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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.
Soil A combination of mineral and organic matter, water, and air; that portion of the regolith that
supports plant growth.
Solar nebula The cloud of interstellar gas and/or dust from which the bodies of our solar system
formed.