Understanding Convergent Plate Boundaries

Convergent boundaries. Convergent plate boundaries are boundaries where two plates come together. There are three different types of convergent boundaries depending on which type of lithosphere is on either of the plates that are converging, whether it's oceanic or continental. The first type is an ocean-continent convergent boundary, where oceanic lithosphere is converging with continental lithosphere. Now remember that the oceanic crust is thin, but it's very dense because it contains a lot of heavier elements like iron, as opposed to the continental crust and the continental lithosphere, which is much, much thicker, but buoyant or low in density. So when a dense thin oceanic plate comes together with a continental plate, the more dense oceanic plate is going to dive down beneath the continental plate and make its way down into the asthenosphere. This process is called subduction. right? So when oceans and continents converge, the oceanic plate will always be the one that subducts because it is higher in density. Now the subducting plate, when it makes its way down into the hot, weak asthenosphere, when it gets to somewhere around 150 to 200 kilometers in depth below that continent, there's some melting that's going to happen. So this is again an example of partial melting, like what we talked about when we talked about what happens at the divergent boundaries. But here the reason for the partial melting is different. This oceanic plate, now remember that the oceanic crust is forming out at the mid-ocean ridge, at the bottom of the ocean. And so the rocks of the oceanic crust contain water in them. So some of the minerals will take water into their structure and there's a layer of sediment over the top of the oceanic crust that is full of water. So when this oceanic plate makes its way over to the convergent boundary here with the continent, and as it makes its way down into the asthenosphere, when it gets down to the depth of about 150 to 200 kilometers, a lot of that water that was in that oceanic plate starts to be released. Okay, so you can kind of think of it as the water starts to get squeezed out of the oceanic plate as it is going down into the asthenosphere. So that water is lighter than the rock around it, and so the water starts to rise into the mantle just above that subducting plate. This causes the mantle... to partially melt. But this style of partial melting is called hydration melting. Because in this case, the... Melting temperature of the mantle is being lowered by the addition of water to the mantle. Okay. So by adding that water from the subducting plate into that hot asthenosphere above the plate, it allows it to melt at a lower temperature than it normally would if there was no water in the system. Okay. So remember that at divergent boundaries. The partial melting is decompression melting due to the plates pulling apart from each other. In the case of subduction zones, it's due to hydration melting, due to the release of water from an oceanic subducting slab. into the mantle above, lowering the melting temperature and allowing magma formation. Okay, so coming back to this diagram. So we're going to have melting occurring above the subducting slab. And it's important to note that it's not the slab itself that is melting, it's the asthenosphere above it due to that water moving up into the asthenosphere. So what's going to happen here is you're going to start making magma. And the magma, because it's a liquid, it's less dense than the solid mantle around it. And so it starts to move up towards the surface. So some of it may pool at the base of the crust, like you see here. Some of it might make its way into the crust and then cool and crystallize. That's what these are showing. So these are called plutons, essentially magma that gets... Stuck inside the crust and then cools very slowly over a long period of time. This forms rocks like granite But some of that magma is going to be able to make its way all the way through the crust, eventually erupting out onto the surface, forming a whole series of volcanoes. And that is called a continental volcanic arc. It's an arc of volcanoes because it tends to have a kind of a curved shape. It's going to follow all along this subduction zone. And you're going to have a whole series of volcanoes on the continent due to the subduction and the partial melting that's happening down here. The other feature to notice here before we move on is this oceanic trench. So this is the place where, because you have the contact of the two plates right here, that subducting oceanic plate kind of starts to pull that leading edge of the continental plate down and it makes a very deep trench all along the edge of that continent. So an example of a series of continental arc volcanoes are the Andes Mountains, as seen here. So here is South America. So if you remember, the little teeth mean that it is a convergent boundary. So in the case of South America, you have the Nazca Plate, which is converging with the South American Plate and forming subduction zones all along the coast. So here, these are all active volcanoes, active volcanoes all along. due to that process that we just discussed of the subduction. And here you can also see if you look kind of just off the coast of South America that really deep blue line that is the oceanic trench right off the coast that follows along this entire subduction zone. So it's called the Peru-Chile Trench here. This animation shows an ocean continent convergent boundary or a subduction zone. So you have the oceanic plate which is subducting beneath the the continental lithosphere. You can see that it is showing the release of water from that subducting plate once it gets down into the asthenosphere, which causes partial melting due to hydration. And that magma makes its way up, up through the crust, eventually erupting out in a whole chain of continental arc volcanoes. The second type of convergent boundary is an ocean-ocean convergent boundary, where two oceanic plates are converging together. Now in this case we also develop a subduction zone, but since both of these are thin oceanic plates, Which one subducts? So whichever one is more dense, because remember, the density is what's driving the plate to be able to subduct down into the mantle. So whichever one of these