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Tectonic activity | Middle school Earth and space science | Khan Academy — Transcript

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0:00Have you wondered where mountains come
0:01from or what causes volcanoes or how do
0:05entire islands appear in the middle of
0:08an ocean and what causes earthquakes?
0:11These all seem like very different
0:14questions but a lot of them are caused
0:17by the movements of tectonic plates. So,
0:20let's explore how these moving plates
0:22shape our planet. But before we begin,
0:25what exactly are tectonic plates you
0:27ask? Well, let's quickly review the
0:29Earth's layers. Remember that the
0:31outermost rigid layer of the Earth is
0:33called the lithosphere. It includes the
0:36crust along with the rigid upper part of
0:40the mantle. But the lithosphere is not
0:42one giant shell. It is broken into many
0:45large pieces called tectonic plates. So,
0:49think of tectonic plates as giant
0:51fragments of lithosphere and these are
0:54moving. What makes them move? Well, the
0:58heat from deep inside the Earth makes
1:00the material in the mantle slowly move
1:03in circulating fashion which we call
1:06convection currents. This is a very
1:08simplified model and scientists are
1:10still debating on how exactly these
1:12currents are but here's what's
1:14important. Since just below the
1:16lithosphere we have a softer layer
1:20called the asthenosphere which can move,
1:23the convection currents makes the
1:25asthenosphere slowly flow like hot
1:28plastic. And this flowing asthenosphere
1:32takes the fragments of the lithosphere
1:33about, the tectonic plates about, for a
1:36ride. That's what makes the tectonic
1:39plates move. Okay, end of recap and if
1:43you need more clarity we've talked about
1:45this in great detail in our previous
1:47video on plate tectonics. So, feel free
1:49to check that out. But what's important
1:50is these tectonic plates can move in
1:52many ways but there are three main types
1:55of movements. We have plates that are
1:58moving away from each other producing
2:00what we call the divergent boundaries.
2:03Then we have the plates moving towards
2:05each other producing what we call the
2:06convergent boundaries and then we have
2:08plates moving parallel to each other
2:10producing what we call the transform
2:13boundaries. Let's start with divergent
2:16boundaries. This happens when plates are
2:18moving away from each other, right?
2:20Okay, what happens? Well, as they do
2:23they create space in between them
2:25exposing the asthenosphere below. So hot
2:27materials can rise up and melt as magma
2:31and then eventually ooze out as lava
2:33which solidifies to form new crust and
2:37ends up forming a ridge.
2:40Since this is constantly happening, new
2:42crust is being produced at the center
2:45and then moving away from it. We call
2:46this the seafloor spreading.
2:49So we can expect a lot of magma and
2:52volcanoes near the divergent boundaries.
2:55A famous example of this is found in the
2:57Atlantic Ocean. One half of the Atlantic
2:59seafloor is moving this way towards
3:02North America. The other half is moving
3:04this way towards Europe and Africa
3:06causing a divergent boundary. As a
3:08result, we have this massive underwater
3:11mountain chain which we call the
3:12Mid-Atlantic Ridge.
3:15And that passes through Iceland. So if
3:18you go there in the Thingvellir National
3:20Park, you can actually walk over pieces
3:23of Earth that are moving away from each
3:26other which is pretty awesome.
3:28Now most of these divergent boundaries
3:30are found under the ocean meaning when
3:32ocean floors are moving away from each
3:34other. But they can also happen on land.
3:36One example of this is the East African
3:39Rift. Africa is being slowly ripped
3:42apart. And near that boundary, you have
3:45the crust sinking below causing the rift
3:49valley. And so again we have magma
3:51rising up so we can expect volcanoes
3:53over here. But, what's more interesting
3:55is we can predict the future. As time
3:58passes by, the plates move further and
4:00further apart, and so the mass sinks
4:02further and further down. Eventually,
4:05ocean water will start filling in. This
4:09is how new ocean basins are formed. And
4:12after millions of years, we will have
4:15completely new oceans. And that boundary
4:19now will be a new mid-oceanic ridge. And
4:23so, future humans might talk about how
4:25this was still part of Africa long time
4:27ago. Now, let's talk about convergent
4:29boundaries, when plates move into each
4:32other. Now, what happens here depends on
4:34which plates we're dealing with. If you
4:36have two thick continental plates moving
4:39and crashing into each other, the crust
4:41just crumples and gets pushed upwards
4:44over millions of years, forming giant
4:47mountain ranges. The most famous example
4:50is the Indian plate moving into the
4:53Asian plate, causing the Himalayas.
