A crater is a bowl-shaped depression, or hollowed-out area, produced by the impact of a meteorite, volcanic activity, or an explosion.

Impact Craters

Craters produced by the collision of a meteorite with Earth (or another planet or moon) are called impact craters. The high-speed impact of a large meteorite compresses, or forces downward, a wide area of rock. The pressure pulverizes the rock. Almost immediately after the strike, however, the pulverized rock rebounds. Enormous amounts of shattered material jet upward, while a wide, circular crater forms where the rock once lay. Most of the material falls around the rim of the newly formed crater.

Earth’s moon has many craters. Most were formed when meteors, bodies of solid matter from space, slammed into the lunar surface millions of years ago. Because the moon has almost no atmosphere, there is hardly any wind, erosion, or weathering. Craters and debris, called ejecta, from millions of years ago are still crystal-clear on the moon’s surface. Many of these craters are landmarks. Craters on the moon are named after everyone from U.S. astronaut Buzz Aldrin to ancient Greek philosopher Zeno.

Many impact craters are found on Earth’s surface, although they can be harder to detect. One of the best-known craters on Earth is Meteor Crater, near Winslow, Arizona, U.S.A. The crater was created instantly when a 50-meter (164-foot), 150,000-ton meteorite slammed into the desert about 50,000 years ago. Meteor Crater is 1.2 kilometers (0.75 miles) in diameter and 175 meters (575 feet) deep.

The Chicxulub Crater, on Mexico’s Yucatan Peninsula, was most likely created by a comet or asteroid that hit Earth about 66 million years ago. The crater is 180 kilometers (112 miles) wide and 900 meters (3,000 feet) deep. The object that created the Chicxulub Crater was probably about 10 kilometers (six miles) wide.

The impact was so powerful the crater is called the Chicxulub Extinction Event Crater. Scientists say half the species on Earth—including the nonavian dinosaurs—went extinct as a result of the impact. The event was more than a billion times more explosive than all the atomic bombs ever detonated on Earth.

Impact craters are found on most of the solar system’s rocky planets and moons. The so-called “gas giants” of the solar system—Jupiter, Saturn, Uranus, and Neptune—don’t have craters. These planets are made up almost entirely of gases, so there is no hard surface for a meteor to impact. Meteors entering the atmosphere of a gas giant simply break up.

Cratering is a rare occurrence in the solar system today. Planets, moons, comets, and other celestial bodies have fairly stable orbits that do not interact with each other. Meteors do collide with planets—including Earth—every day. However, most of these meteors are the size of a speck of dust and do not cause any cratering. Most meteors burn up in the atmosphere as “shooting stars” before ever colliding with the surface of Earth.

Volcanic Craters
Volcanic activity often creates craters. Some volcanic craters are deep and have steep sides. Others are wide and shallow.

A crater is not the same thing as a caldera. Craters are formed by the outward explosion of rocks and other materials from a volcano. Calderas are formed by the inward collapse of a volcano’s magma chamber. Craters are usually much smaller features than calderas, and calderas are sometimes considered giant craters.

Craters at the top of volcanoes are called summit craters. Summit craters are where volcanic material is at or near Earth’s surface. Volcanoes may have one summit crater, such as Mount Fuji in Japan. Or they may have several. Mount Etna, in Italy, has four.

Some volcanoes are calm enough that scientists can get close to the lava in the summit crater. Mount Erebus, a volcano in Antarctica, has a lava lake in its summit crater. Lava lakes are where magma has bubbled up to the surface. Volcanologists can fly over Mount Erebus’ summit crater to see how the lava lake is behaving and predict future behavior.

Volcanic material in some summit craters is near the surface, but not visible. Although Mount Fuji is an active volcano and magma and gases sit below the summit crater, the risk of an eruption is very low. Mount Fuji, Japan’s highest mountain, is one of the most popular places in the country to hike.

