The Alvarez and the Crater of Doom

"It was nothing of this earth, but a piece of the great outside; and as such dowered with outside properties and obedient to outside laws."
"The Colour Out of Space", by H.P. Lovecraft (1927)

Until 50 years ago, it seemed that a characteristic peculiarity of the Cretaceous-Palaeogene (or K–Pg) ) transition, famous for the mass extinction event that "killed" off the dinosaurs, was the apparent lack of a complete stratigraphic record. However, in the decade 1960-1970 the American geologist Walter Alvarez discovered a homogeneous and complete succession of bedded limestone- and marl-layers in the gorge of Gubbio (also Gola del Bottaccione, located in the far north-eastern part of the Italian province of Perugia, Umbria), called the "Scaglia rossa"-formation.

It was during this research that Alvarez discovered the today well-known Iridium anomaly. At first, the origin of this anomalous concentration remained unclear. In 1980, Walter Alvarez and his father, the nobel-prize physicist Luis W. Alvarez (1911-1988), proposed two possible explanations - a very slow sedimentation rate of the Scaglia Rossa, resulting in an apparent concentration of micrometeorites, or the impact of a large mass of extraterrestrial material at once.

But Alvarez could not provide further evidence to confirm or disprove both hypotheses.

In 1981, geologists Antonio Camargo-Zanoguera and Glen Penfield presented during a geophysical conference their research on a geological mystery discovered 30 years earlier, during surveys on the Yucatan peninsula (south-eastern Mexico). The two researchers proposed a new interpretation of a circular structure revealed by seismic investigations and buried under 300 to 1.000 m of sediments, considered until then of volcanic origin. They suggested that the circular structure was a weathered and buried crater, formed by the impact of a large meteorite.

Only ten years later, some researchers from the University of Arizona started to study the crater and obtained the first absolute age - 65-million-years. Research in 1993 revealed that the structure was in fact an impact crater with a total diameter of 180 km. The crater was named after the nearby town of Chicxulub - meaning "the devil's tail".

Alvarez, meanwhile, continued his research on the impact hypothesis and noted the temporal coincidence of the Iridium anomaly, the Chicxulub-impact and the mass extinction at the end of the Cretaceous. The impact could explain the observed rare earth elements anomaly, was an important time marker, and also appeared to be the main culprit to blame for the extinction of the dinosaurs. In 1995 the limit between the Cretaceous and the Palaeogene was therefore defined at the stratotype (the GSSP) of El Kef (Tunisia), coinciding with the peak of Iridium and the mass extinction of foraminifera at the base of a clay layer (also referred as K-Pg boundary clay) deposited after the impact.

Today more than 350 sites worldwide are known to record the Cretaceous-Palaeogene sedimentary transition. Within a radius of 500 km the debris layer of the impact is very thick; around the crater it reaches a thickness of 100 to 80 m. In a radius of 500 to 1.000 km the sediments are typical tsunami deposits - layers containing debris and spherules (molten rocks that subsequently cooled to form droplets) transported by the waves from the impact site. With increasing distance, the layer thins out to form the known Iridium rich clay overlying a layer with small spherules. In a distance over 5.000 km the impact is represented by a single layer of red clay that still contains traces of the material ejected from the crater.

The geological evidence supports the hypothesis that a large extraterrestrial mass collided with earth.

According to the most popular scenario, the mass extinction event at the end of the Cretaceous marking the end of dinosaurs, large marine reptiles and ammonites, was caused by the consequences of the impact. Shock wave and fire storms were soon followed by the release of large quantities of gas from the vaporized rocks, rich in carbonates and sulphates. The gases reacted with the water vapor to form acid rain, and the dust in the atmosphere blocked the sun causing a many years-long nuclear winter. Depending on the locality and the ecological niche that a species occupied - and a good dose of luck - these changes decided a species' fate - to survive or to be doomed.