State of the planet· The columns of time

Magma ocean

4.54 Ga → 4.4 Ga

From 4.54 to 4.4 billion years ago, the surface of the planet was molten, kept so by the energy of accretion and then by the giant impact that formed the Moon.

Regime during which the surface of the planet is molten, held in that state by the energy of accretion and then by the giant impact that forms the Moon.

What the evidence shows

Through its first hundred and forty million years, the Earth has no surface in the ordinary sense. Rock is molten to great depth, held so by the heat of accretion and then by the lunar impact. No trace of that state survives. Its end is dated by a later mineral: the Jack Hills zircons, in Western Australia, around 4.4 billion years, whose oxygen isotopes record interaction with liquid water at low temperature. A temperature compatible with liquid water is equally compatible with the existence of a crust. This band constitutes a backdrop as much as a regime, and the composition accompanying it observes the same restraint: no cones, no vents, no horizon, and nothing in the sky. The bombardment of the early Earth is established, but the trails of falling debris produced suspended lines that the eye read as rain, and an image clearly communicating something false is less useful than one communicating less.

Bounds and conventions

Nothing directly dates the end of the molten state, a molten surface leaving no datable rock. The zircon that bounds it formed afterwards, in a crust already constituted: it supplies a minimum age and no maximum, the magma ocean having possibly solidified a hundred million years before that grain with no data able to establish it.