• Earth is the planet we live on, one of eight planets in our solar system and the only known place in the universe to support life.
     
    Earth is the third planet from the sun, after Mercury and Venus and before Mars. It is about 150 million kilometers (about 93 million miles) from the sun. This distance, called an astronomical unit (AU), is a standard unit of measurement in astronomy. Earth is one AU from the sun. The planet Jupiter is about 5.2 AU from the sun—about 778 million kilometers (483.5 million miles).
     
    Earth is the largest and most massive of the rocky inner planets, although it is dwarfed by the gas giants beyond the Asteroid Belt. Its diameter is about 12,700 kilometers (7,900 miles), and its mass is about 5.97×1024 kilograms (6.58×1021 tons). In contrast, Jupiter, the largest planet in the solar system, has a diameter of 143,000 kilometers (88,850 miles), and its mass is about 1,898×1024 kilograms (2093×1021 tons).
     
    Earth is an oblate spheroid. This means it is spherical in shape, but not perfectly round. It has a slightly greater radius at the Equator, the imaginary line running horizontally around the middle of the planet. In addition to bulging in the middle, Earth’s poles are slightly flattened. The geoid describes the model shape of Earth, and is used to calculate precise surface locations.
     
    Earth has one natural satellite, the Moon. Earth is the only planet in the solar system to have one moon. Venus and Mercury do not have any moons, for example, while Jupiter and Saturn each have more than a dozen.
     
    Planet Earth
     
    Interior
    Earth’s interior is a complex structure of superheated rocks. Most geologists recognize three major layers: the dense core, the bulky mantle, and the brittle crust. No one has ever ventured below Earth’s crust. 
     
    Earth’s core is mostly made of iron and nickel. It consists of a solid center surrounded by an outer layer of liquid. The core is found about 2,900 kilometers (1,802 miles) below Earth’s surface, and has a radius of about 3,485 kilometers (2,165 miles).
     
    A mantle of heavy rock (mostly silicates) surrounds the core. The mantle is about 2,900 kilometers (1,802 miles) thick, and makes up a whopping 84% of Earth’s total volume. Parts of the mantle are molten, meaning they are composed of partly melted rock. The mantle’s molten rock is constantly in motion. It is forced to the surface during volcanic eruptions and at mid-ocean ridges.
     
    Earth’s crust is the planet’s thinnest layer, accounting for just 1% of Earth’s mass. There are two kinds of crust: thin, dense oceanic crust and thick, less-dense continental crust. Oceanic crust extends about 5 to 10 kilometers (3 to 6 miles) beneath the ocean floor. Continental crust is about 35 to 70 kilometers (22 to 44 miles) thick. 
     
    Exterior: Tectonic Activity
    The crust is covered by a series of constantly moving tectonic plates. New crust is created along mid-ocean ridges and rift valleys, where plates pull apart from each other in a process called rifting. Plates slide above and below each other in a process called subduction. They crash against each other in a process called faulting. 
     
    Tectonic activity such as subduction and faulting has shaped the crust into a variety of landscapes. Earth’s highest point is Mount Everest, Nepal, which soars 8,850 kilometers (29,035 feet) in the Himalaya Mountains in Asia. Mount Everest continues to grow every year, as subduction drives the Indo-Australian tectonic plate below the Eurasian tectonic plate. Subduction also creates Earth’s deepest point, the Mariana Trench, about 11 kilometers (6.9 miles) below the surface of the Pacific Ocean. The heavy Pacific plate is being subducted beneath the small Mariana plate. 
     
    Plate tectonics are also responsible for landforms such as geysers, earthquakes, and volcanoes. Tectonic activity around the Pacific plate, for instance, creates “Ring of Fire.” This tectonically active area includes volcanoes such as Mount Fuji, Japan, and earthquake-prone fault zones such as the west coast of the United States.
     
    Revolution and Rotation
    Earth is a rocky body constantly moving around the sun in a path called an orbit. The Earth and Moon follow a slightly oval-shaped orbit around the sun every year. 
     
    Each journey around the sun, a trip of about 940 million kilometers (584 million miles), is called a revolution. A year on Earth is the time it takes to complete one revolution, about 365.25 days. Earth orbits the sun at a speedy rate of about 30 kilometers per second (18.5 miles per second).
     
