Impact event

Impact events are caused by the collision of large meteoroids, asteroids or comets (generically: bolides) with Earth and may sometimes be followed by mass extinctions of life. For discussion of impacts in general, not just on Earth, see impact crater.

The geology of Earth-impact events

In the past, the Western view of history held that the Earth was created a few thousand years ago, and had been shaped since that time by a number of global cataclysms (see catastrophism). In the course of the first half of the 19th century, the new sciences of geology and paleontology supplanted this view, which gradually gave way to a consensus that the Earth was ancient and that its features reflected gradual changes operating over very long periods of time—a view known as uniformitarianism.

Related Topics:
Catastrophism - Geology - Paleontology - Uniformitarianism

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

This view has been amended in recent decades to accommodate the fact that the Earth has in fact also gone through periods of abrupt and catastrophic change, some due to the impact of large asteroids and comets on the planet. A few of these impacts may have caused massive climate change and the extinction of large numbers of plant and animal species. The creation of the Moon is widely attributed to a huge impact early in Earth's history. Impact events even earlier in Earth's history have been credited with creative as well as destructive events; it has been proposed that the water in the Earth's oceans was delivered by impacting comets, and some have suggested that the origins of life may have been influenced by impacting objects bringing organic chemicals to the Earth's surface.

Related Topics:
Extinction - Species - Moon - Huge impact early in Earth's history - Ocean - Origins of life

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

These modified views of the Earth's history did not emerge until relatively recently, chiefly due to a lack of direct observations and the difficulty in recognising the signs of an Earth impact. Large-scale terrestrial impacts of the sort that produced the Barringer Crater in Arizona are rare. Instead, it was widely thought that cratering was the result of volcanism: the Barringer Crater, for example, was ascribed to a prehistoric volcanic explosion (not an unreasonable hypothesis, given that the volcanic San Francisco Mountains stand only 30 miles to the west). Similarly, the craters on the surface of the Moon were ascribed to volcanism. The collision of the comet Shoemaker-Levy 9 with Jupiter in 1994 was the first time a major impact event had been directly observed; to date, no such events have been observed on Earth.

Related Topics:
Barringer Crater - Arizona - Volcanism - San Francisco Mountains - Shoemaker-Levy 9 - Jupiter - 1994

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

It was not until 1903–1905 that the Barringer Crater was correctly identified as being an impact crater, and it was not until as recently as 1963 that research by Eugene Merle Shoemaker conclusively proved this hypothesis. The findings of late 20th-century space exploration and the work of scientists such as Shoemaker demonstrated that impact cratering was by far the most widespread geological process at work on the Solar System's solid bodies. As literally every surveyed solid body in the Solar System was found to be cratered, there was no reason to believe that the Earth had somehow escaped bombardment from space.

Related Topics:
1903 - 1905 - 1963 - Eugene Merle Shoemaker - 20th-century - Space exploration

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Based on crater formation rates determined from the Earth's closest celestial partner, the Moon, astrogeologists have determined that during the last 600 million years, the Earth has been struck by 60 objects of a diameter of five kilometers or more. The smallest of these impactors would release the equivalent of 10 million megatons of TNT and leave a crater 95 kilometers across. For comparison, the largest nuclear weapon ever detonated, the Tsar Bomba, had a yield of 50 megatons.

Related Topics:
Crater - Moon - Astrogeologists - Earth - Megaton - TNT - Nuclear weapon - Tsar Bomba

~ ~ ~ ~ ~ ~ ~ ~ ~ ~