00 · QUICK ORIENTATION
Old does not mean cold
- 1Earth’s internal heat comes mainly from heat left from its formation and heat continually released by radioactive decay.
- 2A planet loses heat through its surface, so great size and poorly conducting rock make cooling a very slow process.
- 3Heat drives mantle motion and helps sustain a liquid outer core, but scientists still refine how much comes from each source.
01 · TWO HEAT BUDGETS
Some heat was inherited; some is still made
Earth’s internal heat comes mainly from heat left from its formation and heat continually released by radioactive decay.
Accreting rocks collided, compressed and differentiated: dense metal sank and gravitational energy became heat. That primordial store has been leaking outward ever since.
02 · A NUCLEAR CLOCK
Slow decay keeps paying into the account
Long-lived isotopes of uranium, thorium and potassium still generate heat inside the planet.
Their nuclei change on billion-year timescales. The released energy is absorbed by surrounding matter and becomes thermal motion; this is not a hidden furnace or burning fuel.
Formation
Impacts, compression and differentiation deposited heat.
4.5B YEARSRadioactive decay
Long-lived isotopes keep releasing energy.
U · Th · KSurface heat flow
Conduction and convection carry energy outward.
CONTINUOUS03 · SIZE CHANGES THE ANSWER
The interior cannot cool all at once
Heat must cross thousands of kilometres before it reaches the surface. Rock conducts poorly, and mantle convection moves material only slowly on human timescales.
Small bodies lose a larger share of heat through each unit of volume. A large planet therefore stays geologically active longer.
04 · HEAT IS NOT MAGMA
Most of the mantle is solid and moving
Hot rock can deform and creep without being liquid. Pressure raises melting temperatures, so temperature alone does not tell us whether a layer is molten.
The outer core is liquid iron alloy; the inner core is solid because pressure is greater there. “Hot” and “liquid” are different questions.
A solid can flow over millions of years without becoming an ocean of magma.
Earth’s internal heat comes mainly from heat left from its formation and heat continually released by radioactive decay.
Researchers estimate residual, radiogenic and core contributions with heat flow, mineral physics and geoneutrinos. The basic sources are established; their exact partition remains model-dependent.
05 · FROM DEPTH TO SURFACE
A slow loss powers a moving planet
Buoyancy and cooling help organize mantle convection, plate motion, volcanism and the transport of heat toward the surface.
The details form a coupled system: composition, phase changes and core cooling matter alongside radioactivity.
Move through the budget. The planet can cool overall while one source still makes new heat.
Formation stores energy
A young Earth starts with a large inherited thermal reservoir.
PRIMORDIAL06 · HOW WE KNOW
No drill reaches the core
Seismic waves reveal layers and physical states. Laboratory mineral physics tests matter at high pressure, while surface heat flow and geoneutrinos constrain the energy budget.
These methods do not provide one perfect thermometer. They combine independent clues into a model that can be revised.
07 · EVIDENCE TRAIL
Geology and planetary heat
Official syntheses establish the heat sources and show where active measurement begins.
- 01USGS · Tapping the Earth’s natural heatOFFICIAL OVERVIEW ↗
USGS overview of deep heat and geothermal flow.
- 02USGS · Some unanswered questions: This Dynamic EarthUSGS SYNTHESIS ↗
USGS discussion of residual and radioactive sources.
- 03USGS Volcano Watch · Heat is deep and magma is shallowUSGS EXPLAINER ↗
USGS explanation separating deep heat from shallow magma.
- 04NASA Astrobiology · Radiogenic heat and rocky planetsNASA CONTEXT ↗
NASA context for long-lived radioactive elements in rocky planets.