abrasion—A process that occurs when particles of sand and other sizes are blown by the wind and impact each other. acid rain—Rain or other form of precipitation that has acidic components leading to a pH of less than seven. The extra acidity in acid rain comes from the reaction of air pollutants, mostly sulfur and nitrogen oxides, with water to form strong acids like sulfuric and nitric acid.

aerosol—Microscopic droplets or airborne particles that remain in the atmosphere for at least several hours. alluvial fans—Coarse-grained deposits of alluvium that accumulate at the fronts of mountain canyons. anthropogenic—Human-induced factors in the environment. aquifers—Any body of permeable rock or regolith saturated with water through which groundwater moves. Archaean—The oldest eon of geological time, ranging from 4.5 billion years ago until 2.5 billion years ago. arête—Knife-edged ridge that forms where two cirques intersect. atmosphere—The disk of air that surrounds Earth, held in place by gravity. The most abundant gas is nitrogen (78 percent), followed by oxygen (21 percent), argon (0.9 percent), carbon dioxide (0.036 percent), and minor amounts of helium, krypton, neon, and xenon. atmospheric pressure—The force per unit area (similar to weight) that the air above a certain point exerts on any object below it. aurora—Glows of light sometimes visible in high latitudes around both the North and South Poles. They are formed by interaction of the solar wind with Earth's magnetic field. beach—Accumulations of sediment exposed to wave action along a coastline.

biofuels—Fuels such as methane that are produced from renewable biological resources, including recently living organisms, such as plants, and their metabolic byproducts, such as manure. biosphere—Collection of all organisms on Earth. calving—The process where large blocks of ice plunge off the front of a tidewater glacier and fall into the sea making icebergs. carbonate—A sediment or sedimentary rock containing the carbonate (CO3-2) ion. Typical carbonates include limestone and dolostone.

carbon sequestration—A group of processes that enable long-term storage of carbon in the terrestrial biosphere, in the oceans, or deep underground, effectively isolating that carbon from the atmosphere.

chemical weathering—Decomposition of rocks through the alteration of individual mineral grains. chlorofluorocarbons—A group of inert, nontoxic, and easily liquefied chemicals that were widely used in refrigeration. When released to the atmosphere they become long-lived greenhouse gases that contain carbon, hydrogen, fluorine, and chlorine, increasing in atmospheric concentration as a result of human activity. Chloro-fluorocarbons are thought to cause depletion of the atmospheric ozone layer.

cirques—Bowl-shaped hollows that open downstream and are bounded upstream by a steep wall. climate—The average weather of an area.

climate change—The phenomena where global temperatures, patterns of precipitation, wind, and ocean currents change in response to human and natural causes. continental shelf—Generally fairly flat areas on the edges of the continents, underlain by continental crust and having shallow water. Sedimentary deposits on continental shelves include muds, sands, and carbonates.

convergent boundaries—Places where two plates move toward each other, resulting in one plate sliding beneath the other when a dense oceanic plate is involved, or collision and deformation, when continental plates are involved. These types of plate boundaries may have the largest of all earthquakes. Coriolis effect—A force that causes any freely moving body in the Northern Hemisphere to veer to the right and in the Southern Hemisphere to the left.

cryosphere—That portion of the planet where temperatures are so low that water exists primarily in the frozen state. cyclone—A tropical storm, equivalent to a hurricane, that forms in the Indian Ocean.

deflation—A process whereby wind picks up and removes material from an area, resulting in a reduction in the land surface. delta—Low flat deposits of alluvium at the mouths of streams and rivers that form broad triangular or irregular shaped areas that extend into bays, oceans, or lakes. They are typically crossed by many distributaries from the main river and may extend for a considerable distance underwater. desert—An area characterized by receiving less than one inch (2.5 cm)

of rain each year over an extended period of time. desertification—Conversion of previously productive lands to desert. desert pavement—A long-term stable surface in deserts characterized by pebbles concentrated along the surface layer. diurnal cycle—Variations in the daily temperature. divergent boundaries—Margins where two plates move apart, creating a void that is typically filled by new oceanic crust that wells up to fill the progressively opening hole. dropstones—Isolated pebbles or boulders in marine sediments, deposited when rocks trapped in floating icebergs melted out of the ice and dropped to the seafloor. drought—A prolonged lack of rainfall in a region that typically gets more rainfall.

