1. Are ALL terranes on the continents coming from the ocean where they are scraped off in continental-oceanic subduction zones?

2. Is the "theory of microplate terranes" suggesting that ALL continents are a patchwork of terranes (from the ocean)?

3. I don't quite get the 3 underlined parts of this paragraph:
"The earth is a convecting system becuase broad upwellings of asthenosphere caused by the sinking of lithospheric plates (why is it sinking? so are we constantly getting closer to the core?) transport heat. Where the plates pull away from each other at the mid-ocean ridges, magma from the asthenosphere rises to the surface, where it cools to from new lithosphere. As the lithosphere moves away from the mid-ocean ridge, it cools and becomes more dense and begins to subduct into the asthenosphere. (how come the lithosphere suddenly subduct without any plate collision?) The downward pull of the subducting lithosphere is probably the main driving force for tectonic plate motion (is this whole paragraph describing the cause of plate motion as convection currents? I don't see how this describes the earth as a convecting system...it seems to me that this paragraph does not relate to convection currents in any way...)"

Can someone explain? I would appreciate!

I'm not an expert on this, but I think the convection refers to the movement of material in the Earth's mantle. The crust sits on top of that moving material, and so is carried along by the molten material.

1. Are ALL terranes on the continents coming from the ocean where they are scraped off in continental-oceanic subduction zones?

2. Is the "theory of microplate terranes" suggesting that ALL continents are a patchwork of terranes (from the ocean)?

3. I don't quite get the 3 underlined parts of this paragraph:
"The earth is a convecting system becuase broad upwellings of asthenosphere caused by the sinking of lithospheric plates (why is it sinking? so are we getting closer to the core forever?) transport heat. Where the plates pull away from each other at the mid-ocean ridges, magma from the asthenosphere rises to the surface, where it cools to from new lithosphere. As the lithosphere moves away from the mid-ocean ridge, it cools and becomes more dense and begins to subduct into the asthenosphere. (how come the lithosphere suddenly subduct without any plate collision?) The downward pull of the subducting lithosphere is probably the main driving force for tectonic plate motion (is this whole paragraph describing the cause of plate motion as convection currents? I don't see how this describes the earth as a convecting system...it seems to me that this paragraph does not relate to convection currents in any way...)"

Can someone explain? I would appreciate!

Yes, Kingwinner, it all relates very much to convection currents. You first need to understand that the crust is actually sitting on top of huge molten ball. As the crust cools, it becomes more dense (heavier per unit volume) than the liquid below and naturally sinks. It then continues to sink as warmer, less dense material around it rises. When it - the part that sank - becomes hot enough it will, in turn, begain to rise again. And that's the very classical form of convection currents. Just exactly like water in a container heated from below.

"The earth is a convecting system becuase broad upwellings of asthenosphere caused by the sinking of lithospheric plates "

But the upwellings of asthenosphere are not caused by the sinking (subducting?) of lithospheric plates at mid-oceanic ridges...the contients are rising instead of sinking there...

"why is it sinking? so are we constantly getting closer to the core?"

Earth is not collapsing into a black hole. "sinking" refers to the oceanic plates' subduction. You may have misinterpreted sinking of the 'lithospheric plates' as sinking of 'continental plates'.

"how come the lithosphere suddenly subduct without any plate collision?"

No, it must to collide at a plate boundary (it can be ocean to ocean, like the Philippine-Pacific boundary). It's true that the ocean crust sinks to a certain extent with age, but it doesn't subduct; the floating continents tow them under for subduction.
Speaking in a hypothetical sense, you probably would be right if the oceans just sank into the asthenosphere, but that assumes that there isn't any convection or thermal gradient in the earth. We don't have any evidence of a plate just sinking completely into the asthenosphere and having a large fissure zone replacing its area, since convection is so active that no ocean crust older than 180my old has ever been found to date.

"is this whole paragraph describing the cause of plate motion as convection currents?"

Yes. When they say downward pull, they're saying the force due to the sinking of the subducting plate is responsible for most plate motion. I'm not really sure on this, but let's just assume I take their word for it. That would mean that the plates would be exerting an overall tension that drags it along, where the mid-ocean ridges would just serve to just replace the 'lost' oceanic crust. It sort of makes sense if you think about it because sinking of a solid plate can pull the entire plate with it since there's enough elasticity near the region of the asthenosphere, and mid-ocean fissures can't really use any sort of magma erupting vertically to push it along. But ultimately this is cycle is preserved from the creation of new ocean crust from convection.

So the convection currents are not only happening in the asthenosphere, but in the lithospheric plates, too!

"The earth is a convecting system becuase broad upwellings of asthenosphere caused by the sinking of lithospheric plates transport heat. Where the plates pull away from each other at the mid-ocean ridges, magma from the asthenosphere rises to the surface, where it cools to from new lithosphere..."

