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 the 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.

The 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 American astronaut Buzz Aldrin to ancient Greek philosopher Zeno.

Many impact craters are found on the Earth’s surface, although they can be harder to detect. One of the best-known craters on Earth is Meteor Crater, near Winslow, Arizona. 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 65 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 (6 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 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 the 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 the 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.

Over a long period of time, small and non-explosive 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.
crater
Mars' Happy Face crater was the result of a happy accident—a meteor impact.
Makhteshim
A makhtesh is a type of circular depression only found in the Negev Desert of Israel. Although often called craters, makhteshim are not created by explosions or impacts. They are created by the process of erosion wearing away softer rocks underlying a harder upper layer. The upper layer ultimately collapses under its own weight, forming a bowl-shaped depression that resembles a crater.

Bacteria Will SurviveYou Wont
The impact of a meteorite that would result in the creation of a Chicxulub-sized crater is something astronomers call an extinction-level event (ELE) or biotic crisis. Meteorites are just one possible cause of an ELE. ELEs have happened more than a dozen times in Earths history.

Extinction-level events actually have little effect on Earths biodiversity. Most life on Earth is microbial. Microbes, such as bacteria and algae, are not significantly affected by ELEs. Its only the larger life formstrees, dinosaurs, peoplethat face biotic crises.

Rampart Craters
Some craters on Mars hint that liquid water was probably present at some point in the planet's past. Rampart craters are a type of impact crater found only on Mars. Unlike craters on the moon, where debris, called ejecta, from the impact is spread out in neat lines, rampart craters show ejecta curving out in smooth, flowing lineslike a mudflow. Rampart craters look more like splashes than explosions.

access
Noun

ability to use.

active volcano
Noun

volcano that has had a recorded eruption since the last glacial period, about 10,000 years ago.

aerial
Adjective

existing, moving, growing, or operating in the air.

Noun

the distance above sea level.

ancient
Adjective

very old.

asteroid
Noun

irregularly shaped planetary body, ranging from 6 meters (20 feet) to 933 kilometers (580 miles) in diameter, orbiting the sun between Mars and Jupiter.

astronaut
Noun

person who takes part in space flights.

Noun

layers of gases surrounding a planet or other celestial body.

atomic bomb
Noun

explosive device that draws energy from the interaction of atomic nuclei. Also called an atom bomb, a-bomb, or nuclear bomb.

Noun

solid rock beneath the Earth's soil and sand.

Buzz Aldrin
Noun

(Edwin Eugene Aldrin, Jr., born 1930) U.S. astronaut.

Noun

large depression resulting from the collapse of the center of a volcano.

chasm
Noun

a deep opening in the earth's surface.

Chicxulub Crater
Noun

impact crater on the Yucatan Peninsula, formed about 65 million years ago.

collide
Verb

to crash into.

comet
Noun

celestial object made up of ice, gas, and dust that orbits the sun and leaves a tail of debris.

compress
Verb

to press together in a smaller space.

Noun

bowl-shaped depression formed by a volcanic eruption or impact of a meteorite.

debris
Noun

remains of something broken or destroyed; waste, or garbage.

depression
Noun

indentation or dip in the landscape.

Noun

area of land that receives no more than 25 centimeters (10 inches) of precipitation a year.

detect
Verb

to notice.

detonate
Verb

to cause something to explode.

diameter
Noun

width of a circle.

dinosaur
Noun

very large, extinct reptile chiefly from the Mesozoic Era, 251 million to 65 million years ago.

Noun

tiny, dry particles of material solid enough for wind to carry.

Noun

our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.

ejecta
Noun

material ejected from a crater, usually by an erupting volcano or meteorite impact.

enormous
Adjective

very large.

Noun

act in which earth is worn away, often by water, wind, or ice.

eruption
Noun

release of material from an opening in the Earth's crust.

Noun

process of complete disappearance of a species from Earth.

flank crater
Noun

depression formed by volcanic activity on the sides of a volcano.

gas
Noun

state of matter with no fixed shape that will fill any container uniformly. Gas molecules are in constant, random motion.

gas giant
Noun

one of the four enormous outermost planets in the solar system (Jupiter, Saturn, Neptune, Uranus), composed mostly of gases instead of rock. Also called a Jovian planet.

geologist
Noun

person who studies the physical formations of the Earth.

