Question 1

Why is the Earth's core still hot after billions of years?

Accepted Answer

Two reasons: heat left over from the planet's formation, and heat still being generated now by radioactive decay. Together they account for the roughly **5,000–6,000°C** at the inner core — comparable to the Sun's surface.

**Primordial heat**, roughly half the total. Earth formed about 4.5 billion years ago through **accretion** — countless collisions of rock and dust, each converting kinetic energy into heat. The **giant impact** thought to have formed the Moon added enormously. Then **differentiation** occurred: as the young Earth melted, dense iron sank to the centre, and that gravitational settling released a vast quantity of energy. Some of that original heat is still leaking out.

**Radiogenic heat**, roughly the other half. Long-lived radioactive isotopes distributed through the Earth — principally **uranium-238, uranium-235, thorium-232 and potassium-40** — decay continuously, releasing heat. Their half-lives are measured in billions of years, which is why they are still supplying energy. This is ongoing generation, not a leftover.

**Why it escapes so slowly.** This is as important as the heat sources. Rock is an extremely poor conductor of heat, and Earth has an enormous **volume-to-surface-area ratio**. Heat generated deep inside must travel thousands of kilometres to reach the surface, and the crust acts as an insulating blanket. The planet loses heat at roughly 47 terawatts globally, which is tiny relative to the reservoir.

**Convection in the mantle** carries heat more effectively than conduction, and it is what drives **plate tectonics** — so earthquakes, volcanoes and mountain building are all consequences of the Earth cooling.

**Why it matters.** Heat drives convection in the liquid outer core, generating Earth's **magnetic field**, which shields the atmosphere from solar wind. Mars is thought to have lost much of its atmosphere partly because its smaller core cooled and its field weakened.

**Earth is cooling**, just very slowly.

Question 2

How do we know how old the Earth is?

Accepted Answer

Principally from radiometric dating of meteorites, which give an age of approximately 4.54 billion years — and the reason meteorites are used rather than Earth rocks is itself the interesting part.

Question 3

Why is nuclear fusion so hard?

Accepted Answer

Because you must force atomic nuclei — all positively charged and therefore violently repelling each other — close enough to fuse, which requires conditions comparable to the inside of a star, and

Question 4

What causes ice ages?

Accepted Answer

A combination of orbital variations setting the rhythm and feedbacks within the climate system amplifying them — with the underlying precondition being a continental arrangement that permits ice

Question 5

How does the geological time scale work?

Accepted Answer

A hierarchy of named intervals dividing Earth's history, defined by changes in the rock and fossil record rather than by round numbers — which is why the boundaries fall at irregular dates. The

Question 6

How do stars form, and what happens when they die?

Accepted Answer

Stars form when gravity overcomes pressure in a cloud of gas and dust, and they die when they run out of fuel to resist gravity with. The entire life of a star is a standoff between those two forces,

Question 7

What is an isotope, and why do isotopes matter?

Accepted Answer

Atoms of the same element with different numbers of neutrons — so they share chemical behaviour but differ in mass and, crucially, in nuclear stability. The distinction that defines it. An element is

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