But how can sinking (in this case they mean subducting!?) occurs near mid-ocean ridges which causes upwellings of magma? Mid-ocean ridges are divergent instead of convergent boundaries and subduction doesn't occur, convergent boundary should only exist in a huge distance away, would that still be able to causes magma to travel all the way to the mid-ocean ridges far away?...what causes the magma from the asthenosphere to upwell to the middle of the mid-ocean ridges?

I think that this is the general idea:

http://www.cas.muohio.edu/limpermuseum/InvestigateConceptsInGeology/_images/Forces%20that%20Cause%20PT.jpg

Which would mean IMO that the Core heat of the Earth is generating the convection current However I doubt if the subducting slabs are pulling. you can't pull at rocks. They will be teared apart, you have to push like the convection does. Question is why there are hotter and cooler spots at the core, which is required for convection to happen.

You might be right, Andre, about the stuff with convection directly having influence on plate motion. That's what I was doubting about the pull.

Thank you!

But how are the "broad upwellings of asthenosphere caused by the sinking of lithospheric plates"? Where is this occuring? (mid-ocean ridges? if so, how can the lithospheric plates be sinking (subducting) at a divergent boundary?)

"The earth is a convecting system becuase broad upwellings of asthenosphere caused by the sinking of lithospheric plates "

I think that this is a misleading bit of text. My understanding is that it should state that - the Earth is a convecting system becuase of broad upwellings of asthenosphere caused by the heat of the Earths core, and subduction of oceanic lithospheric plates are the cause of plate motion and oceanic spreading.

The subduction of the oceanic lithospheric plates is part of this convection system and is thought to pull the slabs along, as in a convection current the cooler oceanic lithospheric sinks exerting a pulling force on the plate. It is difficult to logically seperate cause and effect here.

Kingwinner, I do not agree with Facials answer to your question about subduction without collision
"how come the lithosphere suddenly subduct without any plate collision?"

No, it must to collide at a plate boundary As if there is a pulling force then as per Andre's comment you can't pull at rocks. They will be teared apart, this is exactly what happens. As the oceans get wider the stress on the oceanic plates gets greater and they can break, when they do then a new subduction zone is formed, without any collision. Of course, it may look like a collision at a later stage as the ocean crust that was previously subducting along an oceanic-continental convergence now attached to continental crust may be seen to be traveling in the opposite direction to the original oceanic crusts motion. Also, an island arc will begin to form and a new crust will form so it looks like a collision. I would prefer to rerserve the term collision for continental-continental convergence.
There is a theoretical limit as to how wide an oceanic lithospheric plate can be because of this. This is another reason why there is no oceanic crust older than 180my old (taking that figure from Andre's post).

So the lithospheric plates are part of the convection system, I see!

Thanks halucigenia!

Andre's post

GRRRRRRR. :mad: :mad:

But yes, I must concede to your input. Sorry about all the a priori stuff. Then again, halugigenia, how do you explain the humongous size of the Pacific plate? (it's a lot larger than most other plates in existence, maybe only eurasian plate can rival its size)

these plates are sliding around, right? way back they used to be stuck together....
Australia, and Antartica are about the same size and look the same too....makes me wonder if Australia used to be at a rotational axis, like Antartica is now??

does the iron in the rocks indicate anything in Australia formed at the axis point?

...and, er...(excuse my ignorance)... I even think of land and water as being opposite signs like magnetism, i.e. land is maybe a plus, and water a minus....so we have polar opposite to Antartica an ocean at the North Pole....if Australia were once
at the south axis of rotation....there IS a body of water of approximately equal size at its globally opposite location.

Sorry Facial, I did mean taking the figure from your post. :o

In response to your comment how do you explain the humongous size of the Pacific plate? I propose that the distance across the Pacific plate between North America and the Philippine plate is just about at the theoretical limit as to how wide an oceanic lithospheric plate can be. Though I do not have the Palaeogeographic data on the size of the ocean plates during the history of the Earth to back this up. Though if you insist that it’s wrong I guess I could look it up.
How do you explain the subduction between the Pacific plate and the Philippine plate otherwise?
If the answer is that it’s a collision, then just what was it that was between Pacific plate and the Philippine plate before they collided?

fuzzywuz, yes these plates are sliding around, and way back the currently existing continents used to be stuck together. However they were also split apart, different shapes, closer to the north pole, underwater, under ice etc. at different periods in geological history.

Anyway my point is that things do not have to be balanced so you don't have to have water opposite land, whether they are at the poles or not. Australia was close to the South Pole, along with Antarctica at about 150Ma.

Check out WebDoGS Virtual Plates (http://www.uky.edu/ArtsSciences/Geology/webdogs/plates/reconstructions.html) for some cool animations, or ODSN (http://www.odsn.de/odsn/services/paleomap/paleomap.html) for their sources.
or Palaeogeographic globes (http://jan.ucc.nau.edu/~rcb7/paleogeographic.html) for some cool maps.
It's wonderful what you can find on the www