• crater
    The shadowed interior of Mount St. Helens' crater is dominated by Crater Glacier, formed after the 1980 eruption.

    Photograph by James P. Blair, National Geographic

    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.

    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.

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

    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. The forces of wind, rivers, and precipitation can scrape away evidence of a crater. Vegetation can also grow over the area.

    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.

    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. Erta Ale, a volcano in Ethiopia, has a lava lake in its summit crater. Lava lakes are where magma has bubbled up to the surface. Volcanologists can fly over Erta Ale’s 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.

    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.

  • Term Part of Speech Definition Encyclopedic Entry
    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.

    altitude Noun

    the distance above sea level.

    Encyclopedic Entry: altitude
    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.

    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    atomic bomb Noun

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

    Buzz Aldrin Noun

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

    Chicxulub Crater Noun

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

    collide Verb

    to crash into.

    comet Noun

    celestial object of matter surrounded by ice and dust that orbits the sun and leaves a tail of debris.

    compress Verb

    to press together in a small space.

    crater Noun

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

    Encyclopedic Entry: crater
    debris Noun

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

    depression Noun

    indentation or dip in the landscape.

    desert Noun

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

    Encyclopedic Entry: desert
    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.

    dust Noun

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

    Encyclopedic Entry: dust
    Earth Noun

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

    Encyclopedic Entry: Earth
    ejecta Noun

    material ejected from a volcano, such as ash.

    enormous Adjective

    very large.

    erosion Noun

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

    Encyclopedic Entry: erosion
    eruption Noun

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

    extinction 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.

    groundwater Noun

    water found in an aquifer.

    Encyclopedic Entry: groundwater
    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.

    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.

    magma Noun

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

    Encyclopedic Entry: magma
    meteor Noun

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

    Encyclopedic Entry: meteor
    Meteor Crater Noun

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

    meteorite Noun

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

    Encyclopedic Entry: meteorite
    Moon Noun

    Earth's only natural satellite.

    natural gas Noun

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

    Encyclopedic Entry: natural gas
    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.

    orbit Noun

    path of one object around a more massive object.

    peninsula Noun

    piece of land jutting into a body of water.

    Encyclopedic Entry: peninsula
    permafrost Noun

    permanently frozen layer of the Earth's surface.

    Encyclopedic Entry: permafrost
    phenomenon Noun

    an unusual act or occurrence.

    philosopher Noun

    person who studies knowledge and the way people use it.

    planet Noun

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

    Encyclopedic Entry: planet
    pockmarked Adjective

    scarred with many small indentations.

    precipitation Noun

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

    Encyclopedic Entry: precipitation
    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.

    pyroclastic flow Noun

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

    Encyclopedic Entry: pyroclastic flow
    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.

    river Noun

    large stream of flowing fresh water.

    Encyclopedic Entry: river
    Saturn Noun

    sixth planet from the sun.

    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.

    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.

    village Noun

    small human settlement usually found in a rural setting.

    Encyclopedic Entry: village
    visible Adjective

    able to be seen.

    volcanic Adjective

    having to do with volcanoes.

    volcanic ash Noun

    fragments of lava less than 2 millimeters across.

    Encyclopedic Entry: volcanic ash
    volcanic vent Noun

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

    volcano Noun

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

    Encyclopedic Entry: volcano
    volcanologist Noun

    scientist who studies volcanoes.

    weathering Noun

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

    Encyclopedic Entry: weathering
    wind 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.

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