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Author Topic: How did tectonic plates arise, and when?  (Read 28017 times)

Offline chris

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How did tectonic plates arise, and when?
« on: 25/12/2006 20:19:11 »
Recently someone asked me how and when the Earth came by its tectonic plates, why they are where they are, and are they ubiquitous in rocky planets like ours?

Can anyone shed any geological light on these questions?

Chris


 

Offline Karen W.

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Re: How did tectonic plates arise, and when?
« Reply #1 on: 26/12/2006 00:33:55 »
We live within a couple miles of where the San Andreas fault meets the Cascadia Tetonic Plates.. San Andreas empties out in petrolia, ( in Northern California)a few miles away from me. I basicly live right on top of one of the biggest fault lines..We have numerous rattlings dailey!! Most unnoticeable , but we get real good shakers some times. I would be interested in understanding more about this also!!
« Last Edit: 04/01/2007 15:07:19 by Karen W. »
 

Offline eric l

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Re: How did tectonic plates arise, and when?
« Reply #2 on: 26/12/2006 15:25:10 »
I'm not an expert in this field, and I can not explain it any better than Wikipedia. The article (see link) gives a clear and consistent explanation on the subject, including some point on plate tectonics on other planets !  But it does not offer an answer as how or when the plates came into existence.
http://en.wikipedia.org/wiki/Tectonic_plates
« Last Edit: 26/12/2006 15:28:35 by eric l »
 

Offline Karen W.

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Re: How did tectonic plates arise, and when?
« Reply #3 on: 26/12/2006 23:37:08 »
Wow interesting though! But your right nothing about origins etc.
 

Offline JimBob

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Re: How did tectonic plates arise, and when?
« Reply #4 on: 27/12/2006 00:29:41 »
Achtung Klass
Heuta, wir wollen ...

Weeeellllll, let me start over.

Long, long ago and not very far, far away in this galixy and, believe it or not, on this planet, there was very little water. What was de-gassed from the earth soon evaporated. This was not very much but was anyway held in what little atmosphere there was by gravity.

Imagine the earth the size of an onion. The solid part of the earth (at the scale of the onion) is the size of the dry skin that you shuck off before you use the onion. This part - the crust - is where the plates of solid earth are located. The hot inner part of the earth has been moving this solid crust around like the skim that forms on warm milk since it cooled to the point that the crust could form.

The oceans came later. Enough water falls to the earth every 20,000 years to add an inch of water to the earth. The earth itself is cooling slowly and since the earth is about 4.5 billion years old, it had cooled enough by about 3.8 billion years. Since this is probably anerobic bacteria water may not have been involved. BUT it is found in rocks that are some of the oldest rocks known on earth, on the west coast of Greenland. These rocks are already highly deformed and metamorphised (changed from their original state) by plate tectonics.

There is another 4.4 billion year old contender for oldest life from the middle of the Australia, but it too is in rocks that have been subjected to plate tectonics. Both are dependent on the ratio of C-12 to C-13 with the Greenland sandstone samples being a little more convincing.

A small note. The 4.5 Billion year date for the earth is from an earth that has been 'reset' because of a collision with a 1/4 earth size planitary body or asteroid that melted eveything, reseting the atomic clock for rocks and forming two celestial bodies - the earth and the moon. This was about 4.5 billion years ago.

Class dismissed.
 

Offline Karen W.

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Re: How did tectonic plates arise, and when?
« Reply #5 on: 27/12/2006 04:09:39 »
Achtung Klass
Heuta, wir wollen ...

Weeeellllll, let me start over.

Long, long ago and not very far, far away in this galixy and, believe it or not, on this planet, there was very little water. What was de-gassed from the earth soon evaporated. This was not very much but was anyway held in what little atmosphere there was by gravity.

Imagine the earth the size of an onion. The solid part of the earth (at the scale of the onion) is the size of the dry skin that you shuck off before you use the onion. This part - the crust - is where the plates of solid earth are located. The hot inner part of the earth has been moving this solid crust around like the skim that forms on warm milk since it cooled to the point that the crust could form.

The oceans came later. Enough water falls to the earth every 20,000 years to add an inch of water to the earth. The earth itself is cooling slowly and since the earth is about 4.5 billion years old, it had cooled enough by about 3.8 billion years. Since this is probably anerobic bacteria water may not have been involved. BUT it is found in rocks that are some of the oldest rocks known on earth, on the west coast of Greenland. These rocks are already highly deformed and metamorphised (changed from their original state) by plate tectonics.

