Earth's crust

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.