The forensic of lost impact craters on Earth
In a previous post, I discussed the scarcity of the impact crater record on Earth compared to other rocky planetary bodies in the Solar System. Our dynamic planet quickly — geologically speaking — erases topographic evidence of craters through erosion, infilling, and plate tectonic crustal recycling. Only a handful of craters can be identified from aerial view, owing to a combination of favourable geological and climatic conditions.
But not all craters that are invisible on the present-day surface are completely lost. The record of their past existence is simply locked somewhere else — and geological and geophysical data can help reveal it.
What an impact leaves behind
The creation of an impact crater is not merely the formation of a large hole in the ground. It is accompanied by numerous physical modifications of the surroundings: scattering of excavated material (called ejecta) around the crater; pulverisation, melting, and shock metamorphism of rocks and minerals; and pervasive fracturing of surrounding rock. Some of the diagnostic products of impact events are rocks called suevite and shocked quartz.
Buried impact craters also leave signatures detectable in geophysical data — seismic, radar, gravimetric, and magnetic imaging. The figure below shows two examples: a large impact crater beneath Hiawatha Glacier in northwest Greenland, identified through radar sounding (left; from Kjær et al., 2018, open access), and a large impact crater discovered in the Barents Sea using seismic profiles (right; from Gudlaugsson, 1993).
Large impacts can also trigger chains of events — tsunamis, climate shifts — whose evidence is retained in the stratigraphic record. All of this can help build a case for a past impact even when no topographic trace of a crater survives.
The Chicxulub case: piecing together the puzzle
The most famous crater-hunt is the one that led to the discovery of the Chicxulub crater, formed by the asteroid impact that drove the extinction of the dinosaurs. In the late 1970s, it was proposed that the Cretaceous–Paleogene (K-Pg) mass extinction was caused by an impact, based on the discovery of a thin clay layer globally enriched in iridium — an element rare on Earth but common in asteroids.
In the same years, two geophysicists from Pemex (the Mexican state oil company) identified an offshore structure north of the Yucatán peninsula in magnetic and gravity survey data — a circular structure interpreted as a crater around 180 km in diameter. It was only in the 1990s that academic researchers learned of this earlier discovery. They requested analysis of drill samples from Pemex wells and found evidence of shock-metamorphism consistent with an impact event.
A separate team conducting satellite imagery analysis of the Yucatán peninsula identified a ring of sinkholes — called cenotes — which they interpreted as evidence of limestone weakened by the impact and subsequently collapsed. Further evidence came from large ripple-shaped marks scoured by impact-generated waves, found in the sedimentary record beneath Louisiana, and from tsunami deposits and chaotically mixed fossil assemblages found thousands of kilometres away.
After decades of collecting pieces of the puzzle, the scientific community concluded that the Chicxulub impact triggered the mass extinction approximately 66 million years ago. You can download the key paper here.