The temperature of the Earth’s lithosphere involves a vast spectrum, as this outer shell extends from the surface downward into the upper mantle. While the surface temperature is influenced by the atmosphere, the interior heat increases significantly with depth. This increase is driven by immense internal heat sources. The lithosphere is a thermal layer, transitioning from relatively cool rock to rock hot enough to nearly melt.
Defining the Earth’s Rigid Outer Shell
The lithosphere is the Earth’s outermost mechanical layer, forming the rigid shell that comprises the tectonic plates. It incorporates the entirety of the crust, both continental and oceanic, and the uppermost, non-flowing part of the mantle directly beneath it. This structure is defined by its physical state: it is cold enough to be brittle and rigid, tending to fracture rather than bend under stress.
The thickness of this rigid layer is variable across the globe. Under the ocean basins, the lithosphere can be relatively thin, sometimes only a few dozen miles thick. Beneath stable continental interiors, however, it can reach depths of up to 150 miles. This cold, strong layer contrasts sharply with the layer immediately below it, which is warm enough to behave plastically. The distinction between these layers is based on a difference in mechanical properties, governed largely by temperature and pressure.
The Lithosphere’s Temperature Spectrum
The temperature within the lithosphere spans a wide range, starting near the ambient air temperature at the surface. The surface temperature of the rock typically averages around 50 to 100 degrees Fahrenheit, depending on the climate. This cool starting point soon gives way to intense heat as one travels downward.
At the base of the crust, known as the Mohorovičić discontinuity (Moho), the temperature is already substantially elevated. Temperatures here typically range from approximately 750 degrees Fahrenheit to 1,500 degrees Fahrenheit, though this varies greatly depending on the geological setting. In tectonically active areas, for instance, the Moho temperature can sometimes exceed 2,100 degrees Fahrenheit. This vast temperature increase is a direct result of the heat flow from the planet’s interior.
The Role of the Geothermal Gradient
The mechanism responsible for this temperature increase is the geothermal gradient, which is the rate at which temperature rises with depth into the Earth. In stable continental areas, the average gradient is around 72 to 87 degrees Fahrenheit per mile of depth. This rate is not uniform, however, being much steeper in areas like mid-ocean ridges where new crust is forming.
The heat driving this gradient originates from two primary sources within the Earth’s interior. The first is the residual heat left over from the planet’s formation and subsequent accretion billions of years ago. The second, and currently dominant, source is the heat generated by the continuous radioactive decay of elements like uranium-238, thorium-232, and potassium-40 within the crust and mantle.
The thickness and composition of the lithosphere significantly influence the geothermal gradient. Continental lithosphere is typically thick and contains a higher concentration of heat-producing radioactive elements. However, its great thickness acts as an insulator, resulting in a lower heat flow to the surface. Conversely, the thinner oceanic lithosphere allows for a steeper gradient and higher heat flow, as the mantle’s heat source is closer to the surface.
Temperature at the Lithosphere-Asthenosphere Boundary
The deepest and hottest part of the lithosphere is the transition zone where it meets the underlying asthenosphere. This interface, called the Lithosphere-Asthenosphere Boundary (LAB), is not a sharp line but a zone where the rock’s mechanical properties change drastically. At this depth, the temperature reaches its maximum within the rigid layer, typically hovering around 2,400 degrees Fahrenheit.
This temperature is significant because it approaches the solidus—the temperature at which a rock begins to melt under the immense pressure present at that depth. Although the rock remains mostly solid, the heat is sufficient to make it lose its rigidity and become ductile. This ability to slowly flow defines the boundary between the rigid lithosphere and the soft, flowing asthenosphere beneath it. The temperature at the base fundamentally limits the strength of the lithosphere, dictating the shift from brittle to plastic behavior.