The Lithosphere: Definition & Composition

The lithosphere is where we live. It is also the source of many geologic events that affect us. In this lesson, you will learn more about this important part of our planet.
What Is a Lithosphere?
The name ‘lithosphere’ comes from the Greek words lithos, meaning ‘rocky,’ and sphaeros, meaning ‘sphere.’

The word is defined two ways. The first is in the context of studying the Earth as a system, a set of interrelated processes, where the word is used very broadly to identify what goes on within the solid part of the Earth; basically, everything below the ground surface. It is one of the four main components of the Earth system, along with the atmosphere, hydrosphere, and biosphere.

Geologists use the term as the name for the layer of the Earth extending from the surface to a depth of around 80 to 120 miles, depending on location, in which rocks are relatively brittle and rigid. This is the definition we will explore here.

Properties of the Lithosphere
Aside from the fact that we are living on it, the lithosphere is where many of the geologic processes that affect us originate. The movement of large pieces of the lithosphere account for the global locations of volcanoes, earthquakes, and mountain ranges, as well as the shape and location of our modern continents. Interesting, huh? Let’s look at the physical characteristics of the compounds that make up the lithosphere.

The lithosphere is made up of rocks from two of the Earth’s major layers. It contains all of the outer, thin shell of the planet, called the crust, and the uppermost part of the next-lower layer, the mantle. The thickness of the lithosphere varies; it’s thickest below the continents and thinnest at the mid-ocean ridges, raised areas of the seafloor where new seafloor crust is formed.

The thing all of the rocks in the lithosphere have in common is the way in which they respond to forces applied to them. At the relatively low temperatures found near the Earth’s surface, rocks tend to break under stress. Farther down, as temperature and pressure increase, the more likely it is that rocks will be able to accommodate stress by changing shape, or deforming, compressing, stretching, and bending, rather than breaking.

At some critical depth, the temperature will be high enough that rocks actually start behaving like a viscous fluid rather than a brittle solid. That depth is defined as the bottom of the lithosphere. Below the base of the lithosphere, rocks are hot enough that they actually deform by flowing, even though they remain solid due to the high confining pressure produced by the weight of the rocks above. That layer on which the lithosphere rests is known as the asthenosphere.

The physical connection between the lithosphere and the asthenosphere generates a considerable amount of pushing and pulling on the lithosphere as the rocks below move around. In response, the lithosphere has broken into about a dozen large pieces, called lithospheric plates, or simply plates. The movement of the plates away from, towards, and past each other is known as plate tectonics.

Seafloor Lithosphere
Under the ocean basins, the crust and mantle rocks of the lithosphere have more or less the same chemical composition. They are made up of minerals enriched in iron and magnesium and deficient in silica, the combination of silicon and oxygen. For example, one of the most common rocks is the igneous rock that makes up seafloor crust, called basalt.

The thickness of the seafloor lithosphere is rather constant, although near the mid-ocean ridges, mantle temperature is higher than average because of the upward migration of hotter mantle rock from below. At those places, there really isn’t any rigid mantle rock, so, technically, the lithosphere consists only of seafloor crust. What’s more, much of the upper mantle directly below the ridges is hot enough to melt, which is where the magma that forms seafloor crust comes from.

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