Mass Extinction

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Extinction is, quite literally, the end of a particular evolutionary line, the end of a species, a family, or a larger group of organisms. While it may be bad news for the victims, its a natural event in the history of life on earth. Extinctions, mostly at the level of species, have been occurring constantly at a low background rate, usually matched by the rate at which new species appear - with the result that biodiversity is constantly increasing.

But there have been periods in the Earths history when biodiversity crashes. Indeed this has been a powerful force in evolution, wiping the slate clean of up to 6% of all species, and providing the survivors with a world full of opportunities into which they can diversify. These are the mass extinctions, when more than 50% of the Earths species vanish in the geological instant of a few million years.

The cause for the Permian and cretaceous mass extinctions is still unknown by scientists, below is a list of possible causes.

A graph showing past mass extinctions over millions of years

Cheap Custom Essays on Mass Extinction

Speculated Causes of the Permian Extinction

Although the cause of the Permian mass extinction remains a debate, numerous theories have been formulated to explain the events of the extinction. One of the most current theories for the mass extinction of the Permian is an agent that has been also held responsible for the Ordovician and Devonian crises, glaciation on Gondwana. A similar glaciation event in the Permian would likely produce mass extinction in the same manner as previous, that is, by a global widespread cooling and/or worldwide lowering of sea level.

The Formation of Pangea

Another theory, which explains the mass extinctions of the Permian, is the reduction of shallow continental shelves due to the formation of the super-continent Pangea. Such a reduction in oceanic continental shelves would result in ecological competition for space, perhaps acting as an agent for extinction. However, although this is a viable theory, the formation of Pangea and the ensuing destruction of the continental shelves occurred in the early and middle Permian, and mass extinction did not occur until the late Permian.

Glaciation

A third possible mechanism for the Permian extinction is rapid warming and severe climatic fluctuations produced by concurrent glaciation events on the north and south poles. In temperate zones, there is evidence of significant cooling and drying in the sedimentological record, shown by thick sequences of dune sands and evaporates, while in the polar zones, glaciation was prominent. This caused severe climatic fluctuations around the globe, and is found by sediment record to be representative of when the Permian mass extinction occurred.

Volcanic Eruptions

The fourth and final suggestion that palaeontologists have formulated credits the Permian mass extinction as a result of basaltic lava eruptions in Siberia. These volcanic eruptions were large and sent a quantity of sulphates into the atmosphere. Evidence in China supports that these volcanic eruptions may have been silica-rich, and thus explosive, a factor that would have produced large ash clouds around the world. The combination of sulphates in the atmosphere and the ejection of ash clouds may have lowered global climatic conditions. The age of the lava flows has also been dated to the interval in which the Permian mass extinction occurred.

Speculated Causes of the End-Cretaceous Extinction

The End-Cretaceous mass extinction has generated considerable public interest in recent years, in response to the controversial debates in the scientific community over its cause. The more prominent of these new hypoteses invoke extra-terrestrial forces, such as meteorite impacts or comet showers as the causative extinction agent. Older hypotheses cite earthly mechanisms such as volcanism or glaciation as the primary agent behind this mass extinction.

The K-T Boundary

Evidence for catastrophism at the Cretaceous-Tertiary boundary is found in a layer of sediment, which was deposited at the same time that the extinction occurred. This layer contains unusually high concentrations of Iridium, found only in the earths mantle, and in extra-terrestrial meteors and comets. This layer has been found in both marine and terrestrial sediments, at numerous boundary sites around the world.

Meteorite Impact

Some paleontologists believe that the widespread distribution of this Iridium layer could have only been caused by meteorite impact. Further, these researchers cite the abundance of small droplets of basalt, called spherules, in the boundary layer as evidence that basalt from the earths crust that were melted and flung into the air upon impact. The presence of shocked quartz - tiny grains of quartz that show features diagnostic of the high pressure of impact - found in the boundary layer provides additional evidence of an extra- terrestrial impact at the Cretaceous-Tertiary boundary layer. Recent research suggests that the impact site may have been in the Yucatan Peninsula of Mexico.

Volcanic Eruptions

The high concentrations of Iridium in the boundary layer has also been attributed to another source, the mantle of the earth. It has been speculated by some scientists that the Iridium layer may be the result of a massive volcanic eruption, as evidenced by the Deccan Traps - extensive volcanic deposits laid down at the Cretaceous-Tertiary boundary - of India and Pakistan. These lava flows came about when India moved over a hot spot in the Indian Ocean, producing flows that exceeded one hundred thousand square kilometres in area and one hundred and fifty meters in thickness. Such flows would have produced enormous amounts of ash, altering global climatic conditions and changing ocean chemistry. Evidence that volcanism was a primary extinction agent at this boundary is also relatively strong. In addition, and the presence of spherules and shocked quartz worldwide in the boundary layer may also have been the result of such explosive volcanism. Thus at present, both the volcanic and meteorite impact hypotheses are both viable mechanisms for producing the Cretaceous mass extinction, although the latter is more popular.

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