Tag Archives: meteorite

How did the Dinosaurs Die?

Image credit: Don Davis, NASA

Since we are celebrating the discovery of a new dinosaur, the Pegomastax africanus, I thought about the Alvarez (meteorite impact) theory of Dinosaur extinction.  This theory was hotly contested in the scientific community, but it has gone on to be almost universally accepted.  And it doesn’t just describe the demise of the dinosaurs – in fact, the Alvarez team argued that the giant meteorite wiped out half of all species of plants and animals, including the mighty dinosaurs.  Before the Alvarez team published the article in Science in 1980, scientists had given little serious consideration to the demise of the dinosaurs.

R. P. Walker, S.T. Smith, and S. M. Smith, USGS
The K-T boundary layer in Caravaca, Spain

The theory stemmed from the work of Walter Alvarez, a geologist, on the clay layer that separates the Cretaceous and Tertiary geologic eras, commonly referred to as the KT boundary.  This boundary is approximately sixty-five million years old and is defined by a distinct change in the earth’s flora and fauna: it marks the disappearance from the fossil record of about fifty percent of the Earth’s species at that time.   In the mid-1970s geologists had not dated the KT layer precisely, which led to confusion about the amount of time it represented.  Alvarez proposed to determine the rate of the clay’s deposition, in a bid to understand the Cretaceous extinctions.

He enlisted the help of his father, nuclear physicist and Nobel laureate, Luis Alvarez.  Luis and his associate Frank Asaro decided to measure the amount of Iridium in a clay sample from Gubbio, Italy—a location where the KT boundary was exposed.  Iridium is a metal found in trace quantities in Earth’s crust, deposited steadily and slowly as meteorite dust.  If no Iridium was found in the clay layer, the Alvarez team could assume that the layer had been deposited in a relatively short amount of time-less than one thousand years.  A trace amount of Iridium (approximately 0.03 parts per billion) suggests that the clay layer had been deposited over a longer period of time—more than one thousand years.  The test results took the Alvarez team by surprise.  The sample yielded three parts per billion of Iridium.   It seemed incredible that such a thin layer could correspond to a period of one hundred thousand years!

After waiting nine months for the results of the Iridium test, the Alvarez team was finally able to go to work.  Asaro tested layers above and below the KT boundary to make sure that the high Iridium content was unique to the KT layer.  Walter Alvarez looked for other locations where the KT layer was exposed to make sure that the high Iridium content was not an anomaly local to Gubbio.  In time, they determined that high levels of Iridium were unique to the KT layer and not confined to the Gubbio sample.

What did the Alvarez team make of these results?  While they could have adopted several alternative explanations, they concluded that some extra-terrestrial event had produced the Iridium spike and that this event was also the cause of the mass extinction seen in the fossil record.  They put these conclusions in the form of a provocative question—

What extra-terrestrial even could have caused the sudden extinction of half the genera on earth, while depositing the tell-tale Iridium anomaly?

By 1979, the Alvarez team had definitively settled on the impact of a meteorite the size of Mount Everest.
With this answer in hand the Alvarez group focused on how a meteorite could have caused the mass extinction—what became known as the killing mechanism.  The first mechanism they proposed, inspired by the 1883 eruption of Krakatoa, was that the impact had launched a giant dust cloud into the upper atmosphere.  This dust would choke of sunlight, end photosynthesis, and create a deadly rift in the food chain.

However, this was quickly discounted because some of the affected species lived in dark artic regions.   Instead, impact supporters proposed global wildfires and an ‘impact winter’ (similar to nuclear winter), with accompanying acid rain, which would disrupt the food chain causing the mass extinction.   This killing mechanism was accepted because it explained the selectivity seen in the K2 extinction event.  While there is still debate, there is by now a broad, if by no means complete, consensus that a meteorite impact is the likely cause of the mass extinction at the KT boundary.

For more information about the demise of the dinosaurs, check out Alvarez’s book.  In it, he details the many anomalies that supported the theory – including the shocked quartz found at the boundary.  Walter Alvarez, 1997. T. rex and the crater of doom. Princeton University Press, Princeton, N.J