oceanic plates is more dense is the one that's going to subduct. That is usually going to be the one that is older. The reason for that is that the older the crust is, the longer it has had to cool since it was formed at a mid-ocean ridge. And colder rocks... are more dense than warmer rocks. So whichever one is older is going to be the plate that is more dense and thus will be the plate that is subducted. Now the scenario besides that is very similar to the ocean-continent convergent boundary that we just discussed. Okay, so here we have our older, more dense oceanic plate subducting down at about 150 kilometers of depth. We have hydration melting, so the same exact process that we discussed before where water is being released from the oceanic crust. That water lowers the melting temperature of the mantle over the top of the slab and allows it to partially melt. In this case the magma is going to be able to make its way up but it doesn't have have that thick continental crust that it has to get through now. It just has the much thinner oceanic lithosphere and so it's going to end up making its way up. making its way through the thin oceanic crust and starting to erupt as lava on the ocean floor, building up layer after layer of lava flows, forming volcanoes. So in this case, a volcanic island arc is formed because you have the ocean here. And so your volcanoes are forming on oceanic crust. And as they grow, as they continue to erupt, they get larger and larger and eventually become Islands above sea level. You'll notice that we also have a trench in this case. Okay, so just same as with the ocean continent scenario where you've got the two plates coming together, this edge gets dragged down and makes a deep ocean trench. So places where we can see volcanic island arcs, the entire Western Pacific is dominated by these. All right. So here we have our plate map. So this is, again, the teeth means all convergent boundaries. Right. All through the Pacific, the Western Pacific. And here the red dots are active volcanoes. All right. So New Zealand is an island arc. You have Indonesia, the Philippines and then Japan to the north. OK, so all of these places are. island arc volcanoes that are due to the subduction of oceanic crust beneath oceanic crust. We can also see this in North America, where the Pacific plate is diving or subducting underneath the North American plate in this region up here. So the Aleutian Islands are a chain of volcanic islands that are formed due to the subduction of the oceanic Pacific plate underneath the North American plate, which in this region is oceanic crust. Okay, so you've got an ocean-ocean convergent boundary right in here, and that is forming the Aleutian Islands, which are an island arc of volcanoes. Okay, the third and final type of convergent boundary is a continent-continent convergent boundary, where you have two thick slabs of continental lithosphere that are converging together. together. In this case, something different happens. And a good example is what happened when the subcontinent of India made its way north after Pangea broke up and converged into the Eurasian plate. So in this diagram here we're looking at a time period before the two continents actually converged. So between 30 to 50 million years would be where this scenario is, where India was on a plate but it was surrounded with oceanic crust on the same plate. So in this time period here you would have had active subduction. So an active ocean-continent subduction zone occurring here with hydration melting happening under the continent and a continental volcanic arc forming on the edge of the Eurasian Plate. But move forward 20 million years or so, and the two continents finally meet each other. So what happens there is that, so this is kind of the remnant of the oceanic crust here. So... So once the buoyant, thick continental plate here starts to kind of dive under, eventually subduction is going to turn off. And the reason for that is, is that this plate is too low in density. to make its way down into the much more dense asthenosphere below. It just is not going to be able to subduct. And so subduction turns off, which means that the partial melting stops and the volcanism would cease through here. What does happen, though, is that essentially all of these faults, these are all faults that are shown, these lines here, all of these faults are forming. And essentially what's happening is the land is being pushed upwards. Right. This forms the Himalayan mountain range. And this is referred to as orogenesis or mountain building. So here we see the current position of India. Right. This about again, about 10 million years ago is when they when the two continental. lithospheres actually collided with each other. And it's kind of like this, where you've got two solid objects where neither one is going to be able to go underneath the other and things move upward instead. So that forms the highest mountain range on Earth and the thickest crust on Earth. The Indian plate continues to move to the north. So this motion has not stopped. It continues. And so the Himalayan mountain range is continuing and it continues. to grow larger. This animation demonstrates the convergent boundary between the Indian plate and the Eurasian plate. Before the Indian plate made its way to contact the Eurasian plate, there was active subduction. You can see in the animation how the subduction turned off and the land surface built up as the two buoyant continental plates collided. Finally, before we move on to talking about transform boundaries, I just want to make this point that anywhere that there is a divergent boundary where new crust is being formed, there must be a matching convergent boundary where crust is being destroyed or recycled back into the mantle. And we know that this is true because the Earth is not getting any larger. right? The circumference of the earth is staying the same. So divergent boundaries are considered to be constructive boundaries, meaning that crust is constructed there. Convergent boundaries are considered to be destructive. plate boundaries because that is where a crust is being destructed or destroyed or recycled back into the mantle. Since the earth is not changing size, any formation of new crust has to be matched somewhere else by the destruction of crust at a convergent boundary.

