5. Earth's Internal Heat & the Magnetic Field

Earth's interior is astonishingly hot — hot enough at the center to rival the Sun's surface. Two sources keep it that way over billions of years:

1. Primordial heat — leftover heat from Earth's violent formation and differentiation.
2. Radiogenic heat — heat constantly released by the radioactive decay of elements like uranium, thorium, and potassium-40 in the rock.

That heat slowly escapes toward the surface (you can measure it: temperature rises with depth — the geothermal gradient). The escaping heat drives convection: hot material rises, cooler material sinks, in slow loops in the mantle and in the liquid outer core.

That convection of liquid iron in the outer core, combined with Earth's rotation, acts like a giant electric generator — the geodynamo. It produces Earth's magnetic field, which is shaped roughly like a bar magnet's (a dipole). The magnetic poles sit near, but not exactly at, the geographic poles, and they slowly wander; over geologic time the field has even reversed direction many times.

Why care? The magnetic field forms a protective bubble (the magnetosphere) that deflects much of the solar wind and cosmic radiation, helping shield the atmosphere and life — and it is why a compass needle points north.

⚠️ Common misconception: Earth's magnetism is not from a giant permanent magnet inside. At these temperatures iron is far too hot to stay permanently magnetized (it is above the Curie point). The field is generated by moving liquid metal — a dynamo — and would fade if the core stopped churning.

💡 Tip: heat (from formation + radioactivity) → convection of liquid iron → spinning dynamo → magnetic field → shields the planet.