Earth systems and geophysics

Oxisols

Oxisols, known as Ferralsols in the World Reference Base, are the most highly weathered of the twelve USDA soil orders, forming in hot, humid tropics with low natural fertility.

The oxygen cycle is the biogeochemical cycle through which oxygen moves among the atmosphere, ocean, biosphere, and rocks, produced mainly by photosynthesis and consumed by respiration and weathering.

P-wave tomography

P-wave tomography is a seismic imaging method that inverts compressional wave travel times from millions of earthquakes to build 3-D models of P-wave velocity variations inside the Earth, from dense arrays to the whole mantle.

Pacific Northwest Seismic Network

The Pacific Northwest Seismic Network (PNSN), run by the University of Washington, monitors earthquakes and volcanoes across Washington and Oregon and feeds the USGS ShakeAlert early warning system.

Paleointensity

Paleointensity is the study of Earth's ancient magnetic field strength, recovered from rocks and fired clays by comparing their natural magnetization with one induced in a known laboratory field.

Paleomagnetism

Paleomagnetism is the study of the Earth's past magnetic field, recorded in rocks, sediments, and fired materials; it underpins the geomagnetic polarity timescale and continental reconstructions.

Paleosol

A paleosol is a fossil soil that formed on a landscape of the past and is preserved in the geological record; its study is called paleopedology, and buried soils define geosols in stratigraphy.

Passive seismology

Passive seismology is the imaging of Earth's subsurface using natural seismic sources, earthquakes and ambient noise, instead of controlled artificial sources, at much lower cost.

Peak ground acceleration

Peak ground acceleration, or PGA, is the largest ground acceleration recorded at a point during an earthquake, usually expressed as a percentage of the acceleration of gravity, g.

Pedotransfer functions

Pedotransfer functions are mathematical models, typically regressions or machine learning, that estimate hard-to-measure soil hydraulic properties from routinely available data like texture and bulk density.

Permafrost

Permafrost is ground, soil, or rock that stays at or below 0 °C for at least two years, underlying much of Alaska, Canada, Siberia, and Antarctica and storing large amounts of carbon.

Persistent scatterer interferometry

Persistent scatterer interferometry, developed at the Politecnico di Milano and first reported in 2000, is a radar technique measuring millimeter-scale ground deformation from stacks of satellite SAR images.

Physical geography

Physical geography, also known as physiogeography, is the branch of geography that studies Earth's natural environment and the processes shaping its surface, including landforms, climate, vegetation, soils, and hydrology.

Piney Woods

The Piney Woods, also called the South Central Plains, is a temperate pine forest ecoregion covering East Texas, southern Arkansas, western Louisiana, and southeastern Oklahoma, and Texas's wettest natural region.

Polar night

Polar night is the winter period north of the Arctic Circle and south of the Antarctic Circle when the Sun does not rise for more than 24 hours.

Polarization analysis (seismology)

Polarization analysis in seismology characterizes the direction and shape of ground particle motion recorded by three-component seismic stations, supporting event detection, direction finding, and filtering.

Pollutant

A pollutant is any substance introduced into the environment that may harm human health or ecosystems, typically from human activities; some, like ground-level ozone, form in the air.

Pollution

Pollution is the introduction of contaminants, whether substances or energy, into the environment at rates that harm health, living systems, or air, water, and soil quality.

Post-perovskite

Post-perovskite is a high-pressure form of magnesium silicate, formed when bridgmanite transforms near Earth's core–mantle boundary; discovered in 2004, it explains the D″ seismic discontinuity.

Pre-stack depth migration

Pre-stack depth migration, or PSDM, is a seismic imaging technique that migrates unstacked traces into a depth image using a velocity-depth model, correctly positioning reflectors under salt and complex structure.

Precise point positioning

Precise point positioning (PPP) is a GNSS technique that estimates centimeter-level positions at a single receiver using precise satellite orbit and clock corrections, without nearby reference stations.

Probabilistic seismic hazard analysis

Probabilistic seismic hazard analysis (PSHA) is a method that estimates the annual frequency of earthquake ground shaking exceeding specified levels at a site, formalized by C. Allin Cornell in 1968.

Quasistatic approximation

The quasistatic approximation is a geophysics modeling simplification that neglects inertial terms in the momentum balance, underpinning poroelastic consolidation, crustal deformation, ice-sheet flow, and atmospheric dynamics.

Radar interferometry

Radar interferometry (InSAR) is a remote sensing technique that combines radar signals from two antennas, or one antenna on repeated passes, to measure terrain elevation and surface deformation.

Rangeland

Rangeland is land with natural grasses, shrubs, or scattered trees suitable for grazing, managed through ecological processes rather than cultivation, and covering over half of Earth's land.

Rayleigh wave tomography

Rayleigh wave tomography is a seismological imaging method that measures Rayleigh wave dispersion along earthquake–station paths and inverts it for 3-D shear-wave velocity models of the crust and upper mantle.

Receiver function

A receiver function is a time series computed from teleseismic seismograms by deconvolution, imaging crustal and upper-mantle discontinuities such as the Moho beneath a seismic station.

Reflection seismology

Reflection seismology, also called seismic reflection, is a geophysical method that maps subsurface rock layers from reflected seismic waves and is the petroleum industry's principal imaging tool.

Renewable resource

A renewable resource, also called a flow resource, is a natural resource like water, forests, fish, or sunlight that regenerates on a timescale relevant to human use.