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.