Lithosphere

Earth has a hard, rocky shell.

This shell is called the lithosphere. It is made of the crust and the top part of the mantle. It is very strong and stiff.

The shell is broken into big pieces. These pieces are called plates. Some plates are under the ocean. Other plates are under the land.

The ocean plates are heavy and sink. The land plates are lighter. They stay on top for a long time. Some land parts are billions of years old!

These moving pieces change our world.

The Earth has a hard, rocky shell. Scientists call this the lithosphere.

This shell is made of two parts. First, there is the crust. Second, there is the lithospheric mantle. This is the very top part of the mantle. The lithosphere is strong and stiff. It sits on top of a warmer layer. This layer is called the asthenosphere. The asthenosphere is weaker and can flow.

The lithosphere is broken into big pieces. These are called tectonic plates. Some plates are under the ocean. We call these oceanic lithosphere. They are very dense, which means they are heavy. As they get older, they get thicker and heavier.

Other plates are under the land. These are called continental lithosphere. They are less dense than ocean plates. Because they are light, they do not sink easily. This makes the land a nearly permanent part of Earth. Some parts of the land are billions of years old. Ocean plates are much younger. The oldest ocean plates are about 170 million years old.

The Earth has a hard, rocky shell called the lithosphere.

This outer layer is very important for our planet. It is made of the crust and the lithospheric mantle. The mantle part is the topmost part of the upper mantle. This layer behaves elastically over thousands of years. It stays rigid and strong for a very long time. This shell is what we live on every day.

How does this shell work? The lithosphere sits on a different layer called the asthenosphere. The asthenosphere is hotter and deeper in the upper mantle. It is weaker and can flow like a thick liquid. The lithosphere stays rigid when it feels stress. However, the asthenosphere deforms through plastic deformation. This means it can change shape slowly. Scientists use the temperature of a mineral called olivine to find the boundary. Olivine is the weakest mineral in the upper mantle.

People have studied this shell for a long time. An English mathematician named A. E. H. Love described it in 1911. Later, an American geologist named Joseph Barrell used the term "lithosphere." He noticed gravity differences over the continents. He thought a strong layer must sit above a flowing layer. In 1940, a Canadian geologist named Reginald Aldworth Daly expanded these ideas. His work helped explain the theory of plate tectonics.

There are two main types of lithosphere. Oceanic lithosphere is found under the ocean basins. It is made of mafic crust and ultramafic mantle. This type is very dense and heavy. It is much younger than the land. The oldest oceanic lithosphere is about 170 million years old. Continental lithosphere sits under the continents. It can be much thicker than the ocean type. Some parts of the continental lithosphere are billions of years old.

These plates are always moving in different ways. New oceanic lithosphere forms at mid-ocean ridges. As it moves away, it cools and gets thicker. Eventually, it becomes so dense that it sinks. This sinking happens at places called subduction zones. The heavy oceanic plate slides under another plate. Continental lithosphere is different because it is less dense. It does not sink easily into the mantle. This makes the continents a nearly permanent part of Earth.

The lithosphere is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, this layer serves as the planet's hard outer skin. It is composed of two distinct parts: the crust and the lithospheric mantle. The lithospheric mantle is the topmost portion of the upper mantle. This specific part of the mantle behaves elastically over thousands of years or more.

The crust and the upper mantle are categorized separately based on their chemistry and mineralogy.

To understand how the lithosphere works, we must look at the layer directly beneath it. This lower layer is called the asthenosphere. The asthenosphere is the hotter, deeper, and weaker part of the upper mantle. Unlike the lithosphere, the asthenosphere is able to convect, which means it can flow. The boundary between these two layers is defined by how they respond to stress. The lithosphere remains rigid for long geologic periods. It deforms through brittle failure or elastic deformation. In contrast, the asthenosphere deforms viscously through plastic deformation.

Scientists identify the lithosphere-asthenosphere boundary using a specific temperature marker called an isotherm. This isotherm marks the transition between brittle and viscous behavior. Geologists often use the temperature at which the mineral olivine becomes ductile to set this boundary. Olivine is generally considered the weakest mineral in the upper mantle. Because the lithosphere is divided horizontally into tectonic plates, it is a dynamic system. These plates often include terranes that were once part of other plates but have been accreted.

The concept of the lithosphere has evolved through the work of several key scientists. In 1911, the English mathematician A. E. H. Love described the lithosphere as Earth's strong outer layer. Later, the American geologist Joseph Barrell introduced the actual term "lithosphere." Barrell observed significant gravity anomalies over continental crust. He inferred that a strong, solid upper layer must exist above a weaker, flowing layer. In 1940, Canadian geologist Reginald Aldworth Daly expanded these ideas in his work, "Strength and Structure of the Earth." These concepts are now essential to the theory of plate tectonics.

There are two primary types of lithosphere: oceanic and continental. Oceanic lithosphere exists in ocean basins and is associated with oceanic crust. This crust has a mean density of about 3.0 g/cm³. Oceanic lithosphere is composed mainly of mafic crust and ultramafic mantle, known as peridotite. It is much denser than continental lithosphere. Continental lithosphere underlies the continents and continental shelves. It has a lower mean density of about 2.7 g/cm³.

Oceanic lithosphere undergoes constant change through a cycle of creation and destruction. New oceanic lithosphere is produced at mid-ocean ridges. As it moves away from these ridges, it undergoes conductive cooling. This cooling converts hot asthenosphere into lithospheric mantle. Consequently, the lithosphere thickens and becomes denser as it ages. The thickness of the mantle part can be approximated as a thermal boundary layer. This thickness increases as the square root of time. While young lithosphere is thin, the oldest oceanic lithosphere is about 170 million years old.

Because mature oceanic lithosphere becomes very dense, it eventually sinks at subduction zones. At these zones, the oceanic lithosphere invariably sinks underneath an overriding plate. This overriding plate might be oceanic or continental. Continental lithosphere behaves differently because it is less dense. It cannot subduct much further than about 300 km before resurfacing. Therefore, continental lithosphere is not recycled like oceanic lithosphere. Instead, it remains a nearly permanent feature of the Earth. Some parts of the continental lithosphere are billions of years old.

Recent geophysical studies have revealed how deep the lithosphere can travel. Some large pieces of the lithosphere have been subducted as deep as the core-mantle boundary. Other pieces appear to "float" in the upper mantle. Some pieces stick down into the mantle as far as 660 km but remain attached to the continental plate above. Even in these deep reaches, subducting lithosphere remains rigid, which is demonstrated by deep earthquakes along the Wadati-Benioff zone. This rigid shell also provides a massive habitat for microorganisms, some of which live more than 5 km below the surface.