drumlins—Teardrop-shaped accumulations of till that are up to about 150 feet (50 m) in height and tend to occur in groups of many drumlins. These have a steep side that faces in the direction that the glacier advanced from and a back side with a more gentle slope. Drumlins are thought to form beneath ice sheets and record the direction of movement of the glacier. dune—Low wind-blown mounds of sand or granular material, with variable size and shape depending on sand supply, vegetation, and wind strength.

eccentricity—An astronomical measure of how elliptical the orbit of

Earth (or other planet) is around the Sun. ecosystem—A collection of the organisms and surrounding physical elements that together are unique to a specific environment. El Niño—One of the better-known variations in global atmospheric circulation patterns that causes a warm current to move from the western Pacific to the eastern Pacific and has global consequences in terms of changes in weather patterns. Full name is El Nino-Southern Oscillation (ENSO). epeirogenic—A term referring to the vertical movements of continents.

eustatic sea level changes—Sea levels may rise or fall for local reasons, such as tectonic subsidence, or global reasons, such as melting of glaciers. When the sea level change can be shown to be global in scale it is called eustatic. fiords—Glacially carved steep-sided valleys that are open to the sea. firn—Frozen water that is in transition in density between snow and ice.

flash flood—Floods that rise suddenly, typically as a wall of water in a narrow canyon.

flood basalt—Anomalously thick accumulations of dark lava, variously known as flood basalts, traps, or large igneous provinces. glacial drift—A general term for all sediment deposited directly by glaciers or by glacial meltwater in streams, lakes, and the sea. glacial erratic—Glacially deposited rock fragments with compositions different from underlying rocks. glacial marine drift—Sediment deposited on the seafloor from floating ice shelves or bergs. It may include many isolated pebbles or boulders that were initially trapped in glaciers on land, then floated in icebergs that calved off from tidewater glaciers. glacial moraine—Piles of sand, gravel, and boulders deposited by a glacier.

glacial period—A time in Earth history characterized by great ice sheets that moved across the continents. glacial rebound—When an ice sheet melts, the weight of the ice is removed from the continent, which then rises upward (rebounds) in response to the reduced weight. glacial striations—Scratches on the surface of bedrock, formed when the glacier dragged boulders across the bedrock surface. glacier—Any permanent body of ice (recrystallized snow) that shows evidence of gravitational movement. global warming—A gradual warming of the planet surface (land, air, and sea) typically in response to increases in greenhouse gases in the atmosphere.

Gondwana—A late Proterozoic to late Paleozoic supercontinent of the Southern Hemisphere that included present day Africa, South

America, Australia, India, Arabia, Antarctica, and many small fragments.

great Ice Ages—A term to refer to the late Pleistocene glaciation. greenhouse effect—A phenomenon where Earth's climate is sensitive to the concentrations of certain gases in the atmosphere and heats up when the concentration of these gases is increased. greenhouse gases—Gases such as CO2 that when built up in the atmosphere tend to keep solar heat in the atmosphere, resulting in global warming.

groundwater—All the water contained within spaces in bedrock, soil, and regolith.

Hadley cell—Belts of air that encircle Earth, rising along the equator, dropping moisture as they rise in the tropics. As the air moves away from the equator at high elevations, it cools, becomes drier, and then descends at 15-30°N and S latitude where it either returns to the equator or moves toward the poles. heinrich Events—Specific intervals in the sedimentary record showing ice-rafted debris in the North Atlantic. horn—Peak that forms where three cirques meet. hothouse—A time when the global climate was characterized by very hot conditions for an extended period. hot spot—An area of unusually active magmatic activity that is not associated with a plate boundary. Hot spots are thought to form above a plume of magma rising from deep in the mantle. hurricane—A tropical cyclone in which an organized group of thunderstorms rotates about a central low pressure center and has a sustained wind speed of 74 miles per hour (118 kph) or greater. hydrosphere—A dynamic mass of liquid, continuously on the move between the different reservoirs on land and in the oceans and atmosphere. The hydrosphere includes all the water in oceans, lakes, streams, glaciers, atmosphere, and groundwater, although most water is in the oceans. inselbergs—Steep-sided mountains or ridges that rise abruptly out of adjacent monotonously flat plains in deserts. interglacial period—The present climate epoch is one characterized as being between advances and retreats of major continental ice sheets. The glaciers could return, or global warming could melt the remaining polar ice taking the planet out of the glacial epoch. intergovernmental panel on climate change—The IPCC is a scientific intergovernmental body set up by the World Meteorological

