• Earth
    For more breathtaking images of Earth, check out the "Image of the Day" from NASA's Earth Observatory.

    Courtesy of NASA Goddard Space Flight Center

    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 degrees Celsius (77 Kelvin, 59 degrees 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 balancednot 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 thickthe planets surface temperature is more than 460 degrees Celsius (860 degrees Fahrenheit).

    Jupiters 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 Europas 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.

    Earth is the planet we live on, part of a solar system of eight planets and one star. A planet is a celestial body that does not produce its own light, like stars do. The sun is a star.

    Earth is a rocky body constantly moving around the sun in a path called an orbit. As far as scientists know, our planet is the only one that can support life.

    Earth is the third planet from the sun, after Mercury and Venus and before Mars. The Earth is about 150 million kilometers (about 93 million miles) from the sun. This distance is a standard measure of distance in astronomy. It is called an astronomical unit (AU). Earth is one AU from the sun. The planet Jupiter is about 5.2 AU from the sun.

    Earth is the fifth-largest planet in the solar system. Its diameter is 13,000 kilometers (8,000 miles). Jupiter, the largest planet in the solar system, has a diameter of 143,000 kilometers (88,850 miles).

    Earth is spherical in shape, but not perfectly round. It has a slightly greater diameter at its Equator, the imaginary line running horizontally around the middle of the Earth. In addition to bulging in the middle, Earth’s poles are slightly flattened.

    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, while Jupiter and Saturn have more than a dozen.

    Revolution and Rotation

    The Earth and Moon follow a slightly oval-shaped orbit around the sun every year. The Earth and Moon orbit the sun as one unit. And even though we think of the Moon revolving around the Earth, they actually revolve around each other at a point called the barycenter. The barycenter is the center of mass, or balance, between the Earth and the Moon. The Earth-Moon barycenter is about 1,700 kilometers (1,050 miles) below the Earth’s surface.

    Each journey around the sun, a trip of about 940 million kilometers (584 million miles), is called a revolution. Earth’s year, the time it takes to complete one revolution, is about 365.25 days long. Earth orbits the sun at a speed 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. This line is called an axis. (Basketballs and figure skaters also rotate on their own axes as they spin.) The Earth’s axis is vertical, running from the North Pole to the South Pole.

    The Earth rotates unevenly. It spins faster at the Equator than at the poles or other lines of latitude. Latitude is the distance north or south of the Equator. At the Equator, the Earth rotates at about 1,670 kilometers per hour (1,040 miles per hour). At 45 degrees north, the approximate latitude of Green Bay, Wisconsin, the Earth rotates at 1,180 kilometers per hour (733 miles per hour).

    The Earth makes one complete turn, or rotation, about every 24 hours, causing 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. Earth rotates from west to east, so the sun appears to rise in the east and set in the west.

    As the Earth spins, each area gets a turn to be warmed by the sun. This is necessary for life on Earth because life depends on the sun for light and heat. The sun also determines the weather on Earth, making the water cycle and carbon cycle possible. 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.

    Another reason we experience light and darkness is because Earth’s axis is not exactly straight up and down. Earth’s axis of rotation is tilted 23.5 degrees. This tilt causes changes from season to season.

    It gets dark earlier in the winter than in summer, for instance. This is partly because the tilt of the Earth causes the angle of the sun’s rays to shine at different latitudes throughout the year. Since the Earth is tilted, the latitude at which the sun appears directly overhead at noon changes as the Earth orbits the sun. The direct rays of the sun reach the most northern latitude at about 23.5 degrees north—the Tropic of Cancer. The sun’s direct rays reach their most southern latitude at about 23.5 degrees south—the Tropic of Capricorn. For this reason, the Earth’s tropics (the areas between the Tropic of Cancer and the Tropic of Capricorn) are warmer than the poles—the tropics receive direct sunlight all year round.

    Earth’s Beginnings

    Earth formed about 4.6 billion years ago. Earth and the rest of the solar system formed from a huge, spinning cloud of gas and dust.

    The cloud began to contract, or come together, as it continued spinning. Over a period of 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.

    As objects collided and gravity drew them together, Earth was formed. This process generated a lot of heat.  As the number of collisions dropped, the Earth began to cool. The materials that make up the Earth began to separate. Lighter materials floated upward and formed a thin crust. This crust is where all life exists. Heavier materials sank toward the Earth’s center. Eventually, three main layers formed: the core, the mantle, and the crust.

    As the Earth’s internal structure developed, gases released from the interior mixed together, forming a thick, steamy atmosphere around the planet. Water vapor condensed, and rain began to fall. Water slowly filled basins, or depressions, in Earth’s crust, forming a primitive ocean that covered most of the planet. Today, ocean waters cover nearly three-fourths of the Earth.

