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Plate tectonics The theory that proposes Earth’s outer shell consists of individual
plates, which interact in various ways and thereby produce earthquakes, volcanoes,
mountains, and the crust itself.
Porosity is the volume of open spaces in rock or soil.
Porphyritic texture An igneous rock texture characterized by two distinctively
different crystal sizes. The larger crystals are called phenocrysts, and the matrix of
smaller crystals is termed the groundmass.
Pyroclastic flow A highly heated mixture, largely of ash and pumice fragments,
travelling down the flanks of a volcano or along the surface of the ground.
R
Rainshadow desert A dry area on the lee side of a mountain range; many middle-
latitude deserts are of this type.
Regolith The layer of rock and mineral fragments that nearly everywhere covers Earth’s
land surface.
Relative dating Rocks are placed in their proper sequence or order; only the
chronological order of events is determined.
Reservoir rock is a rock that can both store and transmit fluids.
River A general term for a stream that carries a substantial amount of water and has
numerous tributaries.
Rock A consolidated mixture of minerals.
Rock cycle A model that illustrates the origin of the three basic rock types and the
interrelatedness of Earth’s materials and processes.
Rockslide The rapid slide of a mass of rock downslope along planes of weakness.
Runoff Water that flows over the land rather than infiltrating into the ground.
S
Saturation is the relative amount of the water and oil or gas sharing the pores of the
reservoir which vary from reservoir to reservoir.
Seafloor spreading The hypothesis first proposed in the 1960s by Harry Hess,
suggesting that new oceanic crust is produced at the crests of mid-ocean ridges, which
are the sites of divergence.
Sediment Unconsolidated particles created by the weathering and erosion of rock, by
chemical precipitations from solution in water, or from the secretions of organisms, and
transported by water, wind, or glaciers.
Sedimentary rock Rock formed from the weathered products of preexisting rocks that
have been transported, deposited, and lithified.
Seismic contractor is a company that owns and operates the seismic equipment and
runs the seismic survey
Sheeting A mechanical weathering process characterized by the splitting off of slablike
sheets of rock.
Shield A large, relatively flat expanse of ancient metamorphic rock within the stable
continental interior.
Shield volcano A broad, gently sloping volcano built from fluid basaltic lavas.
Silicate Any one of numerous minerals that have the siliconoxygen tetrahedron as their
basic structure.
Slide A movement common to mass-wasting processes in which the material moving
downslope remains fairly coherent and moves along a well-defined surface.
Slip face The steep, leeward surface of a sand dune that maintains a slope of about 34
degrees.
Slump The downward slipping of a mass of rock or unconsolidated material moving as
a unit along a curved surface.
Snowfield An area where snow persists year-round.
Source rock is a rock that can generate natural gas and/or crude oil.

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