4:57But, notice we wouldn't expect any
4:58volcanoes because there are no magma
5:00rising up over here, right? So,
5:02Himalayas don't have any volcanoes.
5:04Okay, but what do you think would happen
5:06if an oceanic plate was moving into a
5:10continental plate? Well, now we need to
5:12think about their densities. See,
5:14oceanic plates are more dense compared
5:17to continental ones. What this means is
5:19that if you were to take equal volumes
5:21of both the plates, then the oceanic
5:23plates would be heavier. They would have
5:24more mass. That's what density is. It's
5:26a measure of how much mass you have per
5:28unit volume. So, because the oceanic
5:30plates are denser, and couple with that
5:32with the fact that they are thinner,
5:34when these two collide, the oceanic
5:36plates sink in. This process is called
5:40subduction. And this region is called
5:43the subduction zone. And several
5:46dramatic things can happen over here.
5:48First of all, the crust can crumple and
5:50form mountains. But secondly, as the
5:52denser plate subducts, it carries some
5:55water below, and that water causes the
5:59mantle to melt and rise up as magma and
6:04causes volcanoes.
6:07A famous example of this is the Andes.
6:10The South American plate is moving into
6:13an oceanic plate called the Nazca plate.
6:15This creates the Andes mountain ranges,
6:18which also has volcanoes.
6:20But that's not all. Due to the
6:22subduction, the ocean floor close to the
6:25boundary is much deeper than farther
6:27away. In other words, we have a trench.
6:29So, we would also expect a trench over
6:32here. And we have one, the Peru-Chile
6:35Trench.
6:36But there's even more. Because the two
6:39plates are grinding into each other,
6:41there's a lot of friction, and it's
6:43possible that sometimes the plates can
6:46get stuck. But they are still moving
6:49into each other, which means gradually
6:51stress builds up. And then suddenly, one
6:54day, it would slip and release a lot of
6:59built-up energy, causing earthquakes.
7:02So, we would also expect this region to
7:04be prone to earthquakes. And it is. One
7:07of the highest recorded earthquakes is
7:09in Chile. All right, but what if you
7:11have two oceanic floors crashing into
7:13each other? Well, something very similar
7:16happens. Again, the one which is denser
7:19will undergo subduction, and so again,
7:22you will have hot magma rising up,
7:24causing volcanoes. This time, you will
7:26have volcanic islands being formed. A
7:28classic example of this is the
7:30Philippine plate moving into the Pacific
7:33plate. Notice both are oceanic plates
7:35over here. And as a result, we can
7:36expect some volcanic islands over here
7:39at the convergent boundary. And there
7:41are. The Mariana Islands are just that.
7:44And again, we can expect a trench
7:47somewhere over here, the Mariana Trench,
7:50the deepest trench in the world. And
7:52just like before, we can expect a lot of
7:54earthquakes over here. Okay, before
7:56moving on, if you're wondering whether
7:58the Hawaiian Islands were also formed
8:00this way, no, we will get to that in a
8:02second, okay? But now, moving on, the
8:04last kind of boundary is the transform
8:06boundary. This happens when plates are
8:09moving sort of parallel to each other. A
8:11cool example of this is in the west
8:13coast of North America. See, the entire
8:16North America does not lie on a single
8:18plate. A small sliver of it lies on the
8:21Pacific plate. So, San Francisco, for
8:23example, lies on the North American
8:25plate, but Los Angeles, for example,
8:27lies on the Pacific plate. And these two
8:30plates are moving parallel to each
8:32other, producing a transform boundary.
8:34So, you can expect the Earth to fracture
8:37along this boundary, which we call
8:38faults. A famous one is the San Andreas
8:42Fault. This time, you wouldn't expect
8:44any mountains or volcanoes. But, again,
8:47as the plates are grinding against each
8:49other due to a lot of friction and heat,
8:51the plates can get stuck. And so, energy
8:55and stress gets built up, and eventually
8:57when it becomes too great, the plates
9:00suddenly slip. And this can release
9:03enormous amounts of energy, causing
9:06earthquakes. So, you definitely expect a
9:08lot of earthquakes near your transform
9:10boundaries. All right. So, a lot of
9:12interesting things happen near the
9:14boundaries of the two tectonic plates
9:16moving, right? But now, let's talk about
9:19the Hawaiian Islands. These are also
9:21volcanic islands, but guess what? These
9:23islands are nowhere near any boundaries
9:26of the tectonic plates. So, how did
9:28these volcanic islands form?