    Earth’s Composition

    No one has ever ventured below Earth’s crust. One way geologists have gathered information about what lies below the planet’s surface is by studying seismic waves, or vibrations, associated with earthquakes.

    When rocks in the crust shift, seismic waves move away from the place where the shifting occurred and pass through the entire planet. Seismic waves move across and through the planet like circular ripples on a pond. As seismic waves move through materials of varying density, they change speed, arriving at the surface at different times. Scientific instruments record the arrival of the waves. From seismic data, geologists have learned much about the composition and thickness of the Earth’s layers.

    Earth’s super-hot core is mostly made of iron and nickel. It consists of a solid center surrounded by an outer layer of liquid. The solid inner core is about 1,220 kilometers (760 miles) thick. The liquid outer core is about 2,250 kilometers (1,410 miles) thick.

    A mantle of heavy rock surrounds the core. It has a maximum thickness of about 2,900 kilometers (1,800 miles). Recent studies indicate that the boundary between the outer part of the core and the mantle is bumpy and irregular, with high peaks and low valleys. The mantle is molten, meaning it is composed of partly melted rock. This molten rock is forced to the surface at volcanic eruptions and mid-ocean ridges.

    The mantle’s molten rock is constantly in motion. The hot rock close to the core is constantly pushing upward, while the slightly cooler rock near the crust sinks downward. This circular process is called convection. As convection drives the mantle, it shifts slabs of hard material—tectonic plates—on the Earth’s crust.

    Earth’s crust is the planet’s thinnest layer. There are two kinds of crust: oceanic crust and continental crust. Oceanic crust is thinner than continental crust. Oceanic crust is 5 to 10 kilometers (3 to 6 miles) thick. Continental crust is 35 to 70 kilometers (22 to 44 miles) thick. However, oceanic crust is denser and heavier than continental crust.

    Tectonic Activity

    The crust is covered by a series of constantly moving tectonic plates. Tectonic plates are hard slabs of material that slide around the crust like a surfboard on the ocean. New crust is created along mid-ocean ridges, where tectonic plates pull apart from each other in a tectonic activity called rifting. Plates slide above and below each other in a tectonic activity called subduction. They crash against each other in a tectonic activity called faulting.

    Over millions of years, tectonic activity 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. Mount Everest continues to grow every year, as subduction drives the Indo-Australian tectonic plate below the Eurasian tectonic plate. The Earth’s deepest point is at the bottom of the Mariana Trench, 11 kilometers (6.9 miles) below the surface of the Pacific Ocean. The Mariana Trench is also in a subduction zone. The heavy Pacific plate is being subducted beneath the small Mariana plate.

    Plate tectonics are responsible for the eruption of volcanoes. Many volcanoes exist on the boundaries between tectonic plates. The so-called Ring of Fire, for instance, is a series of active volcanoes circling the Pacific Ocean. Really, they circle the Pacific tectonic plate. The Ring of Fire (sometimes called the Pacific Ring of Fire) indicates where molten material from the mantle is forcing its way to the crust. Volcanoes in the Pacific Ring of Fire include Mount St. Helens in the U.S. state of Washington; Popocatepetl, Mexico; Chaiten, Chile; Tongariro, New Zealand; Mount Pinatubo, Philippines; and Mount Fuji, Japan.

    Areas in the Ring of Fire also experience earthquakes. In the U.S. state of California, for instance, tectonic activity reshapes the land along the San Andreas Fault. The San Andreas Fault is not a subduction zone, however. The Pacific plate is moving north, while the North American plate is moving south. The plates are grinding next to each other in what is called a transform fault.

    Tectonic activity can also create islands and other geologic landforms, such as geysers. The Kamchatka Peninsula, for instance, is part of the Pacific Ring of Fire in Russia. The Kamchatka Peninsula was home to the “Valley of Geysers.” Geysers are places where molten magma heats water deep in the Earth’s crust until it is hot enough to boil. The boiling water erupts and shoots into the air. In 2007, an earthquake caused a major mudslide in the Valley of Geysers, burying most of the geysers. Plate tectonics helped create the Valley of Geysers, and it helped destroy it.

    The Spheres

    Earth’s physical environment is often described in terms of spheres: the atmosphere, or air; the hydrosphere, or water; and the lithosphere, or Earth’s rocky shell. Parts of these three spheres make up the biosphere, the area of Earth where life exists.


    The atmosphere is a mixture of gases that includes water vapor, oxygen, and carbon dioxide. This blanket of gases enveloping the Earth acts as a gigantic filter. It blocks most of the sun’s harmful, ultraviolet radiation (UV) while allowing healthy sunlight to heat the Earth.