    At the same time that it revolves around the sun, the Earth rotates on its own axis. Rotation is when an object, such as a planet, turns around an invisible line running down its center. Earth’s axis is vertical, running from the North Pole to the South Pole. Earth makes one complete rotation about every 24 hours.
     
    Earth rotates unevenly, spinning faster at the Equator than at the poles. At the Equator, the Earth rotates at about 1,670 kilometers per hour (1,040 miles per hour), while at 45° north, for example, (the approximate latitude of Green Bay, Wisconsin) the Earth rotates at 1,180 kilometers per hour (733 miles per hour).
     
    Earth’s rotation causes the periods of light and darkness we call day and night. The part of the Earth facing the sun is in daylight; the part facing away from the sun is in darkness. If the Earth did not rotate, one-half of the Earth would always be too hot to support life, and the other half would be frozen. Earth rotates from west to east, so the sun appears to rise in the east and set in the west. 
     
    In addition to Earth’s revolution and rotation periods, we experience light and darkness due to Earth’s axis not being straight up-and-down. Earth’s axis of rotation is tilted 23.5°. This tilt influences temperature changes and other weather patterns from season to season. 
     
    The Spheres
     
    Earth’s physical environment is often described in terms of spheres: the magnetosphere, the atmosphere, the hydrosphere, and the lithosphere. Parts of these spheres make up the biosphere, the area of Earth where life exists.
     
    Magnetosphere
    Earth’s magnetosphere describes the pocket of space surrounding our planet where charged particles are controlled by Earth’s magnetic field. 
     
    The charged particles that interact with Earth’s magnetosphere are called the solar wind. The pressure of the solar wind compresses the magnetosphere on the “dayside” of Earth to about 10 Earth radii. The long tail of the magnetosphere on the “nightside” of Earth stretches to hundreds of Earth radii. The most well-known aspect of the magnetosphere are the charged particles that sometimes interact over its poles—the auroras, or Northern and Southern Lights.
     
    Atmosphere
    Earth’s atmosphere is a blanket of gases enveloping the Earth and retained by our planet’s gravity. Atmospheric gases include nitrogen, water vapor, oxygen, and carbon dioxide. 
     
    The atmosphere is responsible for temperature and other weather patterns on Earth. It blocks most of the sun’s ultraviolet radiation (UV), conducts solar radiation and precipitation through constantly moving air masses, and keeps our planet’s average surface temperature to about 15° Celsius (59° Fahrenheit). 
     
    The atmosphere has a layered structure. From the ground toward the sky, the layers are the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Up to 75% of the total mass of the atmosphere is in the troposphere, where most weather occurs. The boundaries between the layers are not clearly defined, and change depending on latitude and season.
     
    Hydrosphere
    The hydrosphere is composed of all the water on Earth. Nearly three-fourths of the Earth is covered in water, most of it in the ocean. Less 3% of the hydrosphere is made up of freshwater. Most freshwater is frozen in ice sheets and glaciers in Antarctica, the North American island of Greenland, and the Arctic. Freshwater can also be found underground, in chambers called aquifers, as well as rivers, lakes, and springs.
     
    Water also circulates around the world as vapor. Water vapor can condense into clouds and fall back to Earth as precipitation.
     
    The hydrosphere helps regulate the Earth’s temperature and climate. The ocean absorbs heat from the sun and interacts with the atmosphere to move it around the Earth in air currents. 
     
    Lithosphere
    The lithosphere is Earth’s solid shell. The crust and the upper portion of the mantle form the lithosphere. It extends from Earth’s surface to between 50 and 280 kilometers (31 to 174 miles) below it. The difference in thickness accounts for both thin oceanic and thicker continental crust.
     
    The rocks and minerals in Earth’s lithosphere are made of many elements. Rocks with oxygen and silicon, the most abundant elements in the lithosphere, are called silicates. Quartz is the most common silicate in the lithosphere—and the most common type of rock on Earth.
     
    Cycles on Earth
     
    Almost all materials on Earth are constantly being recycled. The three most common cycles are the water cycle, the carbon cycle, and the rock cycle.
     
