posted December 11, 1997 New References Added
|Damage by Impact|
the case at Meteor Crater, Arizona
Written by Linda M.V. Martel
Hawai'i Institute of Geophysics and Planetology
50,000 years ago mammoths, sloths, bison, and camels likely roamed the grassy rolling hills and woodlands of the Colorado Plateau in an area known today as northeastern Arizona. But in an instant, a meteorite impact disturbed this peaceful scene. What were the environmental effects? What happened to the animals living near the impact? According to David Kring of the University of Arizona, damage would have been swift and extensive. Casualties resulted from vaporization, burial by the ejected bedrock, and from the destructive air blast moving across the landscape. Kring calculated the magnitude and radial extent of the air blast produced by the Meteor Crater impact event using scaling relationships from nuclear explosions. His estimates show immediate vaporization of plants and animals at ground zero. Winds in excess of 1000 km/hour scoured the land within 3 to 5 km of the point of impact and led to swift devastation of the local population of plants and animals. Just how often does this sort of impact event occur on Earth? We'll examine the potential hazards.
|A major phenomenon|
However, traces of some craters still remain. Using satellite images, aerial photographs, field observations, maps, and underground geophysical data, geologists have documented over 150 impact craters on Earth. Since no one has ever observed the production of an impact crater on any rocky planet, we must use the size and shape of the craters, the presence of excavated bedrock, the shape and extent of the ejecta, and over-turned rock layers in the crater rims to understand the cratering process. Researchers are gaining more understanding of the interplay between impact energy, target rock strength and structure, presence or absence of volatiles, and gravity from their studies of impact craters throughout the Solar System.
Impact cratering on Earth has an additional profound implication: catastrophic loss of life. Local and regional environmental changes and even global climatic variations on Earth have been attributed to impact events. The Chicxulub Crater on the Yucatan coast of Mexico gained celebrity status in the early 1990s as the most likely impact event possibly associated with the extinction of the
|dinosaurs at the end of the Cretaceous period, about 65 million years ago (also known as the K-T impact.) It is known, however, from estimates of cratering rates that smaller impact events occurred more frequently and that there has been a decline in the rate of cratering through time.|
|On the Meteor Crater rim, some of the ejected blocks tower over the trail. The tallest block in this photograph is about 10 meters high. There's a 2-meter-tall hiker with a blue shirt standing to the right of this block. In the background is the gently undulating plain. (See enlargement.)|
Using current geologic and paleontologic evidence (such as lake sediments and packrat middens), Kring assessed what the landscape, vegetation, and animal life was like 50,000 years ago in this area of the Colorado Plateau. The flat to slightly rolling landscape had an average relief of about 5 to 10 meters over distances of about 0.25 to 1.0 kilometers. A shallow drainage system was already established carrying water to the northeast and toward what we now call the Little Colorado River. Basaltic cinder cones and lava flows existed about 11 to 29 kilometers to the south, west, and northwest of the impact site. With the exception of Sunset Crater (of volcanic origin; erupted less than 1,000 years ago) and possibly two other volcanic craters known as Strawberry Crater and O'Neil Crater, all of the topographic features seen today near Meteor Crater were present 50,000 years ago.
Fossils are rare in the area, but available data suggest that mammoths, sloths, bison, and packrats were probably on the Colorado Plateau at the time of the impact. Mastodons, mountain goats, camels, horses, and tapirs also may have been there. Quite a menagerie!
|The air blast and its effects|
|20 megatons||40 megatons|
|Source: Kring, D. A., 1997.|
At the point of impact, the plants and animals, rock, and most of the meteorite were vaporized. Underlying bedrock was ejected and overturned, burying the land and anything else not already blown away by the air blast, out to a distance of between 1 and 2 km. The animals within 3 to 4 km of the impact site would have been subjected to winds exceeding 2000 km/hour and killed. A 50% casualty rate would occur between 9 and 14 km of the impact site due simply to bodies being picked up by the air blast and accelerated to a few to tens of kilometers per hour before being slammed back down again.
Overpressures (the pressure above normal atmospheric pressure) would cause death to anything living within a radius of 2.7 to 3.2 km of the impact site and cause lung damage within a radius of 6.5 to 9.3 km for a 20 megaton explosion. In the case of a 40 megaton explosion, these distances would increase by an additional 1 to 2 km. Animals as far away as 16 to 24 km would have been injured severely. Vegetation would have been almost completely destroyed over an area of 800 to 1500 km2 around the Meteor Crater impact site. Fortunately, as Kring points out, the impact effects would have been severe only within that 800 to 1500 km2 area. No global extinction would have resulted.
Kring's work is important because it provides insight into Earth's impact record and the potential consequences to the inhabitants of the area. In this case, a 40-km diameter region around Meteor Crater corresponded roughly to the mean of severe to moderate woodland damage calculated for 20 and 40 megaton blasts. Peak overpressures greater than 1 psi would have been felt 80 kilometers away from the actual impact site.
In 1992, "The Spaceguard Survey" was proposed by the NASA International Near-Earth Object Detection Workshop to coordinate international observations to increase the rate of discovery of near-Earth asteroids. The Survey was never funded, nevertheless smaller groups of scientists are making routine searches for objects whose paths may cross the Earth's. Tracking and cataloging the orbits of asteroids and comets could lead to advanced warnings of a threatening impact strike. Evacuations, similar to those issued for floods or hurricanes, may be all that's needed for a Meteor Crater-sized impact event. The graph below shows estimates of the frequency of impacts of varying sizes and is a compilation of information from sources listed in the "Additional Resources" section at the end of this article.
An impact large enough to produce a crater the size of Meteor Crater occurs every 1000 to 2000 years somewhere on Earth. Taking 1500 years as a reasonable estimate, this means that the chances of such an impact occurring this year is 1 in 1500, an uncomfortably large probability. Fortunately, the chances of you being demolished by an impact are much smaller. The area devastated around Meteor Crater was 800-1500 km2; let's say about 1000 km2. The surface area of Earth is 510 million km2. So, the chances of the 1000 km2 you happen to be standing in being destroyed is only about 1 in 500,000. Combining the frequency (1 in 1500) with this results in a low probability of any one of us being killed by a Meteor Crater-sized impact -- only 1 in 7.5 billion. Whew!
It turns out that the chances of a much larger impact doing you in is much higher, in spite of their less frequent occurrence. A 100,000 megaton impact, about the threshold for a global catastrophe, occurs about once every 500,000 years. Assuming that 1 in 4 people would perish in such a global catastrophe, the chances of any one of us dying in such an event during the next year is 1 in 2 million. For comparison, your chances of being killed in a car accident is about 1 in 5,000.
An uncertainty in these scary calculations is how large an impact leads to a global catastrophe. The "Spaceguard Survey" suggests that the threshold is about 100,000 metagons, which would make a crater 10-20 km in diameter. However, this is highly uncertain. There is no geological evidence for global catastrophes associated with the formation of many well dated craters 20-30 km in diameter. So, the threshold for global catastrophe may be much higher, making the odds of one affecting you that much lower.
Can anything be done to prevent or lessen the damage from a meteorite impact? Studies like Dave Kring's are an important first step as they detail the effects of an impact. Beyond that, we need to find as many Earth-crossing asteroids as possible, as advocated by the "Spaceguard Survey." If we found an asteroid on a collision course, then perhaps we would have a chance to deflect it away from Earth. Given enough warning, a small nudge, perhaps from a solar-powered motor attached to the asteroid, could cause a significant shift in the asteroid's orbit and take Earth out of harm's way.
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