Pacific Rim - the countries at the margins of the Pacific ocean.

Pacific Ring of Fire -the ring of volcanoes to be found around the Pacific Rim. Many of the Rim coastal areas are subduction zones, hence the pattern of activity.

Palaeoclimate - evidence of past climate that can be seen in the present e.g. glacial landforms in non-glacial areas, oxygen isotopes in ice deposits.

Palaeomagnetism - evidence of past differences in the alignment of the Earth magnetic field. Provides some of the strongest evidence for sea-floor spreading

Paleozoic -an era of geologic time lasting from 570m to 245m years ago.

Palustrine -of bogs, marshes or swamps.

Pan - see hardpan.

Pandemic -disease spread over a wide geographical area.

Pangaea -the class="d-title" name that Wegener gave to his proposed single supercontinent in his theory of continental drift.

Parent-material -rock or regolith from which the inorganic, mineral component of a soil is derived.

Pastoral farming -the rearing of animals for meat, milk, wool, skins etc.

Patterned ground -in periglacial areas, the appearance of lines and polygonal layouts of stones on the surface produced by the sorting of different sizes of material during the expansion and contraction of the soil with diurnal temperature variation.

Peak flow -the highest discharge found in a river channel in response to a particular rainfall or snowmelt event.       

Peak land value point -the location of the highest land value in an urban area.

Peat -a type of soil formed in cold wet conditions which inhibit full and proper decomposition of the litter layer. Semi decomposed leaves and other litter material remain recognizable within the soil which can be cut, dried and burned as fuel in marginal areas where other more efficient fuels may not be readily available. 

Pebble - a smooth, rounded fragment of rock that is larger than gravel but smaller than a boulder, in the range of 10-100mm in diameter.

Ped -a structured unit of soil created when particles become grouped and bound together. Different shapes of unit occur under different conditions: crumb, block, plate, column and prism.

Pedestrianisation -the temporary or permanent blocking of streets to vehicular traffic.

Pedestrian flow – a measure of the number of pedestrians passing a certain point in a designated timeframe and at designated times of day/week/month/year. To avoid distortions, a pedestrian flow survey should be conducted on a straight piece of pavement away from any junctions [where people tend to bunch]. May be uni- or bi-directional on the chosen stretch of path.

Pediment -concave, rock-cut, slope found between a cliff face and a valley floor in arid and semi-arid areas.

Pedogenesis -lit. soil formation.

Pelean eruption -after the 1902 eruption of Mount Pele in Martinique, an extremely violent eruption that begins with an explosive pyroclastic cloud.

Pennsylvanian -a period of geologic time lasing from 320m to 286m years ago.

Percolation -downward movement of water through soil and bedrock.

Perennial -lit. lasting all year or for several years.

Periglacial -the area at the edge of a glacier or ice sheet which is not covered by ice but experiences very cold conditions. May also be used to refer to high altitude environments or cold high latitudes as well. 

Periphery -lit. the edge or margins. In human geography, those areas which have a poor economic status and thus suffer from the associated social conditions. Unemployment and crime are high, incomes and general living standards are low. The area may be geographically marginal to a core region at a number of scales.

Permafrost -frozen ground. Varies widely in scope and depth and in the period of the year for which it exists. A common feature of periglacial environments. See continuous permafrost, discontinuous permafrost and sporadic permafrost.

Permeable -a rock which can absorb water (porous) or allow water to pass through cracks and joints (pervious).

Permian -a period of geologic time lasting from 286m to 245m years ago.

Pervious -allowing water to flow along cracks or joints.

Pesticide -any chemical sprayed on crops to prevent disease or to kill pests which attack the plants. In modern times, concerns have been raised as to their safety as studies have not had enough time to see the full long-term effects of consuming foods treated this way.

Phanerozoic -the current eon of geologic time that began 2500m years ago.

Phosphates -phosphorous-based fertilizers. Traditional societies dumped unwanted or unusable fish catches on top of the soil. In modern times, more phosphates have been derived from rocks which are quarried or mined. 