    Water Cycle
    The water cycle involves three main phases, related to the three states of water: solid, liquid, and gas. Ice, or solid water, is most common near the poles and at high altitudes. Ice sheets and glaciers hold the most solid water.
     
    Ice sheets and glaciers melt, transforming into liquid water. The most abundant liquid water on the planet is in the ocean, although lakes, rivers, and underground aquifers also hold liquid water. Life on Earth is dependent on a supply of liquid water. Most organisms, in fact, are made up mostly of liquid water, called body water. The human body is about 50% to 60% body water. In addition to survival and hygiene, people use liquid water for energy and transportation.
     
    The third phase of the water cycle occurs as liquid water evaporates. Evaporation is the process of a liquid turning into a gas, or vapor. Water vapor is invisible and makes up part of the atmosphere. As water vapor condenses, or turns back into liquid, pockets of vapor become visible as clouds and fog. Eventually, clouds and fog become saturated, or full of liquid water. This liquid water falls to Earth as precipitation. It can then enter a body of water, such as an ocean or lake, or freeze and become part of a glacier or ice sheet. The water cycle starts again.
     
    Carbon Cycle
    The carbon cycle involves the exchange of the element carbon through the Earth’s atmosphere, hydrosphere, and lithosphere. Carbon, essential for all life on Earth, enters the biosphere many ways. Carbon is one of the gases that make up the atmosphere. It is also ejected during the eruption of volcanoes and ocean vents. 
     
    All living or once-living materials contain carbon. These materials are organic. Plants and other autotrophs depend on carbon dioxide to create nutrients in a process called photosynthesis. These nutrients contain carbon. Animals and other organisms that consume autotrophs obtain carbon. Fossil fuels, the remains of ancient plants and animals, contain very high amounts of carbon.
     
    As organisms die and decompose, they release carbon into the ocean, soil, or atmosphere. Plants and other autotrophs use this carbon for photosynthesis, starting the carbon cycle again. 
     
    Rock Cycle
    The rock cycle is a process that explains the relationship between the three main types of rocks: igneous, sedimentary, and metamorphic. Unlike water in the water cycle and or carbon in the carbon cycle, not all rocks are recycled in different forms. There are some rocks that have been in their present form since soon after the Earth cooled. These stable rock formations are called cratons.
     
    Igneous rocks are formed as lava hardens. Lava is molten rock ejected by volcanoes during eruptions. Granite and basalt are common types of igneous rocks. Igneous rocks can be broken apart by the forces of erosion and weathering. Winds or ocean currents may then transport these tiny rocks (sand and dust) to a different location.
     
    Sedimentary rocks are created from millions of tiny particles slowly building up over time. Igneous rocks can become sedimentary by collecting with other rocks into layers. Sedimentary rocks include sandstone and limestone.
     
    Metamorphic rocks are formed when rocks are subjected to intense heat and pressure. The rocks change (undergo metamorphosis) to become a new type of rock. Marble, for example, is a metamorphic rock created from rock that was once limestone, a sedimentary rock.
     
    Earth’s Evolution
     
    Earth and the rest of the solar system formed about 4.6 billion years ago from a huge, spinning cloud of gas and dust. 
     
    Over a period of about 10 million years, the dense center of the cloud grew very hot. This massive center became the sun. The rest of the particles and objects continued to revolve around the sun, colliding with each other in clumps. Eventually, these clumps compressed into planets, asteroids, and moons. This process generated a lot of heat. 
     
    Eventually, Earth began to cool and its materials began to separate. Lighter materials floated upward and formed a thin crust. Heavier materials sank toward the Earth’s center. Eventually, three main layers formed: the core, the mantle, and the crust.
     
    As Earth’s internal structure developed, gases released from the interior mixed together, forming a thick, steamy atmosphere around the planet. Water vapor condensed, and was augmented by water from asteroids and comets that continued to crash to Earth. Rain began to fall and liquid water slowly filled basins in Earth’s crust, forming a primitive ocean that covered most of the planet. Today, ocean waters continue to cover nearly three-quarters of our planet.
     