Organization (WMO) and by the United Nations Environment Program (UNEP). The IPCC is open to all member countries of WMO and UNEP. Governments participate in plenary sessions of the IPCC where main decisions about the IPCC work program are taken and reports are accepted, adopted, and approved. They also participate in the review of IPCC Reports. The IPCC includes hundreds of scientists from all over the world who contribute to the work of the IPCC as authors, contributors, and reviewers. As a United Nations body, the IPCC work aims at the promotion of the United Nations human development goals. jet stream—High-level, narrow, fast moving currents of air that are typically thousands of miles (km) long, hundreds of miles (km) wide, and several miles (km) deep. landslide—A general name for any downslope movement of a mass of bedrock, regolith, or a mixture of rock and soil, commonly used to indicate any mass wasting process. lava—Magma, or molten rock that flows at the surface of Earth. leeward—The side of a mountain facing away from oncoming winds. limestone—A sedimentary carbonate rock made predominantly of the mineral calcite (CaCO3). lithosphere—Rigid outer shell of Earth that is about 75 miles (125 km) thick under continents and 45 miles (75 km) thick under oceans. The basic theorem of plate tectonics is that the lithosphere of Earth is broken into about twelve large rigid blocks or plates that are all moving relative to one another. loess—Silt and clay deposited by wind.

magma—Molten rock at high temperature. When magma flows on the surface it is known as lava. mass extinction—Times when large numbers of species and individuals within species die off. Mass extinction events are thought to represent major environmental catastrophes on a global scale. In some cases these mass extinction events can be tied to specific likely causes, such as meteorite impact or massive volcanism, but in others their cause is unknown. Earth's biosphere has experienced five major and numerous less-significant mass extinctions in the past 500 million years (in the Phanerozoic era). These events occurred at the end of the Ordovician, in the late Devonian, at the Permian/ Triassic boundary, the Triassic/Jurassic boundary, and at the Cretaceous/Tertiary (K/T) boundary.

mesosphere—Region of the atmosphere that lies above the stratosphere, extending between 31 and 53 miles.

mid-ocean ridge system—A 40,000-mile- (65,000-km-) long mountain ridge that runs through all the major oceans on the planet. The mid-ocean ridge system includes vast outpourings of young lava on the ocean floor and represents places where new oceanic crust is being generated by plate tectonics.

Milankovitch cycles—Variations in Earth's climate that are caused by variations in the amount of incoming solar energy, induced by changes in Earth's orbital parameters including tilt, eccentricity, and wobble.

monsoon—A wind system that influences large regions and has seasonally persistent patterns with pronounced changes from wet to dry seasons.

moraine—Ridge-like accumulations of glacial drift deposited at the edges of a glacier. Terminal moraines mark the farthest point of travel of a glacier, whereas lateral moraines form along the edges of a glacier.

mudflow—A downslope flow that resembles a debris flow, except it has a higher concentration of water (up to 30 percent), which makes it more fluid, with a consistency ranging from soup to wet concrete. Mudflows often start as a muddy stream in a dry mountain canyon, which as it moves it picks up more and more mud and sand, until eventually the front of the stream is a wall of moving mud and rock.

outwash plain—A broad plain in front of a melting glacier, where glacial streams deposit gravels and sand.

pangaea—A supercontinent that formed in the Late Paleozoic and held together from 300 to 200 million years ago. Pangaea contained most of the planet's continental land masses.

passive margin—A boundary between continental and oceanic crust that is not a plate boundary, characterized by thick deposits of sedimentary rocks. These margins typically have a flat shallow water shelf, then a steep drop off to deep ocean floor rocks away from the continent.

pediments—Desert surfaces that slope away from the base of a highland and are covered by a thin or discontinuous layer of alluvium and rock fragments.

permafrost—Permanently frozen subsoil that occurs in polar regions and at some high altitudes.

