Meteorite Impacts in Space and Time Geol 117
What are the Environmental Effects of
on Land and in the Atmosphere?
May 11, 2001
This paper includes a discussion of the effects a meteorite impact large enough to seriously effect earth's global environment would have. The effects of such an impact on both the land and the atmosphere will be the focus. Such effects include ejecta dispersal, landslides, global wildfires, ozone destruction, acid rain, clouds of dust, and overall atmospheric cooling. The result of these effects would be, and has been in the past, the devestation of many of the the orginisms living at the time of the impact. Such a major and sudden change in the conditions of the planet would result in a huge depletion of biomass, what is termed a mass extinction.
Sixty-five million years ago dinosaurs ruled the world. Then in a geologic blink of an eye, they were gone. What could cause such a massive extinction in such a short period of time? Until recently this question remained a mystery. In recent years, scientists have begun to piece together an answer. Evidence exists which points to the dinosaurs having been killed by a large meteorite impact. A crater, which could be associated with such an event, was found off of the Yucatan Penisula in Mexico. This crater, the Chicxulub Crater, dates back 65 million years to the Cretaceous-Tertiary boundary and has a diameter of more than a hundred miles across. A crater this large could only be the result of a meteorite striking the earth's surface with enough force to completely obliterate a large percentage of life on earth, including the dinosaurs and many, many other organisms.
A meteorite falling to the earth can have many disastrous effects on the land. The most noticeable result of a meteorite crashing into the land is an impact crater. "In the immediate vicinity of the crater, the shock wave, air blast, and heat produced by the impact explosion killed many plants and animals' (Kring, 2000). A meteor entering into earth's atmosphere will first send a shock wave into the air. The friction the meteor creates while traveling through the air will cause its temperature to dramatically increase, and it will begin to glow a hot red color. The high temperature often causes smaller meteorites to completely burn up before they reach land. If the meteorite is large enough, only the outer part will burn, and it will then hit land (Nelson, 2000: 4). Even when meteors actually hit the land, they are hard to find. The meteorites that do manage to leave behind pieces are quickly weathered by the climate into which they fall (Bland, 2000:131).
When a meteorite hits the land it will compress and displace rocks to form a depression. The compressed, partially melted, and broken rock forms a layer of breccia inside the depression. The meteorite is now under extreme pressure and releases it in a large explosion (Norton, 1998:126). A great deal of ejecta gets strewn about. Ejecta is a term used to describe the large amounts of dust and rock fragments that are forced into the atmosphere. When the rock fragments fall back to earth, they form an ejecta blanket in the area surrounding the depression. Rocks adjacent to the impact are thrown into the air resulting in upturned and tilted strata around the edge of the crater. The impact sends shock waves through the land resulting in shatter cones in surrounding rocks (Nelson, 2000: 4).
A meteorite impact changes the composition of some minerals when it comes in contact with them. Glass spherules are formed when an impact occurs. The rocks heat up and melt as a result of the meteorite's force, thus forming tinny glass beads that are dispersed throughout the impact area. When minerals such as quarts are hit, shock metamorphism takes place. Quarts is transformed into coesites and stishovites. Quarts absorbs such a large amount of pressure from impacts that the mineral actually changes its structure after melting and recrystallization (Norton, 1998: glossary).
(Nelson, 2000: 4)
There are two types of craters, simple and complex. Simple craters can be easily identified; they are defined by their smooth bowl shape. Complex craters have a central peak and multiple rings. The central peak is formed when the ground in the center of the crater recoils after the initial impact shock. A complex crater is defined as having a rim diameter of greater than or equal to 4 km. Any crater less than 4 km in diameter is considered a simple crater (Koeberl and Sharpton: 1-2).
(Koeberl and Sharpton: 1-2).