The first type is an ocean-continent convergent boundary, where oceanic lithosphere is converging with continental lithosphere. Now remember that the oceanic crust is thin, but it's very dense because it contains a lot of heavier elements like iron, as opposed to the continental crust and the continental lithosphere, which is much, much thicker, but buoyant or low in density. So when a dense thin oceanic plate comes together with a continental plate, the more dense oceanic plate is going to dive down beneath the continental plate and make its way down into the asthenosphere. This process is called subduction. right?

So when oceans and continents converge, the oceanic plate will always be the one that subducts because it is higher in density. Now the subducting plate, when it makes its way down into the hot, weak asthenosphere, when it gets to somewhere around 150 to 200 kilometers in depth below that continent, there's some melting that's going to happen. So this is again an example of partial melting, like what we talked about when we talked about what happens at the divergent boundaries.

But here the reason for the partial melting is different. This oceanic plate, now remember that the oceanic crust is forming out at the mid-ocean ridge, at the bottom of the ocean. And so the rocks of the oceanic crust contain water in them.

So some of the minerals will take water into their structure and there's a layer of sediment over the top of the oceanic crust that is full of water. So when this oceanic plate makes its way over to the convergent boundary here with the continent, and as it makes its way down into the asthenosphere, when it gets down to the depth of about 150 to 200 kilometers, a lot of that water that was in that oceanic plate starts to be released. Okay, so you can kind of think of it as the water starts to get squeezed out of the oceanic plate as it is going down into the asthenosphere.

So that water is lighter than the rock around it, and so the water starts to rise into the mantle just above that subducting plate. This causes the mantle... to partially melt. But this style of partial melting is called hydration melting.

Because in this case, the... Melting temperature of the mantle is being lowered by the addition of water to the mantle. Okay. So by adding that water from the subducting plate into that hot asthenosphere above the plate, it allows it to melt at a lower temperature than it normally would if there was no water in the system. Okay.

So remember that at divergent boundaries. The partial melting is decompression melting due to the plates pulling apart from each other. In the case of subduction zones, it's due to hydration melting, due to the release of water from an oceanic subducting slab. into the mantle above, lowering the melting temperature and allowing magma formation. Okay, so coming back to this diagram.

So we're going to have melting occurring above the subducting slab. And it's important to note that it's not the slab itself that is melting, it's the asthenosphere above it due to that water moving up into the asthenosphere. So what's going to happen here is you're going to start making magma. And the magma, because it's a liquid, it's less dense than the solid mantle around it.

And so it starts to move up towards the surface. So some of it may pool at the base of the crust, like you see here. Some of it might make its way into the crust and then cool and crystallize. That's what these are showing.

So these are called plutons, essentially magma that gets... Stuck inside the crust and then cools very slowly over a long period of time. This forms rocks like granite But some of that magma is going to be able to make its way all the way through the crust, eventually erupting out onto the surface, forming a whole series of volcanoes. And that is called a continental volcanic arc. It's an arc of volcanoes because it tends to have a kind of a curved shape.

It's going to follow all along this subduction zone. And you're going to have a whole series of volcanoes on the continent due to the subduction and the partial melting that's happening down here. The other feature to notice here before we move on is this oceanic trench. So this is the place where, because you have the contact of the two plates right here, that subducting oceanic plate kind of starts to pull that leading edge of the continental plate down and it makes a very deep trench all along the edge of that continent.

So an example of a series of continental arc volcanoes are the Andes Mountains, as seen here. So here is South America. So if you remember, the little teeth mean that it is a convergent boundary. So in the case of South America, you have the Nazca Plate, which is converging with the South American Plate and forming subduction zones all along the coast.

So here, these are all active volcanoes, active volcanoes all along. due to that process that we just discussed of the subduction. And here you can also see if you look kind of just off the coast of South America that really deep blue line that is the oceanic trench right off the coast that follows along this entire subduction zone. So it's called the Peru-Chile Trench here. This animation shows an ocean continent convergent boundary or a subduction zone.

So you have the oceanic plate which is subducting beneath the the continental lithosphere. You can see that it is showing the release of water from that subducting plate once it gets down into the asthenosphere, which causes partial melting due to hydration. And that magma makes its way up, up through the crust, eventually erupting out in a whole chain of continental arc volcanoes.

The second type of convergent boundary is an ocean-ocean convergent boundary, where two oceanic plates are converging together. Now in this case we also develop a subduction zone, but since both of these are thin oceanic plates, Which one subducts? So whichever one is more dense, because remember, the density is what's driving the plate to be able to subduct down into the mantle.

