Glossary - Discovering Physical Geography - page 580

oxidation A form of chemical weathering in which oxygen
chemically combines with metallic iron to form iron oxides,
resulting in the loss of electrons. 378
oxisols Mineral soils in tropical and subtropical environ-
ments that form through laterization and thus have an oxic
horizon within 2 m of the surface. 286
oxygen isotope stages Periods of time that have distinct
O-18/O-16 ratios, which are used to reconstruct prehistoric
climate change. 222
ozone hole The decrease in stratospheric ozone observed
on a seasonal basis over Antarctica, and to a lesser extent over
the Arctic. 67
ozone layer The layer of the atmosphere that contains high
concentrations of ozone, which protect the Earth from ultra-
violet (UV) radiation.

Physical geography Spatial analysis of the physical
components and natural processes that combine to form the
environment. 6
Pinnacle A steep-sided, narrow tower that is the final
remnant of a plateau, mesa, or butte. 483
Pixel The smallest definable area of detail on an image; short
for pixel element. 34
Plane of the ecliptic The flat plane on which the Earth trav-
els as it revolves around the Sun. 45
Plant succession The predictable vegetation transition that
occurs within a landscape over a period of time. 254
Plate tectonics The theory that the Earth's crust is divided
into a number of plates that move because they float on the
asthenosphere. 334
Plateau A very broad, horizontal surface that is upheld by
resistant caprock. 483
Playa A dried lake bed that forms when runoff collects in
closed topographic depressions in arid regions. 483
Pluton An extremely large mass of intrusive igneous rock
that forms within the Earth's crust. 311
Podzolization A regional soil-forming process in cool,
humid environments that results in the eluviation of iron,
aluminum, and organic acids to form well-developed E and Bs
horizons. 290
Polar easterlies

66

P
Pacific ring of Fire The chain of volcanoes that occurs
along the edge of the Pacific lithospheric plate. 362
Pangaea The hypothetical supercontinent, composed of
all the present continents, that existed between 300 and 200
million years ago.

334

Lines of latitude. 17 Parallels

Parent material

The mineral or organic material in which

soil forms. 271
Passive margin A place where the continental crust and the
oceanic crust are on the same tectonic plate and thus do not
move relative to each other. 338
Pedogenic processes The natural processes of soil forma-
tion that involve additions, translocations, transformations, and
losses. 268
Perched water table A localized area of saturated sediment
that is elevated above the regional water table by a zone of
impermeable rock or sediment. 395
Periglacial processes The suite of processes involving frost
action, permafrost, and ground ice that occurs in arctic envi-
ronments or along the margins of ice sheets. 473
Perihelion The point of the Earth's orbit where the distance
between the Earth and Sun is least (~147 million km or 91.5
million mi). 47
Permafrost Ground that is permanently frozen. 474
Petroleum Naturally occurring oily liquid that consists of
ancient hydrocarbons. 318
Photosynthesis The conversion of solar radiation into
chemical energy. Sugars and starches are produced from
carbon dioxide and water through the interaction of light and
chlorophyll in green plants. The process releases oxygen into
the atmosphere. 237
pH The measure of acidity or alkalinity of a solution,
ranging from 0 to 14, based on the activity of hydrogen ions
(H + ).

Band of easterly winds at high

latitudes. 123
Polar front The contact in the midlatitudes between warm
tropical air and colder polar air. 119
Polar front jet stream River of high-speed air in the upper
atmosphere that flows along the polar front. 120
Polar High Zone of high atmospheric pressure at high
latitudes. 122
Potential evapotranspiration (potential ET) A measure
of the maximum possible water loss from a given land area
assuming sufficient water is available. 203
Potential natural vegetation The vegetation that would
occur naturally within a specific area if no human influence
occurred. 241
Pressure gradient force The difference in barometric pres-
sure that exists between adjacent zones of low and high pres-
sure that results in airflow. 111
Prime Meridian The arbitrary reference point for longitude
that passes through Greenwich, England. 17
Process A naturally occurring series of events or reac-
tions that can be measured and that result in predictable
outcomes. 6
Progradation Outward extension of the shoreline through
deposition of sediment. 519
Proxy data Indirect evidence of an event. For example, fossil
pollen is a proxy indicator of climate change because vegeta-
tion reflects climate. 278 218