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The nitrogen cycle has important implications for air quality and climate in addition to fisheries and marine ecosystems. Anthropogenic activities have perturbed the natural nitrogen cycle, with major effects on the atmosphere and marine systems.…

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from article Atmospheric chemistry

Components of the carbon cycle showing the movement of carbon between land, the atmosphere, and oceans. Yellow numbers are natural fluxes, and red are human contributions in gigatons of carbon per year. White numbers indicate stored carbon. (NASA:…

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from article Atmospheric chemistry

Processes that determine aerosol concentrations.

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from article Life forms of plants

Life forms of plants according to C. Raunkiaer. (After P. Dansereau, Biogeography: An Ecological Perspective, Ronald Press, 1957)

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from article Life forms of plants

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from article Echolocation

Echolocation is a physiological tool used by gray bats (Myotis grisescens). (Credit: Amareta Kelly)

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from article Echolocation

Sound spectrograms representing sonar signals of (a) Brazilian or Mexican free-tailed bats (Tadarida brasiliensis), (b) big brown bats (Eptesicus fuscus), (c) Wagner's mustached bats (Pteronotus personatus), and (d) greater horseshoe bats…

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from article Hydrosphere

Dissolved (a) sodium and (b) sulfate in rain (in parts per million) over the continental United States (After R. M. Garrels and F. T. Mackenzie, Evolution of Sedimentary Rocks, W. W. Norton, 1971)

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from article Hydrosphere

Accommodation of cations in aqueous solution. (After H. Blatt, G. Middleton, and R. Murray, Origin of Sedimentary Rocks, Prentice Hall, 1972)

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from article Hydrosphere

Distribution of dissolved gaseous oxygen (low values) and nutrient species (high concentrations) of orthophosphate ions at 26°22′.4N and 168°57′.5W in the Pacific Ocean. 1 m = 3.3 ft.

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from article Red Sea

Division of the Red Sea into regions representing different stages in the formation of an ocean basin. (After F. Martinez and J. R. Cochran, Structure and tectonics of the northern Red Sea: Catching a continental margin between rifting and drifting,…

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from article Precipitation (chemistry)

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from article Deforestation

View of an area of deforestation in Central Kalimantan, Indonesia. In this case, the land is being cleared to develop areas for intensive rice production. (Credit: Australian National University)

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from article Deforestation

The relationship between forest and nonforest (other) land. Preservation of forest lands includes reforestation and natural regeneration. However, deforestation and natural events can lead to nonforest land. Afforestation (the act or process of…

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from article Deforestation

Natural forest areas (in hectares; 1 hectare = 2.47 acres) in various climatic domains (boreal/polar, temperate, subtropical, and tropical) from 1990 to 2015. The data are derived from the Global Forest Resources Assessment 2015 by the Food and…

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from article Deforestation

Former rainforest in northern Queensland, Australia, that was deforested in the early 1900s and is now used for pasture. (Credit: Rodney Keenan)

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from article Deforestation

Aerial view of deforestation in the Amazon region of Brazil. Clearing of trees in the Amazon rainforest has resulted in the elimination of natural resources and numerous species. (Credit: Luoman/Getty Images)

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from article Polystyrene

Polystyrene foam. (Credit: Raywoo/123RF)

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from article Basalt

Thin section of upper crust of lava about 1.2 in. (3 cm) from top; black is microcrystalline groundmass. 0.5 mm = 0.02 in.

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from article Basalt

Thin section of interior of lava shown in Fig. 1, but 26 ft (7.9 m) below the top of the flow. Clear plagioclase crystals are ophitically enclosed in large pyroxene grains up to 0.04 in. (1 mm) in diameter. Crystals of black magnetite are minor. A…

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from article Basalt

Growth of crystals as a function of temperature in basalt in Hawaiian lava. °F = (°C × 1.8) + 32. (After D. L. Peck, T. L. Wright, and J. G. Moore, Crystallization of tholeiitic basalt in Alae lava lake, Hawaii, Bull. Volcanol., 29:629–656, 1966)

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from article Basalt

Diagrams illustrating the compositions of basalts in terms of normative mineral formulas. Customarily, CaAl2Si2O8 (anorthite) is grouped together with NaAlSi3O8 (albite), and Fe2SiO4, FeSiO3, and CaFeSi2O6 are grouped with the corresponding Mg…

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from article Vacuum fusion

Vacuum-fusion methods of analysis.

