Why Are Craters Always Round? Nothing to Do With the Angle

An object arriving at orbital speed carries kinetic energy comparable to a large explosive charge of its own mass, and on striking it is stopped within roughly its own diameter, releasing all of that energy into a tiny volume almost instantly. The result is an explosion, and an explosion expands as a sphere regardless of what direction anything was travelling, so the excavated hole is circular. Only impacts at a very shallow angle, below about fifteen degrees, produce elongated craters, and those are rare. A gouge shaped like the arriving object would require a slow collision, which is what people intuitively imagine and which does not happen at these speeds.

Distinguishing an impact structure from a volcanic or collapse feature requires specific evidence:

• •Shocked quartz, with internal features produced only by extreme pressure
• •High pressure forms of silica that form nowhere else on Earth naturally
• •Shatter cones, distinctive fractured rock radiating from the impact point
• •Melted rock in sheets and droplets
• •A raised rim of overturned strata around the hole
• •Traces of the impacting body itself in the melt, detectable chemically

The Arizona crater was regarded as volcanic through the nineteenth century, and a mining engineer named Daniel Barringer argued from 1903 that it was an impact and spent decades and a fortune drilling for the enormous iron mass he expected to find buried beneath it. He never found it and died believing the search had failed. The resolution came from Eugene Shoemaker in the 1960s, who identified high pressure silica minerals at the site that can only form under impact conditions, establishing the origin conclusively, and who explained why Barringer found nothing, since the impacting body is largely vaporised by the energy released rather than buried intact. That work founded the scientific study of impacts.

The impact associated with the end of the dinosaurs was inferred from chemistry long before anybody located the hole. A layer of clay at the boundary was found in 1980 to contain far more iridium than Earth's crust normally does, and iridium is abundant in meteorites, which suggested a large impact. The structure itself was identified in the early 1990s beneath the Yucatan peninsula, buried under a kilometre of sediment and recognised from gravity and magnetic surveys conducted years earlier by an oil company that had not been looking for it. It is around one hundred and eighty kilometres across. Drilling into it in 2016 recovered core through the ring of uplifted rock at the centre, which confirmed the structure in detail.

The moon is covered in craters and Earth shows around two hundred confirmed structures, which is not because Earth is struck less often. Erosion removes surface features continuously, so anything older than a few tens of millions of years is degraded or gone. Plate tectonics destroys ocean floor entirely on a cycle of roughly two hundred million years, and most impacts land in the sea. Sediment buries structures, so many are known only from geophysical survey rather than from anything visible. Vegetation conceals them. The moon has no atmosphere, no water, no plate movement and almost no erosion, so it retains everything that has ever hit it, which is why its surface is a record and Earth's is not.

An object arriving at orbital speed releases its energy almost instantly into a small volume, so the event is an explosion and the hole is circular regardless of the arrival angle. Shocked quartz, high pressure silica and shatter cones identify a genuine impact structure. Barringer drilled for decades for a buried mass that had been vaporised, and Shoemaker settled the question in the 1960s.