There is another 4.4 billion year old contender for oldest life from the middle of the Australia, but it too is in rocks that have been subjected to plate tectonics. Both are dependent on the ratio of C-12 to C-13 with the Greenland sandstone samples being a little more convincing.

A small note. The 4.5 Billion year date for the earth is from an earth that has been 'reset' because of a collision with a 1/4 earth size planitary body or asteroid that melted eveything, reseting the atomic clock for rocks and forming two celestial bodies - the earth and the moon. This was about 4.5 billion years ago.

Class dismissed.


Jim Bob, I am not very scientificly minded and must ask; What is C-12 and C- 13??
 

Offline JimBob

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Re: How did tectonic plates arise, and when?
« Reply #6 on: 28/12/2006 01:30:48 »
C-12 and C- 13 are carbon isotropes.

An isotrope is:
                       One of two or more forms of a chemical
                       element having the same number of
                       protons, or the same atomic number, but
                       having different numbers of neutrons, or
                       different atomic weights.
 

Offline eric l

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Re: How did tectonic plates arise, and when?
« Reply #7 on: 28/12/2006 09:07:16 »
Quite interesting, JimBob, but it still leaves me puzzled, not on the "when" but on the "why".
You compare the plates to the skin formed on milk when you leave it standing or heat it up (causing evaporation at the surface).  But from what I understand of plate tectonics, one plate is slowly creeping below an other, and is disappearing again in the more or less liquid core, while on an other spot new crust is formed.  And I do not see anything like that happening with the skin on the milk.  Is there so much difference in the temperature of the core that it creates convection flow ? Do we have any proof of that ?  Or if the cause of this movement of the plates is related to the earth's rotation, then why are the faults not parallel to the equator which would make the movement more or less comparable to passat winds.
« Last Edit: 28/12/2006 09:08:54 by eric l »
 

Offline Karen W.

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Re: How did tectonic plates arise, and when?
« Reply #8 on: 28/12/2006 13:14:25 »
C-12 and C- 13 are carbon isotropes.

An isotrope is:
                       One of two or more forms of a chemical
                       element having the same number of
                       protons, or the same atomic number, but
                       having different numbers of neutrons, or
                       different atomic weights.

Thanks Jimbob, I will read that several times slowly, LOL and then I will understand what they are!! Kind of how I learn.. Thanks!!!
 

Offline chris

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Re: How did tectonic plates arise, and when?
« Reply #9 on: 28/12/2006 13:27:18 »
Thanks for your answer Jim, but I'm not sure you've completely addressed my questions. I had asked why tectonic plates formed in the first place? Also, what drives their migration / movement? Do all rocky planets have tectonics?

Chris
 

Offline JimBob

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Re: How did tectonic plates arise, and when?
« Reply #10 on: 29/12/2006 03:34:15 »
Plates have formed as the earth has cooled ever since the earth started cooling. It is the very slow convection currents of mantle that drives the plate movements. (This is starting to be discussed again among earth scientists.)

Yes, there are planets that do not have plate tectonics - Mercury for sure, perhaps Venus for a long, long time geologically speaking. Mars probably has but also not for a long time but not as long as Venus. I may have the two mixed up. But then ..... [:o)]
 

Offline daveshorts

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Re: How did tectonic plates arise, and when?
« Reply #11 on: 01/01/2007 22:33:28 »
In my geophysics lectures (about 6 years ago) the impression I got was that the mantle convection is largely independent of the motion of the plates - upwellings in the mantle cause particularly hot parts of the mantle which produce loads of lava - like in Hawaii or Iceland, and can cause one plate to split  into two by melting the middle while it is in tension, but don't actually drive the motions themselves.

The motion is driven by a convection current involving the creation and destruction of crust.

basically:
Hot oceanic crust forms at the mid ocean ridges, if the pressure is released allowing the mantle to melt.
It then slowly cools and therefore gets more dense over a period of 60-70 million years, until it reaches a subduction zone where it is dense enough to sink in the mantle, which it does, pulling the  rest of the plate along with it.

You also get some effect of the plate surfing down the mid ocean ridge - but this force is smaller than the slab that is subducting pulling the plate at the other end.

The continents are made of much less dense rock that is very different from the mantly and just float around like a scum on the surface, getting pulled around and distorted by the oceanic plates they are attached to.
 

