Plate boundaries

TonyTT

Hi. I am confused about plate boundaries. In school we always drew diagrams where the continental crust sat on top of the oceanic crust. With that in mind, what is a collision boundary? Is this where continental crust sits on top of the mantle? How did India collide with Asia? Is the Indian sub-continent sat on top of oceanic crust and the approach to the Luarentia is a normal destructive plate boundary. If so, why are there no volcanoes in the Himalayas?

Simon Bridge

Keep that picture - now imagine that the entire crust (continental and oceanic - the whole thickness) has cracks in it that go vertically through from the surface to the mantle. The cracks all join up so the crust is divided into large slabs that float on the mantle. The slabs are the plates.

Since they float they can move about, and hit each other. Where they hit, they can can do one of three things: they can buckle upwards, buckle downwards, or one can slide under the other (they can also slide along each other while they do this).

Google "continental drift".

davenn

Hi Tony

welcome to PF

just a little test question for you to set the scene ....
Do you know why the continental crust sits above the oceanic crust in a collision zone ?

Simon's response about doing some info digging on continental drift is good and it will show you a lot about where the different continents started from.

But to get you started .... Gondwana consisted of several distinct continents as we see them today Antarctica, South America, Australia, Africa and India. With rifting that occurred along the "continental margins" those continents moved and are still moving northwards relative to Antarctica.
The Indian continent broke away from Antarctica ~ 220 ma (million years ago) and finally collided with eastern Asia. There wasnt any significant subduction as the Indian continent for the most part is continental crust rather than oceanic crust. Its full on frontal collision, (with only a small amount of subduction) that has caused the raising up of the Tibetan Plateau. If you would like an analogy --- think of the dirt that builds up in depth and width in front of a bulldozer blade, with India being the bulldozer. It is only the continuing collisional forces that are keeping the plateau up. If the collisional forces stopped ie. India stopped its northwards motion, then the Tibetan Plateau would start to subside until it reached isostatic equilibrium There's another term you can google ;)

Its the lack of significant subduction in the India - Asia collisional boundary that is the reason why there are no volcanoes in the Himalayas.

cheers
Dave

Mk

Is it true that the ocean plates tend to go down because they are lower and heavier than continental plates, both because they have water on them and because their minerals are more basaltic or full of magnesium and iron than minerals on continents (granitic and felsic)?

davenn

hi MK

yes thats the primary reason ... basaltic rocks are denser than continental rocks.
I have never thought about the effect of any of the mass of water sitting on top of the seafloor. That may also help with the subduction .... mite have to do some research on that one :)

Dave

billiards

The water on top thing :-- water is on top more as a consequence of the fact that ocean crust is lower, rather than a cause.

davenn

? that didnt really make any sense ?
we werent blaming the water for making the ocean floor lower in general... more so if it has an effect at the subduction zone in particular in aiding the subduction process.
I suspect it doesnt ... but its an interesting idea

Dave

Simon Bridge

... besides, effects can also be (contributary) causes.
I though you could have a subduction zone between two oceanic plates?

davenn

I was thinking about that earlier. ... say the Tonga - Kermadec Trench, where Pacific Plate oceanic crust is subducting under Australian Plate oceanic crust. I wonder what determined the way that subduction zone formed ? that is, was it just pure chance that the Pacific Plate went under the Australian Plate rather than the other way around ?

cheers
Dave

Simon Bridge

It may be - though I can imagine one end of a plate trying to go one way and the other trying to go another way - getting you a twist in the middle (maybe a tear and a new plate forming?) We are talking about a looong time here.

I always figured that the bit of crust that ends up on top by whatever means will tend to end up, over a long time frame, as a continental plate anyway.

davenn

actually I know of one place where thats happening, back in my home country of New Zealand. there's some seriously tectonics happening in the central NZ area.
As you come south down the Tonga - Kermadec Trench, it becomes the Hikorangi Trench off the east coast of the North Island of NZ. The Pacific Plate is subducting down under the Nth Is. and is what fuels the Taupo Vocanic Zone in the central Nth Is. This subduction continues further south and into the top end of the upper South Island.
Then there is a significant change in tectonics and the Pacific Plate changes from subduction to overriding the Australian Plate. It is this oblique slip motion that has given rise (pun intended) to the Southern Alps one of the youngest mountain ranges anywhere on earth at only ~ 5 million years old. This oblique slip motion has a 3:1 ratio, that is for every 3 metres of horizontal motion there is 1 metre of vertical motion. In a mere 5 million years there has been some 500km of horizontal motion aling the Alpine Fault ( the Pacific - Australian Plate boundary) running most of the length of the South Island.
To top all that off, as you leave the SW corner of the Sth Is., subduction again becomes the predominant tectonics BUT now its the Australain Plate subducting under Pacific Plate, a total role reversal!!! and this continues further south past Macquarie Island.



the above image shows that in a visual way
The next image shows the rotation poles that produce that motion and the magnitudes of the motion as it varies along the length of the plate boundary



in my next post I will put a couple of images showing the crossections across the South and North Islands

Dave

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davenn

OK the second part of the post
These are a couple of drawings I did years ago showing the crosssection views across the North Is and South Island of New Zealand. It shows the subduction under the Nth Is and the uplift of the Sth Is.






cheers
Dave

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billiards

opps

my mistake

btw. i think water does "lubricate" the subduction process.

billiards

Certainly. One of the most interesting examples I know of is that of the Molucca sea plate, an ocean plate being swallowed up from both sides. I guess the Earth is hungry!

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Simon Bridge

I see in the NZ example that the "twist" area occurs in a region of continental plate and there is oceanic plate subducting on either side of it.

I suspect that the simple models of plate tectonics being considered at the start of the thread are actually not quite up to handling the nuances of the actual situation.

BTW: small world: I live in Auckland NZ. Kiwi's seem to be more active online than the size of the country would indicate.

davenn

That may be true but it still makes for good discussion :)

ah hah for some reason I had you pegged as in the UK.
I'm originally from the Dunedin area, Edinbrugh of the South ;) I'm 4th gen Scot.
Did Geology at otago Univ.
Have been in Sydney for the last 12 yrs or so, this side of the Tasman isnt quite as geologically interesting

Dave

Simon Bridge

Over a dram or three aye ;)

kia ora.