The arrangement of the continents; ancient geography.
paleontologist
A scientist who studies Earth’s past life forms.
parallax
A method used by astronomers to calculate the distance to nearby stars, using the apparent shift relative to distant stars.
parasitism
A symbiotic relationship between two species in which one species benefits and one species is harmed.
parent isotope
An unstable isotope that will undergo radioactive decay.
partial melting
The melting of some, but not all, of the minerals in a rock, depending on temperature.
pathogen
Disease causing organisms.
pedalfer
Fertile, dark soil that forms in mid latitude, forested regions.
pedocal
Less fertile soil that forms in drier, grassland regions.
penumbra
Outer part of shadow that remains partially lit during an eclipse.
permafrost
Permanently frozen ground that is found in the polar regions.
permeability
The interconnectedness of the pores within a rock or sediment.
permeable
A material with interconnecting holes so that water can move through it easily.
permineralization
Fossilization in which minerals in water deposit into empty spaces in an organism.
pesticide
A chemical that kills a certain pest that would otherwise eat or harm plants that humans want to grow.
pH scale
A scale that measures the acidity of a solution. A pH of 7 is neutral. Smaller numbers are more acidic and larger numbers are more alkaline.
photic zone
The upper 200 m of the ocean, where sunlight penetrates.
photochemical smog
This type of air pollution results from a chemical reaction between pollutants in the presence of sunshine.
photon
A particle of light.
photosphere
The visible surface of the Sun.
photosynthesis
The process in which plants produce simple sugars (food energy) from carbon dioxide, water, and energy from sunlight. Photosynthesis uses carbon dioxide and releases oxygen.
physical model
A physical representation of an object or system.
phytoplankton
Tiny plants that photosynthesize and create food energy and oxygen.
placer
Valuable metal found in modern or ancient stream gravels.
planet
A round celestial object orbiting a star that has cleared its neighboring region of planetesimals.
planetary rings
Rings of dust and rock encircling a planet in a thin plane.
plankton
A diverse group of tiny animals and plants that freely drift in the water.
plasma
A high energy, high temperature form of matter. Electrons are removed from atoms, leaving each atom with a positive electrical charge.
plastic deformation
Strain in which the rock deforms but does not return to its original shape when the strain is removed.
plate
A slab of Earth’s lithosphere that can move around on the planet’s surface.
plate boundary
A location where two plates come together.
plate tectonics
The theory that the Earth’s surface is divided into lithospheric plates that move on the planet’s surface. Plate tectonics is driven by convection currents within Earth’s mantle.
platform
A craton and its overlying younger sedimentary rocks.
plucking
Removal of blocks of underlying bedrock as meltwater seeps into cracks and freezes.
pluton
An igneous intrusive rock body that has cooled in the crust.
polar front
The meeting zone between cold continental air and warmer subtropical air at around 50oN and 50oS.
polar molecule
A molecule with an unevenly distributed electrical charge.
polar orbit
A satellite orbit that goes over the North and South Poles, perpendicular to Earth’s spin.
polar orbit satellite
Orbit that moves over Earth’s North and South poles as Earth rotates underneath so that the entire planet can be viewed in less than one day.
polar stratospheric clouds (PSC)
Clouds that form in the stratosphere when it is especially cold; PSCs are necessary for the breakup of chlorofluorocarbons (CFCs).
pond
A small body of freshwater, with no stream draining it; fed by an underground spring.
pool
A deep, slow-moving part of a stream, usually wider than elsewhere on the stream.
population
All the individuals of a species that occur together in a given place and time.
porosity
The small holes that exist between grains in a rock or sediment.
porphyritic
Igneous rock texture in which visible crystals are found in a matrix of tiny crystals.
potential energy
Energy stored within a physical system that has the potential to do work.
precipitate
Solid substance that separates out of a liquid to form a solid, usually when the liquid evaporates.
precipitation
Water that falls from the sky as rain, snow, sleet, or hail.
predator
An animal that kills and eats other animals.
prey
An animal that could be killed and eaten by a predator.
primary productivity
The creation of food energy.
producer
An organism that converts energy into chemical energy that it can use for food. Most producers use photosynthesis but a very small number use chemosynthesis.
projection
A way to represent a 3-dimensional surface in two dimensions.
prokaryote
An organism that lacks a cell nucleus or membrane-bound organelles.
proton
A positively charged particle in a nucleus.
pyroclastic flow
Hot ash, gas, and rock that race down a volcano’s slopes during an eruption.