What specific morphological challenge did the prevalence of circular craters pose to the long-standing volcanic hypothesis?

Answer

They looked consistently like nested bowls with central peaks

A significant challenge for volcanists was explaining why the numerous circular structures looked so consistently like nested bowls that possessed central peaks, unlike the more varied shapes typically associated with terrestrial volcanoes.

What specific morphological challenge did the prevalence of circular craters pose to the long-standing volcanic hypothesis?
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