I want to know why Mauna Loa has sunk into the Earth's crust creating a trench that surrounds the base of the volcano, and Olympus Mons doesn't have a similar trench.

rocky Cathie Batbie

Dear Cathie,

Your question brings out some interesting points about volcanoes and planets. The moat around the Hawaiian volcanic chain (including Mauna Loa) is the result of the weight of the volcanoes pushing the crust of the underlying oceanic plate down.

The plate is about 65 million years old and about 40 km thick (it is thinner than most plates of that age because it has been reheated by the hot spot). In contrast, the plate beneath Olympus Mons is about 1-2 billion years old and is about 200 km thick. In fact, the outer most layer of Mars is a single, thick lithospheric plate. It has enough strength to support the weight of a giant like Olympus Mons.

As I looked at images of Olympus Mons I wondered if there might be a moat there and it just cannot be detect with our present observations. Perhaps it has been filled with sediments or lava. Chuck Wood mentioned to me that some lava flows move down the scarp at the edge of the volcano and then move parallel to the scarp, a pattern one might expect if this area was topographically low. He also noted that there are some concentric fractures around Olympus Mons, perhaps caused by the weight of the volcano. Maybe the moat is there and our sensing techniques will someday detect it.

A final note: trenches are associated with subduction of one plate beneath another. Hawaii sits in the middle of the Pacific Plate, far from the nearest trench. Therefore, the depression around Hawaii is best described as a moat, not a trench.

Thanks for a great question.

Steve Mattox and Chuck Wood, University of North Dakota 

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