jetty
A structure built along the bank of a stream channel or tidal outlet to direct the flow of a stream or tide and keep the sediment moving so that it cannot build up and fill the channel. Jetties are typically built in parallel pairs along both banks of the channel. Jetties that are built perpendicular to a coast tend to interrupt longshore drift and thus widen beaches.
joint
A fracture dividing a rock into two sections that have not visibly moved relative to each other. See also fault.
juvenile water
The steam that accompanies volcanic eruptions.
karst
A topography characterized by caves, sinkholes, disappearing streams, and underground drainage. Karst forms when groundwater dissolves pockets of limestone, dolomite, or gypsum in bedrock.
kerogen
A solid, waxy, organic substance that forms when pressure and heat from the Earth act on the remains of plants and animals. Kerogen converts to various liquid and gaseous hydrocarbons at a depth of 7 or more kilometers and a temperature between 50º and 100ºC.
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.
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.
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.
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.
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
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.
lopolith
A saucer-shaped intrusive body of igneous rock. Lopoliths are typically mafic in composition.
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.
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.
