Magma ocean

Magma oceans are periods during Earth's or any planet's accretion where it is, either in whole or in part, molten. In the early solar system, energy to melt objects came largely from the decay of radioactive aluminum-26. As planets grew larger, the energy was supplied from large or giant impacts. During its formation, the Earth likely suffered a series magma oceans resulting from giant impacts, the final one being the Moon-forming impact.

Magma oceans are integral parts of planetary formation as they facilitate formation of a core through metal segregation and an atmosphere and hydrosphere through degassing. Magma oceans may survive for millions to tens of millions of years, interspersed by relatively clement conditions.

Magma oceans are widely accepted to have existed on Earth, and the best chemical evidence for them is the abundance of certain siderophile elements in the mantle that record magma ocean depths of approximately 1000 km during accretion. A magma ocean also occurred on the Moon during and following its formation.

...
Wikipedia