A massive and widespread mound discovered at our moon's south pole, with a mineral makeup different from surrounding formations, may have resulted from long-ago volcanic processes, scientists say.

It is distinct in its form from any other mound or upwelling seen anywhere else on the lunar surface, they say.

Stretching almost 50 miles across, the formation dubbed Mafic Mound sits in the very center of the South Pole-Aitken Basin, a huge impact crater, and the volcanic event that formed it may have itself been triggered by the massive impact that created the basin, researchers at Brown University say in a study appearing in Geophysical Research and Letters.

"If the scenarios that we lay out for its formation are correct, it could represent a totally new volcanic process that's never been seen before," says doctoral student Daniel Moriarty in Brown's Department of Earth, Environmental and Planetary Sciences.

The mound, discovered in the 1990s by Brown planetary geologist Carle Pieters, is named for a term denoting rocks that are rich in mineral traces such as olivine and pyroxene.

There are a couple of possibilities for how the impact that formed the basic could have triggered the formation of the extensive mound in its middle, the researchers say.

Some scientists have suggested the giant impact formed a vat of melted rock some 30 miles deep. As the cauldron containing it began to cool and shrink, it may have forced molten material out of  its center, like toothpaste being squeezed out of a tube.

Such material would be rich in calcium-rich pyroxene, which matches what has been identified in the mound.

In another possible scenario, the massive impact could have excavated huge amounts of rock from out of the basin, creating an area of low gravity in its center that would have allowed underlying mantle to force itself upward, partially melting and creating the mound.

"This unusual structure at the very center of the basin begs the question: What is this thing, and might it be related to the basin formation process?" Moriarty says.

Either of the two possibilities fits with the data gathered on the mound so far, the researchers say, so a lunar mission to retrieve and return samples from the basin might help identify the exact processes that created the mound.

The South Pole Aitken Basin has long intrigued scientists studying the moon.

"It's the largest confirmed impact structure in the solar system and has shaped many aspects of the evolution of the moon," Moriarty notes. "So a big topic in lunar science is studying this basin and the effects it had on the geology of the moon through time."