September 4, 2013

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Three Unique Volcanic Centers

image by Ross Sackett

Three of the largest volcanic centers on the Moon occur along a visual line in the middle of northern Oceanus Procellarum. At the northern end (right) is Rümker, the smallest and least conspicuous center. But Rümker is, like the other two volcanic centers, a unique landform, in this case a dome pile, a collection of overlapping volcanic domes. The individual domes are similar in size to stand-along domes such as those near Hortensius, but the collection rises about 800 m above the mare. Next in line is perhaps the most unique structure on the Moon. The Aristarchus Plateau is probably mostly a structurally uplifted feature - like the nearby Harbinger Mountains - that is covered by mare lavas and veneered with pyroclastic deposits erupted from the Cobra Head source vent of Schröter's Valley. The final unique volcanic center is the Marius Hills (left), another collection of volcanic vents, but this time mostly spread laterally rather than on top of each other. The 200 plus steep-sided cones rest on and some are partially embayed by a broad elevated area whose eastern and northern limits are clearly seen here extending past the cones. Back in the late 1960s the colorful planetary scientist pioneer Hal Marsursky speculated that these three anomalously big and exotic volcanic centers were along the equivalent of a terrestrial mid-ocean spreading ridge. That would be a fundamental fracture in the lunar crust, and the three volcanic centers could be the equivalents of Iceland, a unique volcanic island on Earth because of its formation on a spreading center. Ross' image does show that a series of mare ridges connect the three volcanic centers, but the likely origin of these ridges is as low angle thrust faults marking rings due either to the putative Procellarum Impact Basin or an obliquely-formed Imbrium Basin. A basin ring fracture probably extends through the depth of the mare that it cuts but not necessarily entirely through the crust to some source of magma. Forty years after Hal's suggestion we known much more about the Moon, but still don't have a good explanation for the origins of these thrre volcanic centers.

Chuck Wood

Technical Details
Sept 1, 2013. 11" Celestron SCT with an ASI120MM camera; mosaic of three images, each a stack of 1000 frames.

Related Links
21st Century Atlas chartS 27 & 28.

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