Page 103 - 4571

Intrusive rock Igneous rock that formed below Earth’s surface.
J
Joint A fracture in rock along which there has been no movement.
L
Laccolith A massive, concordant igneous body intruded between preexisting strata.
Lahar Mudflows on the slopes of volcanoes that result when unstable layers of ash and
debris become saturated and flow downslope, usually following stream channels.
Laminar flow The movement of water particles in straight-line paths that are parallel to
the channel. The water particles move downstream without mixing.
Lava Magma that reaches Earth’s surface.
Lava dome A bulbous mass associated with an old-age volcano, produced when thick
lava is slowly squeezed from the vent. Lava domes may act as plugs to deflect
subsequent gaseous eruptions.
Law of superposition In any undeformed sequence of sedimentary rocks or surface
deposited igneous materials, each layer is older than the one above it.
Light silicate Silicate minerals that lack iron and/or magnesium. They are generally
lighter in color and have lower specific gravities than dark silicates.
Lithification The process, generally by cementation and/or compaction, of converting
sediments to solid rock.
Lithosphere The rigid outer layer of Earth, including the crust and upper mantle.
Lower mantle The part of the mantle that extends from a depth of 660 km to the top of
the core, at a depth of 2900 km.
Luster The appearance or quality of light reflected from the surface of a mineral.
M
Mafic Because basaltic rocks contain a high percentage of ferromagnesian minerals,
they are also called mafic (from magnesium and ferrum, the Latin name for iron).
Magma A body of molten rock found at depth, including any dissolved gases and
crystals.
Magnitude (earthquake) The total amount of energy released during an earthquake.
Mantle The 2885-km thick layer of Earth located below the crust.
Mass wasting The downslope movement of rock, regolith, and soil under the direct
influence of gravity.
Maturity is the degree to which petroleum generation has occurred in a source rock.
Mechanical weathering The physical disintegration of rock, resulting in smaller
fragments.
Metamorphic rock Rock formed by the alteration of preexisting rock deep within
Earth (but still in the solid state) by heat, pressure, and/or chemically active fluids.
Metamorphism The changes in mineral composition and texture of a rock subjected to
high temperature and pressure within Earth.
Migration is the vertical and lateral flow of the petroleum from the source rock.
Mineral A naturally occurring, inorganic crystalline material with a unique chemical
structure.
Mineral resource All discovered and undiscovered deposits of a useful mineral that can
be extracted now or at some time in the future.
Mohs scale A series of 10 minerals used as a standard in determining hardness.
N
Nebular theory A model for the origin of the solar system that assumes a rotating
nebula of dust and gases that contracted to produce the Sun and planets.
Nonconformity An unconformity in which older metamorphic or intrusive igneous
rocks are overlain by younger sedimentary strata.
Nonmetallic mineral resource Mineral resource that is not a fuel or processed for the
metals it contains.

102