Photochemical smog -a heavy, brown coloured, air pollution formed by the reaction of vehicle and factory emissions with sunlight to produce ozone which is harmful to humans, animals and plants. 

Photovoltaic cells -a unit, usually of silicon, which is able to collect and store, temporarily, energy from the sun to produce electricity.

Phreatic divide -an underground watershed. Marks the outer source of groundwater flow to a particular river. May be caused by sloped bedding planes for example.

Physical weathering -(also mechanical weathering) a process of weathering which results in smaller pieces of the same rock material being produced.

Phytoplankton -a miniscule plant which grows and lives in the upper layers of the ocean and which forms the basis of the marine food chain or web.

Piedmont -where a mountain range abruptly falls into a lowland area.

Piedmont glacier -a glacier formed when a valley glacier spills from an alpine range onto a lowland plain.

Pingo -  a dome-shaped hill in a flat tundra plain, often having a depression in the summit. Closed system pingos occur in areas of continuous permafrost where unfrozen water beneath lakes becomes overly saturated and then expands as the permafrost advances. Open system pingos occur when water within sediments rises towards the surface and then freezes pushing those sediments upwards.

Pioneer -used to define a species or community of plant(s) that is first to colonise a previously barren area.

Plagioclimax -the combination of plant species in an area brought about by human interference. Much of the European landscape is plagioclimax.

Plankton -any drifting biota inhabiting the pelagic (non-bottom) zones of seas and lakes. Important food source for marine organisms.

Plantation -a large-scale form of agriculture in which a tree or bush species is planted from which a fruit, leaf or sap is harvested. Crucially the plants are not cut and replanted each year but remain in situ for a long period of time while having the relevant part harvested from them.

Planned economy -see command economy.

Plant succession -the sequence of plant species that inhabit an area from the pioneers that first arrive, through to the climax community.

Plateau - in geology and earth science, a plateau, also called a high plateau or tableland, is an area of highland, usually consisting of relatively flat terrain.

Plate tectonics -the theory concerning the structure of the earth and the processes occurring which explain the configuration of the continents and the location of earthquakes and volcanic activity.  There are three types of plate margins, or boundaries between plates, depending on the two types of crusts that are interacting: oceanic with oceanic, continental with continental, or continental with oceanic.  The rift valleys of the Atlantic are an example of an oceanic margin where divergence has occurred, while oceanic convergence is illustrated by a striking example in the Pacific. There, subduction of the Philippine Plate by the Pacific Plate has created the Mariana Trench, which at 36,198 ft. (10,911 m) is the deepest depression on Earth.

Playa - a temporary shallow lake with extremely high salinity. Forms in arid and semi-arid areas and usually evaporates away relatively quickly leaving behind a flat deposit of salts, clays and silts.

Pleistocene -first epoch of the Quaternary, 2 m. years BP to 10,000 BP. Characterised by glacials and inter-glacials.

Plucking -a form of glacial erosion in which ice freezes on to rock and pulls it away when the glacier moves on.

Plunge pool -the pool found at the base of a waterfall created by the additional erosional energy that falling water and load has.

Pluton - intrusive igneous rock of any mass.

Pluvial -wet period during the Pleistocene evidenced by fluvial features In currently arid areas.

PLVI - see peak land value intersection (point).

Podsol - soil type found in cold, wet climatic areas in high latitudes. As precipitation exceeds evapotranspiration the soil becomes waterlogged and minerals are leached from the upper layers into the lower layers. Low organism presence means minimal mixing and therefore highly defined horizons.

Point-bar -gently sloping inner bank of a meander. The shorter distance around the inside of the bend requires a lower velocity and therefore deposition. Leads to the encroachment of the inner bend across the channel.

Polar cell -one of the cells of atmospheric circulation in the tri-cellular model.

Polarization -the disparate development of wealth around a core leaving a much poorer peripheral area.