Craters that form on the sides of volcanoes are called flank craters. Eruptions from flank craters can be much more dangerous than eruptions from summit craters. Flank craters can form at lower altitudes than summit craters, near hillside towns. Lava, gas, rocks, and other material ejected from a flank crater can rush down the side of a mountain in a phenomenon called a pyroclastic flow. Mount Etna, one of the most active volcanoes in Europe, has had a number of dangerous eruptions. In 1928, the eruption of a flank crater completely destroyed the village of Mascali, Italy.

Over a long period of time, small and nonexplosive eruptions may fill a volcanic crater with new material. At Mount St. Helens, in the U.S. state of Washington, for example, a large crater formed when a major eruption in 1980 tore off 400 meters (1,300 feet) of the mountaintop. Soon after, smaller eruptions began piling up lava and volcanic ash on the crater floor, slowly rebuilding the mountain.

Volcanoes can also create craters when the magma comes into contact with water. Magma flowing or bubbling beneath a volcano can sometimes interact with groundwater in the area. When this happens, a small explosion occurs and a crater forms around the explosion. This type of volcanic crater is called a maar.

Often, a maar will fill with water and become a shallow crater lake. The thin floors of these lakes are actually the roofs of volcanic vents, waiting to come into explosive contact with water once again. The Seward Peninsula, in the U.S. state of Alaska, is filled with maars that form as magma encounters not groundwater, but permafrost.

Explosion Craters
A third type of crater is formed by an explosion. When materials or chemicals explode, the explosion displaces all the material around it. The debris often lands in a circular pattern around the site of the explosion, creating a crater.

Explosions can be natural or artificial. The explosion that creates a maar, for example, occurs naturally when water interacts with superhot magma from a volcano. Maars are a type of explosion crater as well as a volcanic crater.

Artificial explosions that form craters usually happen underground. The explosion pulverizes or vaporizes material underground, and the earth above sinks. Craters formed by underground explosions are called subsidence craters. (Craters formed by explosions at or near the surface of the Earth are simply called explosion craters.)

Drilling underground for oil and natural gas can lead to explosions and subsidence craters. Machinery can sometimes encounter a pocket of natural gas that is under extremely high pressure. When drilling machinery punctures the pocket of natural gas, the overlying rock layers may not be able to contain it. Like an enormous balloon, the gas pocket pops. As the gas is released in the explosion, a crater forms in the empty space.

A specific type of subsidence crater is formed by an underground nuclear explosion. Most nuclear testing is conducted in underground facilities. As the explosion displaces massive amounts of material, the earth above it sinks. In fact, subsidence craters caused by underground nuclear explosions are sometimes called sinks. The Nevada Test Site, in the remote deserts of the U.S. state of Nevada, is pockmarked with nuclear subsidence craters.

The debris in and around nuclear subsidence craters often comes into contact with radioactive material. For this reason, access to these sites is restricted.

Finding Craters

Although impact craters are found all around the world, they can be very hard to detect. Before the widespread use of aerial and satellite imagery, many craters went undetected. One of the reasons Meteor Crater is so well-known is because the stark Arizona desert makes it an obvious feature of the area’s physical geography. The forces of wind, rivers, precipitation, for instance, can scrape away evidence of a crater. Some areas are also geologically complex, where a meteor’s impact may be more pronounced among some rocks and less by others. Some rocks are also more vulnerable to the forces of weathering and erosion. These forces can mask the traditional circular pattern of an impact crater. Spider Crater, Western Australia, for example, is an unusual feature that puzzled geologists until the 1970s. The winding series of chasms and ridges radiating from a central area resemble a giant arachnid, not a circular crater. Geologists were able to identify Spider as an impact crater only after looking at rocks unearthed from the feature’s central region. There, they discovered shatter cones, rare rocks that only form in the bedrock beneath impact craters. Landscape and vegetation can also hide impact craters. The largest impact crater in the United States, for instance, was unknown until the 1980s. The Chesapeake Bay impact crater was hidden beneath the muddy waters of the Chesapeake Bay and the Atlantic Ocean for about 35 million years. The submarine crater, discovered through oil drilling exploration, was, like Spider Crater, marked by rocks only found at impact craters.