In the late 1970's Walter and Luis Alvarez discovered an unexplainably high level of iridium at the K-T boundary at Gubbio, Italy. The level of iridium at this location was 30 times higher than in the other layers of rock, below and above the boundary. The Alvarez team claimed that some 65 Ma ago, the earth was impacted by a celestial body, probably a meteorite, which was responsible for the high iridium level.
One theory put forward was that the boundary could have been formed by sediment accumulating 30 times more slowly than normal. However, since the discovery by the Alvarez team, further discoveries in other parts of the world, at the K-T boundary, which all show iridium concentrations of 30 times the norm, ruled out the slow sedimentation theory.
Massive celestial objects traveling at great speeds would have high kinetic energy, and if they hit the earth, which they do quickly, the rate of energy conversion would be enormous. The impact would have excavated rock, provided intense heat, which would melt the rock and spray it as lava into the atmosphere. Tsunamis would also occur at the point of impact, or points of impact, if more than one piece of meteorite hit the earth at that one time, which has been suggested.
Evidence supporting the impact theory includes the following:
1. The iridium excess in the 65 My-old soil layer has been confirmed at many points around the world.
2. The same soil layer contains grains of quartz that were deformed by high shock pressures, as would occur in a giant explosion. (The deformation is a microscopic structure called "twinning," in the crystals).
3. There are occasional minute diamonds formed under extreme high pressures.
4. Tiny glass spheres were formed as chilled droplets of molten rock were ejected into the atmosphere.
5. The same soil layer contains enough soot to correspond to burning down all of the forests of the world. This suggests that massive fires were touched off at the time of impact.
6. The same soil layer, especially around the Gulf of Mexico, contains massive deposits of tumbled boulders, as would be generated in a large tsunami, or "tidal wave." The geographic distribution of tsunami deposits suggest the impact was in the Caribbean area.
7. After a decade of searching, scientists in 1990 identified the crater associated with this material. It is no longer visible on the surface of the Earth, but is buried under sediments. It straddles the coast of Yucatan, in the Mexican province of Chicxulub. It was revealed by mapping the strength of the gravity field over that area, and by drilling; it has been dated to 65 Ma old. It was identified as an impact crater.
8. Astronomers have charted numerous asteroids that cross Earth's orbit. From studies of orbit statistics, it is estimated that asteroids of 10 km size can hit the earth roughly every 100 My or so -- which fits with the idea that we actually did get hit 65 My ago by an object this size. (Smaller hits are much more common).
9. At the K-T boundaries throughout the world, there is a rich variety of fossilised plankton below the boundary (older than 65 Ma), but very few fossils at the layer, giving further evidence to mass extinction at that time.
Another cause of the K-T boundary and mass extinction has been suggested, rather than the impact theory. That of flood basalts. Detailed radiometric testing has revealed that large magma flows occurred either side of the K-T boundary. It was the result of India drifting over a hot spot, during continental drift. The vast amounts of pyroclastic material and gases emitted by the flood basalts would have caused a significant extinction of many species of life. Sulphur dioxide would have given rise to acidic rain, which would decrease the Ph of ocean surface water. Carbon dioxide released during this event would cause global warming. The dust released by the activity would increase the Earth's albedo and result in cooling. Sulphur dioxide, carbon dioxide and dust affect the atmosphere and so would spread around the earth quickly, a prerequisite for mass extinction. Foraminifera, with their calcium carbonate shells, would not have survived in the acidic conditions of the sea. The dust would have prevented photosynthesis of phytoplankton, which would become extinct and would no longer be a food chain item for higher life forms, which would also die out.
No comments:
Post a Comment