    Solar heat in the atmosphere drives the Earth’s weather. Heat moves air masses through the atmosphere. This creates moving pockets of warmth and cold—air masses. The planet’s average surface temperature is 15 degrees Celsius (59 degrees Fahrenheit).

    Moving air masses also create wind and storms. This drives the atmosphere’s water cycle.

    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 percent 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 they change depending on the latitude and the season.

    The hydrosphere is composed of the Earth’s water. Nearly three-fourths of the Earth is covered in water, most of it in the ocean. Less than three percent 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.

    Water also circulates around the world as vapor. Water vapor condenses into clouds before falling back to Earth as precipitation.

    The hydrosphere helps regulate the Earth’s temperature and climate. The ocean absorbs heat from the sun and moves it around the Earth in currents. Storms and precipitation help determine a region’s climate.

    The lithosphere is the Earth’s solid shell. The crust and the upper portion of the mantle form the lithosphere. It extends from Earth’s surface to about 97 kilometers (60 miles) below it. Oceanic and continental crust are part of the lithosphere.

    The rocks and minerals in Earth’s lithosphere are made of many elements. Oxygen and silicon are the most abundant elements in the lithosphere. Rocks with both oxygen and silicon are called silicates. Quartz is the most common silicate in the lithosphere. It is also the most common type of rock on Earth.

    The lithosphere constantly interacts with the hydrosphere and atmosphere. For instance, the processes of erosion and weathering are constantly molding the Earth’s lithosphere. Water, in the form of floods, rain, and glaciers, erodes rocks. Atmospheric winds, too, work to erode, or wear away, parts of the lithosphere.


    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 is the process all water on Earth undergoes. It 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 percent 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 air. 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 is essential for all life on Earth. Carbon 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 plants obtain carbon.

    As organisms die and decompose, they release carbon into the ocean, soil, or atmosphere. Fossil fuels, the remains of ancient plants and animals, contain very high amounts of carbon. 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 the water cycle and the carbon cycle, not all rocks are recycled in different forms. There are some rocks that have stayed 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 wind and water. 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 (metamorphize) to become a new type of rock. Marble, for example, is a metamorphic rock created from rock that was once limestone, a sedimentary rock.


    Paleontologists, geologists, and other scientists divide the Earth’s history into time periods. The largest time period is the supereon. Eons, eras, and periods are smaller units of the geologic timeline.

    The first supereon was called the Precambrian. 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 on Earth began in the Precambrian, with bacteria and other single-celled organisms. Fossils from the Precambrian are rare and difficult to study. We are living in Earth’s second supereon, which does not have a name.

    Supereons are broken into time periods known as eons. The Precambrian is usually broken into three eons: the Hadean, the Archaean, and the Proterozoic. The current supereon has a single eon, the Phanerozoic. The Phanerozoic began about 542 million years ago, and continues today.

    Eons are broken into time periods known as eras. The first major era of the Phanerozoic was called the Paleozoic Era. It was followed by the Mesozoic, when most dinosaurs flourished. We currently live in the Cenozoic era, which began about 65 million years ago.

    Eras are broken down into time frames known as periods. The Cambrian period was 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 from archaea, bacteria, algae, fungi, plants, and animals that lived during the Cambrian period. We currently live in the Quaternary period, which began about 2.5 million years ago. All ancestors of homo sapiens (modern humans) evolved during the Quaternary.

  • Term Part of Speech Definition Encyclopedic Entry
    absorb Verb

    to soak up.

    abundant Adjective

    in large amounts.

    air Noun

    the layer of gases surrounding Earth.

    Encyclopedic Entry: 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
    associate Verb

    to connect.

    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.

    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
    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.

    barycenter Noun

    center of mass between two or more objects.

    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
    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.

    celestial body Noun

    natural object in space, such as a planet or star. Also called an astronomical object.

    Cenozoic Noun

    (65 million years ago-present) current geologic era

    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
    condense Verb

    to turn from gas to liquid.

    consist Verb

    to be made of.

    consume Verb

    to use up.

    continental crust Noun

    thick layer of Earth that sits beneath continents.

    contract Verb

    to shrink or get smaller.

    convection Noun

    transfer of heat by the movement of the heated parts of a liquid or gas.

    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
    current Noun

    steady, predictable flow of fluid within a larger body of that fluid.

    Encyclopedic Entry: current
    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.

    density Noun

    number of things of one kind in a given area.

