Glossary of Rock and Mineral Terms - L

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

labradorescence

Effect which causes dark, metallic-like color shimmers, commonly blue and green, to be displayed on a few minerals. The name is derived from Labradorite, a mineral which is the best example of this effect.

laccolith

A large concordant pluton that is shaped like a dome or a mushroom. Laccoliths tend to form at relatively shallow depths and are typically composed of granite. The country rock above them often erodes away completely.

lahar

A flow of pyroclastic material mixed with water. A lahar is often produced when a snow-capped volcano erupts and hot pyroclastics melt a large amount of snow or ice.

lamellar

Synonym of scaly: Aggregate of small, flattened, overlapping crystals, as seen in fish scales.

laminated rock

Sedimentary rocks consisting of many thin layers.

lanthanides

Group of elements, very similar in chemical properties, once thought to be extremely rare. They take up numbers 57 through 71 on the periodical table. Also called rare earth elements.

lapidary

An individual who facets gemstones as a trade, and the shop of such an individual. Also used in adjective form when relating to gems.

lattice

The arrangement of atoms in a crystal, giving each crystal its distinct shape.

lava

Magma that comes to the Earth's surface through a volcano or fissure.

leach

To dissolve from a rock. For example, when acidic water passes through fractured rocks, soluble minerals leach, or dissolve, from the rocks.

lenticular

Lens shaped. When applied to minerals it refers to concretions or nodules that have a flattened, lens-like shape.

levee

A protective barrier built along the banks of a stream to prevent flooding. See also natural levee.

lichen

Plant-like colonies of fungi and algae that grow on the exposed surface of rocks. Lichen grow at a constant rate within a single geographic area.

lichenometry

A method of numerical dating that uses the size of lichen colonies on a rock surface to determine the surface's age. Lich-enometry is used for rock surfaces less than about 9000 years old.

light

Type of electromagnetic radiation that oscillates between electric and magnetic fields, and can be detected by the human eye.

lignite

A soft, brownish coal that develops from peat through bacterial action, is rich in kerogen, and has a carbon content of 70%, which makes it a more efficient heating fuel than peat.

limestone

A sedimentary rock composed primarily of calcium carbonate. Some 10% to 15% of all sedimentary rocks are limestones. Limestone is usually organic, but it may also be inorganic.

liquefaction

The conversion of moderately cohesive, unconsolidated sediment into a fluid, water-saturated mass.

lithification

The conversion of loose sediment into solid sedimentary rock.

lithosphere

A layer of solid, brittle rock making up the outer 100 kilometers of the Earth, encompassing both the crust and the outermost part of the upper mantle. See also asthenosphere.

lithostatic pressure

The force exerted on a rock buried deep within the Earth by overlying rocks. Because lithostatic pressure is exerted equally from all sides of a rock, it compresses the rock into a smaller, denser form without altering the rock's shape.

litmus paper

Paper with powder extracted from certain plants that tests the pH of a substance. Acid turns red while base turns blue; neutral remains white. The stronger the acid or base, the more intense red or blue the color of the litmus paper turns.

locality

Area where a specific mineral was found or occurs.

lode

Vein of precious metal.

loess

A load of silt that is produced by the erosion of outwash and transported by wind. Much loess found in the Mississippi Valley, China, and Europe is believed to have been deposited during the Pleistocene Epoch.

longitudinal dune

One of a series of long, narrow dunes lying parallel both to each other and to the prevailing wind direction. Longi-tudinal dunes range from 60 meters to 100 kilometers in length and from 3 to 50 meters in height.

longshore current

An ocean current that flows close and almost parallel to the shoreline and is caused by the rush of waves toward the shore.

longshore drift

1. The process by which a current moves sediments along a surf zone. 2. The sediments so moved. Longshore drift typically consists of sand, gravel, shell fragments, and pebbles. See also beach drift.

longwave

A wavelength of ultraviolet light. Some minerals display fluorescence when exposed to longwave ultraviolet light. Commonly abbreviated as LW.

lopolith

A saucer-shaped intrusive body of igneous rock. Lopoliths are typically mafic in composition.

loupe

Small magnifying lens used to observe certain aspects of a gemstone. Loupes usually have a magnification level of 10x.

low-velocity zone

An area within the Earth's upper mantle in which both P waves and S waves travel at markedly slower velocities than in the outermost part of the upper mantle. The low-velocity zone occurs in the range between 100 and 350 kilometers of depth.

lubricant

Material that can reduce friction. Used as a coating on objects that are subject to friction.

luminesce

To give off light under certain conditions.

luminescence

General term describing the capability of a mineral with a habit of giving off light when put under certain conditions. Three examples of luminescence are fluorescence, triboluminescence, and thermoluminescence.

luminescent

General term describing any mineral capable of giving off light when put under certain conditions. Three examples of minerals that are luminescent are minerals that exhibit fluorescence, triboluminescence, and thermoluminescence.

luster

1. The reflection of light on a given mineral's surface, classified by intensity and quality.
2. The appearance of a given mineral as characterized by the intensity and quality with which it reflects light.