Glossary Starting with the letter: P


Primary or compressional seismic wave. It is the fastest of the seismic waves, traveling 5.5–7.2 kilometers per second in the crust and 7.8–8.5 kilometers per second in the upper mantle.

Pacific Rim

The countries and landmasses surrounding the Pacific Ocean.


The past climate.


The study of the plants and animals of the past.


The study of past earthquakes.


The method or substance used to determine temperatures at a given time in the past.


The colored film or thin layer on the surface of a rock produced by chemical weathering.


A coarse-grained igneous or metamorphic rock composed primarily of olivine, with or without other minerals such as pyroxenes, amphiboles, or micas.


Permanently frozen soil or subsoil, occurring throughout the polar regions and locally in perennially frigid areas. Its thickness ranges from thirty centimeters to over 1,000 meters and underlies approximately one-fifth of Earth’s land area.


The mineral CaTiO3. At high pressure the mineral pyroxene (MgSiO3) transforms to a form having the same structure, or arrangement of atoms, as perovskite. This MgSiO3 perovskite makes up most of the lower mantle.

Photic zone

The layer in a body of water or aquatic system that is penetrated by sunlight sufficient for photosynthesis to occur, extending as much as 150 meters below the surface.


Chemical decomposition of a substance into simpler units as a result of its absorbing light or other radiant energy.


The chemical process in which green plants (and blue-green algae) make carbohydrates from carbon dioxide and water using sunlight or light as an energy source. Most forms of photosynthesis release oxygen as a byproduct, the chief source of atmospheric oxygen.

Physical Weathering

The processes that mechanically break up rocks into fragments, such as the movement of water, wind and ice, and frost action. Physical weathering is also known as mechanical weathering.


The study of matter, energy, motion, force, and their interrelationships.


Any of innumerable small bodies or satellites that are the precursors of a planet during the early stages of the solar system. A planetesimal can also be the fragmented result of a cataclysmic collision between a planet and another celestial body.


An electronically neutral, highly ionized gas composed of ions, electrons, and neutral particles. It is a state of matter distinct from solids, liquids, and normal gases that exists in extreme heat.


One of several large, mobile pieces of the Earth’s lithosphere adjoining other plates along zones of seismic activity.

Plate Spreading

The separation of two lithospheric plates, as occurs at the mid-ocean ridge.

Plate Tectonics

The theory and study of plate formation, movement, interaction and destruction; the attempt to explain seismicity, volcanism, mountain-building, and paleomagnetic evidence in terms of plate motions.


A relatively elevated, comparatively level expanse of land with at least one abruptly steep side. It is higher than a plain and more expansive than a mesa.


The earlier of the two epochs of the Quaternary Period, ending around 11,000 years ago and characterized by the alternate appearance and recession of northern glaciation and the appearance of progenitors of human beings.

Plumb Bob

A pointed, tapering weight attached to a plumb line, used to measure the verticality of objects.


A narrow, focused upwelling of unusually hot solid rock moving upward through the Earth’s mantle.


The largest division (ninety percent) of geologic time, from 4.55 to 0.55 billion years ago. It precedes the Cambrian Period and is subdivided into the Hadean, Archean and Proterozoic Eons.


The transfer of moisture from the atmosphere to the surface of Earth, usually as rain, snow, and ice.


Happening first in a sequence of time. Belonging to or remnant of the first stage of development, as in the formation of the Earth.

Proterozoic Era

The more recent of three divisions of the Precambrian, from 2.5 billion to 543 million years ago.


A stable, positively-charged atomic particle found in the nuclei of matter.


A contracting gas cloud which was the earliest stage of the Sun’s formation. This stage took place before the temperature and pressure of the interior became so high as to cause thermonuclear reactions.


Light-colored, porous, glassy fragments of lava typically with the composition of rhyolite.


A common, brass yellowish-white, metallic, cubic mineral made of iron and sulfur (FeS2). It is used to produce sulfur dioxide for sulfuric acid. Popularly known as “fool’s gold,” pyrite has metallic luster and has been mistaken for gold, which is more yellow, softer, and heavier than pyrite.


A molten fragment of pumice and ash ejected during a volcanic eruption.

Pyroclastic Flow

A high speed avalanche of hot volcanic ash, rock fragments, and gas.