An ancient volcanic eruption on the Red Planet produced so much lava that it caused the entire planet to tip over. This remarkable finding, detailed in a study in the journal Nature, provides another startling example of how volcanoes can change a world.

There have been some pretty powerful volcanic eruptions throughout Earth’s dramatic history. Some are violent, like the cataclysmic two-part eruption of part of the Yellowstone supervolcano 2.1 million years ago that buried much of North America in ash. Some are prolonged and deadly, like the outburst at the Siberian Traps that contributed towards the world’s worst mass extinction event, the “Great Dying.”

It’s unlikely, however, that any volcanic eruption on Earth was powerful enough to cause the crust to collapse in on itself. Incredibly, this is precisely what happened on Mars thanks to the formation of a region called Tharsis.

A huge volcanic plateau near the Martian equator, Tharsis contains some of the largest volcanoes in the Solar System. Martian volcanoes tend to be of the shield variety, very similar in shape and behavior to Earth’s Hawaiian shield volcanoes. These huge, extremely wide but relatively short beasts tend to continuously and slowly erupt lava over extremely long periods of time, normally until the hotspot fueling them from below either dies or moves on.

Tharsis’ volcanoes were no exception to this, but the volume of erupted lava found in this region is staggering: it collectively weighs a billion billion tonnes. Tharsis as a whole is over 5,000 kilometers (3,100 miles) in width and 12 kilometers (7.5 miles) thick. A 3.5-billion-year-old eruption gradually forced this ginormous amount of lava to the surface over the space of 2 million years.

^{The formation of Tharsis caused at least a 20-degree shift in the axial tilt of the Red Planet. Bouley et al./Nature}

“The Tharsis dome is enormous, especially in relation to the size of Mars. It's an aberration,” Sylvain Bouley, a geomorphologist from Universite Paris-Sud and lead author of the study, said in a statement.

A previous study in 2010 demonstrated that if Tharsis was removed from Mars, the planet would shift on its rotational axis to compensate for the sudden weight loss. Using computer simulations, Bouley’s team worked out what Mars would have been like both before and soon after this massive volcanic eruption occurred.

The gargantuan movement of molten material from the depths to the surface temporarily flipped part of Mars’ upper geology: The solid crust of the planet swiveled around the partly-molten mantle layer beneath it. Incredibly, this chaotic, relatively quick eruption of a vast volume of lava caused the entire planet to tilt downwards by 20 to 25 degrees.

In other words, the geographical north and south poles were in a very different place from where they are today. “If a similar shift happened on Earth, Paris would be in the Polar Circle,” said Bouley. “We'd see Northern Lights in France, and wine grapes would be grown in Sudan.”

Previously unexplained features of the Martian surface make much more sense in light of this research. For example, sizeable underground reservoirs of frozen ice on Mars are today oddly close to the warm equator. Now we know that, prior to the formation of Tharsis, these icy caches would have once resided under the frigid poles.