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from article Ordovician

Ordovician, the second period of the Paleozoic Era.

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from article Ordovician

Early Ordovician (Arenig) paleogeography, major rock suites and potential ocean surface currents; based on remanent magnetism data supplemented by plots of the positions of shelf sea carbonates and reefs.

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from article Soil chemistry

Proper soil chemistry helps a green seedling to sprout out from the soil. Soil chemistry controls the availability of plant nutrients within the soil and thus influences plant growth. (Credit: amenic181/Shutterstock)

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from article Soil chemistry

Extensive chemical weathering can remove essential minerals and elements from soil. Typically, only the most insoluble compounds remain in the soil. For example, iron oxides may persist, imparting a reddish-brown color to the soil. (Credit: Stephen…

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from article Soil chemistry

Factors influencing the chemistry of a soil solution. (Credit: Based on an original illustration created by J. F. Hodson)

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from article Supercritical-fluid chromatography

Block diagram of supercritical-fluid chromatograph. With some detectors (for example, flame ionization), the pressure/flow restrictor is mounted in the detector.

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from article Supercritical-fluid chromatography

High-resolution separation of the oligomers of a methyl silicone by pressure-programmed supercritical-fluid chromatography; each peak in the chromatogram is due to an oligomer with one more Si(CH3)2O group than the previous peak. Pressure is held at…

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from article Ferret

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from article Granite

Classification of granitic rocks by the IUGS scheme. All proportions are by volume percentages.

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from article South America

Map of South America.

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from article South America

Andes Mountains, Chile. The Andes, which parallel the Pacific coast, constitute one of the Earth's most majestic features. Only the Himalayas exceed them in elevation and mass. (United Press International)

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from article South America

Representative view of the hilly uplands and low mountain country of the Guiana Highlands. One of five waterfalls on the Canaima River, Gran Sabana in southern Venezuela. (Venezuelan Information Service)

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from article South America

Uplifted beaches on desert coast, east side of San Juan Bay, Departmento de Ica, Peru. The sea cliff is about 100 ft (30 m) high. Each horizontal line represents a former beach. (Photograph by F. Atchley)

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from article South America

Port of Matarani on the desert coast of southern Peru. This oblique aerial view looks coastwise southward over one of the world's driest and most barren coastal zones. (Servicio Aerofotográfico Nacional)

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from article Calcium

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from article Calcium

Calcium carbonate towers, called tufa, in the saline waters of Mono Lake, California, United States. (Credit: Steve Heap/Shutterstock)

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from article Bytownite

Bytownite, from Crystal Bay, Minnesota. (Specimen from Department of Geology, Bryn Mawr College)

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from article Californium

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from article Quantum chemistry

Methylenecyclopropane structure.

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from article Quantum chemistry

Comparison of self-consistent-field transition states for the HONC → HOCN reaction using four different basis sets.

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from article Quantum chemistry

Example of a sizable molecule for which complete DFT geometrical optimization with a substantial basis set became routine in 1998.

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from article Metamorphic rocks

Fabrics of metamorphic rocks under microscope. (a) Mortar fabric; (b) flaser or mylonitic fabric; (c) granoblastic fabric (after E. E. Wahlstrom, Petrographic Mineralogy, John Wiley and Sons, 1955). (d) Lepidoblastic fabric; (e) nematoblastic…

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from article Metamorphic rocks

Diagram showing the general relationship between deformation folding and slaty cleavage caused by pressure PP or the couple ScSc. Heavy black lines denote the original bedding deformed into folds. Thin lines indicate slaty cleavage which may grow…

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from article Metamorphic rocks

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from article Metamorphic rocks

Mineral facies groups of regional metamorphic rocks showing the temperature and pressure of metamorphism. °F = (°C × 1.8) + 32.1 km = 0.6 mi.

