The Lithosphere: Facts About Earth’s Outer Shell

The lithosphere is the outermost layer of the Earth, made up of the crust and the fragile part of the upper mantle. The term lithosphere is derived from the Greek words “lithos”, which means stone, and “sphaira”, which means globe or ball.

Sandwiched between the atmosphere above and the asthenosphere below, the lithosphere can reach depths of up to 190 miles (300 kilometers), according to The Geological Society.

At the lithosphere-asthenosphere boundary – when temperatures reach 2,400 degrees Fahrenheit (1,300 degrees Celsius) – the rocks take on a viscous nature and flow, albeit very slowly. The rocks remain solid due to the high pressure caused by the miles of mantle and crust above, but move at a rate of one or two inches (2.5 to 5 centimeters) per year, depending Science.com. This change in ductility – the ability of a material to deform or stretch under stress – marks the boundary between the lithosphere and asthenosphere in the upper mantle.

The perpetual “slip and slide” movement of the lithosphere over the asthenosphere has caused it to fragment into several large sections called tectonic plates.

Related: What makes Earth unique?

Varieties of the lithosphere

The lithosphere can be divided into two varieties: oceanic and continental.

Oceanic crust is relatively thin and dense, according to Science.com. The oceanic crust is mainly composed of basalt rocks rich in silica and magnesium. Its thickness varies from just a few kilometers in oceanic centers such as the Mid-Atlantic Ridge, to 60 to 90 miles (100 to 150 km) below mature ocean basins, according to the Geological Society.

The continental crust, on the other hand, is mainly composed of granitic rocks rich in silica and aluminum and can reach thicknesses of up to 190 miles (300 km). Continents are at a higher elevation than the ocean floor because continental crust is less dense than oceanic crust and therefore “floats” higher on the mantle, according to the University of California, Santa Barbara.

The lithosphere and plate tectonics

The lithosphere is divided into sections called tectonic plates. There are seven major plates and eight minor plates according to the Geological Society. Earth is unique in this regard, we have yet to find another planet that has a lithosphere divided into true plates, according to the Lunar and Planetary Institute.

The lithosphere can be divided into large sections called tectonic plates. These plates move slowly above the asthenosphere layer. (Image credit: blueringmedia via Getty Images)

The asthenosphere acts as a lubricant for the slabs of the lithospheric plates, allowing them to slide, bump and rub against each other, resulting in geological events such as volcanic eruptions and earthquakes.

Plate tectonics is also responsible for some of Earth’s most striking landforms, such as the Himalayas, which stretch 1,800 miles (2,900 km) along the border between India and the Tibet. According to United States Geological Survey.

Did you know?

Movement along the San Andreas fault line brings Los Angeles 1.8 inches (4.6 cm) closer to San Francisco each year?

According to The National Oceanic and Atmospheric Administration (NOAA), large ocean ridges such as the Mid-Atlantic Ridge form at diverging plate boundaries – when two tectonic plates move away from each other. When molten rock rises through the asthenosphere to the seafloor, it produces large basalt eruptions. As the plates diverge, a new ocean floor is created and the plates continue to move apart. When an oceanic plate collides with a “lighter” plate like a continental plate, it plunges underneath in a process called subduction, according to Science.com. Tectonic plates likely evolved very early in our planet’s 4.6 billion year history and have been playing in slow motion with bumper cars ever since.

An illustration of plate tectonics and subduction when two plates of different densities collide. (Image credit: CHRISTOPH BURGSTEDT/SCIENTIFIC PHOTO LIBRARY via Getty Images)

How do we know the lithosphere and asthenosphere are there?

We know the lithosphere exists because it is where we live and we can see the direct effects of plate tectonics through dramatic volcanoes and tall mountain ranges. But how do we know what is happening below the surface?

Earthquakes and seismic waves can tell us a lot about the Earth’s interior, including the location of the lithosphere and asthenosphere.

During an earthquake, both primary (P) and secondary (S) waves propagate inside the Earth, according to Colombia University. Special stations located around the world detect these waves and record their speeds, which says a lot about the composition, temperature and pressure of the material through which the waves pass.

Seismic waves travel faster through dense materials like solid rocks and slow down in liquids. At depths of around 60 to 90 miles (100 to 250 km), seismic waves begin to slow down, indicating that they have entered a partially melted zone (around 1%) – the asthenosphere. Rocks in the low-velocity seismic zone – the asthenosphere – partially melt due to increased temperature or reduced pressure. Such partial melting is more common in hotspots and plate boundariesaccording to the Geological Society.

Additional Resources

Explore the most detailed map ever made of the tiny magnetic signals generated by the Earth’s lithosphere with ESA. Visit the volcanoes, earthquakes, impact craters and plate tectonics of the world with this incredibly detailed map from the United States Geological Survey. Take a peek below the surface and discover the Earth’s interior with the Open University.

Bibliography

Boden, David R. Geological foundations of geothermal energy. CRC Press, 2016.

Bartzsch, Stefan, Sergei Lebedev and Thomas Meier. “Resolution of the lithosphere-asthenosphere boundary with seismic Rayleigh waves.“Geophysical Journal International 186.3 (2011): 1152-1164.

Artemieva, Irina. Lithosphere: an interdisciplinary approach. Cambridge University Press, 2011.

Fischer, Karen M., et al. “The lithosphere-asthenosphere boundary.“Annual Review of Earth and Planetary Sciences 38 (2010): 551-575.

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