November 20, 2025
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) cancels topographic evidence of craters, through erosion and filling of the cavity or through plate tectonic crust recycle. Only few craters can be identified from aerial view, due to a combination of favourable geological and climatic conditions.
But not all the craters that are not visible on the present-day surface are completely lost. The record of their past existence is just locked somewhere else. Geological and geophysical data can help revealing this record.
The creation of an impact crater does not merely mean the formation of a big hole in the ground. This is accompanied by numerous other physical modifications of the surroundings: scattering of excavated material (called ejecta) around the crater; pulverization, melting, and shock metamorphism of rocks and minerals; pervasive fracturing of the rocks surrounding the crater. Some of the diagnostic products of impact events are rocks called suevite and shocked quartz
Buried impact craters also leave signatures that can be identified in geophysical data, such as seismic, radar, gravimetry and magnetic imaging. For example, the figure below shows 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, no open acces)
In addition, large impacts can trigger a series of chain of events, from tsunamis to climate changes, whose evidence can be retained in the stratigraphic record. All this can help building a case for a past impact, even if there is no longer topographic evidence of a crater.
The most famous crater-hunt is the one that led to the discovery of the Chicxulub crater, formed by an asteroid impact that led to the exinction of the dinosaurs. In the late 70's, it was proposed that the Cretaceous-Paleogene (K-Pg) mass exinction was caused by an impact event based on the discovery of a thin layer of clay rich in Iridium, an element rare on Earth but common in asteroids [Below is a photo I took as a geology student in 2009 of a K-Pg (formerly known as K-T) boundary outcrop in the Marche region, Italy].
This Iridium-rich thin layer has been found globally. In the same years, using magnetic and gravity data as part of a survey of the Gulf of Mexico, two geophysists from Pemex (the Mexican state-owned oil company Petróleos Mexicanos) identified an offshore structure north of the Yucatán peninsula that they interpreted to be a crater 180 km in diameter. It's only in the 90's that academic researchers came to know about the earlier discovery of a possible impact crater. They requested the analysis of drill samples from the Pemex wells and found evidence of material being deformed by shock waves and heated (shock-methamorphism) during an impact event.
Another team of researchers conducted satellite imagery analysis of the Yucatán peninsula and identified several sinkholes (called cenotes) forming a ring. This was interpreted as evidence of limestone rocks that were weakened by the impact and subsequently collapsed. Additional evidence of the huge impact is provided by large ripple-shape marks (in the sedimentary record below Lousiana) scoured by impact-generated waves. The impact caused catastrophic devastation that is recorded in tsunami deposits and chaotic mixing of fossil carcasses found thousand of kilometers away.
After decades of collecting pieces of the puzzle, the scientific community concluded that the impact at Chicxulub triggered the mass extinctions ~66 million years ago. [Download the paper here]