Ice - A frozen form of the water molecule. Ice has a specific gravity (0.9166), which is slightly less than water. This difference in specific gravity causes ice to float on water.
Ice Age - A time when glaciers dominated the landscape of the Earth. The last major Ice Age was the Pleistocene Epoch.
Ice Apron - A relatively thin layer of snow and ice covering a mountainside.
Ice Barrier - The outer edge of ice associated with a glacial ice sheet.
Ice Cap - A large dome-shaped glacier found covering a vast expanse of land. Smaller than an ice sheet.
Ice Cluster - A relatively large area of ice floating on a water body.
Ice Dome - Elevated areas of glacial ice accumulation for ice caps and ice sheets.
Ice Edge - The zone between the margin of a mass of sea ice and the open ocean.
Ice Fall - An area of crevassed ice on a glacier. Caused when the base of a glacier flows over steep topography.
Ice Field - A large level area of glacial ice found covering a substantial expanse of land. Similar in size to an ice cap, but does not have a dome shape.
Ice Floe - A piece of sea or lake ice that is floating unconnected to the land margin. In the Arctic and Antarctic, ice floes can be quite large, up to several kilometers (miles) in width and about 3 meters (9 feet) thick.
Ice Flow - Refers to the movement of ice associated with glaciers and permafrost ground ice by plastic deformation and/or basal sliding.
Ice Fog - A fog that is composed of small suspended ice crystals. Common in Arctic locations when temperatures are below -30°C (-22°F), and an abundant supply of water vapor exists.
Ice Front - A vertical cliff of ice associated with a glacier or the seaward margin of an ice shelf.
Ice Jam - The accumulation of ice at a specific location along a stream channel. Ice jams can reduce stream flow downstream and cause flooding upstream.
Ice Lense (Ice Lens) - A horizontal accumulation of permanently frozen ground ice.
Ice Pellets - A type of frozen precipitation. Ice pellets or sleet are transparent or translucent spheres of frozen water that fall from clouds. To form, this type of precipitation requires an environment where raindrops develop in an atmosphere where the air temperature is above freezing. These raindrops then fall into a lower layer of air with freezing temperatures. In this lower layer of cold air, the raindrops freeze into small ice pellets. Like freezing rain, an air temperature inversion is required for the development of ice pellets.
Ice Rind - A term that describes an initial phase in the formation of sea ice when the accumulating ice crystals come together to produce a thin, fragile ice layer.
Ice Sheet - A dome-shaped glacier that covers an area greater than 50,000 square kilometers (19,300 square miles). Greenland and Antarctica are considered ice sheets. During the Pleistocene glacial advances, ice sheets covered extensive areas of North America, Europe, and Asia. This mass of glacial ice is larger than an ice cap.
Ice Shelf - A large flat layer of ice that extends from the edge of the Antarctic Ice Cap into the Southern Ocean. A source of icebergs.
Ice Storm - A mid-latitudinal cyclone storm characterized by precipitation in the form of freezing rain, rather than rain and/or snow. Ice storms can disrupt road and air travel and cause significant damage to vegetation, such as trees, and human-made structures.
Ice Stream - A relatively fast-moving zone of glacial ice found embedded within an ice sheet or ice cap. These glacial features are believed to form when a subsurface basal water layer lies between two ice layers. Along the outer edges of ice streams, crevassing is common.
Ice Wedge - Wedge-shaped, ice body composed of vertically oriented ground ice that extends into the top of a permafrost layer. These features are approximately 2 to 3 meters (4 to 6 feet) wide at their top and extend into the soil about 8 to 10 meters (26 to 33 feet). Ice wedges form in cracks that develop in the soil during winter because of thermal contraction. In the spring, these cracks fill with liquid water from melting snow, which subsequently refreezes. The freezing process causes water to expand, increasing the size and depth of the crack. The now-larger crack fills with more water, and again it freezes, causing the crack to enlarge. This process continues for many cycles until the ice wedge reaches its maximum size.
Iceberg - A large mass of ice found floating in an ocean or a lake. Icebergs often form when ice calves from land-based glaciers into the water body. Icebergs are very dangerous to shipping in the high- and mid-latitude regions of the ocean because 90 percent of their mass is submerged below the ocean surface.
