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Ocean trench
Ocean trench
A deep, steep-sided trough in
the ocean floor formed where a slab, or plate, containing oceanic crust pushes beneath another plate. Most ocean trenches are found parallel to, and just offshore of, strings of islands (island arcs). These trenches are the deepest places in the oceans. (See also: Subduction zone.)
Olivine
A group of magnesium iron silicate minerals that have an olive color. They mostly form small crystals
in dark-colored igneous rocks such as basalt.
P
Pacific Ring of Fire
The ring of volcanoes and volcanic activity that circles the Pacific Ocean (see: Island arc). The pattern is created by the collision of the Pacific Ocean plate with its neighboring plates.
Volcanoes along the Ring of Fire are found throughout the Andes Mountains, in western North America, on the islands off the coast of Asia, such as Japan and the Philippines, and across Indonesia and Papua New Guinea to New Zealand.
Because so many of the volcanoes are active, and therefore send out plumes of fiery lava, the ring of volcanoes is often called the Pacific Ring of Fire.
The pattern of volcanoes provides many clues about the way that the Earth works. Because volcanoes only erupt where there is a line of weakness in the
crust, the pattern of volcanoes identifies major lines of weakness in the crust. The pattern around the Pacific Ocean provides evidence
that one of the Earth’s great plates lies under the Pacific Ocean. The Ring of Fire is also a place
of intense earthquake activity— 90% of the world’s earthquakes occur on the Ring of Fire.
Pahoehoe lava
A form of basalt lava that flows in thin sheets and has a smooth surface. Compare with aa lava.
Lava moves fairly quickly.
 Pahoehoe lava—This form of basaltic lava forms a smooth surface.
A thin crust forms on the surface.
Parasitic cone
A side cone on a volcano.
Pegmatite
An igneous rock (for example, a dike) of extremely coarse crystals.
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 Pacific Ring of Fire—The Ring of Fire includes both island arcs and continental mountain ranges, active volcanoes, and frequent earthquakes. On this map the pattern of the Ring of Fire is marked by earthquakes.
Depth of earthquake
0–69km 70–299km 300–800km