A Tale of Two Rocks

Retrieved from a mile beneath the earth’s surface 65 million years after their creation, they bear witness to a cataclysm - and the death of the dinosaurs

Samples of rock from Chicxulub Crater
Samples of rock from Chicxulub Crater Wikimedia Commons

They are two small chunks of humble gray rock. Amid the glittering array of exhibits in the Hall of Geology, Gems and Minerals at the National Museum of Natural History (NMNH) they are easy to overlook. Yet they bear witness to a shattering event in the life of our planet, and to one of the hottest scientific debates of this century--exactly what did (or did not) do in the dinosaurs.

The rocks are breccias, their name derived from the word Italian stonemasons use to describe bits of broken stone held together like pebbles in concrete. Today we know they were blasted into existence about 65 million years ago when an object more or less the size of Washington, D.C. crashed to earth near what is now Mexico's Yucatán Peninsula.

The crater, now known as Chicxulub (cheek-shoe-lube), was not the only effect. There were fires worldwide, a tsunami more than half a mile high and storms of acid rain. In the resulting devastation, the sky went dark. The sun did not shine through for perhaps a year because of a killing cloud cover of dust. As much as 70 percent of all plant and animal species on earth appear to have been wiped out--including, most spectacularly, the dinosaurs, whose disappearance would long puzzle modern scientists. The two breccias are pieces of evidence of that earth-wrenching but only lately understood event. And they have figured in the great scientific detective story that resulted: the discovery of the crater itself, some 65 million years after it was formed.

Temperatures may have reached 18,000 degrees F--by comparison, the sun's surface is a chilly 10,000 degrees F. The rocks that were hit directly were instantly vaporized, and the underlying rock was rapidly melted or pulverized.

At that time only a few scientists theorized that the massive biological extinctions (suggested by fossil remains) at the end of the Cretaceous period were caused by the impact of an extraterrestrial object. It was regarded as a radical theory. Most scientists figured that the dinosaurs had been done in by a change of climate or a change in sea level. Others thought it unlikely that in such a large, empty solar system, an asteroid or comet could actually have managed to hit the earth--much less have wreaked global destruction on impact.

Curiously enough, the two NMNH breccia samples had already been wrested from the earth and stored in Mexico, not for science but for purposes of commerce. They came from sample cores drilled during the 1950s and '60s by the Mexican national oil company, PEMEX, not far from the northern Yucatán hamlets of Chicxulub Pueblo and Sacapuc.

They looked a lot like breccias of volcanic origin: melted rock holding together angular chunks of unmelted rock. Their presence in the drill cores did not bode well for the oil company's exploration of the area, since volcanic rock usually means that oil, even if present, is not easy to extract. The area from which the cores were taken did show a strange feature unlike that of a volcano--it was apparently part of a huge, semicircular ring with a high gravity field at the center. But because so few scientists took seriously the likelihood of a large asteroid's impact on earth, it seemed eminently sensible to assume the breccias were the products of a volcano.

In 1978 a young geophysicist named Glen Penfield, who was working with PEMEX, found himself assigned to fly over the Gulf of Mexico. Using a magnetometer, he was to measure the magnetic field of rocks on the Gulf floor--specifically off the coast near Chicxulub Pueblo. Like the findings of earlier PEMEX geologists, Penfield's were intended to map out the rock composition beneath the surface and determine the likelihood of finding oil.

But what Penfield's magnetometer let him see was very odd. More than a mile below the surface of the Yucatán Peninsula, and for 70 miles out into the Gulf of Mexico, was a saucer-shaped underground structure with a magnetic field different from that of any known volcanic terrain. It also had a most un-volcano-like symmetry. Put together, the old land data and the new underwater data indicated the existence of a huge ring, about 120 miles in diameter, half on land, half under the Gulf of Mexico. It was ten times the size of any volcano, with an upward bulge at its center similar to those seen on known--though much smaller--impact craters.

Penfield and PEMEX geophysicist Antonio Camargo-Zanoguera concluded that it could not be the result of a volcano; it was probably an impact crater. But proving that scientifically was a major problem. For one thing, the data upon which their conclusion rested were held in confidence by the oil company. Worse yet, the warehouse in Mexico where all the core samples were supposed to have been stored and catalogued had burned down, apparently destroying everything.

Just why those core samples and the breccias they contained were so important was the result of some relatively recent research involving the effects of known asteroid impacts upon assorted rocks. It was only in the 1960s that scientists discovered that one of the most important of these effects is the production of what is known as "shocked" quartz. Common in earth's crust, quartz is present in most impact breccias (including the ones on display at NMNH). Normally, quartz crystals are unmarked. But when an asteroid hits the ground, its powerful shock waves, passing through the tiny quartz grains at a rate of three to six miles per second, leave a unique, indelible mark: microscopic parallel lines scored through the quartz, like three-dimensional cross-hatching. The presence of these features provides certain proof of an asteroid impact.

It wasn't until spring of 1990 that Penfield got a call from a graduate student, Alan Hildebrand, who had examined a 65-million-year-old rock layer in Haiti, only 300 miles from the Yucatán, and determined that the still-hypothetical asteroid impact of that time must have occurred somewhere around the Caribbean. Now Hildebrand wanted to see if he and Penfield could complete the puzzle by locating rock samples from Penfield's Chicxulub structure.

Working determinedly, they began their search for samples. By marvelous chance, they found that a few breccia samples, part of the original oil drill cores, had been distributed here and there in Mexico and the United States, thus escaping destruction in the Mexican warehouse fire. Penfield and Hildebrand were fortunate enough to get hold of a few of these, including one breccia from the 14th core of the PEMEX drill site called Yucatán 6. And that did it. Shocked quartz samples from the Yucatán 6 breccia clinched the fact that Penfield's underground saucer was not a volcano, but rather an asteroid impact crater--the elusive smoking gun.

Within a year, it turned out that many of the supposedly destroyed Chicxulub area cores were available. In the kind of coincidence that one might expect in an Indiana Jones adventure, a scientist at PEMEX, intrigued by the anomalous structure from which the cores were taken, had stored the cores in an office in Mexico City for eventual study. The breccias on display at NMNH are from that cache.

Today, most of those scientists who still think that the decline of the dinosaurs occurred gradually also grant that the great asteroid hastened the end of their existence--and the sudden birth of a very different world.

To understand why, you have only to imagine the effect on the modern world of a similar asteroid impact. Apart from the incredible physical destruction--and the human violence that might ensue--imagine the earth with the sun blocked out for a year: there would be no harvests, and deprived of the warmth of the sun, the earth would grow dramatically colder, perhaps by as much as 30 degrees.

The extremes of temperature and pressure that created these breccias literally changed the earth. Millions of years later, the breccias are a reminder of the new kind of life that resulted, one in which large mammals like us could evolve because dinosaurs were extinct. Like these rocks, we might not be here today if it weren't for the asteroid that formed the Chicxulub crater.

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