photosynthesis—The process in green plants of trapping solar energy and using it to drive a series of chemical reactions that result in the production of carbohydrates such as glucose or sugar. plate tectonics—A model that describes the process related to the slow motions of more than a dozen rigid plates of solid rock on the surface of Earth. The plates ride on a deeper layer of partially molten material that is found at depths starting at 60-200 miles (100-320 km) beneath the surface of the continents, and 1-100 miles (1-160 km) beneath the oceans. playa—Dry lake bed in desert environment.

precambrian—The oldest broad-grouping of geological time, stretching from the formation of Earth at 4.5 billion years ago and including the Archaean and Proterozoic eons, ending at 540 million years ago.

radiation—Energy in the form of waves.

radiative forcing—The net change in downward minus the upward irra-diance at the tropopause, caused by a change in an external driver such as a change in greenhouse gas concentration. rain shadow—The area on the leeward side of a mountain where the air is descending as a dry air mass, causing little rain to fall. reef—Wave-resistant framework-supported carbonate or organic mounds generally built by carbonate-secreting organisms. In some usages the term may be used for any shallow ridge of rock lying near the surface of the water. regolith—The outer surface layer of Earth, consisting of a mixture of soil, organic material, and partially weathered bedrock. regression—Retreat of the sea from the shoreline, caused by eustatic sea level fall or local effects. Rossby Waves—Dips and bends in the jet stream path. saltation—Movement of sand or particles in a series of small jumps and bounces by wind or currents. seafloor spreading—The process of producing new oceanic crust as volcanic basalt pours out of the depths of Earth, filling the gaps generated by diverging plates. Beneath the mid-oceanic ridges, magma rises from depth in the mantle and forms chambers filled with magma just below the crest of the ridges. The magma in these chambers erupts out through cracks in the roof of the chambers and forms extensive lava flows on the surface. As the two different plates on either side of the magma chamber move apart, these lava flows continuously fill in the gap between the diverging plates, creating new oceanic crust. sea ice—Ice that has broken off an ice cap or polar sea ice, or calved off a glacier and is floating in open water. sea level rise—The gradual increase in average height of the mean water mark with respect to the land. seasons—Variations in the average weather at different times of the year.

Snowball Earth—A time in Earth history when nearly all the water in the planet is frozen, and glaciers exist at low latitudes. storm surge—A mound of water that moves ahead of and with tropical cyclones and hurricanes, formed by the low pressure in the center of the storm and winds in front of the storm. stratosphere—Region of the atmosphere above the troposphere that continues to a height of about 31 miles (50 km). subsidence—The sinking of one surface, such as the land, relative to another surface, such as sea level. supercontinent cycle—The semi-regular grouping of the planet's landmasses into a single or several large continents that remain stable for a long period of time, then disperse, and eventually come back together as new amalgamated landmasses with a different distribution.

talus—The entire body of rock waste sloping away from the mountains is known as talus, and the sediment composing it is known as sliderock. This rock debris accumulates at the bases of mountain slopes, deposited there by rock falls, slides, and other downslope movements.

thermohaline circulation—Ocean currents that are driven by differences in temperature and salinity of ocean waters. thermosphere—Region of the atmosphere above the mesosphere that thins upward and extends to about 311 miles (500 km) above the surface.

tides—The periodic rise and fall of the ocean surface, and alternate submersion and exposure of the intertidal zone along coasts. tidewater glaciers—Glaciers that are partly floating on the ocean, often in steep walled fiords. till—Glacial drift that was deposited directly by the ice. tilt—An astronomical measure of how much Earth's rotational axis is inclined relative to the perpendicular to the plane of orbit.

transform boundaries—Places where two plates slide past each other, such as along the San Andreas fault in California, and often have large earthquakes. transgression—Advance of the sea on the shore, caused by either a eus-

tatic sea level rise or local effects. troposphere—The lower 36,000 feet (11 km) of the atmosphere. urban heat island—An effect where cities tend to hold heat more than the countryside.

windward—The side of a mountain facing the oncoming prevailing winds, typically the wet side of a mountain range. wobble—An astronomical measure of the rotation axis that describes a motion much like a top rapidly spinning and rotating with a wobbling motion, such that the direction of tilt toward or away from the Sun changes, even though the tilt amount stays the same. This wobbling phenomenon is known as precession of the equinoxes. yardangs—Elongate streamlined wind-eroded ridges, which resemble an overturned ship's hull sticking out of the water.

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