The young Earth's mantle harboured two magma oceans which were layered like a pudding cake, scientists say.
Using the world's most brilliant X-ray source, scientists have for the first time peered into molten magma at conditions of the deep Earth mantle.
The analysis at Deutsches Elektronen-Synchrotron (DESY)'s light source PETRA III revealed that molten basalt changes its structure when exposed to pressure of up to 60 gigapascals (GPa), corresponding to a depth of about 1400 kilometres below the surface.
At such extreme conditions, the magma changes into a stiffer and denser form, the team led by first author Chrystele Sanloup from the University of Edinburgh said.
The findings support the concept that the early Earth's mantle harboured two magma oceans, separated by a crystalline layer.
Today, these presumed oceans have crystallised, but molten magma still exists in local patches and maybe thin layers in the mantle.
"Silicate liquids like basaltic magma play a key role at all stages of deep Earth evolution, ranging from core and crust formation billions of years ago to volcanic activity today," Sanloup said.
To investigate the behaviour of magma in the deep mantle, the researchers squeezed small pieces of basalt within a diamond anvil cell and applied up to roughly 600,000 times the standard atmospheric pressure.
The team used two strong infrared lasers that each concentrated a power of up to 40 Watts onto an area just 20 micrometres (millionths of a metre) across - that is about 2000 times the power density at the surface of the Sun.
"For the first time, we could study structural changes in molten magma over such a wide range of pressure," said co-author Zuzana Konopkova from DESY.