Noun

water found in an aquifer.

impact crater
Noun

circular surface depression made by the impact of a meteorite.

Jupiter
Noun

largest planet in the solar system, the fifth planet from the Sun.

landmark
Noun

a prominent feature that guides in navigation or marks a site.

Noun

the geographic features of a region.

lava
Noun

molten rock, or magma, that erupts from volcanoes or fissures in the Earth's surface.

lunar
Adjective

having to do with Earth's moon or the moons of other planets.

maar
Noun

depression formed as magma reacts with groundwater.

machinery
Noun

mechanical appliances or tools used in manufacturing.

Noun

molten, or partially melted, rock beneath the Earth's surface.

magma chamber
Noun

underground reservoir that holds molten rock.

Noun

rocky debris from space that enters Earth's atmosphere. Also called a shooting star or falling star.

Meteor Crater
Noun

impact crater near Winslow, Arizona, formed about 50,000 years ago.

Noun

type of rock that has crashed into Earth from outside the atmosphere.

Moon
Noun

Earth's only natural satellite.

Noun

type of fossil fuel made up mostly of the gas methane.

Neptune
Noun

eighth planet from the sun in our solar system.

Nevada Test Site
Noun

testing site for nuclear weapons and other military products in the southern Nevada desert. Nuclear weapons testing was discontinued there in 1992.

nuclear explosion
Noun

large release of energy as a result of a reaction between atomic nuclei or nuclear particles.

oil
Noun

fossil fuel formed from the remains of marine plants and animals. Also known as petroleum or crude oil.

oil drilling
Noun

process of digging below the surface of the Earth for oil.

orbit
Noun

path of one object around a more massive object.

Noun

piece of land jutting into a body of water.

Noun

permanently frozen layer of the Earth's surface.

phenomenon
Noun

an unusual act or occurrence.

philosopher
Noun

person who studies knowledge and the way people use it.

physical geography
Noun

study of the natural features and processes of the Earth.

Noun

large, spherical celestial body that regularly rotates around a star.

pockmarked
Adjective

scarred with many small indentations.

Noun

all forms in which water falls to Earth from the atmosphere.

predict
Verb

to know the outcome of a situation in advance.

pulverize
Verb

to crush and make into dust or powder.

puncture
Verb

to penetrate or poke through.

Noun

current of volcanic ash, lava, and gas that flows from a volcano.

radioactive
Adjective

having unstable atomic nuclei and emitting subatomic particles and radiation.

rare
Adjective

unusual or uncommon.

remote
Adjective

distant or far away.

restrict
Verb

to limit.

ridge
Noun

long, narrow elevation of earth.

Noun

large stream of flowing fresh water.

satellite imagery
Noun

photographs of a planet taken by or from a satellite.

Saturn
Noun

sixth planet from the sun.

shatter cone
Noun
conical structure produced in rock by intense mechanical shock, usually associated with a meteor impact.
shooting star
Noun

rocky debris from space that enters Earth's atmosphere. Also called a meteor.

solar system
Noun

the sun and the planets, asteroids, comets, and other bodies that orbit around it.

species
Noun

group of similar organisms that can reproduce with each other.

stark
Adjective

severe, striking, or clear-cut.

subsidence crater
Noun

depression formed as the result of an underground explosion.

summit
Noun

highest point of a mountain.

Uranus
Noun

large, gaseous planet in the solar system, seventh from the sun.

vaporize
Verb

to turn into gas, or vapor.

vegetation
Noun

all the plant life of a specific place.

Noun

small human settlement usually found in a rural setting.

visible
Adjective

able to be seen.

volcanic
Adjective

having to do with volcanoes.

Noun

fragments of lava less than 2 millimeters across.

volcanic vent
Noun

opening in the Earth's crust where lava and gases escape to the Earth's surface or atmosphere.

Noun

an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

volcanologist
Noun

scientist who studies volcanoes.

Noun

the breaking down or dissolving of the Earth's surface rocks and minerals.

Noun

movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.

Zeno
Noun

(340-265 BCE) Greek philosopher.