Icelandic Low - Subpolar low pressure system typically found near Iceland. Most develop during the winter season. The Icelandic Low spawns many mid-latitude cyclones, which influence western Europe.
Icing - The build-up of ice on objects because of the presence of certain weather conditions. Icing is a hazardous situation for aviation.
Ideal Gas Law - This law describes the physical relationships that exist between pressure, temperature, volume, and density for gases. Two mathematical equations are used to describe this law:
Pressure x Volume = Constant x Temperature
and
Pressure = Density x Constant x Temperature
Igneous Intrusion - A mass of igneous rock that formed from solidified magma beneath the Earth's surface.
Igneous Rock - Rocks formed by solidification of molten magma either beneath (intrusive igneous rock) or at (extrusive igneous rocks) the Earth's surface.
Ignimbrite - A volcanic igneous rock made of consolidated pumice fragments and volcanic ash that came from a pyroclastic flow.
Illite - A non-expanding type of clay composed of phyllosilicate minerals.
Illuvial Horizon - A subsurface soil horizon where the process of illuviation dominates, leading to the deposition of silicate clay, oxides of iron and aluminum, and organic substances. Commonly forms in the B horizon.
Illuviation - The deposition of humus, chemical substances, and fine mineral particles in the lower layers of a soil from upper layers because of the downward movement of water through the soil profile. Compare with eluviation.
Immature Soil - A relatively young soil that has not been given enough time for soil formation processes to develop typical A horizon and B horizon layers down its soil profile.
Immigrant Species - A species that migrates into an ecosystem or that is deliberately or accidentally introduced into an ecosystem by humans. Some of these species can be beneficial, whereas others can interfere with and eliminate native species. Compare with indicator species, keystone species, and native species.
Immigration - The migration of an organism into an area with the end result of changing its home permanently. Compare with emigration.
Impeded Dunes - A type of dune whose location is determined by another feature in the landscape that influences the movement and deposition of sand particles. Features that can create impeded dunes include vegetation, a localized sand source, and a topographic obstacle.
Impermeable - The inability to allow for the vertical or horizontal movement of a gas or liquid because of structural or textural properties found in a soil or rock formation. This term is commonly used in Hydrology and Ecology.
Impervious - A substance that cannot let radiation, gases, or liquids pass through it. This term can be synonymous with impermeable.
Inceptisols - A soil order (type) of the United States Natural Resources Conservation Service Soil Classification System. These soils have horizons in the early stages of pedogenesis. Inceptisols are often found on glacial drift deposits.
Incised Meander - A meander that is carved deep into the landscape surface because land uplift has enhanced the erosion by a stream.
Inclination - The angle at which a rock stratum or some other geological feature dips.
Inclusion - (1) Crystals of one mineral found inside a larger mass of another mineral. (2) A mass of older rock found inside a larger mass of younger rock.
Incompetent Bed - A stratum of rock that deforms plastically instead of bending because of the stresses associated with folding.
Inconsequent Drainage - A stream drainage network that is not influenced by the underlying geologic structure of the landscape.
Independent Variable - A variable in a statistical test that is thought to be controlling through cause and effect the value of observations in another dependent variable modeled in the test.
Index Contour - A contour line that is emphasized in terms of thickness and is often labeled with the corresponding measure of elevation. Index contours usually occur every fourth or fifth contour interval. Index contours help individuals to read elevations on a map quickly.
Index Cycle - The cyclical variation in the zonal index. The period of each variation is about 3 to 8 weeks.
Index Fossil - A fossil of a particular organism or organisms that can be used to date a stratum of rock. For example, the presence of trilobite fossils of the genus Paradoxides suggests that the bed containing them was deposited during the middle of the Cambrian period.
Index Map - A small-scale ancillary map found on a larger-scale map that provides identification information about other associated large-scale maps adjacent to the map being viewed.
Indian Ocean - The Indian Ocean covers about 14% of the Earth's surface area and has a total area of about 68,556,000 square kilometers (26,463,000 squaremiles). The Indian Ocean has an average depth of 3,900 meters (12,800 feet). This ocean region is enclosed on three sides by the landmasses of Africa, Asia, and Australia. The southern border of the Indian Ocean is open to water exchange with the much colder Southern Ocean.
Indian Summer - (1) A fall season that usually has mild and sunny weather in the USA and Canada. (2) A period of uncharacteristically mild and sunny weather in Britain.
