banner

IMPACT

-----

-----We stand awestruck at the base of the cliff. It looks ordinary enough. It is, however, a scene of mass destruction. The calamity preserved in the rocks is on a scale incomprehensible to us. There were no human witnesses. Almost all of our continent was swept by fire and then by a wall of water, annihilating virtually every living land creature - plant or animal. The whole planet shuddered from the horrendous impact and its devastating effects surround us today.

IMPACT

-----We stand looking up at a cliff carved open by the nearby Cumberland River. A few cars trundle by us - folks on the way to work. A Carolina Wren sings from the nearby shrubbery. The winter hills around us are tan from the castoff leaves of fall. The trunks of the oaks create dark vertical stripes that merge upward into a piebald canopy of branches and sky. The sun is warm on our backs and a light zephyr reminds us that spring is approaching far western Kentucky. Halcyon rests comfortably a half-mile away at Green Turtle Bay. It is a beautiful day - in the world after the holocaust.

-----It must have been such a day sixty-five million years ago. The sky above was blue and the sun warm. Little else would be recognizable - it was the world before. Had we stood at the same place, we would be on a muddy beach. To the west of our beach, lies the broad shimmering water of the Bearpaw Sea. It is a shallow sea, teeming with fish. Abruptly the water surface boils as a giant reptile thrashes the surface in pursuit of fish. Huge crocodile-like creatures lie sunning on our beach.

-----We are on the western edge of a low landmass, blanketed by thick vegetation. The land extends far to the east over the rounded knobs of the deeply eroded Appalachian Mountains. It ends along the shores of the Atlantic Ocean. In the other direction, far across the Bearpaw Sea, lies a western landmass. Its eastern shore is about where Colorado is today. That land extends westerly from the low plain beside the Bearpaw. It slowly rises to the rumbling volcanic highlands of the Sierra Nevada. The west slope of the Sierra descends steeply to the Pacific.

-----We are in a steamy hot, verdant, long-ago world. Tropical conifers tower above us where we stand on our muddy shore. Wide muddy trails, pocked with huge water-filled footprints, lead inland through the thick undergrowth of ginkgo, palms, and broad-leafed flowering plants.

-----Small furry things scurry through the brush, and scaly lizards glide on feathered legs between the tree trunks. In the distance, the screams of a young triceratops mingle with the thundering roars from the attacking tyrannosaurus.

-----It is a day like all days in that ancient time - except, it is the end - the end of an Era - the end of the Mesozoic and almost the end of all life larger than a microbe on the surface of Earth.

-----During the earliest days of our planet, long before the evolution of life, it was a cold chunk of rock drifting through a rock-filled corner of cosmic space. There was nothing special about it, except it was larger than the other rocks. Therefore, it had a little more gravitational pull than its rocky neighbors. With gravity advantage, it attracted nearby asteroids. Collisions were common - in fact, those impacts built earth from a smallish mass to a planet. As Earth grew, so did its gravity. After the first billion years or so, Earth's gravitational 'vacuum cleaner' had swept its cosmic surroundings of almost every rock - even most of the specks of dirt and dust. But not all. Many times during the year, we travel through a 'cloud' of dust and meteors shower through our atmosphere. Dust we can live with. Big hunks can be fatal. Fortunately, most are gone.

-----However, a few big ones remain. One had skimmed past Earth in close encounters, probably, many times. Each time its velocity carried it through and past our gravitational pull - but not on that last day of the Mesozoic. From our place on the beach, we notice something different. Low on the southern horizon, there is a bright light in the sky. It looks like a star - somewhat like Venus on those days when it is visible through our sunlight. Curious, we stare at the bright spot and realize it is getting bigger - and brighter. In less time than it takes to tell, we, and everything living around us, disappear in a blinding flash of heat.

v

-----The bright spot is a large rocky asteroid, about the size of Manhattan Island. It spins through space, closing on Earth at over 15 miles/second. The blue planet slowly rotates before it. Blindly the asteroid streaks toward it, approaching from the south over the green continents of a joined Australia and Antarctica. The inexorable pull of Earth gravity sucks the jagged rock lump lower and it barely skims over the bulge of the Equatorial forests of South America. For a heart-stopping moment, it looks like it will miss - perhaps merely be a forest-leveling near miss. But no, the gravitational drag bends the trajectory of hurtling rock. Ahead of it is the sparkling turquoise water of the Bearpaw Sea - to the left and right are the twin green landmasses of North America.

