The meteorite hurtled toward the big, blue planet, reaching speeds of as much as 150,000 miles per hour. Its surface white-hot from the friction caused by its plunge through the atmosphere, the giant crashed into the sea on what is now the northern coast of the Yucatán Peninsula of Mexico. The impact of the six-mile-wide space maverick created an underwater crater more than 25 miles deep and 112 miles in diameter – farther across than the distance between Austin and Waco.
Material was blasted out of the crater at 50 times the speed of sound. About 400 cubic miles of debris were carried upward by the resulting fireball. (The 1980 eruption of Mount St. Helens, by comparison, released less than a third of a cubic mile of ash.) After several months of drifting around in the atmosphere, the finer particles began settling back to Earth, covering the entire planet with a thin layer of dust.
Another 5,000 cubic miles of melted and crushed rock was ejected from the crater, then fell back to Earth in a matter of hours within 3,000 miles of the impact in all directions. Called "ejecta," this melt rock was thickest near the point of impact, becoming patchier farther away.
The enormous energy generated by the impact also created huge tidal waves radiating out from the area of collision – giant versions of the ripples that form on the surface of a lake when someone tosses a rock into the water. These "ripples" were 150 to 300 feet high – up to as tall as a football field is long. Churning up the seabed to a depth of 40 feet, one of the monster tsunamis roared across what we now call the Gulf of Mexico. The wave-from-hell tore rocks, sharks' teeth, sand and boulders from the bottom of the sea as it went. The giant wave finally deposited its burden of trash more than 150 miles inland from today's coastline.
The Texas Connection
One of the sites where this antique debris has been found is along the Brazos River in Falls County about 30 miles southeast of Waco, where the rock layer has been exposed by water erosion. This area was under a shallow sea at the time of the probable impact and tidal wave. Paleontologist Thor Hansen began studying the Brazos River site in 1985. He found a layer of mud clumps as much as three feet thick, along with fist-size chunks of sandstone. This layer is in a deep bed of mudstone that had been produced by a million years of otherwise quiet accumulation of water-deposited silt. Similar layers of what is most likely tidal-wave trash dating from the same geological period have been found in Mexico, Arkansas, Cuba, and off the coasts of Haiti and North Carolina.
Some scientists believe that the debris in question can be explained by a large volcanic eruption. But each new report by scientists studying the phenomenon adds weight to the collision theory.
The cosmic collision occurred about 65 million years ago at the boundary between two geological periods: the Cretaceous, when dinosaurs flourished on the planet and few mammals existed, and the Tertiary, when the dinosaurs had mostly vanished and mammals began to proliferate. Did the enormous amounts of material thrown into the atmosphere block out the sun and destroy parts of the ozone layer to the extent that the Earth's ecological systems shut down? And did that shutdown cause more than half the planet's species to die off? This is the scenario suggested by a number of scientists who have been exploring the probable Big Crash.
The asteroid/comet collision idea is not a recent one. French scientist Pierre de Maupertuis proposed as early as 1750 that comets striking the Earth had caused mass extinctions by altering the atmosphere and the oceans.
But the first solid evidence linking a cosmic catastrophe with the wholesale eradication of species was suggested in the late 1970s by Walter Alvarez, a geologist at the University of California at Berkeley, and his father, physicist Luis Alvarez. The layer of fine debris from the impact shows up today as a stratum of grayish clay at the Cretaceous/Tertiary boundary (K/T boundary). The clay layer appears to be world-wide: It has been found at more than 100 sites scattered over the globe. This clay contains an unusually high amount of the element iridium – up to 30 times more than could normally be expected. Iridium, a heavy, brittle, metallic chemical element, is found in the Earth's core. It is rare on the Earth's surface, but comets and asteroids are relatively rich in it. The high concentration of iridium in the K/T boundary clay stratum could be explained by the collision theory.
The Brazos River site reveals not only the jumbled anomalous sandstone rocks and sharks' teeth from the tidal wave, but also this overlying, iridium-rich layer of clay.
The discovery of a thick layer of glassy particles at the K/T boundary in Haiti in the early 1990s provided what many geologists feel is the last piece of evidence needed to support the collision theory. Chemical analysis of the glass drops, called tektites, confirms their impact origin and indicates that they are probably part of the ejecta layer.
The most likely candidate for ground zero is the Chicxulub crater, just north of Mérida, Mexico, on the coast of the Yucatán Peninsula. The huge crater, which was underwater at the time of the probable impact, contains deposits of rocks, such as highly shocked quartz, which can be produced only by such an impact or by a nuclear explosion. Analysis of rocks from drill cores taken from the Chicxulub crater indicate an age of 65 million years, identical to the age of the tektites in Haiti.
The consensus appears at this time to be that an asteroid did splash down in the Yucatán and that it did produce the ejecta layer with its glassy particles, the global layer of clay, and the tsunami trash in Texas and elsewhere – and that the dinosaurs died off after the collision.
But the jury is still out on whether the first caused the second.
— written by Mary G. Ramos, editor emerita, for the Texas Almanac 1994-1995.