Impact Craters on Earth: Meteor Crater to Chicxulub

How an impact crater forms

A fast rock hitting the ground does not push the surface aside like a stone in mud. It creates a shock wave. The USGS says the iron meteorite that made Arizona's Meteor Crater struck at an approximate speed of 12 to 20 km/s. The Lunar and Planetary Institute's handbook on shock effects, written by geologist Bevan French, gives the conditions near an impact point as pressures of 100 to 400 gigapascals and temperatures up to 10,000 degrees Celsius. One gigapascal is about 10,000 times normal air pressure, so those pressures run from roughly a million to four million atmospheres. Rock near the center vaporizes.

Farther out, the shock wave breaks and melts rock without vaporizing it, and an expanding release wave starts an excavation flow that throws the broken material outward. That material builds the rim and the blanket of debris around the bowl. French's tables say that even a large structure 100 km across forms in under an hour. The shock effects it leaves in the rock are what geologists read later.

Not every impactor reaches the ground. A 1993 study by Christopher Chyba, Paul Thomas and Kevin Zahnle, hosted in the NASA Technical Reports Server, says typical iron objects reach and crater the surface, while stony asteroids tens of meters across typically break up in the air, as happened over Siberia in the 1908 Tunguska airburst. The 2013 Russian airburst in the Chelyabinsk case was also a breakup in the air.

Why Earth has so few visible craters

The Smithsonian article calls plate tectonics the major reason. Oceanic crust is recycled into the planet, and the oldest oceanic crust is only about 200 million years old, compared with about 4 billion years for the oldest continental crust. Almost all of Earth's known craters sit on continental crust. Water in some form adds to the loss by breaking down rock and carrying it across the surface, which hides craters or erases them. Many craters on Earth can only be recognized by studying the rocks around them.

The Moon makes the contrast clear. It has no atmosphere and no flowing water, so it does not weather the way Earth does, and its surface is older, which gives its craters time to pile up. Earth's small crater count, the Smithsonian article says, reflects a geologically active planet.

Recognition can come late. The Smithsonian notes that the Ries crater in Germany, where the town of Nördlingen now stands, was identified as an impact structure only in the 1960s, because weathering had obscured its origin. It also mentions Tenoumer crater in Mauritania, about 1.9 km (1.2 miles) across in the Sahara, and the Sudbury structure in Ontario, Canada, which hosts some of the world's largest copper and nickel deposits.

Meteor Crater, Arizona

The USGS describes Meteor Crater as a bowl-shaped crater 180 m (about 590 feet) deep and 1.2 km (0.75 mile) across, formed about 50,000 years ago by an iron-nickel meteorite. NASA's Earth Observatory gives somewhat different numbers, so treat the values below as a range, not a single answer.

NASA's Earth Observatory says the rim rises about 50 m above the plain and is made of smashed, jumbled boulders. It also says more than 30 tons of meteoritic iron have been found scattered across an area 12 to 15 km in diameter. The USGS says Eugene Shoemaker trained NASA astronauts at the crater in the 1960s, and credits his work, along with Daniel Barringer's, for the evidence of its impact origin. Iron meteorites like the one that formed the crater are described in the main types of meteorites.

Chicxulub, the buried giant

Chicxulub lies on and under the Yucatán Peninsula in Mexico, and it is far larger. NASA JPL gave its width as 180 km (112 miles) in a 2003 article, which also says it lies buried under about 1 km of younger rock and sediment. That cover hid it for decades. The impactor was about 10 km (6.2 miles) across, according to JPL's Don Yeomans.

The story of its discovery shows how craters are found indirectly. JPL says a Mexican petroleum company's magnetic and gravity surveys had shown a large circular structure, and that in 1990 NASA researchers noticed a ring of sinkholes called cenotes across the northern tip of the peninsula in satellite images. Luis and Walter Alvarez had first proposed an impact as the cause of the extinction in 1980, and a 1991 analysis of magnetic, gravity, rock and ocean sediment data helped convince the scientific community.

Drilling went inside. Expedition 364 of the International Ocean Discovery Program drilled into the crater's peak ring, the ring of uplifted rock inside it, and recovered a continuous core from 505.7 to 1,334.7 m below the seafloor, with the top of the ring at about 618 m, according to a 2021 paper in Frontiers in Microbiology. That paper treats the impact as about 66 million years old, while the 2003 NASA page says about 65 million years ago, so the age is best given as roughly 65 to 66 million years. JPL calls the crater a smoking gun for the extinction of more than 70% of species, including the dinosaurs, and says how the impact killed so many species is still debated.

How geologists confirm an impact

A round hole is not proof. Volcanoes, sinkholes and mining all make round holes. The handbook by French says the recognized shock effects are unique to the pressures that only impacts produce on Earth's surface. Geologists look for the evidence below.

• Shatter cones, cone-shaped fracture patterns, form at relatively low shock pressures of about 2 to 10 gigapascals.
• Planar deformation features, sets of fine parallel lines inside quartz grains, record higher pressures.
• Diaplectic glass, a glass made from quartz or feldspar without full melting, is another accepted criterion.
• High-pressure minerals such as coesite and stishovite point the same way. In the shallow crust where craters form, French says, only a shock wave produces such pressures.

French's handbook adds that these effects survive long after meteorite fragments have eroded away, which is why old craters can still be identified. At Meteor Crater, the handbook shows shocked sandstone with highly fractured quartz grains and dark glass containing coesite. How near-Earth objects are found and watched today is a separate subject, covered in planetary defense and the DART test.

Common mistakes and myths

• "Every impact leaves a crater." Many do not. Tunguska and Chelyabinsk were airbursts, and a stony object that breaks up high in the air may leave only a shock wave.
• "A crater is always an obvious bowl." Chicxulub is buried under about 1 km of sediment, and Ries holds a town.
• "A round depression means a meteorite hit." The shock evidence above is what counts. Without it, the cause stays unproven.
• "The meteorite is still buried at the bottom." NASA's Earth Observatory says more than 30 tons of meteoritic iron were found scattered across an area 12 to 15 km in diameter, not sitting in one lump.
• "The dinosaur impact is fully explained." The impact itself is well supported, but JPL says the exact kill mechanism is unresolved, and it adds that whether the impactor was an asteroid or a comet is still unclear, though most scientists favor an asteroid.

Frequently asked questions

How many impact craters are confirmed on Earth?

The Smithsonian's April 2025 article gives about 190. The count changes as new structures are confirmed, so check a current catalog for the latest figure.

How old is Meteor Crater?

About 50,000 years, according to the USGS and NASA. NASA's wording is that scientists now believe the crater formed around that time.

How big is Chicxulub?

NASA JPL gives about 180 km (112 miles) in a 2003 article. It is buried under about 1 km of sediment, so its outline was mapped with gravity and magnetic surveys and later checked by drilling.

Why did Tunguska leave no crater?

The object broke up in the air before it reached the ground. The 1993 study cited above says typical iron objects reach the surface and dig craters, while stony asteroids of that size tend to explode high up, as the Chelyabinsk object did in 2013.