Passage I

Unlike most impact craters, lunar impact basins (LIBs) develop far from typical meteorite impacts. Two students discuss the origin and properties of LIBs.

Student 1
In the lunar crust beneath an LIB, at depths between 50 km and 150 km, molten impact melt rises toward the Moon’s surface in one large pool called a melt sheet. The cooling melt sheet causes thermal expansion and creates networks of large fractures in crustal rocks. Formation of these fracture networks enables melt to reach the surface more efficiently, which is why the frequency of volcanic activity at an LIB typically increases over time. Melt that does not breach the Moon’s surface will cool and eventually solidify back down into the crust.

LIBs erupt silicate-rich lavas that are chemically similar to crustal rocks. Plagioclase and pyroxene are the most abundant minerals in crustal rocks and in the lavas erupted at LIBs. The lavas at LIBs also retain a lot of titanium from the crust. By weight, titanium dioxide accounts for 75% of the total mineral output at LIBs, while magnesium oxide accounts for only 15%–20%. All other minerals combined never account for more than 10%–15% of the total mineral output. Mare Imbrium, on the Moon, erupts this way.

Student 2
In certain places near the edge of the crust, at depths of less than 50 km, a high concentration of dissolved aluminum allows crustal rocks to melt at lower temperatures than they normally would. This is how the impact melt that fuels an LIB forms. The melt then rises toward the Moon’s surface in small isolated bodies that melt through the entire thickness of the crust, sometimes causing small fractures (less than 5 km long) in crustal rocks. Each eruption at an LIB depletes some of the excess aluminum, which is why eruption frequency at an LIB will typically slow down over time.

LIBs erupt lavas in which olivine is the most abundant mineral. These aluminum-rich lavas contain much less titanium than most crustal rocks. By weight, magnesium oxide and aluminum oxide each account for 30% of the total mineral output at LIBs. Titanium dioxide accounts for 25% of the total mineral output, and all other minerals combined account for only 15%. Mare Orientale, on the Moon, erupts this way.