Indicator Species - A species that can be used as an early signal of environmental degradation to a community or an ecosystem. Compare with immigrant species, keystone species, and native species.
Indomalaya - One of eight biogeographic realms found on Earth, where many species have a unique evolutionary history because of geographic isolation. Geographically, this realm includes India, Southeast Asia, and southern China. The climate of this realm is mainly tropical to subtropical.
Induction - A form of logical speculation where the explanation or hypothesis of how a cause-and-effect process works is derived from the observation of empirical facts. Compare this form of logic with deduction.
Induration - The hardening of sedimentary deposits into a rock by the processes of pressure caused by compaction, drying, and cementation.
Industrial Revolution - A time of major change in the economy and society of humans because of the use of machines and the efficient production of goods. Many historians argue that this period in human history began in England in the late 18th century and continues to this day. After its start in England, the Industrial Revolution quickly spread to continental Europe and then to other places around the world. The Industrial Revolution is a significant turning point in history, and its effects have altered many aspects and routines of daily life.
Industrial Smog - A form of air pollution that develops mainly in urban areas. This type of air pollution consists of a combination of sulfur dioxide, suspended droplets of sulfuric acid, and various suspended solid particles. Also see photochemical smog.
Inferential Statistics - A statistical test that makes generalizations about a population based on the numeric information obtained from a random data sample.
Infiltration - The absorption and downward movement of water into the soil layer.
Infiltration Capacity - The ability of a soil to absorb surface water.
Infiltration Rate - The rate of absorption and downward movement of water into the soil layer.
Infiltrometer - A scientific instrument used to measure the rate of water infiltration into soil or material covering the ground surface.
Inflorescence - A group of flowers on a branch or network of branches on a flowering plant.
Influent - (1) A tributary of a stream. (2) A stream that supplies water to an area of groundwater.
Influent Stream - A stream that fails to retain its discharge and has a net reduction in stream flow downstream because of evaporation, ground seepage, and irrigation withdrawal. Influential streams are found mainly in arid climates; examples include the Nile, Colorado, and Snake rivers. Also called a losing stream. Compare with an effluent stream.
Infrared Photography - A form of photography where the instrument or film used is specially designed to capture infrared radiation being reflected or emitted from an imaged object.
Infrared Radiation - There are several different definitions for this term. (1) Form of electromagnetic radiation with a wavelength between 0.7 and 100 micrometers (µm). (2) Form of electromagnetic radiation with a wavelength between 0.74 and 300 micrometers (µm). Also called longwave radiation.
Ingrown Meander - A form of an incised meander where the slope of the stream valley is steep on one side and gentle on the opposite side.
Inhibition Model of Succession - This model of plant succession suggests that changes in plant species dominance over time are caused by death and small-scale disturbances, as well as by variations in plant species longevity and dispersal ability. Over time, species turnover favors plant species that have long lifespans.
Initial Landform - A young landform produced by folding, faulting, tectonic uplift, or volcanic activity that is relatively unaltered by the landscape processes of erosion and deposition.
Inland Sea - A large body of saline or brackish water that is found in the central area of a landmass and not connected to an ocean. This water body may have formed when higher sea levels caused seawater to flow inland.
Inlet - A narrow watercourse that connects a small water body to a larger one.
Inlier - An outcrop of older rock isolated within a formation of younger rocks. Commonly forms with the erosion of anticlines.
Inner Core - The inner spherical-shaped region of the Earth's core. The inner core is thought to be composed of iron and nickel and has a density of about 12.6 to 13.0 g/cm3 - grams per cubic centimeter (roughly 0.46 to 0.47 pounds per cubic inch). The inner core has a diameter of about 1,220 kilometers (760 miles). Scientists believe the inner core is mainly solid and is surrounded by the liquid outer core. The surface of the inner core is estimated to be very hot, about 5400° Celsius (9750° Fahrenheit). Also see outer core.
Inorganic - Something that is not living. Usually refers to the physical and chemical components of an organism's environment. Also called abiotic.
Inosilicate - Subclass of the silicate class of minerals. Inosilicates have two distinct forms: single- and double-chain silicates.
Input - Addition of matter, energy, or information to a system. Also see output.
