Laccolith – Sill-like igneous intrusion that forces apart two strata and forms a round, lens-shaped body many times wider than it is thick.
Lag – Distance a nuclear logging probe moves during one time constant.
Lahar – Mudflow of unconsolidated volcanic ash, dust, breccia, and boulders mixed with rain or the water of a lake displaced by a lava flow.
Laminar Flow – Flow regime in which particle paths are straight or gently curved and parallel.
Landslide – Rapid downslope movement of soil and rock material, often combined or under-washed by groundwater over a shear zone.
Lapilli – Fragment of volcanic rock formed when magma is ejected into the air by expanding gases such as those expelled by a volcano. The size of the fragments ranges from granular- to stone-size.
Last Reading – Depth of the shallowest value recorded on a log.
Lateral Log – Multielectrode, resistivity-logging technique that has a much greater radius of investigation than the normal techniques although requires thick beds and produces an unsymmetrical curve.
Lateral Moraine – Moraine formed along the side of a valley glacier and composed of rock removed or fallen from the valley sides.
Laterolog – Resistivity logging technique that is highly focused.
Laurasia – Ancient continent that split apart in Mesozoic time to form Europe, Asia, North America and Greenland.
Laurentia – Ancient North American continent that lay on the west side of the Iapetus, or proto-Atlantic Ocean in Cambrian time, approximately 500 million years ago.
Lava – Molten, liquid rock that comes out of a volcano, has reached the surface, and solidified by cooling. The rocks that form from this molten lava are classified as extrusive.
Lava Tube – Sinuous, hollow tunnel formed when the outside of a lava flow cools and solidifies and the molten material passing through it is drained away.
Leach – Method of extracting gold from ore by a chemical solution usually containing cyanide.
Leach Pad – Large, impermeable foundation or pad used as a base for ore during heap leaching. The pad prevents the somewhat poisonous leach solution from escaping out of the circuit and into the surrounding soil.
Leaching – Chemical process for the extraction of minerals such as gold and other precious metals from a soil by dissolution in water moving downward in the ground.
Left-Lateral Fault – Strike-slip fault on which the displacement of the far block is to the left when viewed from either side.
Lens – Body of ore or rock that is thick in the middle and converges toward the edges, resembling a convex lens.
Levee – Low ridge along a stream bank, formed by deposits left when floodwater decelerates on leaving the channel. This can also refer to an artificial barrier to floods built in the same form.
Light-Non-Aqueous Phase Liquids – LNAPLs are organic fluids that are less dense than water and are capable of forming an immiscible layer that floats on the water table.
Limb – Relatively planar part of a fold or of two adjacent folds.
Limestone – Sedimentary rock type composed mostly of calcium carbonate which was formed by chemical precipitation from sea water or by accumulation of fossils with carbonate shells.
Limy – Sediments, soils, or rocks that contain a significant amount of lime..
Lineation – Linear arrangement of features found in a rock.
Lithification – Processes that convert a sediment into a sedimentary rock.
Lithology – Systematic description of rocks, in terms of mineral composition and texture.
Lithosphere – Outer rigid shell of the earth, situated above the asthenosphere and containing the crust, continents, and plates.
Lode – Large vein or set of veins containing concentrations of valuable ore minerals.
Loess – Widespread loose deposit consisting mainly of silt. The majority of loess deposits formed during the Pleistocene time as an accumulation of wind-blown dust carried from deserts, alluvial plains, or glacial deposits.
London Metals Exchange – LME is the world’s single most important metals exchange for copper and other base metals.
Long Normal Log – Resistivity log with AM spacing usually 64 in.
Longitudinal Dune – Long dune parallel to the direction of the prevailing wind.
Longitudinal Profile – Cross section of a stream from its mouth to its head displaying elevation versus distance to the mouth.
Longshore Current – Current that moves parallel to a shore and is formed from the momentum of breaking waves that approach the shore obliquely.
Longshore Drift – Movement of sediment along a beach by swash and backwash of waves that approach the shore obliquely.
Long Term Price – Price for product sold or purchased under contract for multiple deliveries beginning and ending at specified dates.
Lopolith – Large bowl-shaped laccolith that is depressed in the center, possibly by subsidence of an emptied magma chamber beneath the intrusion.
Lowland – Land of general low relief at the lower levels of regional elevation.
Low-Velocity Zone – Area in the earth such as a planar layer that has lower seismic-wave velocities than the area immediately above it.
Luster – General textural impression of a mineral surface affected by the light reflected from it. Often leads to references such as metallic.