Polder - reclaimed land created by the construction of an embankment or dyke to hold back water that would otherwise flood the new surface.

Pollarding -in forest/woodland management, the technique of cutting tree growth back to the main trunk and thereby encouraging new growth from the top of the main trunk.

Pollen analysis -the study of preserved pollen particles used to reconstruct past climatic conditions based on the types of plants prevalent at the time.

Population density -number of people per unit area, usually people per square kilometre.

Population distribution -the variation in population densities over wide areas.

Population policy -government interventions to try and control high population growth, stimulate low population growth or control the distribution of people within a country.

Population structure -the ratio of age groups, by sex, within a population.

Pore spaces -the space between particles in a rock or soil, usually filled with air or water.

Porosity -the degree to which a rock or soil is porous.

Porous - the ability to allow water to occupy pore spaces.

Positive feedback -in a system, those changes which serve to increase the effect. 

Potential evapotranspiration -theoretical amount of moisture that could be lost from the surface to the atmosphere if it were available.

Pothole -two types:

• bowl-shaped holes found in river beds formed by abrasion by load caught in eddies.

• tunnels in limestone areas.

Precambrian -a span of geologic time lasting from 4600m to 570m years ago.

Precipitate -a solid that has formed out of dissolved state.

Precipitation -two types:

• movement of water from the atmosphere to the earth surface.

• solidification of a substance from a liquid where it has been held in solution.

Pressure gradient -in the atmosphere, the rate at which pressure changes across space. Higher rates of change will create faster winds.

Pressure melting point -under a glacier, the pressure brought to bear by the mass of ice above can lower the freezing/meting point of water.

Pressure release -the removal of overlying rock which releases pressure on underlying strata causing them to expand and crack.

Prevailing wind -the direction from which wind most frequently blows in a particular place.

Primacy -when the primate city is overly large compared to the size expected according to the rank-size rule.

Primary consumers -the herbivores which feed on plants and are available as food to carnivores.

Primary sector -in industry, those activities concerned with extraction and or exploitation of raw materials.

Primary succession -the colonization of a previously barren area by a plant community and its development to a climax community.

Prime meridian - ˚

of longitude. The line joins the north and south poles and passes through the Royal Observatory at Greenwich in east London, England.

Primogeniture -inheritance of an estate to the eldest son, or eldest child, only. 

Prisere -the stages that make up a primary succession.

Private sector -the firms in an economy owned by shareholders or individuals.

Privatization -the change of ownership of a firm or industry sector from government to private hands. 

Producer -two types:

in human geography, a company making physical goods from raw materials.

physical geography, see autotroph.

Production -the conversion of raw materials into usable products.

Production-line -the organization of a factory so that each stage of the manufacture is physically next to the one before and a good moves along the line being added to as it goes until it is complete.

Productivity -the efficiency with which a company turns inputs into outputs.

Proportional symbols -data presentation where the size of a symbol, or the divisions of a symbol, are proportional to the size of the data being represented.

Protectionism -the placing of barriers to trade used by a government to try and promote domestic products over imports.

Proterozoic -an eon of geologic time lasting from 2500m to 570m years ago.

Psammosere - succession of plants in a sandy environment such as coastal dunes.

Public corporation -an economic enterprise that has been nationalized.

Public sector -all enterprises and activities owned and funded by the government.

Pull factor -an attractive quality of a place which pulls migrants to it.

Push factor -a negative quality of a place which pushes people to migrate away from it.

P-wave - a seismic wave released during an earthquake which travels in a collision fashion and can thus travel through gases, liquids and solids.

Pyramidal peak -the classic, pyramid-shape of a mountain-top formed when three or four corries form on different sides of the mountain and erode backwards towards each other.

Pyroclastic cloud -the cloud of gas, ash, dust, stones and rocks emitted during a highly explosive volcanic eruption.

Pyrophytes -plants adapted to withstand fire, and to require fire for regeneration.