Insect - A group of organisms that are relatively small and simple animals with a rigid chitinous external skeleton, three body sections, three pairs of legs, and antennae. These organisms are the most abundant group of eukaryotes on the Earth. See the Encyclopedia of Life for more information on this group of organisms. https://eol.org/pages/344
Insecticide - A chemical substance that causes reduced growth and/or reproduction or the death of insects.
Inselberg - A term of German origin used to describe a steep-sided hill composed of rock that rises from a pediplain.
Insequent Stream - A stream whose path is not the outcome of the slope of the land surface or by the erosion characteristics of the underlying bedrock.
Insolation - Direct and diffused shortwave solar radiation that is received in the Earth's atmosphere or at its surface.
Insolation Weathering - A form of physical weathering. Involves the physical breakdown of minerals and rock due to thermal expansion and contraction.
Instability - An atmospheric condition where a parcel of air is warmer than the surrounding air in the immediate environment. This condition causes the parcel to rise in the atmosphere. Also see unstable atmosphere.
Insulator - (1) A substance that has a relatively slow rate of heat energy flow. (2) A substance that resists the flow of electrical energy.
Interaction (Biological) - Mutual or reciprocal influence between two or more similar organisms or individuals of different species. Major biotic interactions are: competition, mutualism, predation, parasitism, amensalism, and commensalism.
Interbedded - The sandwiching of a rock bed of a particular type between strata of a similar type.
Intercardinal Direction - The four secondary navigational directions (Northeast, Southeast, Southwest, and Northwest) found on a compass or a map. For navigational purposes, these directional names are given the following values relative to the degrees found in a circle: Northeast = 45°, Southeast = 135°, Southwest = 225°, and Northwest = 315°. Compare with cardinal direction.
Interception - Is the capture of precipitation by the plant canopy and its subsequent return to the atmosphere through evaporation or sublimation. The amount of precipitation intercepted by plants varies with leaf type, canopy architecture, wind speed, available radiation, temperature, and atmospheric humidity.
Interference - A form of competition where an individual directly prevents the physical establishment of another individual in a portion of a habitat.
Interflow - (1) Another term used to describe the process of throughflow. (2) A component of stream flow that represents water that infiltrates into the ground and then is transported laterally beneath the ground surface until it reaches a stream channel.
Interglacial - A time during an ice age when glaciers melted and retreated because of milder temperatures. Compare with glacial.
Intergovernmental Panel on Climate Change (IPCC) - Is an intergovernmental scientific body created by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP). The purpose of this group of scientists is to produce scientific assessments of scientific, technical, and socio-economic information regarding the risk of human-mediated climate change. Further, the body determines the potential environmental and socio-economic consequences of human-caused climate change and suggests options for adapting to these consequences or mitigating its effects. See the following website for more information: https://www.ipcc.ch.
Intermediate Magma - A type of magma that has between 53% to 62% silica and significant quantities of magnesium and iron content. Intermediate magma solidifies to form dark-colored igneous rocks rich in magnesium, iron, and silica.
Intermediate Rocks - Types of igneous rocks with roughly equal mixtures of felsic minerals (mainly plagioclase) and mafic minerals (primarily hornblende, pyroxene, and/or biotite). In intermediate rocks, the mineral quartz is absent or in low quantity.
Intermittent Stream - A stream that has water flow only for short periods over a year. Intermittent stream flow events are usually initiated by rainfall. Compare with a perennial stream and an ephemeral stream.
Intermontane - Something located in between mountains or mountain ranges.
International Date Line - A line drawn almost parallel to the 180 degree longitude meridian that marks the location where each day officially begins. The location of the International Date Line was decided upon by international agreement.
International System of Units (SI) - A set of standard measurement units used in science and commerce, originally set up in 1960 and adopted by most countries. In France, this system is known as Le Système International d'unités. Also known as the metric system. For more information, see the Bureau International des Poids et Mesures website - https://www.bipm.org/en/measurement-units
International Union for Conservation of Nature (IUCN) - Is an international organization that works to protect our planet's natural world. In particular, the IUCN is concerned with conserving biodiversity, and the organization suggests that this goal is linked to addressing other environmental issues, including human-caused climate change, achieving energy sustainability, creating a green economic system worldwide, and improving human well-being. As part of its mandate, IUCN publishes the Red List, which evaluates the extinction risk of over 40,000 species of plants and animals. See the following website for more information: https://www.iucn.org
Interpluvial - A period of relatively drier climatic conditions sandwiched between two wetter climatic phases. Compare with pluvial.
