ICELAND-VOLCANO-BARDABUNGA

Volcanoes have been a persistent feature on Earth since the planet condensed out of the primordial nebula of our solar system. The scale and style of that volcanism has changed dramatically over that 4.5 billion years–heck, after Thera bumped into proto-Earth to form the Moon, we probably had a planet-wide lava lake as the molten Earth coalesced and cooled from the collision. However, we lack much of a record of that tumultuous time beyond a few zircon found in younger sediments. Figuring out what exactly the volcanism might have been like that far back is a little bit of scientific storytelling.

If we look at the first few billion years of the planet, we can guess that we might have seen some very different kinds of volcanic eruptions. During the Archean Eon (~3.8 to 2.5 billion years ago), a type of lava that has been rarely seen since erupted in many places across the early Earth: komatiite, a type of magma that is hotter, more liquid, and denser than any lava that erupts today.

Basalt, which is common in volcanic eruptions across the globe, is the most mafic (that is, lowest silica and highest magnesium) lava erupting on modern Earth. It is usually 45 to 52 percent silica by weight and full of plagioclase feldspar, olivine, and pyroxene. When basalt erupts, it is typically over 1100oC and has a relatively low viscosity so that gas can escape, leading to lava flows like what we see in Hawaii.

But early-Earth komatiites are even more mafic–actually, we’re talking ultramafic (yes, that’s a real geologic term). Komatiites typically are less than 45 percent silica by weight and are chock full of olivine and pyroxene, making them much more magnesium and iron-rich (and denser) than basalts. A typical komatiite is 18 percent magnesium by weight, roughly double that of your typical basalt. Those changes in composition means that komatiites are hot, erupting at temperatures between 1300-1600oC. Some komatiite lavas even have chromite crystals, betraying how much chromium they have.

This composition, with abundant magnesium, iron, and chromium, indicates that komatiites are a product of melting the Earth’s mantle. The composition of komatiite lava is pretty close to what we might expect if we were able to sample the mantle underneath our feet, so if you melt a good portion of it (maybe 50 to 60 percent!), you’d get a komatiite composition. Basalt is also derived from melting the mantle, but in that case, only 10 to 20 percent is typically melting thanks to fractional melting (the lowest temperature minerals melt first, leaving behind the more mafic minerals/elements).