Lunar lava layers and their Hawaiian analogs

Post contributed by Dr. M. Elise Rumpf, Astrogeology Science Center, US Geological Survey.

Images of the lunar surface reveal layered deposits presumed to be sequences of basaltic lava flows. These sequences have been imaged since the Apollo astronauts acquired both orbital and surface photographs in the 1960s and 1970s. Apollo 15 astronauts visited Hadley Rille, a 130 km long, 200 m deep sinuous feature that was formed by flowing lava, similar to lava channels or tubes on Earth. Photographs taken by the astronauts (such as Image 1) show that the rille cut into the underlying substrate, revealing sequences of layered material. The layers are believed to be basaltic lava flows, based on outcrop morphologies and nearby samples. The thicknesses of ancient lava flows provide insight into the emplacement, dynamics, and history of volcanism on the Moon.

Image 1: Apollo 15 surface image of the interior wall of Hadley Rille ( Inset highlights layered deposits presumed to be basaltic lava flows with possible intercalated regolith deposits. Outcrop is approximately 8 meters thick.

Basalt erupted onto large impact basins on the near side of the Moon during the last of several phases volcanism, peaking around 3.5 to 3.3 Ga, and perhaps lasting until as recently as 1.2 Ga. Estimates of the thicknesses of lava flows have varied widely, from 1 to 100s of meters, representing ranges in eruption conditions, as well as available data resolution and measurement techniques. Over the last decade, the Lunar Reconaissance Orbiter (LRO) Narrow Angle Camera (NAC) has provided panchromatic images at ~0.5 m/pixel, greatly improving our ability to interpret meter-scale features on the lunar surface, including layers of lava flows. Layered sequences have been observed in lunar pit craters, which are skylights above void spaces such as lava tubes, likely created by impacts into the surface. Layers have also been observed in the walls of large (greater than several km in diameter) impact craters. Image 2 shows a wall of Bessel Crater, which exposes layers presumed to be basaltic lava flows. LRO NAC observations  found a mean layer thickness of 9.7±2.9 m at Bessel Crater.

Image 2: Layered sequences of basaltic lava deposits in the walls of Bessel Crater. Discernible layers are separate outcrops of basalt, each may contain more than one individual lava flow. LRO image number M135073175R [NASA/GSFC/Arizona State University].

Hawaiian localities are commonly used as terrestrial analogs to geological features on other planetary bodies, particularly as analogs for lava flows and other volcanic features. In panchromatic satellite images, layered sequences of basalt in valley and cliff walls on O‘ahu are visibly similar to those seen on the Moon (Image 2). Comparison between layer thickness measurements made from WorldView-2 images (~ 0.5 m/pixel) and field-based measurements on O‘ahu found that satellite images did not reveal the thickness of individual lava flows. Instead, they revealed outcrop thickness, and roughly half of the outcrops contained more than one lava flow layer.

Image 3: Field photographs of lava flow contacts in valley and cliff walls on O‘ahu, Hawai‘i. Lines indicate contacts between individual lava flows and/or ‘a‘ā clinker layers. Small scale contacts such as these are not visible in satellite images. Redrawn after Rumpf et al., 2020.

As shown in Image 3, individual flow contacts are too small to be discerned at current satellite imaging resolutions. Layer contacts can be discerned where there are breaks in slope between outcrops. Thus, measurements of the thicknesses of layered sequences of basalt in terrestrial and planetary satellite images should be considered as the thicknesses of outcrops containing multiple lava flows, or the maximum thicknesses of individual flows. Therefore, it remains the case that identification of geological unit contacts is best accomplished by human or rover observations on a planetary surface.

Further reading:

Chen, Y., Li, C., Ren, X., Liu, J., Wu, Y., Lu, Y., Cai, W., Zhang, X., (2018), The thickness and volume of young basalts within Mare Imbrium. J. of Geophys. Res.: Planets 123, 630–645.

Howard, K.A., and J. W. Head III, (1972), Regional geology of Hadley Rille. in: Apollo 15: Preliminary Science Report. NASA SP-289, 53–58.

Mouginis-Mark, P.J., Rowland, S.K., (2008), Lava flows at Arsia Mons, Mars: insights from a graben imaged by HiRISE. Icarus 198, 27–36.

Robinson, M.S., Ashley, J.W., Boyd, A.K., Wagner, R.V., Speyerer, E.J., Ray Hawke, B., Hiesinger, H., and van der Bogert, C.H. (2012), Confirmation of sublunarean voids and thin layering in mare deposits. Planetary and Space Science, 69, 18–27.

Rumpf, M. E., H. Needham, and S. A. Fagents, (2020), Thicknesses of Lava Flows in Satellite Images: Comparison of Layered Mare Units with Terrestrial Analogs, Icarus 350, 113853.

Vondrak, R., Keller, J., Chin, G., Garvin, J., (2010), Lunar Reconnaissance Orbiter (LRO): observations for lunar exploration and science, Space Sci. Rev. 150, 7–22.

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