EARTH

When the Whole Planet Was Molten

~4.5 billion years ago

Hadean

For its first tens of millions of years the Earth had no solid ground. The energy of its own assembly, then the collision that formed the Moon, kept rock molten hundreds of kilometres down. In that planetary melt, iron, densest of all, sank to the centre and pooled there as the core, while the rock left above it froze from the bottom of the mantle upward. At the very base of the mantle, seismologists still map pockets where their waves drag oddly; these may be what the crystallisation of that buried ocean left behind.

Why it matters

The planet came out of that ocean in layers, and each has kept its function: the iron core generates the magnetic field, the mantle drives the drift of the continents, and the lightest rock, risen to the surface, became the first crust.

Dating & uncertainty

Radioactive clocks place the making of the core within the planet's first thirty million years or so. The ocean of molten rock above it left nothing to date: how deep it ran and how long it took to freeze are known only through models and chemical budgets, because no rock survives from it.

Sources

1. Elkins-Tanton (2012), Annual Review of Earth and Planetary Sciences · Magma oceans in the inner solar system
2. Kleine et al. (2002), Nature · Rapid accretion and early core formation on asteroids and the terrestrial planets from Hf-W chronometry
3. Hu et al. (2024), National Science Review · Earth's core-mantle boundary shaped by the crystallization of a hydrous terrestrial magma ocean

Earth history as one day

00:12:4123 h 47 min before midnight

if Earth’s 4.54 billion years were compressed into a single day