P wave
The fastest of the seismic body waves generated by an earthquake. The ‘P’ stands for primary and its particle movement is parallel to the direction of the propagation of the wave. Also called compressional, push-pull or longitudinal wave.
Paleoseismicity
Refers to earthquakes recorded geologically, most of them unknown from seismograms. Geologic records of past earthquakes can include faulted layers of sediment and rock, injections of liquefied sand, landslides, abruptly raised or lowered shorelines and tsunami deposits.
Paleotsunami
A tsunami that occurs prior to the written records of civilized history.
Pelean style eruption
A type of eruption typified by phreatic/phreatomagmatic explosions followed by dome growth which is continually interrupted by pyroclastic emissions through and around the dome, and gravitational collapse of the dome. The type example of this eruption is the 1902-05 eruption of Mt. Pelée in Martinique.
Petrology
The study of the composition, occurrence, and origin of rocks
Phenocryst
A usually large crystal embedded in igneous rock.
Phreatomagmatic eruptions
Phreatomagmatic eruptions occur when magma comes into contact with water causing the water to flash to steam. The expanding steam disrupts not only the pre-existing solid rock but also the magma itself so that the fragments thrown out are a mixture of broken-up old rocks and fragmented magma.
Phreatic eruptions
Phreatic eruptions occur when confined, sub-surface geothermal waters are heated to temperatures above their boiling point and flash to steam, thereby expanding to form an explosion. Such eruptions eject abundant hot steam, hot water, mud and old rock debris in to the air. In some cases the mud and water ejected may be acidic. No new magma is involved in a phreatic eruption, although the heat needed to flash water to steam and thus generate a phreatic eruption is often provided by an underlying magma body.
Pillow Lave
Interconnected, sack-like bodies of lave formed underwater.
Plate tectonics
The theory supported by a wide range of evidence that considers the earth’s crust to be composed of the several large slabs of relatively rigid rocks called tectonic plates. These plates move relative to one another due to the convectional currents in the mantle below. Along the boundaries of these plates, different interactions occur leading to either convergence, divergence, subduction and or transform faults.
Plinian
An explosive eruption in which steady, turbulent stream of fragmented magma and magmatic gases is released at a high velocity from a vent. Large volumes of tephra and tall eruption columns are characteristic of these types of eruptions.
Plug
Solidified lava that fills the conduit of a volcano. It is usually more resistant to erosion than the material making up the surrounding cone and can remain standing after the original structure has eroded away. E.g. The Pitons, Saint Lucia.
Plug Dome
The steep-sided, rounded mound formed when viscous lava wells up into a crater and is too rigid to flow away.
Precambrian
All geologic time from the beginning of Earth history to 570 million years ago. Also refers to the rocks that formed in that epoch.
Prediction (of earthquakes)
The forecasting in time, place, and magnitude of an earthquake. Currently, there is no reliable method of predicting earthquakes.
Prediction (of volcanoes)
A comparatively precise statement of the time, place and, ideally, the nature and size of impending activity. A prediction usually covers a shorter time period than a forecast and is generally based dominantly on interpretations and measurements of ongoing processes and secondarily on a projection of past history.
Probability of exceedance
The odds that the size of a future earthquake will exceed some specified value.
Pumice
Light-colored, frothy volcanic rock, formed by the expansion of gas in erupting lava. Commonly seen as lumps or fragments of pea-size or larger. Can also float when placed in water.
Pyroclastic
The word pyroclastic is derived from two Greek words meaning “fire” and “broken”. It refers to rocks which have been broken into fragments by the action of heat. The smallest fragments (less then 2 mm in diameter) are called ash. Fragments between 1mm and 1 cm in diameter are called lapilli. Larger fragments are called ‘blocks’ if they were solid at the time they were fragmented and ‘bombs’ if they were liquid.
Pyroclastic flow
A hot (100-600oC) fast-moving (>100 km/hr) mixture of ash and pumice fragments in a turbulent gas cloud travelling down the flanks of the volcano or along the ground surface. Such flows form when an eruption column or a lava dome collapses. They usually travel down valleys and cause total devastation of the area over which they flow. People in the path of a hot pyroclasticflow can be killed by asphyxiation, heat and noxious gases.
Pyroclastic surge
A highly-fluidised, low density, fast-moving turbulent flow. These are cold and wet when they form at the base of an eruption column (base surge) or hot and dry when they form on top or
at the front of a pyroclastic flow (ash-cloud and ground surge, respectively).