-----Thirty-five years ago, I stood in frozen desert ten miles from one of the largest explosions ever experienced by a human. An evacuation helicopter hovered above, if something went wrong. The radio in my icy hand counted down …5…4…3…2…1… For what seemed two heartbeats - nothing. Then, in the distance, the desert rose in a gigantic bulge and I watched, transfixed, as waves - waves! - came toward me across the land surface. I had to brace my legs, as though standing in a dinghy rocked by a wake, to keep from losing balance. As the frozen rock groaned and squealed beneath my feet, lifting and falling with the waves, I kept my eyes focused on a distant mountain to maintain equilibrium. Then, in stomach-wrenching shock, I watched the mountain shake like a dog. Huge boulders cascaded down its flanks, trailing plumes of dust rising in their wake. I had witnessed the effects of an underground one-megaton thermonuclear detonation - the largest ever detonated in the continental United States.

-----Sixty-five million years ago, that asteroid impact was one hundred million times greater - one hundred million times! It plowed into the mouth of the Bearpaw Sea at a low angle - generating far more heat than a vertical impact. The impact crater, now buried under thousands of feet of later oceanic sediments, lies below the northwestern tip of Mexico's Yucatan Peninsula, near the town of Chicxulub. The superheated shock waves of impact incinerated every living thing as far north as the Pole. In a blinding flash, all the forests - the scurrying furry things - the tyrannosaurus - every land dweller of the twin landmasses were carbonized. Their ashes filled the atmosphere and settled slowly everywhere around the globe. The blue earth, after the dust settled, was grey and brown.

-----The Chicxulub crater was more than a hundred miles across, and three times the depth of the Grand Canyon. Where is all the debris that once was an asteroid and that filled that immense hole? Forty-five years ago, I asked the same question. I stood on the rim of another impact crater. This one was in southeastern Utah and mapping it in detail was my Masters thesis. 'My' crater was only a mile across and 700 feet deep, but, crisscrossing the desert, I could find no evidence of any ejected material. Eugene Shoemaker, of the US Geological Survey, was the world's leading expert on impact craters, having recently mapped Meteor Crater in northern Arizona. We met in Grand Junction and had a fine day comparing 'our' craters. Gene explained that a meteor impact is actually an explosion. The force of impact so compresses the rock that, after the mass has stopped, it (and surrounding material) blasts back outward and leaves a deep crater. 'His' crater, the result of a vertical impact, not only blew material skyward, but folded back the surface layers of rock, like lifting the edge of a carpet and folding it back atop itself. 'My' crater was apparently much older so ejected material had been eroded and removed.

-----The asteroid that blasted the massive hole in Yucatan collided at a very low angle. Double a fist and slam it into a sandbox - at a low angle - and you can visualize the Chicxulub impact. As your fist plows a trench, and ultimately a hole, most of the displaced sand flies forward from the crater. This was exactly the case with Chicxulub. A great mass of superheated and molten debris blasted up the Bearpaw Seaway and the two continental landmasses, devastating everything in its path with the shock wave, intense heat, and smothering rubble.

--

-----Such a quantity of debris was pulverized and suspended in the atmosphere that sunlight couldn't penetrate it and reach the land surface. The entire earth was covered with a black, choking, freezing blanket - it was darker and colder than a cave. Slowly, through decades, the dust settled. Slowly twilight returned to the frozen surface of Earth. In the muted light, little life remained anywhere on the land. Most plants and animals in the northern hemisphere died - either from the deep freeze or from the inability of plants to photosynthesize. It was immensely cold and there was little to eat.

-----The asteroid impact was in a sea, connected with the world's oceans. Shortly after impact, there was an immense hole in the ocean. An incredible flow of seawater must have cascaded into the crater, draining most of the water from the shallow sea. Huge sea waves first resulted from the impact and roared up the Bearpaw. Just as the ground rolled in giant swells from the thermonuclear explosion I witnessed, so must the sea and land have rolled from the shock of this explosion many hundreds of millions of times larger. Sea waves, tsunamis, must have been at least a half-mile high.

--

-----As the huge crater filled, more tsunamis must have roared up the seaway. When ocean water overtopped the crater, waves roared up the bed of the Bearpaw. The sediments that record the events of that day preserve a layer of fragmented ejecta overlain by cobbles, pebbles, and sand carried by those gigantic sea waves.

----- El and I stand below the cliff in western Kentucky. Fragments of Mexico lie about our feet, and cobbles, borne by sea waves thousands of feet high, cap the cliff. We stare in awe, thinking of that long ago time. We are overwhelmed by emotion, driven by the knowledge of what those pebbles witnessed. Above us, a pebble tumbles down the steep slope of gravel - the last time that pebble moved was on the last day of the Mesozoic.

(1/03)

Top | Home