Meteorite clues to giant volcanoes on Mars

Without plate tectonics, Martian volcanoes grew huge. New research explains why, reports Andrew Masterson. 

An image of a piece of nakhilite meteorite about 1 mm across, taken in cross-polarised light. Different colours represent different volcanic minerals.
An image of a piece of nakhilite meteorite about 1 mm across, taken in cross-polarised light. Different colours represent different volcanic minerals.
Benjamin Cohen

Mars endured a much more volcanically active past than previously thought, but its volcanoes grew at a rate 1000 times slower than those on Earth, new research shows.

The fresh estimate for volcanic activity, published in the journal Nature Communications, is derived from an analysis of the composition of a group of meteorites known as the nakhilites, which are all thought to be the products of a single, long-lived Martian volcano.

Most volcanoes on Earth arise because of the pressures exerted by tectonics, with hotspots arising where the plates comprising the planet’s crust either collide or diverge.

A few, however, are caused by a different process, wherein a magma “plume” is pushed directly up from deep in the Earth’s mantle. This is especially the case with the Hawaiian island chain, which were (and are still being) created by a plume of molten rock.

The Hawaiian chain, however, is also influenced by plate tectonics. Research shows that as a volcano forms, the Pacific plate moves it inexorably away from the plume that is pushing it from below. Volcanoes in the Hawaiian chain grow older as they move away from the source plume.

In geologic time, too, the entire Hawaiian chain is remarkably young. A study published last year estimated the initial plate movement that uncovered the plume occurred only around three million years ago.

Mars, in stark contrast, does not have plate tectonic movements that influence the landscape. Instead the planet has a “stagnant lid”, an outer crust that never changes position.

It does, however, have magma plumes. This means that when such a plume ruptures the crust and erupts, depositing lava and other ejecta and thus catalysing the creation of a volcanic mountain, the rupture and the above-ground result will always stay in the same relationship to each other – for billions of years.

As a result, Mars has the largest and oldest volcanoes known in the solar system. Just how old and just how fast these grew has until now remained poorly understood.

To try to shed light on the matter, a team led by Benjamin Cohen of the Scottish Universities Environmental Research Centre in the UK, turned to meteorites.

The nakhilites are a group of 18 meteorites that over a period of time landed on Earth after a single large object slammed into a Martian volcano about 10.7 million years ago.

The meteorites comprise mostly basalt, interlaced with other minerals including clinopyroxene, olivine, feldspar and volcanic glass. They are all similar to each other but, crucially, not identical.

These small differences allowed Cohen and his colleagues to estimate when each was created, using a combination of laser step-heating and argon-based dating.

“The data show that the nakhilites were not all formed during a single cooling event, but instead reveal a protracted volcanic eruption history on Mars,” the scientists report.

Using the results, the team found that the volcano was the result of four discrete eruptions over a period of 93 million years. The volcano itself, they calculated, grew at a rate of only 400 to 700 millimetres every million years – orders of magnitude slower than plume-driven volcano growth on Earth.

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