April 20, 2008
Ignore the Domes
image by Jim Phillips, Charleston, SC
All the little volcanoes in the Marius Hills divert us from noticing what else is going on there. The Hills is a low plateau of a few hundred separate volcanoes, each presumably fed from an a reservoir of magma perhaps at the base of the lunar crust, some tens of kilometers below the surface. The cones and domes are located on western Oceanus Procellarum, which has its own origin and history. What is the relationship between the Hills and the mare? USGS mapper Jack McCauley in 1966 considered the Hills to be Eratosthenian, which is younger than the surrounding Upper Imbrium mare lavas. More recent studies show that individual eruptions in the Hills occurred over a long time, with some being as old or perhaps older than the surrounding lavas. Interesting thoughts provoked by Jim's image concern the interaction between the mare ridges and the Hills. The most prominent ridge - perhaps because it is closest to the terminator - appears to pass completely through the Hills from south to north. Another ridge, just west of the crater Marius, extends at least half way through the Hills. A third ridge, with the crater Reiner on top of it, seems to stop just at the southern edge of the Hills. Lunar Orbiter 2 and 5 acquired some very high resolution images of this area, showing two rilles, called A and B by Greeley, which cut the prominent ridge, and to the north a cone seems to be superposed on the ridge. These are evidence that at least some of the volcanic features are younger than the mare and its ridges. But the fact that some of the smaller ridges do not seem to penetrate the Hills suggests a more complex relation. Also, we don't know the origin of these ridges. In a normal circular impact basin the ridges are associated with subsidence and lateral compression of the hardened mare lavas. But the Procellarum ridges may have a different origin, perhaps related to the giant Procellarum Basin. None-the-less, the ridges and the Hills have a relationship, but its apparently not simple.
Chuck Wood
Technical Details
April 18, 2008, 2:05 - 2:12 UT. TMB 8" F/9.
Related Links
Rükl plates 18 & 29
Yesterday's LPOD: New Mysteries in Familiar Territory
Tomorrow's LPOD: A Forgotten Corner
COMMENTS
(1) Chuck--I received an e-mail from you regarding Plato and the Alpine Valley that I believe was intended for Jim Mosher. Thought you'd like to know. Sorry, but I don't know how to access the e-mail functions on this site, so I'm posting my message here. --Bill
(2) Bill - I actually sent it to everyone on the Moon Wiki list so everyone would see the great new pages. You receive an email but you also get the message through the Wiki, which is where you can easily respond.
Thanks,
Chuck
(3) I too wonder about those ridges. In looking at my own notes when I observed this region I could not help but notice them over the domes. I had noted that they seem to form a valley like shape. But looking at the picture above its clearly more complex then a simple valley. Could such ridges form if they were covering some feature like a thrust fault? -Aethrae, Andrew Martin SFO
(4) Andrew - where we have seen ridges up close - especially near Ap 17 in eastern Serenitatis they are definitely made by thrust faulting, so we assume that all or most ridges are everywhere. At the Marius Hills I'd assume the ridges came first, but perhaps ridging and volcanism overlapped in time.
Chuck
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