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#44649 - 10/17/01 05:35 PM Re: Earthquake? Landslide?
Capital Kiwi Offline
Carpal Tunnel

Registered: 11/13/00
Posts: 3146
Loc: Northamptonshire, England
The magma on which the continental plates "float" has currents which appear to remain relatively stable in terms of direction and velocity for long periods of time. A bit like the Gulf Stream, I guess. The force of these currents is great enough to force the plates together for what appears to us to an amazing amount of time. The Pacific plate subducts under the Australian plate, from memory and New Zealand kind of trembles on the brink, so to speak. Incidentally, the Himalayas and the Tibetan plateau are all the result of the plate on which India sits pushing up at the southerly end of the Asian plate. Some force, huh?

It's driven by magma being forced up between the plates in the centre of the major oceans - the Pacific and the Atlantic. The mid-Atlantic ridge is highly volcanic. New plate material is formed by the magma and is pushed away from the scene of its creation. Or something like that, anyway.

It's all been proved by the study of changes in magnetic polarity in the material on the seabed. Turns out that the patterns of magnetic change are mirror images of each other on each side of the mid-Atlantic ridge, showing that the material on both sides was created at the same time.

I dunno, one too many Discovery Channel programs, I guess!

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#44650 - 10/18/01 07:12 AM Re: Earthquake? Landslide?
milum Offline
old hand

Registered: 09/03/01
Posts: 872
Loc: Birmingham, Alabama
Dear Max,
That "Bang" you heard at the time of the earthquake might have been the audible manifestation of a phenomenon that has been many times reported but never photographed - a flash of electromagnetic light caused by the sudden release of energy by the earthquake. A direct transfer of energy, as Stales suggests- from medium (rock) to medium (air), would seem likely to produce a rumble more so than a bang. A thunderclap produces a similar "Bang" as air rushes in to fill the vacuum left by a lightning strike.


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#44651 - 10/18/01 07:57 AM Re: Earthquake? Landslide?
wsieber Offline
old hand

Registered: 03/15/00
Posts: 1027
Loc: Switzerland
Hi Milum,
A thunderclap produces a similar "Bang" as air rushes in to fill the vacuum left by a lightning strike..
Unfortunately this is an old erroneous myth. The lightning strike does not leave a vacuum. It simply heats up the air incredibly quickly, and the resulting expansion makes itself felt as a bang. The crackling sparks of static created when you pull off your acrylic sweater, are miniature models of the phenomenon.







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#44652 - 10/18/01 08:10 AM Re: Earthquake? Landslide?
maverick Offline
Carpal Tunnel

Registered: 09/15/00
Posts: 4757
an old erroneous myth

Ever the careful and judicious scientist, eh, Werner? ~ distinguishing neatly between this and the *new erroneous myths, the old *true myths, and...


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#44653 - 10/18/01 08:25 AM Re: Earthquake? Landslide?
of troy Offline
Carpal Tunnel

Registered: 10/17/00
Posts: 5400
Loc: rego park
*new erroneous myths, the old *true myths, and...

aren't new erroneous myths called urban legends? and all are identified by having happened to "a friend of a someone i work with cousin" ? (there being six degrees of separation between strangers in the modern world but only three degrees of separation in an legend.. this being one of its major characteristics.)

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#44654 - 10/18/01 02:42 PM
Max Quordlepleen Offline
Carpal Tunnel

Registered: 08/12/00
Posts: 3409

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#44655 - 10/18/01 05:14 PM Re: Earthquake? Landslide?
belMarduk Offline
Carpal Tunnel

Registered: 09/28/00
Posts: 2891
milum, I've never heard of this. Where would this flash happen? If people can hear it over several kilometres is it the same as lightning (which can cover a lot of space)?


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#44656 - 10/21/01 07:30 AM Re: Earthquake? Landslide?
stales Offline
old hand

Registered: 11/13/00
Posts: 866
Loc: Perth, Western Australia
Ah - you take a few days off to go bush and everyone horns in on your special subject. Never mind - thanks for all the learned input.

Must however correct one slight inaccuracy:

"It's driven by magma being forced up between the plates in the centre of the major oceans - the Pacific and the Atlantic. The mid-Atlantic ridge is highly volcanic. New plate material is formed by the magma and is pushed away from the scene of its creation. Or something like that, anyway."

The subduction of one plate under another isn't really "driven" by the magma at the opposite side of the plate forcing its way up. As mentioned in an excellently descriptive post, the convection currents within the mantle (driven, it is postulated, by the heat generated from the radioactive decay of the minerals present) carry the crustal plates across the surface of the earth - rather like a conveyor belt.

Where they collide, the crust on one side of the impact zone buckles and heaves - building mountain chains like the Himalayas and the Andes. The other plate is "subducted" (ie pushed under the other) - the line along which this happens often forming a deep ravine ("trench") in the ocean (eg the Mariana Trench off the east coast of Japan). The crustal material that melts has a lower density than the surrounding mantle and rises slowly through it, eventually cooling at or near the surface as granitic or related rocks on the far side of the mountain range.

Crustal material delivered into the subduction zone at one side of the plate must be replaced - this happens at the Mid Ocean Ridges (MOR's). The magma that comes to the surface at the MOR's is filling the void left by the plates as they move apart along the line of the MOR. We've all seen pictures of this happening - especially the Mid Atlantic Ridge where it comes to the surface in Iceland.

Not all plates meet head on. The movement between the Pacific plate (upon which half of NZ's North Island sits) and the Australian plate is lateral and, as mentioned in the previous post, they've been locked together for a long time. Simply put, the rate of movement at the join isn't keeping up with the movement away from there. It has to release sooner or later - and all Kiwis should be hoping for a lot of smaller quakes, rather than one big mutha.

Enough of the rocks - on with the words!

stales



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#44657 - 10/21/01 10:20 AM Re: Earthquake? Landslide?
Jazzoctopus Offline
old hand

Registered: 07/03/00
Posts: 1094
Loc: Cincinnati & Loveland, Ohio, U...
Mid Atlantic Ridge

I've seen time projection maps that say that because all of the continents are moving they will again form one big continent where the Pacific ocean is now. Could it be possible, though, that while the plates are moving they keep pushing the Mid Atlanic Ridge up and it eventually rises into a new continent?


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#44658 - 10/21/01 10:45 AM Re: Earthquake? Landslide?
of troy Offline
Carpal Tunnel

Registered: 10/17/00
Posts: 5400
Loc: rego park
Jazzo, anything is possible given enough time. the east coast mountains, the Appalachians, now eroded, and derided as meer hills, used to be as high as the himalayas! almost everything south of the hills of arkansas is mississippi delta-- about 1000 miles! (the result of the erosion of the same mountains into hills! )

some of the rocks found in Marin county, north of Frisco, are from area near LA. I like geology, i have trouble comprehending the vast number of years, (eons!) it has taken to create, say the 1000 mile mississippi delta, but a good geological map, gives lots of information.
I'll look for a link on Monday and send it to you as PM-- the US geological service has some great ones. I don't know of any really great world ones, but each country has good ones of there own-- maybe stales can get one of down under.. and so on..
and for further information, i would recommend The Map that Changed the World, by simon winchester, the author of The Professor and the Madman. a short history of the first geological map, and other information about the beginnings of the science of geology.

PS. you should know some geology-NYC's skyline is shaped by the underling geology of manhattan island... in NY case,geology is destiny! after all the Bronx is gneiss, Manhattan is schist*, and NY is not with out its faults!
*bronx natives have at tendency to mispronounce this word rudely.

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