    The end of the Earth will come with the end of the sun. In a few billion years, the sun will no longer be able to sustain the nuclear reactions that keep its mass and luminosity consistent. First, the sun will lose more than a quarter of its mass, which will loosen its gravitational hold on Earth. Earth’s orbit will widen to about 1.7AU. But the sun will also gain volume, expanding to about 250 times its current size. The sun in this red giant phase will drag Earth into its own fiery atmosphere, destroying the planet.
     
    Eras on Earth
    Paleontologists, geologists, and other scientists divide the Earth’s history into time periods. The largest time period is the supereon, and only applies to one unit of time, the Precambrian. Eons, eras, and periods are smaller units of geologic time.
     
    Most of Earth’s history took place in the Precambrian, which began when the Earth was cooling and ended about 542 million years ago. Life began in the Precambrian, in the forms of bacteria and other single-celled organisms. Fossils from the Precambrian are rare and difficult to study. The Precambrian supereon is usually broken into three eons: the Hadean, the Archaean, and the Proterozoic.
     
    We are currently living in the Phanerozoic eon. 
     
    The first major era of the Phanerozoic is called the Paleozoic, and the Cambrian is the first period of the Paleozoic era. “The Cambrian Explosion of Life” was the rapid appearance of almost all forms of life. Paleontologists and geologists have studied fossils of archaea, bacteria, algae, fungi, plants, and animals that lived during the Cambrian period. The Cambrian was followed by the Ordovician, Silurian, Devonian, Carboniferous, and Permian periods. 
     
    The Mesozoic era began about 251 million years ago. This was the era when dinosaurs flourished. The Mezozoic has three periods: the Triassic, the Jurassic, and the Cretaceous.
     
    We currently live in the Cenozoic era, which began about 65 million years ago. The Cenozoic is generally marked by three periods: the Paleogene, the Neogene, and the Quaternary. We live in the Quaternary period, which began about 2.5 million years ago. All ancestors of Homo sapiens (modern humans) evolved during the Quaternary.
    Earth
    For more breathtaking images of Earth, check out the "Image of the Day" from NASA's Earth Observatory.

    Earth by the Numbers

    • Surface Gravity: 1 (1 kilogram on Earth)
    • Orbital Period: 365.256 days
    • Satellites: 1 (the Moon)
    • Atmosphere: nitrogen (78%), oxygen (21%), argon, carbon dioxide, neon
    • Average Temperature: 15° Celsius (77 Kelvin, 59° Fahrenheit)

    Ingredients for Life
    Scientists have gathered enough information about other planets in our solar system to know that none can support life as we know it. Life is not possible without a stable atmosphere containing the right chemical ingredients for living organisms: hydrogen, oxygen, nitrogen, and carbon. These ingredients must be balanced—not too thick or too thin. Life also depends on the presence of water.

    Jupiter, Saturn, Uranus, and Neptune all have atmospheres made mostly of hydrogen and helium. These planets are called gas giants, because they are mostly made of gas and do not have a solid outer crust.

    Mercury and Mars have some of the right ingredients, but their atmospheres are far too thin to support life. The atmosphere of Venus is too thick—the planet's surface temperature is more than 460 degrees Celsius (860 degrees Fahrenheit).

    Jupiter's moon Europa has a thin atmosphere rich with oxygen. It is likely covered by a huge ocean of liquid water. Some astrobiologists think that if life will develop elsewhere in the solar system, it will be near vents at the bottom of Europa's ocean.

    Earth to Earth
    Earth is the only planet in the solar system not named for a Greek or Roman god or goddess. "Earth" originally meant the soil and land of our planet. (This is still what it means when the word is lowercase.) Eventually, Earth came to mean the planet itself.

  • Term Part of Speech Definition Encyclopedic Entry
    absorb Verb

    to soak up.
    abundant Adjective

    in large amounts.
    air current Noun

    flowing movement of air within a larger body of air.
    air mass Noun

    a large volume of air that is mostly consistent, horizontally, in temperature and humidity.

    		 Encyclopedic Entry: air mass
    algae Plural Noun

    (singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds.
    altitude Noun

    the distance above sea level.

    		 Encyclopedic Entry: altitude
    ancestor Noun

    organism from whom one is descended.
    aquifer Noun

    an underground layer of rock or earth which holds groundwater.