Offline paulat

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Re: How did tectonic plates arise, and when?
« Reply #12 on: 01/01/2007 22:46:32 »
Dave, I think that in future there will be a mass market for "Dave Ansell Science Cartoons", which critics will describe as "unique in their hastily scribbled yet colourful and effective mechanism of communcation"!
 

Offline daveshorts

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Re: How did tectonic plates arise, and when?
« Reply #13 on: 02/01/2007 00:27:52 »
Glad you like them, I hate describing things without diagrams.. which is why I wrote the diagram feature for the forum. One of these days I will get round to adding some features to it which will mean that they will be a little less scribbled ;)
 

Offline JimBob

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Re: How did tectonic plates arise, and when?
« Reply #14 on: 02/01/2007 03:44:07 »
In my geophysics lectures (about 6 years ago) the impression I got was that the mantle convection is largely independent of the motion of the plates - upwellings in the mantle cause particularly hot parts of the mantle which produce loads of lava - like in Hawaii or Iceland, and can cause one plate to split  into two by melting the middle while it is in tension, but don't actually drive the motions themselves.

Plates have formed as the earth has cooled ever since the earth started cooling. It is the very slow convection currents of mantle that drives the plate movements. (This is starting to be discussed again among earth scientists.)


Yes, construction and destruction of the plates themselves is now considered to be one valid reason by some geologists. There is also a school that believe plates and the upwelling of crustal material MAY be controlled by ephemeral (geologically) topographic ridges in the upper core. There is so little firm data that I didn't want to mention this reason or any of the other two or three other theories to cause confusion. I am leaning but by no means convinced, that this ridge possibility may be worth considering because it helps explain SOME crustal magnetic changes with time.

DIAGRARAM'S
1. Smoothing function for lines
2. Buy myself a plotting tablet

Oh well, next Christmas
 

Offline Karen W.

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Re: How did tectonic plates arise, and when?
« Reply #15 on: 02/01/2007 08:33:23 »
Glad you like them, I hate describing things without diagrams.. which is why I wrote the diagram feature for the forum. One of these days I will get round to adding some features to it which will mean that they will be a little less scribbled ;)

Hey Dave, I was playing with the diagram tool earlier in the chat 2007 new year thread, and I figured out the color and the posting, but it did another wierd thing this last round and sent the sript code for the piccy I drew into my post, but it failed to turn it into a picture.. I did another and it worked fine could you look at it and see if there is anyway to get it to open as the picture ??
 

Offline moonfire

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Re: How did tectonic plates arise, and when?
« Reply #16 on: 02/01/2007 15:59:52 »
Ditto Dave....it happened to me too...sniff, sniff...but i didn't get a piccy
 

Offline JimBob

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Re: How did tectonic plates arise, and when?
« Reply #17 on: 03/01/2007 03:00:01 »
Dave said on 01/01 (with diagram, of course)



In my geophysics lectures (about 6 years ago) the impression I got was that the mantle convection is largely independent of the motion of the plates - upwellings in the mantle cause particularly hot parts of the mantle which produce loads of lava - like in Hawaii or Iceland, and can cause one plate to split  into two by melting the middle while it is in tension, but don't actually drive the motions themselves.

Later:

The motion is driven by a convection current involving the creation and destruction of crust.

Edited By JB for this post. See above for original.





Dave, Dave, Dave. Had I been awake, not still main-lining coffee and also not watchiing the n+63 Bowl as I was typing, my trap-like mind may have caught this logic bomb. But as I didn't as it should have done, do I need to fall on my sword (or slide rule)?  Yes, I still have my K&E and NO it is NOT for sale. I would also rather not get the thing bloody, either and my sword is in the shop for a new gryphin-skin custom made grip.

I just have no Idea what to do but to chastize you at the point is like beating a dog after she has gotten the crown roast off the dining table, gotten it to the garden and hidden long enough to eat the whole thing and take a 4 hour nap.

Suggestions from you - or public????????
« Last Edit: 03/01/2007 03:10:24 by JimBob »
 

Offline daveshorts

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Re: How did tectonic plates arise, and when?
« Reply #18 on: 03/01/2007 10:28:58 »
Ok the crustal convection current does involve a return path in the mantle, but there appears to be a seperate mantle convection current system from deep mantle to the surface which produces hot spots, and this convection system doesn't appear to drive the plates around. The second type of current is what is normally meant by convection currents in the mantle,
« Last Edit: 03/01/2007 10:35:30 by daveshorts »
 

Offline JimBob

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Re: How did tectonic plates arise, and when?
« Reply #19 on: 03/01/2007 20:30:51 »
Hi Dave,

I wish I could be concise and give a simple answer that would be as elegant as E=MC2, but alas, our state of knowledge of the deep earth - below 40 Km - is surprisingly vague and the state of the art in this field is finger painting. Were I to be fair to all the good theories I would easily exceed the 20,000 character max post size and leave the first theory unfinished.

