The Earth’s crust is the planet’s thin, outermost solid layer, forming the top component of the rigid lithosphere. Determining its temperature is complex, as thermal conditions vary dramatically across its span. The crust’s temperature ranges from freezing point at the surface to hundreds of degrees Celsius near the boundary with the underlying mantle. This variability is governed by depth, the specific type of crust (continental or oceanic), and the concentration of internal heat-producing elements.
The Geothermal Gradient
The change in temperature with increasing depth beneath the surface is described by the geothermal gradient. This gradient represents the rate at which heat transfers outward from the planet’s interior toward the surface. In most stable continental regions, the temperature increases at an average rate of approximately 25 to 30 degrees Celsius for every kilometer of depth.
This continuous warming exists because Earth’s interior is significantly hotter than its exterior, causing heat to constantly move toward the cooler surface. A deep well or mine will experience a predictable temperature increase as it descends. The gradient measures this outward heat flow, linking surface conditions to the deep thermal engine of the planet.
The surface layer is influenced by weather and seasonal changes, but this effect is transient and shallow. Below 10 to 20 meters, often called the “zone of constant temperature,” daily and yearly fluctuations cease. It is at this depth that the true geothermal gradient begins, and the rock temperature follows the steady increase dictated by internal heat.
The rate of temperature increase is not uniform globally and changes significantly based on local geology. Areas near active volcanic zones or tectonic plate boundaries exhibit a much steeper gradient, meaning temperature rises more rapidly with depth. Conversely, in stable, ancient continental interiors, the gradient is often shallower, reflecting a slower rate of heat transfer.
Temperature Variation by Crust Type and Depth
The specific temperatures found within the crust depend heavily on whether it is continental or oceanic crust. At the surface, temperature is dictated by the environment, ranging from the global average of 14°C to extremes like freezing in polar regions. Below this thin, climate-affected surface layer, the temperature profiles of the two crust types diverge considerably.
Continental Crust
Continental crust is substantially thicker, typically ranging from 20 kilometers to 80 kilometers in mountainous regions. Its thickness allows it to act as an insulating blanket, trapping heat from the mantle and from its own internal heat production. Due to this insulation and a higher concentration of heat-producing elements, the geothermal gradient is generally shallower, but the total temperature reached at the base is much higher.
The temperature at the Mohorovičić discontinuity (Moho), the boundary between the crust and the mantle, beneath continents can vary widely. In stable continental interiors, the temperature at the Moho is commonly estimated to be between 500°C and 800°C. In areas of recent magmatic activity or crustal collision, temperatures can exceed 900°C or even 1000°C, sufficient to cause melting and the formation of new magma bodies.
Oceanic Crust
Oceanic crust is much thinner and denser than continental crust, typically measuring only 5 to 10 kilometers thick. This results in a steeper initial geothermal gradient because mantle heat has a shorter distance to travel to the surface. Despite the steeper gradient, the temperature at the base is generally lower due to the lack of insulation and fewer heat-producing elements.
At the Moho beneath the oceans, the temperature is often estimated to be in the range of 150°C to 400°C in older, stable oceanic plates. This lower temperature reflects that oceanic crust is constantly being recycled and is much younger. An exception occurs at mid-ocean ridges, which are divergent plate boundaries where new crust is created. Here, the upwelling of hot mantle material and magma causes temperatures to be much higher, with the base of the newly formed crust reaching temperatures closer to 600°C or more.
Sources of Internal Crustal Heat
The heat driving the geothermal gradient comes from two primary sources within the planet. The first is the continuous process of radioactive decay occurring within the rocks themselves. Unstable isotopes, particularly Uranium-238, Thorium-232, and Potassium-40, spontaneously break down within the crust and mantle. This decay releases thermal energy, warming the surrounding rock. These elements are concentrated disproportionately in the continental crust, explaining why continental plates are generally hotter and thicker.
The second major contributor is the residual heat left over from Earth’s formation approximately 4.5 billion years ago. This heat originated from planetary accretion and the gravitational differentiation of heavy materials sinking to the core. A substantial amount remains trapped within the deep mantle and core. This residual heat slowly transfers outward, contributing to the overall thermal flow that moves through the crust toward the surface.