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Plate tectonics and the ocean floor

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In the ocean, tectonic processes continuously create and destroy oceanic crust. New crust is created at mid-ocean ridges and moves away from them over millions of years. Old crust is destroyed when it reaches seafloor trenches. Created by Khan Academy.

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Video transcript

- [Instructor] Imagine that all the Earth's oceans disappeared for a day. And imagine that you being the excellent Explorer you are, decided to go investigate. You'd travel across the continental shelf, down the continental slope and across the abyssal plain. You'd see gaping trenches running deep into the ground, and mountains taller than any mountain on the continents. And you might wonder, what causes such dramatic landscapes to be formed? Earth's lithosphere, which is made up of the crust and the upper part of the mantle, is broken up into large puzzle piece like chunks called tectonic plates. These tectonic plates move around slowly over millions of years on the section of the mantle below the lithosphere, which we call the asthenosphere. There are two types of tectonic plates, oceanic plates and continental plates. The continental plates, as you probably guessed, are the ones that make up the continents. The oceanic plates are the ones that make up the sea floor. The main difference between oceanic plates and continental plates is the type of crust found on each plate. Oceanic crust and continental crust are made out of different kinds of rock. The continental crust contains a lot of granite, which is an igneous rock. This means that it was made out of rock that was once molten. The oceanic crust has a lot of basalt in it, which is another kind of igneous rock. The difference in the kinds of rock that the crusts are made out of means that the oceanic crust is denser than the continental crust. If you took a cubic centimeter of the rock from the continental crust, it would be about 2.7 grams. A cubic centimeter from the oceanic crust, on the other hand, would weigh about three grams. While this difference in density might not seem like much, it completely changes how tectonic plates interact. The high density of oceanic crust causes oceanic plates to sink into the asthenosphere a bit more than continental plates do. When an oceanic plate collides with another plate at a convergent boundary, it always follows this rule, the denser plates always dives beneath the less dense plate. When it's an oceanic plates and a continental plate converging, the denser oceanic plates is the one that dips down. And when it's two oceanic plates that are colliding, the older, denser oceanic plate will move under the newer and less dense oceanic plates. And over time, the denser plate will be recycled into asthenosphere. The place where the plates collide is called a subduction zone. This bending of the denser plates under the other creates a trench. The deepest one is the Mariana Trench, which is located where the Pacific plates dives under the Marianna plates. The trench is about 11 kilometers deep. The plate that sinks into asthenosphere often has some water and fluids trapped inside of it. These fluids heat up and bubble to the surface. The hot fluids can cause sections of mantle rock to melt into magma, which then rises to the surface and creates volcanoes. And you might be wondering, if the sea floor is constantly being destroyed then what keeps the earth from shrinking? Well, new sea floor is constantly being created too. This happens when two tectonic plates move away from each other at a divergent boundary. When two plates diverge in the middle of an ocean, it creates a kind of underwater mountain range called a mid-ocean ridge. As the plates move apart at mid-ocean ridges, magma moves up, cools and forms a new younger lithosphere. You can think of this process like a really slow conveyor belt. New ocean sea floor is made at the ridges, and moves away from them over millions of years. This means that the oceanic crust that is closest to the ridge is the youngest. And as it moves away from the ridge, the crust gets older and older. As it ages, the crust becomes cooler and more dense. And eventually it dips back down into the asthenosphere at the trenches. The sea floor is perhaps the most unexplored part of our planet. So if they do drain the ocean and you go for a walk at the bottom of the sea, I hope you'll tell us all what it's like down there.