    		 Encyclopedic Entry: aquifer
    archaea Plural Noun

    (singular: archaeon) a group of tiny organisms often living in extreme environments, such as ocean vents and salt lakes.
    Archaean Noun

    geological time period between 3.8 billion years ago and 2.5 billion years ago.
    Arctic Noun

    region at Earth's extreme north, encompassed by the Arctic Circle.

    		 Encyclopedic Entry: Arctic
    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.
    asteroid belt Noun

    area of the solar system between the orbits of Mars and Jupiter filled with asteroids.
    astronomical unit Noun

    (AU) (150 million kilometers/93 million miles) unit of distance equal to the average distance between the Earth and the sun.
    astronomy Noun

    the study of space beyond Earth's atmosphere.
    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    		 Encyclopedic Entry: atmosphere
    augment Verb

    to enlarge or add to.
    aurora Noun

    brightly colored bands of light, visible around Earth's geomagnetic poles, caused by solar wind interacting with particles in Earth's magnetic field.

    		 Encyclopedic Entry: aurora
    autotroph Noun

    organism that can produce its own food and nutrients from chemicals in the atmosphere, usually through photosynthesis or chemosynthesis.

    		 Encyclopedic Entry: autotroph
    axis Noun

    an invisible line around which an object spins.

    		 Encyclopedic Entry: axis
    axis of rotation Noun

    single axis or line around which a body rotates or spins.
    bacteria Plural Noun

    (singular: bacterium) single-celled organisms found in every ecosystem on Earth.
    basalt Noun

    type of dark volcanic rock.
    basin Noun

    a dip or depression in the surface of the land or ocean floor.

    		 Encyclopedic Entry: basin
    biosphere Noun

    part of the Earth where life exists.

    		 Encyclopedic Entry: biosphere
    body water Noun

    amount of liquid water in an organism's body.
    boundary Noun

    line separating geographical areas.

    		 Encyclopedic Entry: boundary
    brittle Adjective

    fragile or easily broken.
    Cambrian Noun

    (540 million years ago-505 million years ago) first period in the Paleozoic era, noted for the rapid development of many different life forms.
    Cambrian Explosion of Life Noun

    rapid development of almost all major types (phyla) of organisms during the Cambrian time period.
    carbon Noun

    chemical element with the symbol C, which forms the basis of all known life.
    carbon cycle Noun

    series of processes in which carbon (C) atoms circulate through Earth's land, ocean, atmosphere, and interior.
    carbon dioxide Noun

    greenhouse gas produced by animals during respiration and used by plants during photosynthesis. Carbon dioxide is also the byproduct of burning fossil fuels.
    Cenozoic Noun

    (65 million years ago-present) current geologic era
    charged particle Noun

    molecule that has a positive or negative electric charge.
    circulate Verb

    to move around, often in a pattern.
    climate Noun

    all weather conditions for a given location over a period of time.

    		 Encyclopedic Entry: climate
    cloud Noun

    visible mass of tiny water droplets or ice crystals in Earth's atmosphere.

    		 Encyclopedic Entry: cloud
    compress Verb

    to press together in a smaller space.
    condense Verb

    to turn from gas to liquid.
    consist Verb

    to be made of.
    consistent Adjective

    maintaining a steady, reliable quality.
    consume Verb

    to use up.
    continental crust Noun

    thick layer of Earth that sits beneath continents.
    core Noun

    the extremely hot center of Earth, another planet, or a star.

    		 Encyclopedic Entry: core
    craton Noun

    old, stable part of continental crust, made up of shields and platforms.
    crust Noun

    rocky outermost layer of Earth or other planet.

    		 Encyclopedic Entry: crust
    data Plural Noun

    (singular: datum) information collected during a scientific study.
    decompose Verb

    to decay or break down.
    dense Adjective

    having parts or molecules that are packed closely together.
    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

    microscopic particles of rocks or minerals drifting in space. Also called cosmic dust or space 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
    earthquake Noun

    the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.
    eject Verb

    to get rid of or throw out.
    element Noun

    chemical that cannot be separated into simpler substances.
    energy Noun

    capacity to do work.
    eon Noun

    second-largest unit of geologic time, smaller than a supereon and larger than an era.
    Equator Noun

    imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.

