% O2, below 1% CO2 in atmosphere

1,150 to present: life in the sea

1,700 to present:
free oxygen (O2), in atmosphere 438 to 410 Silurian 460 to 430 505 to 438 Ordovician 540 to 505 Cambrian 2,500 to 540 Neoproterozoic

800 to 600 Mesoproterozoic
1,600-1,000 3,500 to 1,150: primitive lifeforms producing an atmosphere; 20% carbon dioxide in atmosphere gradually reducing 3,500: oldest fossil record of life Paleoproterozoic
2,500-1,600 4,600 to 2,000: early atmosphere virtually anaerobic with 80% CO2 concentration;
sun colder than now;
more ultra-violet radiation;
sea 1/3 present salinity 4,600 to 2,500
million years ago archaean
3,800–2,500 2700 to 2300 hadean

the changing face of the earth over time

The past placement of these continenets and countries cannot be thought of as reliable in the sense of modern maps. Placing the changing landmasses is a matter of complex detective work involving geo-magnetism, plant evolution, geology and other clues.

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.

A tectonic plate slip can be a very fast, explosive event. Think in terms of bending a stick, where the tension gradually increases and then suddenly, the stick snaps back. If you want to see this happen, push a fairly thin green branch at an angle, up against a brick or concrete wall and the stick will bend, and then at some point the branch will slip and spring back. (If you are youthful, don’t do this without supervision; and if you’re more experienced, take precautions, because the energy when the branch snaps back can be dangerous.)

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Volcanoes form along the meeting of the tectonic plates, hence the long strings of volcanic activity and associated earthquakes around the planet.

after a volcano blows

“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 notes

  1. 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.

  2. Because the Richter scale is logarithmic, an increase of 0.3 is equivalent to a doubling of the strength of an earthquake.

  3. 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.
      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
    • 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)
    • 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).
    • 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.
    • 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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