Crust (geology)

The Earth has a thin shell.

It is on the very outside. This shell is called the crust. It is not the same everywhere. Earth has land crust and ocean crust.

The crust is broken into big pieces. These pieces are called plates. The plates move around. This movement lets heat escape from inside the Earth. The crust is very thin. It is less than one percent of Earth. It is the top layer of our world.

The crust is the outer shell of a planet.

It is a thin, solid layer. On Earth, it is less than 1% of the planet's volume. The crust is part of the lithosphere. This is a layer that includes the crust and the top of the mantle. The lithosphere is broken into tectonic plates. These plates move around. This movement lets heat escape from the inside of Earth into space.

Scientists group crust into three types. The first is primary crust. This is the original crust of a planet. It forms when a magma ocean cools and hardens. Earth has lost all its primary crust. We study it by looking at other worlds. The Moon has primary crust. It formed when crystals floated to the top of a magma ocean.

Secondary crust is the most common type. It forms when the mantle melts a little bit. This type is found on many planets. The third type is tertiary crust. This type is very different. Earth is the only planet with this type. It forms through many steps. It needs plate tectonics to make it.

The crust is the outer shell of a planet.

It is a thin and solid layer. On Earth, the crust is less than 1% of the planet's volume. It is part of a larger layer called the lithosphere. This layer includes the crust and the upper mantle. The lithosphere is broken into moving pieces called tectonic plates. These plates move to let heat escape from Earth into space.

Scientists group crust into three different types. The first type is called primary crust. This is the original crust of a planet. It forms when a huge magma ocean cools and hardens. Earth has lost all of its primary crust. This happened because of erosion and the recycling of rocks. We can study primary crust by looking at other worlds.

Secondary crust is the second type. It is the most common type in our Solar System. It forms when the mantle melts a little bit. This creates rocks that are mostly basaltic. You can find secondary crust on Mercury, Venus, and Mars. It is also found in the lunar maria on the Moon. On Earth, this crust forms at mid-ocean spreading centers.

Tertiary crust is the third type. It is more chemically changed than the other types. It can form from melting secondary crust or from sediments. Earth has the only known example of tertiary crust. This is the continental crust. Earth is the only planet with plate tectonics. This movement is needed to create this special type of crust.

We can learn a lot by studying the Moon. The Moon has a thick primary crust. It formed between 4.5 and 4.3 billion years ago. It grew from a lunar magma ocean. Crystals called plagioclase floated to the top to form the crust.

The Moon's crust is 50 to 60 km thick on average. This is much thicker than Earth's crust. It shows that even small rocky bodies can have a crust.

In geology, the crust is the outermost solid shell of a planet, dwarf planet, or natural satellite.

It is a thin layer that covers the surface of these celestial bodies. On Earth, the crust is quite small compared to the rest of the planet. It accounts for less than 1% of Earth's total volume. The crust is the top part of the lithosphere. The lithosphere is a layer that includes both the crust and the upper part of the mantle. On Earth, this lithosphere is broken into several moving tectonic plates. These plates move to allow heat to escape from the interior of Earth into space.

Planetary geologists divide crust into three specific categories based on how and when they formed. The first category is called primary crust, or primordial crust. This is the original crust of a planet. It forms during the solidification of a massive magma ocean. Toward the end of planetary accretion, terrestrial planets likely had surfaces made of these magma oceans. As these oceans cooled, they solidified into the primary crust. This crust was likely destroyed by large impacts and reformed many times during the Era of Heavy Bombardment.

Because of Earth's high rates of erosion and crustal recycling, no primary crust survives on our planet today. Plate tectonics has destroyed all rocks older than about 4 billion years. Geologists must look to other worlds to study this stage of planetary history. For example, the anorthosite highlands on the Moon are considered primary crust. These formed as plagioclase crystallized from the Moon's initial magma ocean and floated to the surface. On Mercury, the highlands might represent primary crust, though scientists still debate this. On Mars, the meteorite ALH84001 may represent primary crust, but this is also debated. Venus lacks primary crust because the entire planet has been repeatedly resurfaced.

Secondary crust is the most common type of crust found in our Solar System. It forms through the partial melting of silicate materials located in the mantle. This process usually results in a basaltic composition. Most surfaces of Mercury, Venus, Earth, and Mars consist of secondary crust. The lunar maria on the Moon are also made of secondary crust. On Earth, this type of crust forms primarily at mid-ocean spreading centers. At these locations, the adiabatic rise of the mantle causes the material to melt partially.

Tertiary crust is more chemically modified than both primary and secondary crust. It can form through several different geological processes. One way is through igneous processes, which involve the partial melting of secondary crust. This can be coupled with differentiation or dehydration. Another way is through erosion and sedimentation. This happens when sediments are derived from primary, secondary, or tertiary crust. The only known example of tertiary crust in the Solar System is Earth's continental crust. This is likely because tertiary crust requires plate tectonics to form. Earth is currently the only planet known to have plate tectonics.

We can learn a great deal about crust formation by studying the Moon. The Moon's crust formed through a specific process after a collision. A theoretical protoplanet named "Theia" is thought to have hit the forming Earth. Some of the material ejected by this collision accreted to form the Moon. As the Moon formed, its outer part was likely a molten lunar magma ocean.

Large amounts of plagioclase feldspar crystallized from this ocean and floated toward the surface. These rocks form much of the lunar crust.

The Moon's crust provides interesting data regarding thickness and composition. The average thickness of the lunar crust is between 50 and 60 km. This is significantly thicker than Earth's crust, even though the Moon's radius is only about a quarter of Earth's. The crust on the far side of the Moon averages about 12 km thicker than the near side. The upper part of the crust averages about 88% plagioclase. The lower part may have more ferromagnesian minerals, such as pyroxenes and olivine, but it still averages about 78% plagioclase. Most of this crust formed between 4.5 and 4.3 billion years ago.

Only about 10% or less of the crust consists of igneous rock added after this initial period. The most common later additions are mare basalts, which formed between 3.9 and 3.2 billion years ago. While minor volcanism continued as recently as 1 billion years ago, there is no evidence of plate tectonics on the Moon.