9:31Well, sometimes hot plumes of magma rise
9:36upwards from deep inside the earth. We
9:39call them mantle plumes. Think of them
9:41as superheated rocks rising upwards.
9:45They produce abnormally hot regions just
9:48under the lithosphere. We call them
9:50hotspots.
9:51What do they do? Well, that depends upon
9:54where exactly they form. If they are
9:56formed under the oceanic crust,
9:59the hot material can melt through the
10:02thin oceanic plate rising up as magma
10:05and erupt as lava. The lava cools and
10:08solidifies to form new crust and the
10:10process can repeat over and over again
10:15to form volcanic islands. But, remember
10:19the lithosphere is constantly moving.
10:22So, eventually the island gets carried
10:25away from the hotspot and then the whole
10:29process can repeat creating a new
10:33island.
10:34This is how this hotspot can create a
10:38chain of islands. The youngest island
10:41would be close to the hotspot and it
10:42would be volcanically active. The older
10:45ones would be farther away. They would
10:47be less active or dormant. This is how
10:50the Hawaiian Islands are formed. There's
10:52a hotspot over here. So, this is the
10:55youngest island. It's pretty
10:56volcanically active and these are the
10:59oldest ones. They are pretty dormant.
11:01Finally, what if the hotspot was under a
11:03continental crust? Well, just like
11:06before, the magma can melt through the
11:07crust. But, this time the crust is
11:09thick. There's a lot of pressure from
11:11the above. So, it's very hard to melt
11:14through the whole thing. So, it's
11:15possible that the magma just keeps
11:17accumulating, just keeps building up.
11:20This produces what we call a
11:22supervolcano.
11:24This can also happen over subduction
11:26zones, okay? But, what makes a
11:28supervolcano super is that the magma
11:30just keeps accumulating for tens of
11:33thousands or even millions of years
11:36below. As it accumulates, the pressure
11:39builds up and eventually, one day, the
11:41whole thing erupts. These eruptions can
11:44be so catastrophic, they can change
11:47climates or even cause extinction
11:49events. After such an enormous eruption,
11:52a lot of pressure is released, and so
11:54now the upper chamber collapses under
11:56its own weight, forming a giant
11:58bowl-shaped depression called caldera.
12:02And what's interesting is that this
12:03process can repeat at that same spot.
12:06So, calderas are telltale signs of
12:09supervolcanoes and past eruptions. And
12:12these calderas are also known for still
12:14active volcanic activities, such as
12:16geysers, fumaroles, and magma flows. An
12:19example of this is the Valles Caldera in
12:22New Mexico. This is an aerial footage by
12:24NASA. This caldera is about 20 km wide.
12:28Another famous example is the
12:30Yellowstone National Park. It is sitting
12:32on a giant supervolcano. By the way,
12:35this is not the caldera, these are just
12:37the hot springs. The caldera is huge.
12:40It's hard to identify today because it's
12:42filled with roads and trees and
12:44buildings. But if it erupts again, it
12:46can be catastrophic. But don't worry,
12:49the last time it erupted was more than
12:51600,000 years ago, and we are closely
12:53monitoring it today, and we don't think
12:55it's going to erupt for another at least
12:5710,000 years, at least. So, long story
13:00short, moving plates can create
13:03boundaries. When they move away from
13:04each other, they create divergent
13:06boundaries, which create ridges and
13:09rifts, and you can have volcanoes and
13:11earthquakes over there. Convergent
13:13boundaries are when plates move into
13:15each other. The denser plate usually
13:18undergoes subduction. It can cause the
13:20mantle to melt into magma, rising up,
13:24creating volcanoes and volcanic islands.
13:27They also produce deep trenches and they
13:30have a lot of earthquakes. And when
13:32plates move parallel to each other, they
13:34produce transform boundaries. You don't
13:37expect volcanoes or mountains usually
13:39over here, but you can have earthquakes.
13:42But even far away from boundaries,
13:44interesting thing can happen because of
13:46hot spots. They can create a chain of
13:49islands if they are formed below an
13:51oceanic crust. And if they are formed
13:53under a continental crust or subduction
13:55zones, then magma can build up for
13:58millions of years forming
13:59supervolcanoes.
14:01I find it incredible how lifeless moving
14:04tectonic plates can actually be so
14:07lively on geological scales.

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