Thermal Evolution of the Earth

Since its hot formation the Earth has cooled by convecting away internal heat, a long thermal history that governs the slowing of plate tectonics, the growth of the inner core, and the longevity of the magnetic field.

Definition

The thermal evolution of the Earth is the secular change in the temperature and heat content of its interior over geological time, set by the competition between internal heat generation and convective heat loss, which controls the history of mantle convection, core cooling, and the magnetic field.

Scope

This topic covers the long-term thermal history of the Earth: the balance between heat generated by radioactivity and primordial sources and heat lost at the surface, parameterized models of how mantle temperature and convective vigor evolve, and the cooling of the core that drives inner-core growth and the geodynamo. It treats the constraint that convective heat loss depends on mantle viscosity and hence temperature, the thermal coupling of mantle and core, and the implications for the planet's tectonic and magnetic history. The emphasis is on how the Earth's internal heat has changed over billions of years.

Core questions

• How has the balance of heat generation and loss changed over Earth history?
• How does temperature-dependent viscosity regulate the cooling of the mantle?
• How does core cooling drive inner-core growth and the geodynamo?
• What does thermal evolution imply for the history of plate tectonics?

Key concepts

• Secular cooling and the Earth's heat balance
• Parameterized convection models
• Temperature-dependent viscosity and self-regulation
• Core cooling and inner-core growth
• Thermal history of plate tectonics

Key theories

Parameterized convective cooling

Because convective heat loss increases with mantle temperature through the temperature dependence of viscosity, the mantle self-regulates: a hotter early mantle convected and cooled faster, so models parameterize heat flux against the Rayleigh number to track the planet's cooling history.

Core cooling and the geodynamo

As the mantle extracts heat from the core, the core cools and the inner core crystallizes, releasing latent and gravitational energy that helps power the geodynamo; the timing of inner-core nucleation is a key constraint on the field's long history.

Mechanisms

The Earth began hot from accretion and core formation and has since lost heat faster than radioactivity supplies it, so it is slowly cooling; because mantle viscosity falls with rising temperature, a hotter mantle convects more vigorously and cools more quickly, providing negative feedback that smooths the cooling, while heat drawn from the core lowers its temperature, freezes the inner core, and sustains the convection that drives the magnetic field.

Clinical relevance

Thermal evolution links the Earth's energy budget to the history and future of plate tectonics, volcanism, and the magnetic field, frames comparisons with the divergent histories of Venus and Mars, and bears on long-term planetary habitability.

History

Kelvin's cooling calculation gave a famously low age for the Earth before radioactivity was known to supply internal heat; modern parameterized convection models from the late twentieth century, together with constraints on inner-core growth and core energetics, established the current picture of a self-regulating, slowly cooling planet.

Debates

Age of the inner core and core thermal budget

Revised estimates of the high thermal conductivity of the core imply rapid heat loss and a geologically young inner core, raising the question of how the geodynamo was powered before inner-core nucleation and prompting proposals such as a basal magma ocean or alternative compositional energy sources.

Key figures

• William Thomson (Lord Kelvin)
• Geoffrey Davies
• Stéphane Labrosse

Seminal works

• schubert2001
• jaupart2011
• labrosse2007

Frequently asked questions

Is the Earth cooling down?

Yes, over billions of years the Earth is slowly losing more heat than its radioactivity produces, so its interior is gradually cooling; this cooling drives the freezing of the inner core and will, in the very distant future, weaken the convection that powers plate tectonics and the magnetic field.

How does the Earth's cooling relate to its magnetic field?

Cooling of the core lets the inner core crystallize, releasing heat and light elements that stir the liquid outer core; this convection is what sustains the geodynamo, so the planet's thermal evolution is closely tied to the strength and survival of the magnetic field.