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Plutonic rock
Plutonic rock
An igneous rock that has solidified at great depth and contains large crystals due to the slowness of cooling. The name comes from Pluto, the Roman god of the underworld.
Intrusive rocks may cool relatively quickly if they are simply thin sheets of magma squeezed between layers of cold rock. This happens when dikes and sills are formed. Diorite is an example of this kind of rock. Large masses
of magma—for example, the magma chambers that are
the sources of volcanoes—cool very slowly at the end of their active lives, and that gives time for crystals to grow larger. Granite is an example of this kind of plutonic rock.
Pumice
Any volcanic material that contains so many gas bubbles that it can float on water. Some pumice is thrown out by volcanic eruption; some is formed on the surface of lava flows.
P wave, primary wave, primary seismic wave
A body wave produced during
an earthquake. P waves travel through the ground from the source of the earthquake and shake the surface to and fro in the same direction as the wave is moving (see: First motion).
P waves are the fastest body waves. They can travel through
all layers of the Earth and are generally felt at the surface as a thump before the main earthquake. They give a few seconds notice
of the arrival of larger surface waves. (See also: Seismic waves and S wave.)
Pyroclastic
A general term used to describe anything that is thrown out of
a volcano during an explosive eruption; pyro means “fire” and clast means “broken piece.”
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Pyroclastic flow
Solid material ejected from a volcano, combined with searingly hot gases, which together behave as a liquid, moving very fast and hugging the ground. Pyroclastic flows can do immense damage,
as was the case with Mount
St. Helens (see: Eruption and Glowing avalanche).
N
Blast
 Pyroclastic flow—These pictures show the 1980 pyroclastic flow during the eruption of Mount St. Helens, Washington State. The blast zone was to the right (north), as shown in the inset diagram. Notice that the area to the left of the cone is undisturbed, while a great piece has been blown out of the right-hand side of the cone (a new plug of lava has formed in the center of the cone). A large area of forest was flattened and covered in pyroclastic material.