Lithospheric Cooling and Thermal Evolution of Plates

Core Idea

Oceanic lithosphere cools as it ages, following half-space cooling or plate models. Temperature, density, and seismic velocity change predictably with age. Plate age variations explain bathymetry, heat flow, and isostatic structure across ocean basins and passive margins.

Explainer

From your understanding of heat conduction models and plate tectonics, you know that temperature within the Earth is governed by the balance between heat sources (mantle convection, radioactive decay) and heat loss through conduction to the surface. Lithospheric thermal evolution applies these principles to track how an oceanic plate changes from the moment it forms at a mid-ocean ridge to its eventual subduction, often tens or hundreds of millions of years later.

At the ridge, hot asthenospheric material rises to near the surface, creating new lithosphere at temperatures close to 1,300°C. As this plate moves away from the ridge, it cools from the top down by conduction. The simplest model treating the plate as a half-space cooling from an initially uniform temperature predicts that the depth to any isotherm grows as the square root of age. This means the lithosphere thickens proportionally to √t — a 25-million-year-old plate has a thermal boundary layer roughly twice as thick as a 6-million-year-old one. The model successfully predicts two independently observable quantities: surface heat flow decreases as 1/√t (younger crust loses heat faster), and ocean depth increases as √t (cooler, denser lithosphere sinks isostatically).

The half-space model works well for young oceanic lithosphere (less than ~70 million years), but it overpredicts both subsidence and heat flow decline for older plates. Observations show that bathymetry and heat flow flatten out for ages beyond about 80 Ma, as if the plate reaches a maximum thickness and stops cooling further. The plate model resolves this by imposing a fixed temperature at the base of the lithosphere — effectively assuming that small-scale convection or heat input from the asthenosphere prevents the thermal boundary layer from growing indefinitely. The plate model treats the lithosphere as a slab of finite thickness (typically 100–125 km) with a hot base, and its predictions match the observed flattening of heat flow and bathymetry at old ages.

These thermal models have far-reaching consequences. Because density depends on temperature (cooler rock is denser), the thermal state of the lithosphere controls its buoyancy and therefore the depth of the ocean floor — this is why mid-ocean ridges stand high and abyssal plains are deep. The same cooling governs when oceanic lithosphere becomes dense enough to subduct. At passive margins, the thermal history of rifting and subsequent cooling controls the pattern of subsidence that creates accommodation space for sedimentary basins. Understanding lithospheric thermal evolution thus connects heat flow measurements at the surface to the large-scale dynamics of plate tectonics.