The Earth’s outer layer, the lithosphere, is a rigid shell whose temperature changes dramatically with depth. Understanding its thermal structure requires examining how heat is transferred through the rock and the factors that influence this process. The temperature profile is crucial for determining the rigidity and behavior of the Earth’s outermost shell.
Composition and Scope of the Lithosphere
The lithosphere is the Earth’s stiff, outermost layer. It consists of the entire crust and the uppermost, non-flowing part of the mantle. This rigid unit is broken into the tectonic plates that move across the planet’s surface. The rocks within the lithosphere are relatively cool and strong, allowing them to fracture under stress.
The thickness of this rigid layer is highly variable. Under the oceans, the oceanic lithosphere is generally thinner, often measuring around 40 kilometers deep near mid-ocean ridges. In contrast, the continental lithosphere beneath stable landmasses can be much thicker, sometimes extending to over 200 kilometers in depth. This rigid shell effectively “floats” on the softer, hotter mantle below.
The Geothermal Gradient: Temperature with Depth
The geothermal gradient describes the rate at which temperature rises as one descends into the Earth. Starting at ambient surface temperature, the temperature increases rapidly due to heat flowing outward from the planet’s core and mantle. This heat transfer occurs primarily through conduction, where vibrational energy passes from atom to atom through the solid rock.
In the upper continental crust, the temperature typically increases at a rate of about 25 to 30 degrees Celsius per kilometer of depth. This rapid increase is partially fueled by the decay of radioactive elements, such as uranium and thorium, which are concentrated in the crustal rocks. As depth increases, the rate of temperature rise often slows down. However, temperatures can still reach approximately 600°C to 800°C near the base of a typical lithosphere, just before the transition to the deeper mantle.
Regional Variations in Lithospheric Temperature
The thermal profile is not constant worldwide, but varies based on the specific tectonic setting. In active plate boundaries, the lithosphere is thin and newly formed. Here, hot mantle material rises close to the surface, resulting in high heat flow and a steep temperature gradient near the surface, sometimes exceeding 100°C per kilometer.
Conversely, stable continental interiors, known as cratons, exhibit a significantly different thermal profile. These areas have thick and old lithosphere, which acts as a thermal insulator. The heat flow is much lower, meaning the temperature increases more slowly with depth, resulting in a cooler deep lithosphere.
The Boundary Temperature and the Asthenosphere
The maximum temperature of the lithosphere is reached at its bottom edge, defined as the Lithosphere-Asthenosphere Boundary (LAB). This boundary is characterized not by a change in chemical composition, but by a sudden change in mechanical strength. The lithosphere remains strong and rigid because its temperature is below the point where rock begins to lose its strength and flow.
The transition to the underlying asthenosphere occurs when the mantle rock reaches its solidus—the temperature at which it is just about to melt or becomes weak enough to flow plastically. This transition temperature is estimated to be around 1300°C. Once the rock reaches this temperature, it loses its rigidity and becomes ductile.