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Numerical date Date that specifies the actual number of years that have passed since an
event occurred.
O
Oceanic ridge A continuous mountainous ridge on the floor of all the major ocean
basins and varying in width from 500 to 5000 km. The rifts at the crests of these ridges
represent divergent plate boundaries.
Offshore zone The relatively flat submerged zone that extends from the breaker line to
the edge of the continental shelf.
Oil field brine is very salty water that shared the pores with the oil.
Oil-water contact is the boundary between the oil and water reservoir.
Oil trap A geologic structure that allows for significant amounts of oil and gas to
accumulate.
Ore Usually a useful metallic mineral that can be mined at a profit. The term is also
applied to certain nonmetallic minerals such as fluorite and sulfur.
Organic sedimentary rock Sedimentary rock composed of organic carbon from the
remains of plants that died and accumulated on the floor of a swamp. Coal is the
primary example.
Outer core A layer beneath the mantle about 2270 km thick that has the properties of a
liquid.
Outlet glacier A tongue of ice normally flowing rapidly outward from an ice cap or ice
sheet, usually through mountainous terrain to the sea.
P
Pahoehoe A lava flow with a smooth-to-ropy surface.
Paleoclimatology The study of ancient climates; the study of climate and climate
change prior to the period of instrumental records using proxy data.
Paleontology The systematic study of fossils and the history of life on Earth.
Pangaea The proposed supercontinent that 200 million years ago began to break apart
and form the present landmasses.
Parent rock The rock from which a metamorphic rock formed.
Partial melting The process by which most igneous rocks melt. Because individual
minerals have different melting points, most igneous rocks melt over a temperature
range of a few hundred degrees. If the liquid is squeezed out after some melting has
occurred, a melt with a higher silica content results.
Pegmatite A very coarse-grained igneous rock (typically granite) commonly found as a
dike associated with a large mass of plutonic rock that has smaller crystals.
Crystallization in a waterrich environment is believed to be responsible for the very
large crystals.
Pegmatitic texture A texture of igneous rocks in which the interlocking crystals are all
larger
than one centimeter in diameter.
Peridotite An igneous rock of ultramafic composition thought to be abundant in the
upper mantle.
Permeability is a measure of the ease with which a fluid can flow through a rock.
Phaneritic texture An igneous rock texture in which the crystals are roughly equal in
size and large enough so that individual minerals can be identified with the unaided eye.
Physical geology A major division of geology that examines the materials of Earth and
seeks to understand the processes and forces acting upon Earth’s surface from below.
Pillow lava Basaltic lava that solidifies in an underwater environment and develops a
structure that resembles a pile of pillows.
Pipe A vertical conduit through which magmatic materials have passed.

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