    Encyclopedic Entry: density
    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.

    erode Verb

    to wear away.

    erosion Noun

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

    Encyclopedic Entry: erosion
    essential Adjective


    evaporate Verb

    to change from a liquid to a gas or vapor.

    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.

    faulting Noun

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

    filter Verb

    to remove particles from a substance by passing the substance through a screen or other material that catches larger particles and lets the rest of the substance pass through.

    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.

    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
    gigantic Adjective

    very large.

    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.

    indicate Verb

    to display or show.

    inner core Noun

    deepest layer of the Earth, beneath the outer core.

    internal Adjective

    inside, or having to do with the inner part of something.

    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.

    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
    magma Noun

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

    Encyclopedic Entry: magma
    mantle Noun

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

    Encyclopedic Entry: mantle
    marble Noun

    type of metamorphic rock.

    Mariana Trench Noun

    deepest place on Earth, located in the South Pacific Ocean at 11,000 meters (36,198 feet) at its deepest.

    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.

    metamorphize Verb

    to change into something else.

    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.

    Mount Everest Noun

    highest spot on Earth, 8,850 meters (29,035 feet). Mount Everest is part of the Himalaya range, in Nepal and China.

    mudslide Noun

    rapid, downhill flow of soil and water. Also called a mudflow.

    nickel Noun

    chemical element with the symbol Ni.

    noon Noun

    time of day when the sun is directly overhead: 12 p.m.

    North Pole Noun

    fixed point that, along with the South Pole, forms the axis on which the Earth spins.

    Encyclopedic Entry: North Pole
    nutrient Noun

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

    Encyclopedic Entry: nutrient
    ocean Noun

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

    Encyclopedic Entry: ocean
    oceanic crust Noun

    thin layer of the Earth that sits beneath ocean basins.

    orbit Noun

    path of one object around a more massive object.

    organic Adjective

    composed of living or once-living material.

    outer core Noun

    layer of the Earth between the mantle and the inner core.

    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 542251 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.

    rain Noun

    liquid precipitation.

    Encyclopedic Entry: rain
    rapid Adjective

    very fast.

    recycle Verb

    to clean or process in order to make suitable for reuse.

    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.

    revolve Verb

    to orbit or spin around something.

    rift Noun

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

    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.

    rotate Verb

    to turn around a center point or axis.

    rotation Noun

    object's complete turn around its own axis.

    Encyclopedic Entry: rotation
    San Andreas Fault Noun

    transform fault in western California that marks the boundary between the North American and Pacific tectonic plates.

    sand Noun

    small, loose grains of disintegrated rocks.

    sandstone Noun

    rock formed by grains of sand.

    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.

    seismic wave Noun

    shock wave of force or pressure that travels through the Earth.

    silicate Noun

    most common group of minerals, all of which include the element silicon (Si).

    silicon Noun

    chemical element with the symbol Si.

    solar Adjective

    having to do with the sun.

    solar system Noun

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

    South Pole Noun

    fixed point that, along with the North Pole, forms the axis on which the Earth spins.

    Encyclopedic Entry: South Pole
    spherical Adjective

    rounded and three-dimensional.

    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

    highest 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.

    subduction Noun

    process of one tectonic plate melting or going beneath another.

    subduction zone Noun

    area where one tectonic plate slides under another.

    summer Noun

    time of year when part of the Earth receives the most daylight: The months of June, July, and August in the Northern Hemisphere and the months of December, January, and February in the Southern Hemisphere.

    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.

    tectonic activity Noun

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

    tectonic plate Noun

    large, moveable segment of the Earth's crust.

    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.

    transform fault Noun

    boundary between two tectonic plates, where the plates are moving horizontally or vertically in opposite directions, not against or away from each other. Also called a conservative plate boundary.

    transportation Noun

    movement of people or goods from one place to another.

    Tropic of Cancer Noun

    line of latitude 23.5 degrees north of the Equator.

    Tropic of Capricorn Noun

    line of latitude 23.5 degrees south of the Equator.

    tropics Noun

    region generally located between the Tropic of Cancer (23 1/2 degrees north of the Equator) and the Tropic of Capricorn (23 1/2 degrees south of the Equator).

    Encyclopedic Entry: tropics
    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.

    vapor Noun

    visible liquid suspended in the air, such as fog.

    vent Noun

    crack in the Earth's crust that spews hot gases and mineral-rich water.

    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
    water cycle Noun

    movement of water between atmosphere, land, and ocean.

    Encyclopedic Entry: water cycle
    weather Noun

    state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.

    Encyclopedic Entry: weather
    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.

    winter Noun

    time of year when part of the Earth receives the least daylight: December, January, and February in the Northern Hemisphere and June, July, and August in the Southern Hemisphere.

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