Moon's Mafic Mound Created by Volcanic Process Triggered by Massive Impact

Posted on October 18, 2015

There is a giant mound near the Moon's south pole called Mafic Mound. Geologists from Brown University believe the mound was formed by unique volcanic processes set in motion by a massive impact. This colossal impact also formed the South Pole-Aitken impact.

Daniel Moriarty, a Ph.D. student in Brown's Department of Earth, Environmental and Planetary Sciences and lead author of the study, says in a statement, "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."

The mound was first discovered in the 1990s by Carle Pieters, a planetary geologist at Brown and Moriarty's adviser. The term mafic in geology is used to describe rocks rich in minerals such as pyroxene and olivine. The mound itself has a different mineralogical composition than the rock that surrounds it. Mafic Mound is rich in high-calcium pyroxene while the surrounding rock is low-calcium.

The researchers used mineralogical data from the Moon Mineralogy Mapper, which flew aboard India's Chandrayaan-1 spacecraft to study the unique mound. They also used data from NASA's Lunar Orbiter Laser Altimeter and the GRAIL mission. They determined that Mafic Mound was created by one of two possible unique volcanic processes. Each process was set in motion by the giant South Pole-Aitken impact.

One idea is that the massive impact created a cauldron of melted rock as much as 50 kilometers deep. As the sheet of impact melt cooled and crystallized it shrank and still-molten material in the middle of the melt sheet was squeezed out of the top like toothpaste from a tube forming the mound. Another scenario that fits the data involves the melting of the Moon's mantle shortly after the South Pole-Aitken impact. The impact would have blasted tons of rock and material out of the basin, creating a low-gravity region. This lower gravity condition could have enabled the center of the basin to rebound upward causing partial melting of mantle material, which then erupted to form the mound.

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

A topographical view of the South Pole-Aitken Basin is pictured above. The reddish area in the center is Mafic Mound. It stands 800 meters above the surrounding surface. A research paper of the volcanic processes that formed Mafic Mound was published here in the journal, Geophysical Research Letters.

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