The simple answer to your question is there is only one set of convection currents but we do not fully understand how this works .

The fortunate part is that in the borderlands near the Pictish frontier there exists a place of learning named "University of Durham" where resides the web site http://www.dur.ac.uk/maple.plumes/index.html. Curmudgeons from all over the world post to this site arguing over the width of a quartz crystal so there is a wide field of views to explore.

I wish I could get down and dirty on this question - answer it with the differential equations included - but since this process of plate tectonics is not directly observable, even if you had an asbestos suit, it would not be as well understood as "Sediment load transport during tidal fluctuations in a shallow estuarine environment".

Start with the "Definition of a Plume" and explore from there. Since some of the eccentric gentelmen and gentelwomen, this might make a good topic for a radio broadcast.

On the 29th of last month I tried to warn you all but - well, it seems a prophet is never honoured in his own land - "Plates have formed as the earth has cooled ever since the earth started cooling. It is the very slow convection currents of mantle that drives the plate movements. (This is starting to be discussed again among earth scientists.)"   
Some understatement that.


(Some perspective: it was about 29 or 30 years ago that the last paper I saw rejecting the idea of Continental drift was published.) This subject is still in it's infancy.
« Last Edit: 03/01/2007 20:35:19 by JimBob »
 

Offline daveshorts

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Re: How did tectonic plates arise, and when?
« Reply #20 on: 03/01/2007 22:56:01 »
I can imagine that the guy who was lecturing me was giving a view of how the system works that was simplified towards his own theories to the poor little physicists. I have heard of his tendancies in this general direction.

It looks like it is horribly complex and we are getting to the point where there isn't really enough data in a single planet at one time to understand much better what is going on....
 

Offline JimBob

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Re: How did tectonic plates arise, and when?
« Reply #21 on: 04/01/2007 00:17:40 »
This is also a reflection of how accretion of accelerated learning in any one subject area generates too much data to be explained away simply and/or rigorously examined. Enough data has been accumulated on this one subject by measuring the P-wave, strength, path and velocity from every seismic event over the last 3 years with better recording equipment to keep a team odd 50 grad students busy and earn them a doctorate. But there are not that many interested in this problem, and there is certainly not enough money.
 

Offline Karen W.

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Re: How did tectonic plates arise, and when?
« Reply #22 on: 04/01/2007 15:04:56 »
When the plates shift here where I am, am I on the upper overlaping plate or the lower plate.. Will I be sliding off into the ocean or will the ocean floor be coming up over me. See previous post for me location on the two fault lines..:)
« Last Edit: 04/01/2007 15:11:19 by Karen W. »
 

Offline JimBob

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Re: How did tectonic plates arise, and when?
« Reply #23 on: 05/01/2007 00:04:29 »
Well, a little clarification of the setting: "In the Pacific Northwest, the Juan de Fuca Plate plunges beneath the North American Plate; locally melting at depth the magma rises to feed and form the Cascade volcanoes."

Basiclly, you are in deep caca for earthquake activity.



As you will no doubt note, the Juan de Fuca plate is sliding under the the North American Plate. But where you live, at the boundary of the Pacific and Juan de Fuca plates, the Juan de Fuca plate is also sliding against (not under) the North American plate and this truncates the San Andreas Fault and the Pacific plate. At the Pacific plate boundry, the relative movement on the ocean side of the coast is to the north. All of the plates are in collision at or near Petrolia. If there is an earthquake near you, you will be shaken to bits BUT the good news is that you will not be subducted.
 

Offline FuzzyUK

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How did tectonic plates arise, and when?
« Reply #24 on: 25/01/2007 14:46:14 »
"How did tectonic plates arise, and when?"

There is a series running everyday this week on UK TV History, Freeview CH12:

'Earth Story', BBC Learning Channel, presented by Aubrey Manning:
http://uktv.co.uk/index.cfm/uktv/History.item/aid/528223

Needless to say Cambridge based Dan McKenzie who pioneered the concept of tectonic plate movements is interviewed and expands on the mechanisms involved.
« Last Edit: 25/01/2007 15:03:11 by FuzzyUK »
 

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How did tectonic plates arise, and when?
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