When such a massive meteorite makes contact with the earth, another major effect that occurs takes the form of impact ejecta. As the extraterrestrial object strikes the earth, massive amounts of dust and small pieces of rock are sent up into the atmosphere. This cloud of dust and rock is distributed across the earth. The consequences of this are extremely serious for the biota of the planet. The dust and rock will block sunlight, keeping it from getting through the atmosphere. Day thus becomes as dark as night for months at a time. Freezing conditions occur in the oceans, away from the coastlines. Without sunlight, the life-supporting process of photosynthesis ceases in plants and algae (Paine, 1999). "The cold and darkness would cause the collapse of the food pyramid" (Dott & Prothero, 1994). "The disappearance of plants would break the food chains and the carnage would begin" (Courtillot, 1999). Conditions would be more severe than a winter in the Arctic Circle. The Earth would remain dark and cold for over a year (Paine, 1999). "If a substantial portion of this dust was submicron in size, model calculations suggest the dust may have made it too dark to see for one to six months and too dark for photosynthesis for two months to one year, seriously disrupting marine and continental food chains and decreasing continental surface temperatures" (Kring, 2000).
Another of the major effects of a meteorite impact on the land is global wildfires or firestorms. Firestorms are perpetuated by massive amounts of methane gas that are released from the Earth by the collision. Methane is an extremely flammable substance. Lightning can ignite the released methane gas. When the meteorite hits, it shakes up the earth, rupturing pockets of methane that are trapped in gas hydrates. The impact also allowes slumping to occur (a sliding down of the ocean floor). This also releases massive amounts of methane gas. The fire burns close to the ground and quite high into the atmosphere. Such fires would not be simple forest fires that only burn vegetation. Due to the fact that this fire is fueled by extraordinary levels of methane gas, the atmosphere itself would also be on fire. The fires would incinerate global flora and fauna. The blaze would also decrease oxygen supplies and increase levels of carbon dioxide instigating a run-away greenhouse effect, a major, overall heating of the planet (Paine, 1999).
Ejecta dispersal from a meteorite impact occurs in the form of gas as well as rocks. "On impact, the asteroid […] essentially dig[s] a hole in the atmosphere, and then in the earth’s crust." The gasses which are produced as results of such an event include sulfur and nitrogen. There is evidence that the Chicxulub impact "produced several climatically active gas components, including aerosol-producing SO2 and SO3, greenhouse-warming CO2 and H2O, and ozone-depleting Cl and Br" (Kring, 2000). "Nitric acid [is] generated from atmospheric molecular nitrogen by the shock of the impact" (Kerr, 1988). A more recent study agrees, Nitrogen oxide is produced by the released energy of the impact itself (Courtillot, 1999).
Such gases produce consequences on the order of ozone destruction and acid rain. They are created and added to the atmosphere not only by the force of the impact but also by secondary effects such as forest fires. Soot from these fires combines with water to form "nitric acid aerosols, capable of destroying the protective ozone layer and acid rains that would damage vegetation and even dissolve the calcareous skeletons of microorganisms living in the surface layers of the ocean" (Courtillot, 1999). "Sulfate aerosols [are] converted to sulfuric acid rain, whose effects compound those produced by nitric acid rain… Acid rain [can] defoliate continental vegetation and even aquatic plants in shallow, inadequately buffered lakes or seas whose entire water columns became acidic (Kring, 2000). "In the worst case, the impact of an ice-rich comet newly arrived in the inner solar system generates enough acid to lower the pH of rain from its natural level of 5.5 to 0 to 1 for a number of years" (Kerr, 1988). An impact on the scale of the Chicxulub event does not bode well for the biomass of the planet.
It is plain that, in the aftermath of a massive meteorite impact the Earth would not be a pleasant place. A large percentage of life would cease to exist. "When the effect [of an impact]transcends geographical boundaries and becomes global, the change must be rapid relative to the time scale of evolutionary adaptation or...last longer than the dormant capacity of a species" (Kring, 2000). Then species will die out and an extinction will occur. The myriad of effects listed above are just some of the reasons for the demise of numerous plant and animal species, at the Cretaceous-Tertiary boundary as well as at other times in earth's history.
All, however, would not be lost. Not every species would perish. After the meteorite impact caused the mass extinction at the Cretaceous-Tertiary boundary, many plants and animals did become extinct. But there were survivors. A large impact opens up a space, creating new environments, in which new species can evolve and thrive. "The Chicxulub impact event...also illustrates how a process that destroys some organisms can create opportunities for other organisms, in this case leading to distinctly different ecosystems during the Cenozoic Era. This dual pattern of disaster and opportunity has existed with impact events throughout Earth history (Kring, 2000). Just as the extinction of the dinosaurs cleared the way for modern organisms to evolve, another large meteorite impact in the future could result in another mass extinction.
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