• Craters formed by the impact of a comet or asteroid with Earth have either a simple bowl shape (smaller craters) or a more complex structure featuring a central peak.
• There are over 150 recognized impact craters worldwide.
• Impact events generate a series of associated features including craters, ejecta, shock metamorphism, breccia, and melt rocks.
• The impact of a large comet or meteorite with Earth could devastate the global. environment by generating air blasts, earthquakes, wildfires, and tsunamis, and by blocking sunlight for months and altering the composition of the atmosphere

mpact craters are common on all the rocky terrestrial planets and their moons (Fig. 28). The majority of the craters formed during a period of intense bombardment soon after the formation of the solar system. All of these early craters date from before 3.9 billion years ago. More recent impacts on Earth are preserved in relatively young rocks as older impact craters are either worn away by erosion and weathering or were covered up by later rock layers. Craters are preserved in their original state on the Moon where the lack of atmosphere ensures that they won't be worn away by the action of wind and water.

Figure 28. Left: Manicouagan impact crater, Canada, a 70 km wide circular lake surrounds the crater site formed by an impact 200 million years ago. Much of the original 100 km-wide crater has been obliterated by erosion but melt rocks of the crater floor remain. This is one of the largest terrestrial impact craters. Right: craters are more common on the Moon. Images courtesy of NASA.

Impact craters on Earth come in two basic forms (Fig. 29). Smaller simple craters such as Meteor Crater, Arizona, have a diameter of a few kilometers and exhibit a simple bowl-shaped morphology (Fig. 30). Larger complex craters with diameters of more than 4 km (2.5 miles) are characterized by central peaks and ring-like structures along their margins where the crater rim collapsed inward (Fig. 31). Crater size is largely a consequence of the size and velocity of the impacting meteorite or comet and the character of the impact site.

	Figure 29. Bowl-shaped simple craters exhibit fewer features and a smaller width-to-depth ratio than larger complex craters.

Figure 30. Two simple craters. Left: Meteor crater, Arizona, was the first impact crater recognized on Earth. The crater is 1200 meters across. Right: crater on Mars approximately 2 km across. Note ejecta blanket preserved around Martian crater and on the northeast side of Meteor crater. Images courtesy of NASA.

Craters often contain smashed rocks known as breccia and may be surrounded by a blanket of ejecta, displaced particles thrown outward by force of the impact. Heat from the impact can cause melting of rocks on the crater floor. The impacting body is typically pulverized by the force of the collision although some small fragments may occasionally be preserved. The atomic structures of minerals in the rocks of the impact site will be altered by the extreme force of the collision to form a suite of features that are unique to impact events. These changes, evident only under the microscope, are collectively termed shock metamorphism and are an unmistakable signal of impact events.

Figure 31. Copernicus crater a complex crater on the Moon exhibiting a central peak and ring structures. Note simpler bowl-shaped small craters. Image courtesy of NASA.

Scientists have identified approximately 150 impact sites on the continents (Fig. 32). Impacts that occurred in the oceans may not have been large enough to form craters on the ocean floor or the locations may have been destroyed or obscured by geological processes. The largest craters are formed by meteorites approximately 10 km (6 miles) in width or larger. Such  events are relatively infrequent and are separated by hundreds of millions of years. The most recent such event occurred 65 million years ago, forming the Chicxulub impact structure on the Yucatan Peninsula, Mexico, and is thought to have caused a worldwide extinction that wiped out 70% of species. These large-scale impacts leave a clear imprint in the geologic record that can be readily documented. A meteorite of 1 km (0.6 miles) in diameter is sufficiently large to devastate most nations and objects just 50 to 100 meters (160-330 feet) across could level whole cities. The explosive force of the relatively small meteorite that carved out Meteor Crater, Arizona, was several thousand times greater than the atomic bomb dropped on Hiroshima at the close of World War II.

Figure 32. Locations of impact events discussed in text. Numbers refer to identified craters per continent. Over 150 craters have been recognized. It is likely that exploration of less-accessible regions of Earth will yield many more examples.

Environmental Consequences of a Large Impact Event
It could be a day like any other. The entry of a large asteroid or meteorite into Earth's atmosphere may occur with no warning or it could be predicted decades in advance and watched anxiously by billions of people around the world. It would be accompanied by an atmospheric shock wave and the frictional heating of the speeding object would cause it to glow as it plunged through the atmosphere. For many, this might be their first warning of their fate. The fireball would take just 15 to 30 seconds before making impact, too little time to take any actions that would permit survival for those close to the impact site. The collision would send out a powerful air blast that would flatten everything for hundreds of kilometers in every direction. Anything that survived the air blast would be rocked by a massive earthquake hundreds or thousands of times greater than the largest ever recorded (Fig. 33).

The impact would gouge out a deep crater about 10 to 20 times larger than the colliding meteorite/asteroid. The Chicxulub Crater in Mexico is approximately 200 km (125 miles) in diameter and was formed by a meteorite up to 10 km (6 miles) across. The air blast from the impact event felled forests 2,000 km (1,250 miles) away in the interior of North America. Almost every living thing in southern North America or northern South America would have been killed by the collision. The impact would pulverize rocks, ejecting a massive plume of dust and melted rock fragments upward into the atmosphere. There would be sufficient dust in the atmosphere, potentially for several months, to block sunlight, leading to lower temperatures and a short-term cooling trend. Scientists have estimated that Earth was in darkness for up to six months following the Chicxulub impact which may have been sufficient to prevent photosynthesis for the next year. Vegetation would not survive without the ability to enter a dormant phase until conditions improved sufficiently to once again allow photosynthesis.

Figure 33. Frequency of impact events of contrasting sizes. The largest impacts occur on time intervals measured in hundreds of millions of years. Impacts large enough to destroy a large city or have substantial regional consequences occur every 100 to 1,000 years.

Pieces of molten rock blasted out of the crater would fall back to Earth to generate colossal wildfires that would add smoke to the rapidly darkening skies. Tiny globules of molten material would form glassy spheres known as spherules that are indicative impact events. Some of these particles would travel fast enough to leave the atmosphere and orbit Earth before falling back to the surface. An impact event in the open ocean would generate a giant tsunami that would drown coastal regions and travel far inland. Waves with heights measured in thousands of meters (0.6-2 miles) would be possible from a Chicxulub-sized event in the deep ocean. A 10 km-wide impactor would be over twice the average depth of the ocean floor. The tsunami associated with Chicxulub was muted as only a portion of the impact was located in the shallow waters of the Gulf of Mexico along the margin of the Yucatan Peninsula. The impact generated tsunamis up to 300 meters (1,000 feet) high that pushed into the present Gulf Coast states and created sufficient backwash to carry forest debris up to 500 meters (0.8 miles) offshore.

Finally, atmospheric chemistry would be changed as gases derived from ocean waters or pulverized rocks would be added to the atmosphere. Gases such as sulfur dioxide, carbon dioxide, and water vapor could have residence times in the atmosphere measured in years to decades and could remain after the dust settles and wildfires burn themselves out.  Injections of sufficient sulfur dioxide would result in global acid rain conditions. The potential consequence of these additional greenhouse gases is to trap more solar radiation and generate a warming trend in the decades following the impact. 

Think about it . . .

Draw a diagram or make a concept map that summarizes the consequences of the impact of a large asteroid with Earth.