Interpolation - In statistics, this term refers to the mathematical generation and insertion of a new value within the distribution of known values. The new value is calculated based on the statistical pattern observed in the known values.
Interspecific Competition - A biological interaction where two or more individuals, each of different species, in close proximity to each other require the same resource (e.g., nutrients, food, water, nesting space, and ground space). Their use of this resource causes its supply to be limited. The outcome of this interaction is that all organisms involved have their fitness reduced, as the resources they receive are insufficient for optimal health and survival. This reduction in fitness can cause the death of one or more of the competing individuals, freeing up resources to the remaining competitors and increasing their fitness and reproductive output.
Interspecific Interaction - Any biological interaction between individuals that are from different species of organisms. Compare with intraspecific interaction.
Interstadial - A period of relatively warmer temperatures and glacial retreat within a much larger period when widespread glaciation is dominant in a region's landscape.
Interstices - The pores and small cracks found within a rock. Geologists classify these voids according to how they were created and their size and shape. The most basic classification scheme calls voids created at the time of rock formation primary interstices. Secondary interstices are voids created by forces like metamorphism, tectonic activity, and weathering after the rock initially formed.
Intertidal Zone - The area that is between low tide and high tide along an ocean shoreline. Organisms found living here are adapted to survive in an environment of fluctuating extremes.
Intertropical Convergence Zone (ITCZ) - Zone of low atmospheric pressure and ascending air located at or near the equator. The rising air currents at the ITCZ result from global wind convergence and thermal convection. The ITCZ generally lies at the same latitude as the thermal equator.
Intraspecific Competition - A type of biological interaction where two or more individuals of the same species that are in close proximity to each other, require the same resource (e.g., nutrients, food, water, nesting space, and ground space) which is in limiting supply. The outcome of this interaction is that all of the organisms involved have their fitness reduced to some level, because the limiting resources they receive are less than what is needed for optimum health and survival. This reduction in fitness can cause the death of one or more of the competing individuals, freeing up resources to the remaining competitors and increasing their fitness and reproductive output.
Intraspecific Interaction - A biological interaction occurring between the individuals that make up a single species. Compare with interspecific interaction.
Intrazonal Soil - A soil group that is generally well-developed and can be categorized by characteristics that occur in its profile because of pedogenesis controlled by local factors like parent material, relief, and drainage. Compare with azonal soil and zonal soil.
Intrenched Meander - A stream meander or series of meanders that has become deeply cut into the landscape because of tectonic uplift.
Intrinsic Permeability - A measure of the ability of soil, sediment, or rock to permeate liquids under natural conditions. Intrinsic permeability is usually measured in Darcy units. An intrinsic permeability equal to 1 Darcy allows a flow of 1cm3 per second (1cm3/s) of a liquid with viscosity roughly equal to water at 20°C under a pressure gradient of 1 standard atmosphere per centimeter (1 atm/cm) acting across an area of 1cm2. The rate of a substance's intrinsic permeability is determined mainly by the shape, size, distribution, and density of pores. Also called specific permeability.
Introduced Species - See invasive species. See alien species and an exotic species.
Introduction - A term that refers to the process of accidentally or intentionally introducing an invasive species to a habitat where it is not normally found.
Intrusion - A general term that describes a situation where a quantity of igneous rock becomes embedded via fissures and faults into older igneous rock. Also see batholith, dyke, sill, laccolith, lopolith, and phacolith.
Intrusive Igneous Rock - A mass of igneous rock that forms when magma from the mantle migrates upward and cools and crystallizes beneath the Earth's surface. Also called plutonic igneous rock. Also see batholith, dyke, sill, laccolith, lopolith, and phacolith.
Invasive Species - A non-native species that is introduced to a new habitat either intentionally or by accident and subsequently establishes itself. Also called an alien species, an exotic species, and an introduced species.
Inverse Square Law - This mathematical law models the reduction in the intensity of radiation as it moves from its source in three-dimensional space. It suggests that the amount of radiation passing through a specific area is inversely proportional to the square of the distance of that area from the energy source. Mathematically, the Inverse Square Law is described by the equation:
Intensity = I/d2
where I is the intensity of the radiation at 1 unit distance, and d is the distance traveled in those units.
