Researchers believe that peculiar rock formations observed by NASA's Spirit rover in Gusev Crater, and more recently by Perseverance in Jezero Crater, are the results of some catastrophic explosive eruptions from vast and ancient volcanic calderas. 

There have been long-standing questions about the presence of bedrock on Mars that is rich in olivine, a silicate mineral that comes from magma. Mars has a history of volcanism, including sporting the tallest volcano in the Solar System – Olympus Mons – but it was not clear how these deposits actually formed. Anything is fair game, from lava flows to powerful impacts, to releasing magma from the mantle itself.  

As reported in the journal Icarus, a research team led by Steve Ruff from Arizona State University believes that the deposits are also rich in ignimbrite, a type of rock that defies simple categorization by being both igneous rock and sedimentary. Ignimbrite rock is formed from pyroclastic material ejected from explosive volcanic eruptions. Peculiar images taken by Spirit’s Microscopic Imager showed remarkable similarities to these types of rocks here on Earth.

“That was a eureka moment,” Ruff said in a statement. “I was seeing the same kind of textures in the rocks of Gusev crater as those in a very specific kind of volcanic rock found here on Earth.”

Ignimbrite is not an easy rock to make at all. On Earth, it is the product of huge volcanic explosions where flows of pyroclastic ash, pumice, and other rocks get deposited on thick layers over a relatively short time.

“Imagine a ground-hugging cloud of hot gases and nearly molten ash and pumice flowing through the landscape for dozens of miles and piling up in layers up to hundreds of feet thick in just a few days,” Ruff added.

The hot deposits cool down over months and years after the eruption, producing particular cooling fractures. Patterns in the olivine-rich rocks of Mars are remarkably similar to what we see in ignimbrite rocks found in places like Yellowstone National Park.

“No one had previously suggested ignimbrites as an explanation for olivine-rich bedrock on Mars,” Ruff said. “And it’s possible that this is the kind of rock that the Perseverance rover has been driving around on and sampling for the past year.”

The most recent deposits of ignimbrite on Earth are not particularly rich in olivine but some of the oldest ones are, so it is certainly possible that this might be the case on Mars. Perseverance has collected samples that will be flown back to Earth in a future mission, which will allow testing this hypothesis for sure.