The Earth's Crust: Facts, Layers, Temperature & Composition

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  • 0:02 What Is the Earth's Crust?
  • 0:37 The Two Types of Crust
  • 2:21 What Is the Crust Made of?
  • 3:24 The Temperature of the Earth
  • 3:55 Lesson Summary
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Lesson Transcript
Instructor: Ryan Hultzman
The Earth's crust is just a thin layer surrounding our world. Although it is the only part of the Earth any of us see, it makes up just one percent of the planet's total volume.

What Is the Earth's Crust?

The Earth is not just a solid ball of rock. That rock is made up of layers characterized by different elements and different types of rock that are either solid or fluid. It also possesses different temperatures and thicknesses. The outermost layer of the Earth is the crust. It's thin compared to the other layers, a little bit like the peel of an orange. Unlike a fruit peel, though, the crust is broken up into several pieces, called tectonic plates. These plates move relative to each other so that the surface of the Earth's crust is always changing, although very slowly.

The Two Types of Crust

There are two types of crust. Oceanic crust is the crust that is under the world's oceans, and continental crust is that under the continents and other large land masses, like Greenland and Madagascar. Oceanic crust is denser and heavier than continental crust. It is made mostly of basaltic rock. Basalt is a volcanic rock and it forms the oceanic crust as the mantle, the molten rock under the crust, wells up and hardens. This happens at mid-ocean ridges where the crust is being pulled apart due to the motion of tectonic plates. Oceanic crust is about four miles thick.

Oceanic crust may be heavier and denser, but continental crust is a thicker and older part of the Earth's crust. The depth of continental crust varies more than oceanic crust and can be anywhere between six and 47 miles thick. The rocks that make up continental crust are less dense and heavy than basaltic ocean crust. Granite is a mineral that's a major component of continental crust. The crust of the continents is also much older than ocean crust. Unlike the ocean floor, continental crust is not constantly reforming. You can find rocks as old as four billion years in continental crust.

Because oceanic crust is denser than continental crust, it is always sinking relative to the continents. New ocean crust forms at mid-ocean ridges, but old ocean crust gets recycled in regions called subduction zones. These are tectonic plate boundaries at which two plates are moving toward each other, rather than pulling apart. When ocean crust meets continental crust at a subduction zone, the heavier oceanic crust sinks under the continent and becomes part of the molten mantle again.

What Is the Crust Made of?

The simple answer is that the Earth's crust is made of rock, which is made of minerals. To get a little more detailed, we can talk about specific minerals. Feldspars, a group of minerals are the most abundant in all of the Earth's crust, both continental and oceanic. Feldspar minerals are made of silicon, oxygen, and various metals, most commonly aluminum. Silica, which is silicon dioxide and also called quartz, is the second most abundant mineral overall. However, if we are looking at continental crust only, it is the most abundant and makes up 60% of the continents on Earth.

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