Earth once erupted lava as fluid as water, but it stopped billions of years ago.

Nature · Published 3 October 2026 Swipe more like this →

In short: Komatiites are rare, ancient volcanic rocks that erupted at temperatures exceeding 1600 degrees Celsius. Unlike the thick, honey-like basaltic lava we see today, these ultramafic flows were incredibly fluid, spreading across the early Earth’s surface in layers as thin as 10 millimeters before the planet’s cooling mantle silenced them.

Komatiites are defined by their high magnesium oxide content, typically exceeding 18 weight percent. Named after the Komati River in South Africa, these rocks are almost exclusively restricted to the Archaean Eon, spanning 4.03 to 2.5 billion years ago. Their disappearance from the geological record is largely attributed to the cooling of Earth’s mantle, which was likely 100 to 250 degrees Celsius hotter during the Archaean. This intense heat was fueled by residual energy from planetary accretion and the decay of radioactive isotopes like uranium-235.

The physical behavior of komatiitic lava was extraordinary. With a viscosity approaching that of water, it moved with a speed and fluidity that modern basaltic lavas cannot replicate. This rapid flow often resulted in distinct stratigraphic features, including the famous 'spinifex' texture—named after Australian Triodia grass—which consists of long, bladed crystals formed by quick cooling in a thermal gradient. These flows frequently organized into channelized structures, with high-magnesium olivine accumulating in the center while thinner, spinifex-rich sheets formed at the margins.

Geologists classify these rocks into two geochemical groups based on their aluminum-to-titanium ratios, though recent evidence from fluid inclusions suggests that a single flow might represent a mixture of magmas rather than distinct source depths. Today, all known komatiites are technically 'metakomatiites,' as they have undergone significant alteration through metamorphism. Depending on the partial pressure of carbon dioxide in metamorphic fluids, they transform into either carbonated rocks rich in talc and magnesite or hydrated assemblages dominated by serpentine and chlorite. While rare, these rocks remain vital for understanding the extreme thermal conditions of our planet’s infancy.

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