Tectonic activity

0:00- [Instructor] Have you wondered where mountains come from or what causes volcanoes or how do entire islands appear in the middle of an ocean and what causes earthquakes? These all seem like very different questions, but a lot of them are caused by the movements of tectonic plates.

0:20So let's explore how these moving plates shape our planet. But before we begin, what exactly are tectonic plates, you ask? Well, let's quickly review the earth's layers. Remember that the outermost rigid layer of the earth is called the lithosphere. It includes the crust along with the rigid upper part of the mantle, but the lithosphere is not one giant shell.

0:44It is broken into many large pieces called tectonic plates. So think of tectonic plates as giant fragments of lithosphere, and these are moving. What makes them move? Well, the heat from deep inside the earth makes the material in the mantle slowly move in circulating fashion which we call convection currents.

1:07This is a very simplified model, and scientists are still debating on how exactly these currents are. But here's what's important: Since just below the lithosphere, we have a softer layer called the asthenosphere, which can move, the convection currents makes the asthenosphere slowly flow like hot plastic.

1:29And this flowing asthenosphere takes the fragments of the lithosphere above, the tectonic plates above for a ride. That's what makes the tectonic plates move. Okay, end of recap. And if you need more clarity, we've talked about this in great detail in our previous video on plate tectonics.

1:48So feel free to check that out. But what's important is these tectonic plates can move in many ways, but there are three main types of movements. We have plates that are moving away from each other, producing what we call the divergent boundaries. Then, we have the plates moving towards each other, producing what we call the convergent boundaries.

2:08And then, we have plates moving paddle to each other, producing what we call the transform boundaries. Let's start with divergent boundaries. This happens when plates are moving away from each other, right? Okay, what happens? Well, as they do, they create space in between them, exposing the asthenosphere below so hot materials can rise up and melt as magma, and then eventually ooze out as lava, which solidifies to form new crust and ends up forming a ridge.

2:40Since this is constantly happening, new crust is being produced at the center and then moving away from it. We call this the sea floor spreading. So we can expect a lot of magma and volcanoes near the divergent boundaries. A famous example of this is found in the Atlantic Ocean.

2:58One of the Atlantic sea floor is moving this way towards North America. The other half is moving this way towards Europe and Africa, causing a divergent boundary. As a result, we have this massive underwater mountain chain, which we call the Mid-Atlantic Ridge, and that passes through Iceland.

3:17So if you go there in the Thingvellir National Park, you can actually walk over pieces of earth that are moving away from each other, which is pretty awesome. Now, most of these divergent boundaries are found under the ocean, meaning when ocean floors are moving away from each other, but they can also happen on land.

3:36One example of this is the East African Rift. Africa is being slowly ripped apart. And near that boundary, you have the crust sinking below, causing the rift valley. And so again, we have magma rising up, so we can expect volcanoes over here. But what's more interesting is we can predict the future.

3:57As time passes by, the plates move further and further apart. And so, the mass sinks further and further down. Eventually, ocean water will start filling in. This is how new ocean basins are formed. And after millions of years, we will have completely new oceans, and that boundary now will be a new Mid-Oceanic Ridge.

4:23And so, future humans might talk about how this was still part of Africa long time ago. Now, let's talk about convergent boundaries when plates move into each other. Now, what happens here depends on which plates we are dealing with. If you have two thick continental plates moving and crashing into each other, the crust just crumples and gets pushed upwards over millions of years forming giant mountain ranges.

4:49The most famous example is the Indian plate moving into the Asian plate, causing the Himalayas. But notice we wouldn't expect any volcanoes because there are no magma rising up over here, right? So Himalayas don't have any volcanoes. Okay, but what do you think would happen if an oceanic plate was moving into a continental plate?

5:11Well, now we need to think about their densities. See, oceanic plates are more dense compared to continental ones. What this means is that if you were to take equal volumes of both the plates, then the oceanic plates would be heavier. They would have more mass, that's what density is.

5:26It's a measure of how much mass you have per unit volume. So because the oceanic plates are denser and coupled with the fact that they are thinner, when these two collide, the oceanic plates sink in. This process is called subduction. And this region is called the subduction zone.

5:45And several dramatic things can happen over here. First of all, the crust can crumple and form mountains. But secondly, as the denser plate subducts, it carries some water below and that water causes the mantle to melt and rise up as magma and causes volcanoes.

6:07A famous example of this is the Andes. The South American plate is moving into an oceanic plate called the Nazca plate. This creates the Andes mountain ranges, which also has volcanoes. But that's not all. Due to this subduction, the ocean floor close to the boundary is much deeper than farther away.

6:28In other words, we have a trench. So we would also expect a trench over here and we have one, the Peru-Chile trench. But there's even more. Because the two plates are grinding into each other, there's a lot of friction and it's possible that sometimes the plates can get stuck, but they are still moving into each other, which means gradually stress builds up.

6:53And then, suddenly one day, it would slip and release a lot of built up energy causing earthquakes. So we would also expect this region to be prone to earthquakes, and it is. One of the highest recorded earthquakes is in Chile. All right, but what if you have two oceanic floors crashing into each other?