So whichever one of these oceanic plates is more dense is the one that's going to subduct. That is usually going to be the one that is older. The reason for that is that the older the crust is, the longer it has had to cool since it was formed at a mid-ocean ridge. And colder rocks...

are more dense than warmer rocks. So whichever one is older is going to be the plate that is more dense and thus will be the plate that is subducted. Now the scenario besides that is very similar to the ocean-continent convergent boundary that we just discussed.

Okay, so here we have our older, more dense oceanic plate subducting down at about 150 kilometers of depth. We have hydration melting, so the same exact process that we discussed before where water is being released from the oceanic crust. That water lowers the melting temperature of the mantle over the top of the slab and allows it to partially melt.

In this case the magma is going to be able to make its way up but it doesn't have have that thick continental crust that it has to get through now. It just has the much thinner oceanic lithosphere and so it's going to end up making its way up. making its way through the thin oceanic crust and starting to erupt as lava on the ocean floor, building up layer after layer of lava flows, forming volcanoes. So in this case, a volcanic island arc is formed because you have the ocean here.

And so your volcanoes are forming on oceanic crust. And as they grow, as they continue to erupt, they get larger and larger and eventually become Islands above sea level. You'll notice that we also have a trench in this case. Okay, so just same as with the ocean continent scenario where you've got the two plates coming together, this edge gets dragged down and makes a deep ocean trench.

So places where we can see volcanic island arcs, the entire Western Pacific is dominated by these. All right. So here we have our plate map. So this is, again, the teeth means all convergent boundaries.

Right. All through the Pacific, the Western Pacific. And here the red dots are active volcanoes. All right. So New Zealand is an island arc.

You have Indonesia, the Philippines and then Japan to the north. OK, so all of these places are. island arc volcanoes that are due to the subduction of oceanic crust beneath oceanic crust. We can also see this in North America, where the Pacific plate is diving or subducting underneath the North American plate in this region up here.

So the Aleutian Islands are a chain of volcanic islands that are formed due to the subduction of the oceanic Pacific plate underneath the North American plate, which in this region is oceanic crust. Okay, so you've got an ocean-ocean convergent boundary right in here, and that is forming the Aleutian Islands, which are an island arc of volcanoes. Okay, the third and final type of convergent boundary is a continent-continent convergent boundary, where you have two thick slabs of continental lithosphere that are converging together. together. In this case, something different happens.

And a good example is what happened when the subcontinent of India made its way north after Pangea broke up and converged into the Eurasian plate. So in this diagram here we're looking at a time period before the two continents actually converged. So between 30 to 50 million years would be where this scenario is, where India was on a plate but it was surrounded with oceanic crust on the same plate.

So in this time period here you would have had active subduction. So an active ocean-continent subduction zone occurring here with hydration melting happening under the continent and a continental volcanic arc forming on the edge of the Eurasian Plate. But move forward 20 million years or so, and the two continents finally meet each other. So what happens there is that, so this is kind of the remnant of the oceanic crust here. So...

So once the buoyant, thick continental plate here starts to kind of dive under, eventually subduction is going to turn off. And the reason for that is, is that this plate is too low in density. to make its way down into the much more dense asthenosphere below.

It just is not going to be able to subduct. And so subduction turns off, which means that the partial melting stops and the volcanism would cease through here. What does happen, though, is that essentially all of these faults, these are all faults that are shown, these lines here, all of these faults are forming.

And essentially what's happening is the land is being pushed upwards. Right. This forms the Himalayan mountain range.

And this is referred to as orogenesis or mountain building. So here we see the current position of India. Right. This about again, about 10 million years ago is when they when the two continental.

lithospheres actually collided with each other. And it's kind of like this, where you've got two solid objects where neither one is going to be able to go underneath the other and things move upward instead. So that forms the highest mountain range on Earth and the thickest crust on Earth.

The Indian plate continues to move to the north. So this motion has not stopped. It continues.

And so the Himalayan mountain range is continuing and it continues. to grow larger. This animation demonstrates the convergent boundary between the Indian plate and the Eurasian plate.

Before the Indian plate made its way to contact the Eurasian plate, there was active subduction. You can see in the animation how the subduction turned off and the land surface built up as the two buoyant continental plates collided. Finally, before we move on to talking about transform boundaries, I just want to make this point that anywhere that there is a divergent boundary where new crust is being formed, there must be a matching convergent boundary where crust is being destroyed or recycled back into the mantle.

And we know that this is true because the Earth is not getting any larger. right? The circumference of the earth is staying the same. So divergent boundaries are considered to be constructive boundaries, meaning that crust is constructed there. Convergent boundaries are considered to be destructive.

plate boundaries because that is where a crust is being destructed or destroyed or recycled back into the mantle. Since the earth is not changing size, any formation of new crust has to be matched somewhere else by the destruction of crust at a convergent boundary.

Transcript for:Understanding Convergent Plate Boundaries

Transcript for:
Understanding Convergent Plate Boundaries