Flows of varying sizes erupted at different places at different times; the basalts look similar to one another to the naked eye; and there aren’t any obvious marker horizons, Widdowson says.
These factors complicate efforts to correlate the timing and size of the different flows.
Paleontologists have long recognized from the fossil record that more than half of the species inhabiting Earth perished at the end of the Mesozoic — the most emblematic of course being the remaining nonavian dinosaurs, like T. And for the last quarter-century, since the identity and approximate age of the roughly 200-kilometer-wide Chicxulub crater on Mexico’s Yucatán Peninsula were confirmed, the impact’s devastating blast, and resulting fallout, has been the favored explanation for the K-Pg extinctions.
But before the impact’s ascendance in both the scientific and popular psyches, another prevailing hypothesis invoked to explain the mass die-off described an altogether different global calamity: massive volcanism that heaped layer upon layer of basaltic lava onto the Indian subcontinent and belched climate-modifying gases and aerosols into the atmosphere.
The Siberian Traps erupted about the time of the end-Permian extinction, for example, which is thought to have knocked off about 90 percent of Earth’s species some 252 million years ago.
And the basalts of the Central Atlantic Magmatic Province emerged roughly 200 million years ago, about the time of the Triassic-Jurassic extinction.
Could it have affected global climate enough on its own to account for the scale of the extinction episode seen at the K-Pg boundary?
And was it actually contemporaneous with the major pulse of extinction, or did it happen sometime before or after?
Shoring up this critical part of the story would be a major step, scientists say, in revising our perception of this extraordinary episode in Earth history.
The scale of change leading up to the K-Pg boundary pales in comparison, however, to what happened during the main pulse of extinction, when an estimated 60 to 70 percent of Earth’s species died off in a geological instant.
In addition to the dinosaurs, many other large land animals and terrestrial plants, as well as marine reptiles, mollusks and numerous ocean-dwelling microorganisms, succumbed.
Other evidence from the geologic record also points to a major disturbance on Earth at about the same time as this huge pulse of extinction: A sudden shift in carbon isotopic ratios and a decrease in the amount of calcite observed in marine sediment cores indicate drastic drops in biological productivity and carbonate sedimentation in the oceans — markers traditionally taken to define the K-Pg boundary. At certain locations on land, layers of sediment enriched in iridium and other rare metals, as well as bits of shocked quartz and droplet-shaped particles called tektites — which form when melted rock resolidifies quickly as it flies through the air — indicate that something big struck the planet around the same time that the extinctions were occurring.
Images like this one of an asteroid hurtling toward Earth are often what come to mind when people think about the Cretaceous-Paleogene mass extinction that ended the Age of the Dinosaurs. It was these terrestrial deposits, particularly at Gubbio in Italy’s Apennine Mountains, that prompted a group led by University of California at Berkeley physicist Luis Alvarez and his son Walter, a geologist also at UC Berkeley, to lay out the first detailed hypothesis in 1980 that a bolide about 10 kilometers wide had slammed into Earth, sending large amounts of rocky debris and gases into the atmosphere and perhaps sealing the unfortunate fates of so many species.
Possibly totaling more than 1 million cubic kilometers, these lava flows — known as the Deccan Traps — erupted over several million years, beginning before and ending after the mass extinction.