7:14Well, something very similar happens. Again, the one which is denser will undergo subduction. And so, again, you will have hot magma rising up causing volcanoes. This time, you'll have volcanic islands being formed. A classic example of this is the Philippine plate moving into the Pacific plate.

7:33Notice both are oceanic plates over here. And as a result, we can expect some volcanic islands over here at the convergent boundary, and there are. The Mariana Islands are just that. And, again, we can expect a trench somewhere over here, the Mariana Trench, the deepest trench in the world.

7:52And just like before, we can expect a lot of earthquakes over here. Okay, before moving on, if you're wondering whether the Hawaiian islands were also formed this way, no. We will get to that in a second, okay? But now moving on, the last kind of boundary is the transform boundary.

8:07This happens when plates are moving sort of parallel to each other. A cool example of this is in the West Coast of North America. See, the entire North America does not lie on a single plate. A small sliver of it lies on the Pacific plate. So San Francisco, for example, lies on the North American plate.

8:26But Los Angeles, for example, lies on the Pacific plate. And these two plates are moving parallel to each other, producing a transform boundary. So you can expect the earth to fracture along this boundary, which we call faults. A famous one is the San Andreas fault.

8:43This time, you wouldn't expect any mountains or volcanoes. But, again, as the plates are grinding against each other, due to a lot of friction and heat, the plates can get stuck. And so, energy and stress gets built up, and eventually when it becomes too great, the plates suddenly slip.

9:01And this can release enormous amounts of energy causing earthquakes. So you definitely expect a lot of earthquakes near transform boundaries. All right, so a lot of interesting things happen near the boundaries of the two tectonic plates moving, right? But now let's talk about the Hawaiian Islands.

9:20These are also volcanic islands, but guess what? These islands are nowhere near any boundaries of the tectonic plates. So how did these volcanic islands form? Well, sometimes hot plumes of magma rise upwards from deep inside the earth. We call them mantle plumes.

9:40Think of them as super heated rocks rising upwards. They produce abnormally hot regions just under the lithosphere. We call them hot spots. What do they do? Well, that depends upon where exactly they form. If they are formed under the oceanic crusts, the hot material can melt through the thin oceanic plate rising up as magma and erupt as lava.

10:07The lava cools and solidifies to form new crust, and the process can repeat over and over again to form volcanic islands. But remember, the lithosphere is constantly moving. So eventually, the island gets carried away from the hotspot. And then, the whole process can repeat, creating a new island.

10:34This is how this hot spot can create a chain of islands. The youngest island would be close to the hotspot and it would be volcanically active. The older ones would be farther away. They would be less active or dormant. This is how the Hawaiian Islands are formed.

10:52There's a hot spot over here. So this is the youngest island. It's pretty volcanically active, and these are the oldest ones. They are pretty dormant. Finally, what if the hot spot was under a continental crust? Well, just like before, the magma can melt through the crust.

11:08But this time, the crust is thick, there's a lot of pressure from the above, so it's very hard to melt through the whole thing. So it's possible that the magma just keeps accumulating, just keeps building up. This produces what we call a supervolcano. This can also happen over subduction zones, okay?

11:27But what makes a supervolcano super is that the magma just keeps accumulating for tens of thousands or even millions of years below. As it accumulates, the pressure builds up. And eventually, one day the whole thing erupts. These eruptions can be so catastrophic, they can change climates or even cause extinction events.

11:50After such an enormous eruption, a lot of pressure is released. And so, now the upper chamber collapses under its own weight, forming a giant bull shaped depression called caldera. And what's interesting is that this process can repeat at that same spot. So calderas are telltale signs of supervolcanoes and past eruptions.

12:11And these caldera are also known for still active volcanic activities such as geysers, fumaroles, and magma flows. An example of this is the Valles Caldera in New Mexico. This is an aerial footage by NASA. This caldera is about 20 kilometers wide. Another famous example is the Yellowstone National Park.

12:31It is sitting on a giant supervolcano. By the way, this is not the caldera. These are just the hot springs. The caldera is huge. It's hard to identify it today because it's filled with roads and trees and buildings. But if it erupts again, it can be catastrophic.

12:48But don't worry, the last time it erupted was more than 600,000 years ago and we are closely monitoring it today. And we don't think it's gonna erupt for another at least 10,000 years, at least. So long story short, moving plates can create boundaries. When they move away from each other, they create divergent boundaries which create ridges and rifts, and you can have volcanoes and earthquakes over there.

13:13Convergent boundaries are vent plates move into each other. The denser plate usually undergoes subduction. It can cause the mantle to melt into magma, rising up creating volcanoes and volcanic islands. They also produce deep trenches and they have a lot of earthquakes.

13:32And then, plates move parallel to each other. They produce transform boundaries. You don't expect volcanoes or mountains usually over here, but you can have earthquakes. But even far away from boundaries, interesting thing can happen because of hot spots. They can create a chain of islands if they are formed below an oceanic crust.

13:52And if they are formed under continental crusts or subduction zones, then magma can build up for millions of years forming supervolcanoes. I find it incredible how lifeless moving tectonic plates can actually be so lively on geological scales.