Earth's crust

The Earth has a hard shell of rock.

This shell is called the crust. It is the top part of our world. The crust has two main kinds. One kind makes the land. This land is thick and sits high.

The other kind is under the sea. It is thinner and heavier. Both kinds float on the deep, hot rock below. The rock gets hotter as you go down. This shell helps keep our world together.

The Earth has a hard outer shell of rock. We call this the crust.

There are two main types of crust. The first type is continental crust. This makes the land we walk on. It is very thick, from 25 to 70 km. It is made of lighter rocks like granite. Because it is light, it floats high on the mantle. The mantle is the thick layer of rock below.

The second type is oceanic crust. This is found under the sea. It is much thinner than the land crust. It is made of heavy rocks like basalt. Because it is heavy, it sits lower. This creates deep ocean basins.

New ocean crust is always being made. It forms at mid-ocean ridges. At the same time, old crust sinks back into the mantle. This happens at a subduction zone, which is a deep trench. This moving shell is broken into tectonic plates. These plates move to let heat escape from inside the Earth.

The Earth has a hard outer shell made of rock. This shell is called the crust. It is the top part of a layer called the lithosphere. This layer includes the crust and the upper mantle. The crust is quite thin compared to the whole planet. It makes up less than one percent of Earth's radius and volume.

This shell is broken into large pieces called tectonic plates. These plates move around on the layer below. Their movement helps heat escape from inside the Earth into space. The crust sits on top of the mantle. The mantle is made of a rock called peridotite. This rock is much denser than the crust. Because of this, the crust stays stable on top.

There are two different types of crust on our planet. The first type is the continental crust. This is the thick layer that forms our continents. It is between 25 and 70 kilometers thick. It is mostly made of light rocks like granite. In some places, it is even thicker. Examples include the Tibetan Plateau and the Altiplano.

The second type is the oceanic crust. This type is found under the ocean basins. It is much thinner than the continental crust. It is made of heavy rocks like basalt and gabbro. Because the continental crust is lighter, it floats higher. This is why continents form high ground above the sea.

Earth formed about 4.6 billion years ago. It grew from a disk of dust and gas. Small rocky bodies collided and stuck together to build the planet. This process is called accretion. These collisions created a huge amount of heat. The early Earth even melted completely. As the planet cooled, it formed its first crust. This was called the primary or primordial crust.

None of that first crust is left today. It was destroyed by impacts and erosion. Now, Earth makes new crust through different ways. The oceanic crust is called secondary crust. The continental crust is called tertiary crust.

New oceanic crust forms at mid-ocean spreading centers. This happens when the mantle melts slightly. This creates basaltic magma that rises up. This process is a driving force for plate tectonics. It is called ridge push. As new crust forms, old crust must go away. It sinks back into the mantle at a subduction zone. These are deep trenches in the ocean floor.

Because of this cycle, ocean crust is quite young. The oldest ocean crust is only 200 million years old. The continental crust is much older. Some of the oldest rocks are about 4.28 billion years old. Scientists found these in Western Australia and Canada.

We can see how the crust works by looking at its minerals. The continental crust has many different parts. Feldspars are the most common mineral there. They make up about 41 percent of the mass. Quartz is also common at 12 percent. Pyroxenes make up about 11 percent.

Oxygen is the most abundant element in the crust. It makes up about 46.1 percent of the mass. Silicon is the next most common at 28.2 percent. These elements work together to build the rocks under our feet. Even the movement of these rocks affects us. Moving crust on land can cause earthquakes. Movement under the sea can lead to tidal waves.

Earth's crust is the planet's outermost shell of rock. It is a very thin layer compared to the rest of the Earth. The crust makes up less than one percent of the planet's total radius and volume. It is the top part of the lithosphere. The lithosphere is a solid division of Earth's layers. It includes both the crust and the very top part of the mantle.

The crust sits directly on top of the mantle. This arrangement is stable because of density differences. The upper mantle is made of a rock called peridotite. Peridotite is significantly denser than the crustal rocks. Because the mantle is heavier, the crust stays on top. Scientists identify the boundary between the crust and mantle using the Mohorovičić discontinuity. This boundary is defined by a specific change in seismic velocity. As you go deeper into the crust, the temperature increases. In the upper part of the crust, temperatures can rise by as much as 25 degrees Celsius for every kilometer of depth.

There are two distinct types of crust: continental and oceanic. The continental crust is the thick layer that forms our landmasses. It is between 25 and 70 kilometers thick. In places like the Tibetan Plateau or the Altiplano, it is even thicker. This crust is mostly made of felsic rocks, which are less dense. Granite is a common example of a felsic rock. The oceanic crust is much thinner than the continental crust. It is composed of denser, more mafic rocks. These include basalt, diabase, and gabbro.

Both types of crust float on the mantle because they are less dense. This process is known as isostasy. Because the continental crust is thicker and less dense, it has greater buoyancy. This causes the continents to sit much higher than the ocean floor. The result is high ground surrounded by deep ocean basins. The continental crust has an average density of 2.835 g/cm3. This density increases with depth. It starts at 2.66 g/cm3 in the uppermost crust and reaches 3.1 g/cm3 at the base. The oceanic crust is primarily made of pillow lava and sheeted dikes. These are composed of mid-ocean ridge basalt. A thin layer of sediment sits on top of the oceanic crust.

Earth formed about 4.6 billion years ago. It grew through a process called accretion. During accretion, planetesimals and small rocky bodies collided and stuck together. These collisions generated enormous heat. This heat caused the early Earth to melt completely. As the Earth cooled, it formed a primary or primordial crust. However, none of this original crust survives today. It was destroyed by large impacts, erosion, and plate tectonics. Today, we have secondary and tertiary crust. Secondary crust is the oceanic crust. Tertiary crust is the continental crust.

New oceanic crust forms at mid-ocean spreading centers. This happens through the partial-melting of the underlying mantle. This melting creates basaltic magmas that form new crust. This process is called ridge push. It is a major driving force for plate tectonics. Because new crust is always being made, old crust must be destroyed. This happens at subduction zones. These are deep trenches where an ocean plate sinks back into the mantle. Because of this constant recycling, the oldest ocean crust is only 200 million years old.

The continental crust is much older than the oceanic crust. The oldest continental rocks are between 3.7 and 4.28 billion years old. Scientists have found these in the Narryer Gneiss in Australia and the Acasta Gneiss in Canada. The average age of all current continental crust is about 2.0 billion years. Some very old crust is found in cratons. Cratons are stable regions where the crust is less likely to be destroyed by subduction. This is because the old crust and the underlying mantle are less dense than other areas.

We can understand the crust by looking at its chemical makeup. Oxygen is the most abundant element, making up 46.1% of the mass. Silicon is next at 28.2%. Other important elements include aluminum, iron, calcium, sodium, magnesium, and potassium. In the continental crust, feldspars are the most common mineral at 41% by mass. Quartz makes up 12%, and pyroxenes make up 11%. The continental crust is also enriched in incompatible elements. These are elements that are much more concentrated in the crust than in the mantle. The movement of these crustal plates has big effects. Movement on land can cause earthquakes. Movement under the sea can lead to tidal waves.