    		 Encyclopedic Entry: equator
    era Noun

    time period.
    erosion Noun

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

    		 Encyclopedic Entry: erosion
    essential Adjective

    needed.
    evaporation Noun

    process by which liquid water becomes water vapor.

    		 Encyclopedic Entry: evaporation
    evaporation Noun

    process by which liquid water becomes water vapor.

    		 Encyclopedic Entry: evaporation
    evolve Verb

    to develop new characteristics based on adaptation and natural selection.
    exosphere Noun

    outermost layer of Earth's atmosphere, beginning at an altitude of about 550 kilometers (341 miles) above the Earth's surface.
    fault Noun

    a crack in the Earth's crust where there has been movement.
    faulting Noun

    movement of rocks and tectonic plates beneath the Earth's surface.
    flourish Verb

    to thrive or be successful.
    fog Noun

    clouds at ground level.

    		 Encyclopedic Entry: fog
    fossil Noun

    remnant, impression, or trace of an ancient organism.

    		 Encyclopedic Entry: fossil
    fossil fuel Noun

    coal, oil, or natural gas. Fossil fuels formed from the remains of ancient plants and animals.
    freshwater Noun

    water that is not salty.
    fungi Plural Noun

    (singular: fungus) organisms that survive by decomposing and absorbing nutrients in organic material such as soil or dead organisms.
    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.
    geoid Noun

    spheroid representation of the Earth that takes into account mean sea level and gravity.
    geologic timeline Noun

    scale used by geologists used to divide the Earth's 4.6 billion year history into units of time.
    geologist Noun

    person who studies the physical formations of the Earth.
    geyser Noun

    natural hot spring that sometimes erupts with water or steam.

    		 Encyclopedic Entry: geyser
    glacier Noun

    mass of ice that moves slowly over land.

    		 Encyclopedic Entry: glacier
    granite Noun

    type of hard, igneous rock.
    gravity Noun

    physical force by which objects attract, or pull toward, each other.
    Hadean Noun

    (4.5 billion years ago-3.8 billion years ago) first eon in the Precambrian supereon. Also called the Pre-Archean.
    Himalaya Mountains Noun

    mountain range between India and Nepal.
    Homo sapiens Noun

    (200,000 years ago-present) species of primates (hominid) that only includes modern human beings.
    hydrosphere Noun

    all the Earth's water in the ground, on the surface, and in the air.

    		 Encyclopedic Entry: hydrosphere
    hygiene Noun

    science and methods of keeping clean and healthy.
    ice Noun

    water in its solid form.

    		 Encyclopedic Entry: ice
    ice sheet Noun

    thick layer of glacial ice that covers a large area of land.

    		 Encyclopedic Entry: ice sheet
    igneous rock Noun

    rock formed by the cooling of magma or lava.
    inner planet Noun

    one of the four rocky planets closes to the sun: Mercury, Venus, Earth, or Mars. Also called a terrestrial planet.
    iron Noun

    chemical element with the symbol Fe.
    island Noun

    body of land surrounded by water.

    		 Encyclopedic Entry: island
    lake Noun

    body of water surrounded by land.

    		 Encyclopedic Entry: lake
    landform Noun

    specific natural feature on the Earth's surface.

    		 Encyclopedic Entry: landform
    landscape Noun

    the geographic features of a region.

    		 Encyclopedic Entry: landscape
    latitude Noun

    distance north or south of the Equator, measured in degrees.

    		 Encyclopedic Entry: latitude
    lava Noun

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

    type of sedimentary rock mostly made of calcium carbonate from shells and skeletons of marine organisms.
    liquid Noun

    state of matter with no fixed shape and molecules that remain loosely bound with each other.
    lithosphere Noun

    outer, solid portion of the Earth. Also called the geosphere.

    		 Encyclopedic Entry: lithosphere
    luminosity Noun

    energy emitted by a stellar object (such as a star or galaxy) per unit of time. 
    magma Noun

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

    		 Encyclopedic Entry: magma
    magnetic field Noun

    area around and affected by a magnet or charged particle.
    magnetosphere Noun

    teardrop-shaped area, with the flat area facing the sun, around the Earth controlled by the Earth's magnetic field.
    mantle Noun

    middle layer of the Earth, made of mostly solid rock.

