Impact event
Geological significance

Impacts have had, during the history of the Earth, a significant geological and climatic influence.

The Moon 's existence is widely attributed to a huge impact early in Earth's history. Impact events earlier in the history of Earth have been credited with creative as well as destructive events; it has been proposed that impacting comets delivered the Earth's water, and some have suggested that the origins of life may have been influenced by impacting objects by bringing organic chemicals or lifeforms to the Earth's surface, a theory known as exogenesis.

These modified views of Earth's history did not emerge until relatively recently, chiefly due to a lack of direct observations and the difficulty in recognizing the signs of an Earth impact because of erosion and weathering. Large-scale terrestrial impacts of the sort that produced the Barringer Crater, locally known as Meteor Crater, east of Flagstaff, 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 Peaks stand only 48 km or 30 mi to the west). Similarly, the craters on the surface of the Moon were ascribed to volcanism.

It was not until 1903–1905 that the Barringer Crater was correctly identified as 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. Every surveyed solid body in the Solar System was found to be cratered, and there was no reason to believe that the Earth had somehow escaped bombardment from space. In the last few decades of the 20th century, a large number of highly modified impact craters began to be identified. The first direct observation of a major impact event occurred in 1994: the collision of the comet Shoemaker-Levy 9 with Jupiter.

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 5 km (3 mi) or more. The smallest of these impactors would leave a crater almost 100 km (60 mi) across. Only three confirmed craters from that time period with that size or greater have been found: Chicxulub, Popigai, and Manicouagan, and all three have been suspected of being linked to extinction events though only Chicxulub, the largest of the three, has been consistently considered. The impact that caused Mistastin crater generated temperatures exceeding 2,370 °C, the highest known to have occurred on the surface of the Earth.

Besides the direct effect of asteroid impacts on a planet's surface topography, global climate and life, recent studies have shown that several consecutive impacts might have an effect on the dynamo mechanism at a planet's core responsible for maintaining the magnetic field of the planet, and may have contributed to Mars' lack of current magnetic field. An impact event may cause a mantle plume (volcanism) at the antipodal point of the impact. The Chicxulub impact may have increased volcanism at mid-ocean ridges and has been proposed to have triggered flood basalt volcanism at the Deccan Traps.

While numerous impact craters have been confirmed on land or in the shallow seas over continental shelves, no impact craters in the deep ocean have been widely accepted by the scientific community. Impacts of projectiles as large as one km in diameter are generally thought to explode before reaching the sea floor, but it is unknown what would happen if a much larger impactor struck the deep ocean. The lack of a crater, however, does not mean that an ocean impact would not have dangerous implications for humanity. Some scholars have argued that an impact event in an ocean or sea may create a megatsunami, which can cause destruction both at sea and on land along the coast, but this is disputed. The Eltanin impact into the Pacific Ocean 2.5 Mya is thought to involve an object about 1 to 4 kilometres (0.62 to 2.49 mi) across but remains craterless.
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