"Your chance of dying in a global impact catastrophe is 1/10,000."
-- Thorne Lay
The impact hypothesis for the K-T boundary event can account for the global environmental change that would account for the widespread extinction that occurred. However, is it a unique explanation? It provides an independent punctuation of the relatively gradual decrease in sea level and resulting reduction in shallow water environments that had been occurring for about 5 million years. It can explain many observations, such as the global presence of a thin Iridium-enriched clay layer, shocked quartz, microspherules of glassy materials, large wave deposits, and the presence of a 180km diameter crater in the Yucatan. As a scientific hypothesis, it is viable and testable. But that does not ensure that it is a unique interpretation, nor does it necessarily establish a causal link between the impact and global extinctions.
Another clear geological record of the late Cretaceous is massive layers of basalts, in the form of Flood Basalts, found in several places. Immense deposits of basalt flows are found in India, in the Deccan Traps, and in other countries including Siberia, Brazil, and the western U.S.. At the end of the Cretaceous India had not yet collided with Eurasia, and the Deccan Traps formed rapidly by massive volcanic extrusion over the present position of the island of Reunion. It is speculated that the dust arising from these prolonged and extensive eruptions could have opaqued the atmosphere, much as an Impact Winter would, causing global disruption of the food chain and massive extinction. There is no clear explanation for why the Cretaceous finished with massive volcanism, but presumably deep mantle dynamics and turn-over were involved. The onset of basalt flows in the Deccan Traps appears to have slightly preceded the end of the Cretaceous, but it is striking to plot the position of India at the time, relative to the impact site in the Yucatan. They are almost on opposite sides of the Earth, which has prompted the speculation that the impact sent shock waves through the Earth that accentuated the volcanic activity on the far side of the planet. In this way, the catastrophic effects of the impact and volcanism could be linked.
From our consideration of the frequency of impacts, it appears that a large meteorite or comet may strike the Earth about every 50 million years on average. Is this the explanation for the many dramatic extinctions in the geological record? Some extinctions were in fact much more extensive than the K-T event. 250 million years ago, at the boundary between the Permian and Triassic periods, 75% of all genera and 95% of all oceanic species went extinct rather abruptly. In fact, life on Earth was almost wiped out. This again suggests a global catastrophe. Other mass extinctions took place 360 and 435 million years ago, and lesser extinctions have speckled geological history.
Following the Alvarez hypothesis of a large impact, paleontologists began to scrutinize the record of extinctions for statistical properties. If one plots the rate of genus level extinctions as a function of time over the past 250 million years, a statistically significant periodicity of 26 million years is found. The same is true if one considers extinctions of entire families of taxa. This is extraordinary, and suggests some sort of regularity in what might be assumed to be a totally random process of environmental perturbations.
If one considers terrestrial craters that have 140-200 km diameters and ages over the past 2 billion years (well preserved in some parts of the continents, particularly long stable regions such as in North American, Europe and Australia, a total of 130 are found. Many smaller craters are also found, and if those with a diameter greater than 10 km are considered, there proves to be a 32 million year peak in the time between impacts, which is quite close to the 26 million year extinction number. Variations in low sea level show peaks with periods of 21 and 33 million years, while changes in plate creation (sea-floor spreading) peak at 18 and 34 million years. Are these processes independent or linked? This is very hard to establish, but one can ask the question, What in the Solar System has a 26 million year period? The answer is that nothing does, so any explanation for an extraterrestrial cause must invoke a larger scale phenomenon.
One idea that emerged in the last decade is that there may be a 26 million year perturbation of the Oort Cloud, which is a vast halo of comets that lies at large distances from the sun. Based on the number of comets that penetrate into the inner solar system, Oort proposed in 1950 that there are around 10exp18 comets about 1 light year away from the sun, the relic of the original solar nebula which formed the solar system. These are in a spherical shell of orbits, and some process is needed to periodically perturb them.
Two ideas for how to perturb the Oort Cloud have been proposed:
But, there are many objects in space that could fall to Earth at any time, and they need not all originate by some perturbation of the Oort cloud. By the best estimate now, your chance of being killed in a global catastrophic impact is considered to be 1 in 10,000. This is the same as your chance of dying in an airplane crash. It is about 1/60th of the chance that you will die in an auto accident.
This investigation of extinctions has awakened humans to their vulnerability to debris from space. Ideas for a prevention system have been advanced, including Project Spacewatch, in which near-Earth orbiting comets and asteroids are detected and tracked. Some nuclear physicists (Edward Teller, for example) have advocated setting up nuclear missile defense systems to deflect any infalling objects on a collision course with Earth. We may be able to avoid the fate of the dinosaurs using technology!
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