Inversely Proportional - A cause and effect relationship between two variables where a positive or negative change in the quantity of one causes a predictable opposite change in the quantity of the other.
Invertebrates - Animals that do not have a backbone. Compare with vertebrates.
Inverted Relief - A situation where natural landscape features have reversed their elevation relative to other landforms. Inverted relief can occur when areas of low elevation in the landscape are filled with sediment or lava flows that harden into rock more resistant to weathering and erosion than the surrounding rock. Over time, differential weathering and erosion lower the surrounding rock to an elevation that is below the filled area. These features have also been seen on other planets and moons in our Solar System. Also called inverted topography and a topographic inversion.
Inversion - See temperature inversion.
Ion - An atom, molecule, or compound that carries either a positive (cation) or negative (anion) electrical charge.
Ionizing Radiation - The emission of alpha or beta particles or gamma rays from radioisotopes. These emitted particles can dislodge one or more electrons from atoms they strike. Free electrons can form charged ions in living tissue, which can react with and damage cells.
Ionosphere - An identified region in the atmosphere located 60 to 400 kilometers (40 to 250 miles) above the Earth's surface where relatively large concentrations of ions and free electrons exist. The ionosphere is important for human communications because it is used to redirect AM radio transmissions. This process extends the range of radio transmissions. This is done by bouncing the radio waves off the positively charged ions. The ionosphere also has a role in the creation of aurora.
Iron Age - An age in the technological development of human civilization when processed iron was used to make many tools. The Indian, European, and Near Eastern cultures began their Iron Age at about 1200 BCE.
Iron Pan - A hardpan layer in a soil that usually consists of sand particles that are cemented together with iron oxides.
Iron Pyrite - A common mineral with the chemical formula FeS2. Also called pyrite and fool's gold.
Irradiance - The radiant flux (rate of flow of electromagnetic radiation) received by a surface per unit area. Irradiance is often measured in Watts per square meter.
Isarithm - See isoline.
Island - A piece of land surrounded by freshwater or seawater.
Island Arc - A line of volcanic islands found in the ocean that has been created by the convergence of two tectonic plates and the subsequent subduction of one of the plates beneath the other. Subduction causes magma to rise to Earth's surface, creating volcanic islands.
Island Biogeography - The scientific study of abundance, distribution, spatial pattern, and evolution of organisms on islands. One particularly important contribution to this branch of science is the book by R.H. MacArthur and E.O. Wilson titled the Theory of Island Biogeography. See Ecology and Biogeography.
Isobar - Is a line (isoline) on a map joining points of equal atmospheric pressure.
Isolated System - A system that has no interactions beyond its boundary layer. Many controlled laboratory experiments are this type of system.
Isoline - Lines joining points of equal value for some measurable characteristic shown on a map. Also called an isopleth and an isarithm.
Isopleth - See isoline.
Isostacy - The buoyant condition of the Earth's crust floating in the asthenosphere. The greater the crust's weight, the deeper it sinks into the asthenosphere. When the weight is removed, the crust gradually rises higher.
Isostatic Depression - Large-scale sinking of the crust into the asthenosphere because of an increase in weight on the crustal surface. Common in areas of continental glaciers, where the crust was depressed by the weight of the enormous ice mass.
Isostatic Rebound - The upward movement of the Earth's crust following isostatic depression.
Isotherm - Lines (isolines) on a map joining points of equal temperature.
Isothermal Layer - A vertical layer in the atmosphere where temperature remains unchanged with altitude. In the Earth's atmosphere, three isothermal layers exist below the stratosphere, mesosphere, and the thermosphere.
Isotope - A form of an element where the number of neutrons in its atomic nucleus is different from the number of protons.
Isotopic Dating - Any method of dating used to determine the age of something like a fossil, mineral, or rock through the decay of radioactive elements.
Isotropy - An object or thing that has an attribute whose measurement does not change value with direction. Compare with anisotropy.
Isoyet - Lines (isolines) on a map joining points of equal precipitation.
Isthmus - An area of relatively narrow terrain that connects two larger pieces of land.
ITCZ - See Intertropical Convergence Zone.
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