Volcano

Pahoehoe
Pahoehoe (pronounced "pah-hoy-hoy") is a Hawaiian term for lava that, when solid, has a smooth or rope-like surface.

paleontology
Paleontology is the branch of biology that studies the forms of life that existed in former geologic periods, chiefly by studying fossils.

paleontologist
A paleontologist is a scientist who studies paleontology, the forms of life that existed in former geologic periods, chiefly by studying fossils.

Paleozoic era
The Paleozoic era (540 to 245 million years ago) saw an explosion of new life forms. It ended with the largest mass extinction in history and was followed by the Mesozoic Era.

Pangaea
Pangaea was a supercontinent consisting of all of Earth's land masses. It existed during the Permian and Jurassic period. It began breaking up during the Jurassic period, forming continents Gondwanaland and Laurasia, separated by the Tethys Sea.

Pele's hair
Pele's hair is a Hawaiian term for natural spun glass that can be as much as 2 m long. It is formed from the fountaining of fluid basaltic lava, cascading lava falls, or from turbulent lava flows.

period
The period is the basic unit of geological time in which a single type of rock system is formed, lasting tens of millions of years.

Permian period
Also known as "The Age of Amphibians" (280 to 245 million years ago), this is the time when Pangaea formed and Earth's atmosphere was oxygenated to modern levels. It ended with the largest mass extinction and was followed by the Mesozoic Era. Ancient sharks swam the seas during the Permian period.

phases of matter
Matter can exist in four phases (solid, liquid, gas, and plasma) and a few other extreme phases, like critical fluids and degenerate gases. The phase diagram of water (above) shows its phase at various temperatures and pressures.

pillow lava
Pillow lava is lava that cools under the sea, with an interconnected, sack-like form.

pipe
A pipe is a vertical passageway that runs through the Earth's crust under a volcano. Pipes are formed as magma (molten rock) travels upward through the crust. Pipes are often filled with breccia and other rock debris.

plates
The crust of the Earth is broken into plates. The plates are enormous chunks of rock that float atop the soft mantle. The plates are moving at a speed that has been estimated at 1 to 10 cm per year. Oceanic plates (those that are under the ocean) are thinner and denser than continental plates.

plate tectonics
Plate tectonics is the now-established theory that chunks of the Earth's crust (plates) float on the surface and change both position and size over time.

plutonic formation
A plutonic formation (also called a laccolith or an igneous intrusion) is a formation in which magma (molten rock) is trapped beneath the surface of the Earth and pushes the rock located above it into a dome shape. It has a flat base and a convex upper surface. The magma cools and solidifies, and eventually, it is exposed (as the fractured sedimentary rock above it erodes away).

plug
A plug (also called a volcanic neck) is solidified lava that fills the conduit of a volcano.

Precambrian
The Precambrian is the time 540 before the Cambrian period (before million years ago). It is the time from when the Earth formed until simple life-forms evolved.

precipitation
Precipitation is rain, sleet, hail, snow, or other condensation products falling from the atmosphere. Precipitation on Earth is is key in the water cycle.

prehistoric
Prehistoric refers to the time before people began recording history in writing. This time varies from culture to culture.

pumice
Pumice is a light-weight, spongy rock that is formed in explosive volcanic eruptions. This igneous rock is light and porous, containing a network of gas bubbles in volcanic glass and minerals. Pumice can form from any types of magma, including basalt, andesite, dacite, and rhyolite.

pyroclastic
Pyroclastic (meaning "fire fragmented") refers to broken-up rocks, pumice, ash, and other bits of material that are formed in a volcanic eruption.

pyroclastic flow
A pyroclastic flow is an avalanche of pyroclastic materials (broken rocks, pumice, and ash) and hot gases that erupts from within a volcano. A pyroclastic flow travels at up to 100 miles per hour. Within the flow, temperatures can reach 500 degrees C.