A sprawling rock formation that is scattered across a vast tract of Mars — including the crater where a rover is expected to touch down in 2021 — turns out to be the remnants of ash from an ancient eruption. Geologists can use this discovery to explore the planet’s early volcanic history.

The formation is rich in the mineral olivine, which a leading theory attributed to a lava flow. Christopher Kremer and his colleagues at Brown University in Providence, Rhode Island, mapped the formation using data from several spacecraft, and calculated its thickness.

Examining the formation from this 3D perspective, the scientists concluded that the rock layer probably formed from volcanic ash that wafted across the landscape and then settled and hardened into rock. The formation likely took shape between about 3.96 billion and 3.6 billion years ago, during a time when scientists have proposed that explosive eruptions were more common on Mars.

NASA plans to launch its Mars 2020 rover next year and land it at Jerezo Crater, where the vehicle will be able to visit these rocks.