Encyclopedic Entry

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.

Atmosphere

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.

Hydrosphere
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.

Lithosphere
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.


Cycles

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.

Eras

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.

Vocabulary

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

needed.

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.

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.

Credits

Media Credits

The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.

Writer

Melissa McDaniel
Erin Sprout
Diane Boudreau
Andrew Turgeon

Illustrator

Tim Gunther, Illustrator
Mary Crooks, National Geographic Society

Editor

Jeannie Evers
Kara West

Educator Reviewer

Nancy Wynne

Producer

Caryl-Sue, National Geographic Society

Sources

Dunn, Margery G. (Editor). (1989, 1993). "Exploring Your World: The Adventure of Geography." Washington, D.C.: National Geographic Society.

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