These particular structures, called grabens, consist of blocks of the crust that dropped down due to extension. The extension resulted from the subsidence (sinking) of central Humorum basin under the weight of dense basalt deposits known as Mare Humorum. The grabens curve concentrically along the margin of the basin (26.53°S, 330.16°E).
Volcanic activity in Mare Humorum peaked during the middle Imbrian period (3.3–3.7 Ga), allowing subsidence and cooling to exert prolonged stress on the lithosphere. This stress led to the crustal extension at the basin margins, which formed the grabens. Sinking can occur gradually over millions of years or episodically through large-scale moonquake events. In the case of Rimae Hippalus, the shapes of each of the valleys indicate that they underwent different stages of formation at various times. The lower-relief, less distinct grabens suggest subsequent volcanic episodes and partial lava filling after their initial formation.
These lunar landforms are particularly exciting to find because they resemble analogous structures on Earth. Examining these analogs can provide valuable insights into the processes that shaped the lunar surface. Additionally, scientists use structures such as these expansive grabens to reconstruct the timeline of impact events, mountain formation, and episodes of tectonism and volcanism that intersect these elongated, linear features, applying the principle of cross-cutting relationships.
Lunar Reconnaissance Orbiter Narrow Angle Camera (NAC) image of the westernmost unit, Rimae Hippalus I. Image ID: M150720890L/R - [NASA/GSFC/Arizona State University].
Rimae Hippalus III, the easternmost unit. Image ID: M1169722500L/R [NASA/GSFC/Arizona State University].
The middle unit, Rimae Hippalus II. Image ID: M1382500773L/R [NASA/GSFC/Arizona State University].
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Published by John Pierangelo on 22 July 2025