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from article Geographic information systems

Comparison of (a) raster and (b) vector data models at the boundary of a residential housing subdivision. Raster data files consist of rows of uniform cells coded according to data values. Raster files are easily manipulated, but are less detailed…

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from article Geographic information systems

Overlay analysis using raster data files.

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from article Chaos

Graphic depicting extreme rainfall totals in the state of South Carolina in early October, 2015, which according to National Weather Service statistics represented a 1,000-year rainfall event, meaning such an event is expected to occur about once…

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from article Chaos

Transition from order to chaos (turbulence) in a rising column of cigarette smoke. The initial smooth streamline flow represents order, while the erratic flow represents chaos.

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from article Chaos

Three-dimensional visualization of the Lorenz butterfly as traced out by the trajectory of the original simulation reported by Lorenz in 1963 (σ = 10, b = 8/3, r = 28) with initial conditions (X, Y, Z) = (0, 1, 0). The integration was performed with…

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from article Chaos

Separation of solutions on the Lorenz attractor. The two trajectories starting from the nearby initial points indicated by the balls in the right lobe of the attractor stay together for several circuits around both lobes and then separate as they…

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from article Chaos

Examples of fractal (self-similar) sets. (a) The Mandelbrot set in the complex plane, with origin indicated by the cross. A single point in the complex plane (in this case C = 0.314 − 0.537 i, where 1 = 1 ) generates (b) an associated Julia set.

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from article Chaos

Representations of pendulum motion corresponding to a periodic variation in angular position θ whose amplitude is just less than π radians (180°). (a) Phase-space representation (plot of momentum p versus θ). (b) Time plots of θ and of p = Θ. (c)…

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from article Chaos

Example of a pendulum in which gravity and a torsional ribbon act in opposition. The oscillating mass is a thin semicircular metal plate that is rigidly attached to the ribbon at its midpoint. The ribbon is fixed at each end but twists where the…

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from article Chaos

Simple fractal geometry attractors generated by computer. (a) Attractor generated using the Chirikov map defined by Eqs. (15), with c = 1.6 and initial values p0 = θ0 = 2. (b) Attractor generated using the torsion-gravity pendulum map defined by…

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from article Chaos

Examples of the period-doubling route to chaos for two vastly different iterative expressions. (a) The logistic map, defined by Eq. (18). (b) The Chebyshev root-finding algorithm applied to Kepler's equation, defined by Eqs. (20) and (21) with…

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from article Chaos

Power spectra of acoustic cavitation noise at four different excitation levels, showing the successive filling of the spectrum by period doubling (frequency halving) when the sound input intensity is raised. Numbers at tops of the graphs indicate…

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from article Chaos

Dependence of the scaled threshold field n40 F (10%) that produces an ionization probability of 10% after weakly bound hydrogen atoms are exposed to about 300 microwave field oscillations, on the scaled frequency n30ω.

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from article Berkelium

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from article X-ray fluorescence analysis

Scheme of a crystal spectrometer for x-ray fluorescence. Photons from an x-ray tube excite the sample elements at top left. A parallel beam of fluorescent radiation is filtered out by a collimator and hits a single crystal under angle Θ. Only…

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from article X-ray fluorescence analysis

Atomic shell of iron (Z = 26). Left: initial ionization of the L3-shell, followed by an L3←M5 transition. Right: initial vacancy in the K-shell, followed by a K←L3 transition. The new (secondary) vacancy in L3 is filled by L3←M5. This scheme repeats…

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from article X-ray fluorescence analysis

Measured relative intensities (blue dots, not linear due to interelement effects) and "corrected" values (orange, linear relationship) for Cr-Ni-Fe alloys. Examples of corrections are shown by arrows. (Credit: Michael Mantler, based on data from S.…

