The new rock core drilled at the crater impact site that is thought to have help cause the extinction of the dinosaurs 65 million years ago has helped reveal the crater’s formation and shape, including the existence of an inner ring of mountains which scientists call a peak-ring.
After a decade of planning, the project penetrated 1,335 metres into the sea floor off the coast of Progreso, Mexico, in April and May. Drillers hit the first peak-ring rocks at a depth of 618 metres, and a pinkish granite at 748 metres. Geologists know that the granite must have come from relatively deep in the crust — perhaps 8–10 kilometres down — because it contains big crystals. The size of these crystals suggests that they formed by the slow cooling of deep, molten rock; in contrast, rapid cooling at shallow depth tends to form small crystals. Finding the granite relatively high in the drill core means that something must have lifted it up and then thrown down it on top of other rocks.
That rules out one idea of how craters form, in which the pulverized rock stays mostly in place like hot soup in a bowl. Instead, the core confirms the ‘dynamic collapse’ model of cosmic impacts, in which the asteroid punches a deep hole in the crust, causing the rock to flow like a liquid and spurt skyward. That rock then falls back to Earth, splattering around in a peak ring.
To put it another way, the impact moved the earth like a pebble dropped into a pond of water, causing at least two big circular ripples that flowed just like water but then quickly froze in place to form the two concentric peak-ring mountain ranges.