    		 Encyclopedic Entry: mantle
    marble Noun

    type of metamorphic rock.
    mass Noun

    unit of measurement (abbreviated m) determined by an object's resistance to change in the speed or direction of motion.
    mesosphere Noun

    region in Earth's atmosphere between the stratosphere and the thermosphere, about 50-80 kilometers (31-50 miles) above the Earth's surface.
    Mesozoic Noun

    (250 million years ago-65 million years ago) second era in the Phanerozoic eon. Also called the Age of Dinosaurs.
    metamorphic rock Noun

    rock that has transformed its chemical qualities from igneous or sedimentary.
    metamorphosis Noun

    complete change in form and structure from one part of the life cycle to the next, such as caterpillar to pupa, and pupa to butterfly.
    mid-ocean ridge Noun

    underwater mountain range.
    mineral Noun

    inorganic material that has a characteristic chemical composition and specific crystal structure.
    molten Adjective

    solid material turned to liquid by heat.
    Moon Noun

    Earth's only natural satellite.
    nickel Noun

    chemical element with the symbol Ni.
    nutrient Noun

    substance an organism needs for energy, growth, and life.

    		 Encyclopedic Entry: nutrient
    oblate spheroid Noun

    shape of the Earth, which is larger around the Equator than at the poles.
    ocean Noun

    large body of salt water that covers most of the Earth.

    		 Encyclopedic Entry: ocean
    ocean current Noun

    continuous, predictable, directional movement of seawater.
    oceanic crust Noun

    thin layer of the Earth that sits beneath ocean basins.
    ocean vent Noun

    underwater fissure from which geothermally heated fluid is ejected.

    		 Encyclopedic Entry: ocean vent
    orbit Noun

    path of one object around a more massive object.
    organic Adjective

    composed of living or once-living material.
    oxygen Noun

    chemical element with the symbol O, whose gas form is 21% of the Earth's atmosphere.
    paleontologist Noun

    person who studies fossils and life from early geologic periods.
    Paleozoic Era Noun

    about 541-252 million years ago.
    particle Noun

    small piece of material.
    period Noun

    unit of geologic time, shorter than an era and larger than an epoch.
    Phanerozoic Noun

    (542 million years ago-present) current eon in the geologic timeline, comprising the Paleozoic, Mesozoic, and Cenozoic eras.
    photosynthesis Noun

    process by which plants turn water, sunlight, and carbon dioxide into water, oxygen, and simple sugars.
    planet Noun

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

    		 Encyclopedic Entry: planet
    plant Noun

    organism that produces its own food through photosynthesis and whose cells have walls.
    plate tectonics Noun

    movement and interaction of the Earth's plates.
    pole Noun

    extreme north or south point of the Earth's axis.
    Precambrian Noun

    (4.5 billion years ago-542 million years ago) first supereon in Earth's history.
    precipitation Noun

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

    		 Encyclopedic Entry: precipitation
    primitive Adjective

    simple or crude.
    Proterozoic Noun

    (2.5 billion years ago-542 million years ago) last eon in the Precambrian, noted for the development of bacteria and algae.
    quartz Noun

    common type of mineral.
    Quaternary Noun

    (2.5 million years ago-present) most recent period in geologic time.
    radius Noun

    ray extending from the center of a circle or sphere to its surface or circumference.
    rain Noun

    liquid precipitation.

    		 Encyclopedic Entry: rain
    rapid Adjective

    very fast.
    recycle Verb

    to clean or process in order to make suitable for reuse.
    red giant Noun

    "main sequence" star with huge surface area, low surface temperature, and reddish color.
    regulate Verb

    to determine and administer a set of rules for an activity.
    revolution Noun

    orbit, or a complete journey of an object around a more massive object.
    rift Noun

    break in the Earth's crust created by it spreading or splitting apart.
    rift valley Noun

    depression in the ground caused by the Earth's crust spreading apart.

