Bolide and Impact Crater

Bolide

Any large meteor, but especially in terms of our little world one that smashed into the Earth or the Moon. Real World Examples: Contrary to the once nearly universally accepted tenants of gradualism-uniformitarianism by mainstream geologists everywhere, today considerable evidence has been found indicating that meteors ranging from large to small once pounded the Earth with regularity. The 200-million-year old, 42 miles in diameter crater at Manicouagan, Quebec, is without doubt an eroded impact crater. The Chicxulub and Shiva Craters in the Gulf of Mexico and the Indian Ocean respectively are also very significant but not the only examples extent (see impact crater for more detailed descriptions). Other instances include what has been interrupted as a comet or asteroid string that fell in a straight line across what is now Illinois, Missouri, and Kansas about 320 mya, leaving as evidence eight craters in a row. Those impacts were not sufficiently large to trigger a global catastrophe such as the one that may have wiped out the dinosaurs (see mass extinction), but it would have caused considerable damage on a regional scale. The idea that bolide impacts often come in bunches, as comets or asteroids break up before impact, is exactly what some scientists say happened with the Chicxulub and Shiva Craters and what may have caused the mass extinction at the end of the Cretaceous period.

Impact Crater

Depression formed by the impact of an unspecified projectile, especially a crater formed on the Earth, another planet, or moon surface where the nature of the impacting body is not known. Impact craters form when an asteroid or comet (see bolide) strikes another, usually but not always larger body such as an asteroid, planet, or moon at high speed. These craters are classified as simple (relatively small with a smooth bowl shape and exhibiting a depth-to-diameter ratio of about one-to-six) or complex (large in diameter but shallow in depth with a depth-to-diameter ratio of between one-to-ten and one-to-twenty).

Images gathered by space probes have shown that the surfaces of the other inner planets (Mercury, Venus, and Mars) and their moons have been heavily bombarded by bolides. Because the Earth is very active geologically much of the physical evidence of impact craters has been removed or extensively modified by erosion and later tectonic activity. That said, over one hundred seventy impact craters have been identified on Earth and more are found each year as investigatory techniques improve. The impacting body generally melts (and frequently vaporizes if small enough) and the resultant materials mix with rocks at the impact site. Sometimes the impact site contains numerous siderophile elements such as iridium, osmium, platinum, and palladium as well as siderolites. Author’s Note: Other indicative signs of bolide impact are products of shock metamorphism (see that definition in the general category, metamorphism, types of): shatter cones, planar features in quartz, planar features and bent twins in feldspar, kink bands in biotite flakes, heterogeneous vesicular fused glass, quartz and feldspars that have been completely transformed into diaplectic glass (a type of glass formed solely by shock metamorphism), and a high-pressure form of quartz called stishovite, a mineral that has never been found anywhere on Earth except around impact sites.

Real World Examples: Meteor Crater (also known as Barringer Crater) in northern Arizona was formed between 20,000 and 50,000 years ago when a large meteor of nickel and iron, roughly 150 feet in diameter and weighing more than 60,000 tons, struck the Earth at a speed approaching 40,000 miles an hour, exploded and scattered molten debris for miles. Today, the Crater measures about three-fourths of a mile across and 530 feet deep. Its rim rises nearly 250 feet about the surrounding plain. Other large, complex impact craters include the Vredefort Ring in South Africa (with a diameter of about 155-185 miles), the Sudbury Astrobleme in Ontario, Canada (about 125-155 miles in diameter), and the Chicxulub structure in the Gulf of Mexico (> 110 miles in diameter).

The Manicouagan Ring Structure in northern Quebec, Canada, is one of the largest impact craters still preserved on the surface of the Earth. The diameter of the original rim is approximately 62 miles. A prominent 43-mile diameter, ice-covered, annular lake fills the inner ring where impact-brecciated rock has been eroded by glaciation. The causative impact occurred 214 mya, probably had a diameter of about three miles, and may have produced a mass extinction similar to that at the end of the Cretaceous period.

Wolf Creek Impact Crater in Australia is a relatively well-preserved, small impact crater that today is partly buried under wind-blown sand. The crater is located in an isolated area in the Great Sandy Desert of Western Australia, about 75 miles south of the town of Halls Creek, which is about 1,865 miles northeast of Perth. Its crater rim has a diameter of 2,400 feet, rising approximately 82 feet above the surrounding desert; the crater floor is 165 feet below the rim. Author’s Note: The age of the Wolf Creek crater’s is approximately 10,000-20,000 years, which means that since the first Aboriginals arrived in Australia around 40,000 years ago, their stories about its origin may not be fanciful.

Much closer to home is the Chesapeake Bay Impact Crater. Its discovery started with one of those utterly serendipitous occurrences with which science is blessed all too infrequently. In July 1983, a core sample taken about 90 miles east of Atlantic City, New Jersey, contained an unexpected present: a ten-centimeter-thick layer of late Eocene debris that proved to have been ejected from a massive bolide impact that had occurred somewhere around 35 mya. When the debris layer was analyzed it was found to contain microfossils and argon isotope ratios that revealed the ejecta were part of a broad North American impact debris field, previously known primarily from deposits in the Gulf of Mexico and Caribbean Sea. Since that time a lot has happened, including years of seismic reflection profiling, gravity measurements, and core drilling. The result is the source of that strewn field has been confirmed as a huge impact crater in and around Chesapeake Bay, which turned out to be the largest structure of its kind in the United States and the sixth-largest on Earth. The comet that crashed in what is now Chesapeake Bay most likely ranged from about one to two miles in diameter and was traveling at around 50,000 miles per hour prior to impact. It produced a crater 53 miles wide and fractured/collapsed bedrock to a depth of well over a mile. Today, those shattered rock structures continue to affect the pattern of groundwater flow throughout southeastern Virginia and southern Maryland. The force of the impact ejected enormous amounts of debris into the atmosphere and also generated gigantic seismic sea waves that extended far inland at elevations approaching 2,000 feet above sea level. Both effects probably killed most animals within several hundred miles of the impact site and the global atmosphere may have been so loaded with particulates that the entire Earth experienced a type of nuclear winter for many decades. Fossil and paleo-pollen evidence are still being analyzed, therefore, no definitive paleoclimatic effects have been documented. Today the results of that horrific impact more than 35 mya continue to affect the nearly two million people living in the region through subsidence, faulting, high-salinity groundwater in the coastal plain, and earthquakes around the crater’s perimeter.