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from article Bean

Pods of soybean plants (Glycine max). (Credit: Howard F. Schwartz/Colorado State University)

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from article Bean

Red dry bean pods (Phaseolus vulgaris). (Credit: Howard F. Schwartz/Colorado State University)

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from article Bean

Different stages of maturity of the seeds of light red kidney beans in opened pods. (Credit: Howard F. Schwartz/Colorado State University)

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from article Bean

Symptoms of Pythium disease on Phaseolus vulgaris (common bean) plants. (Credit: University of Georgia Plant Pathology Archive)

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from article Bean

Anthracnose, caused by Colletotrichum lindemuthianum, on bean pods. (Credit: U.S. Department of Agriculture Cooperative Extension Series, Clemson University)

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from article Cerium

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from article Formation

Geological units of Paleozoic age recognized in the walls of the Grand Canyon, Arizona. (Modified from S. S. Beus, Geology along the South Kaibab Trail, eastern Grand Canyon, Arizona, in S. S. Beus, ed., Geological Society of America Centennial…

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from article Chelation

Formulas of hydrated cadmium(II) ion, two cadmium(II) complexes, and two cadmium(II) chelates.

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from article Chelation

Heme, a tetradentate chelate of iron(II).

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from article Chelation

Structural formula of anion of EDTA, a sexadentate ligand.

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from article Chelation

Structural formula of NTA anion, a quadridentate ligand.

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from article Chelation

Structural formula of anion of HBED, a sexadentate ligand.

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from article Chelation

“Tetraphos,” a quadridentate ligand.

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from article Chelation

Chelating ligands indicating the types of donor groups available for coordination.

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from article Chelation

Compounds forming intramolecular and intermolecular hydrogen bonds.

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from article Chelation

Structure of a chelated copper compound.

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from article Chelation

Metal chelates containing four-, five-, and six-membered rings.

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from article Chelation

Optical isomers of chelate complexes.

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from article Chelation

Formula for an optically inactive nonchelate.

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from article Precipitation measurement

Automatic gages. (a) Tipping-bucket rain gage. (b) Weighing rain gage. (Belfort Instrument)

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from article Precipitation measurement

Comparison of rainfall rates measured with gages and radar using (a) the R, Z relation and (b) the R, KDP relation. Each data point (+) is the rainfall rate for a single storm.

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from article Vegetation and ecosystem mapping

Terrestrial major habitat types.

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from article Barometer

Type of mercury barometer used in meteorology.

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from article Taiga

Siberian mountain taiga. (Credit: M. Valerii/Shutterstock)

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from article Taiga

Schematic profile of the three main subzones of the North American taiga, showing the main forest assemblages on three of the more important landform types: (a) northern taiga; (b) middle taiga; (c) southern taiga. (Copyright © McGraw-Hill Education)

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from article West Indies

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from article Geoelectricity

Time variations of the horizontal orthogonal components of the natural (a) magnetic and (b) electric fields, simultaneously measured at one site at the surface.

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from article Geoelectricity

A vectogram representing a few minutes of recording of the natural electric field vector. The band-pass filter peaked at the 20-s period (0.05 Hz).

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from article Geoelectricity

Approximate and schematic frequencies and origins of the natural electromagnetic fields.

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from article Geoelectricity

Diurnal variations (solar plus lunar) of the horizontal component of the magnetic field, December 21, 1933, Huancayo, Peru. (After S. Chapman and J. Bartels, Geomagnetism. 2 vols., Oxford University Press, 1962)

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from article Geoelectricity

Amplitude spectra of the tellurograms shown in Fig. 2.

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from article Geoelectricity

Three components of the magnetic field of a magnetic storm of May 14, 1921, Potsdam, Germany. (After S. Chapman and J. Bartels, Geomagnetism, 2 vols., Oxford University Press, 1962)

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from article Andesine