    		 Encyclopedic Entry: rift valley
    Ring of Fire Noun

    horseshoe-shaped string of volcanoes and earthquake sites around edges of the Pacific Ocean.

    		 Encyclopedic Entry: Ring of Fire
    river Noun

    large stream of flowing fresh water.

    		 Encyclopedic Entry: river
    rock Noun

    natural substance composed of solid mineral matter.
    rock cycle Noun

    processes that explain the relationship between the three rock types: igneous, sedimentary, and metamorphic. Any rock type can become any other.
    rotation Noun

    object's complete turn around its own axis.

    		 Encyclopedic Entry: rotation
    sand Noun

    small, loose grains of disintegrated rocks.
    sandstone Noun

    common sedimentary rock formed by grains of sand compacted or cemented with material such as clay.
    satellite Noun

    object that orbits around something else. Satellites can be natural, like moons, or made by people.
    saturate Verb

    to fill one substance with as much of another substance as it can take.
    season Noun

    period of the year distinguished by special climatic conditions.

    		 Encyclopedic Entry: season
    sedimentary rock Noun

    rock formed from fragments of other rocks or the remains of plants or animals.
    seismic Adjective

    having to do with earthquakes.
    silicate Noun

    most common group of minerals, all of which include the elements silicon (Si) and oxygen (O).
    silicate Noun

    most common group of minerals, all of which include the elements silicon (Si) and oxygen (O).
    silicon Noun

    chemical element with the symbol Si.
    soil Noun

    top layer of the Earth's surface where plants can grow.
    solar radiation Noun

    light and heat from the sun.
    solar system Noun

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

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

    flow of charged particles, mainly protons and electrons, from the sun to the edge of the solar system.
    spherical Adjective

    rounded and three-dimensional.
    spring Noun

    small flow of water flowing naturally from an underground water source.
    star Noun

    large ball of gas and plasma that radiates energy through nuclear fusion, such as the sun.
    storm Noun

    severe weather indicating a disturbed state of the atmosphere resulting from uplifted air.
    stratosphere Noun

    level of Earth's atmosphere, extending from 10 kilometers (6 miles) to 50 kilometers (31 miles) above the surface of the Earth.
    subduct Verb

    to pull downward or beneath something.
    sun Noun

    star at the center of our solar system.
    supereon Noun

    largest unit of geologic time. The Precambrian supereon lasted until 542 million years ago. We are currently in the second supereon, which has no name.
    sustain Verb

    to support.
    tectonic activity Noun

    movement of tectonic plates resulting in geologic activity such as volcanic eruptions and earthquakes.
    tectonic plate Noun

    massive slab of solid rock made up of Earth's lithosphere (crust and upper mantle). Also called lithospheric plate.
    temperature Noun

    degree of hotness or coldness measured by a thermometer with a numerical scale.

    		 Encyclopedic Entry: temperature
    thermosphere Noun

    layer of the Earth's atmosphere located between 80 kilometers (50 miles) and 550 kilometers (341 miles) above the Earth's surface.
    transportation Noun

    movement of people or goods from one place to another.
    troposphere Noun

    lowest layer of the Earth's atmosphere, extending from the surface to about 16 kilometers (10 miles) above.
    ultraviolet radiation Noun

    powerful light waves that are too short for humans to see, but can penetrate Earth's atmosphere. Ultraviolet is often shortened to UV.
    universe Noun

    all known matter, energy, and space.
    vapor Noun

    visible liquid suspended in the air, such as fog.
    venture Verb

    to take a risky or dangerous opportunity.
    volcanic eruption Noun

    activity that includes a discharge of gas, ash, or lava from a volcano.
    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
    volume Noun

    space an object occupies.
    water cycle Noun

    movement of water between atmosphere, land, and ocean.

    		 Encyclopedic Entry: water cycle
    weathering Noun

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

    		 Encyclopedic Entry: weathering
    weather pattern Noun

    repeating or predictable changes in the Earth's atmosphere, such as winds, precipitation, and temperatures.
    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.

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Editors

Melissa MacPhee, National Geographic Society
Jeannie Evers, Emdash Editing

Producer

Caryl-Sue, National Geographic Society

Last Updated

Aug. 26, 2016

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