Glossary
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laccolith
a blister-shaped intrusion that forms when sills dome upward
lacustrine
lake sediments
lag deposit
The coarse sediment left behind in a desert after wind erosion removes the finer sediment.
lagoon
A body of shallow seawater separated from the open ocean by a barrier island.
lahar
A thick slurry formed when volcanic ash and debris mix with water, either in rivers or from rain or melting snow and ice on the flank of a volcano.
laminar flow
The flow pattern in which all parcels of water follow parallel paths and flow downstream.
landslide
A sudden movement of rock and debris down a nonvertical slope.
landslide-potential map
A map that ranks regions according to the likelihood that a mass movement will occur.
landslide-potential map
A map that ranks regions according to the likelihood that a mass movement will occur.
land subsidence
Sinking elevation of the ground surface; the process may occur over an aquifer that is slowly draining and decreasing in volume because of pore collapse.
La Niña
Years in which the El Niño event is not strong.
lapilli
Marble-to-plum-sized fragments of pyroclastic debris.
Laramide orogeny
The mountain-building event that lasted from about 80 Ma to 40 Ma, in western North America; in the United States, it formed the Rocky Mountains as a result of basement uplift and the warping of the younger overlying strata into large monoclines.
latent heat of condensation
The heat released during condensation, which comes only from a change in state.
lateral moraine
A strip of debris along the margins of a glacier.
laterite soil
Soil formed over iron-rich rock in a tropical environment, consisting primarily of a dark-red mass of insoluble iron and/or aluminum oxide.
Laurentia
A continent in the early Paleozoic era composed of today’s North America and Greenland.
Laurentide ice sheet
An ice sheet that spread over northeastern Canada during the Pleistocene ice age.
lava
Molten rock that has flowed out onto the Earth’s surface.
lava dome
A dome-like mass of rhyolitic lava that accumulates above the eruption vent.
lava flows
Sheets or mounds of lava that flow onto the ground surface or sea floor in molten form and then solidify.
lava lake
A large pool of lava produced around a vent when lava fountains spew forth large amounts of lava in a short period of time.
lava tube
The empty space left when a lava tunnel drains; this happens when the surface of a lava flow solidifies while the inner part of the flow continues to stream downslope.
leach
To dissolve and carry away.
leader
A conductive path stretching from a cloud toward the ground, along which electrons leak from the base of the cloud, and which provides the start for a lightning flash to the ground.
lightning flash
A giant spark or pulse of current that jumps across a gap of charge separation.
light year
The distance that light travels in one Earth year (about 6 trillion miles or 9.5 trillion km).
lignite
Low-rank coal that consists of 50% carbon.
limb
The side of a fold, showing less curvature than at the hinge.
limestone
Sedimentary rock composed of calcite.
liquefaction
the process that occurs when the motion of an earthquake causes clay-rich sediment to become a slurry of clay and water.
liquification
The process in which clay flakes in wet sediment unstick from one another in clay-rich sediments so that the sediment becomes a slurry of mud and water; liquification may be triggered by earthquake vibrations.
lithification
The transformation of loose sediment into solid rock through compaction and cementation.
lithologic correlation
A correlation based on similarities in rock type.
lithosphere
The relatively rigid, nonflowable, outer 100–150-thick layer of the Earth; comprising the crust and the top part of the mantle.
little ice age
A period of cooler temperatures between 1500 and 1800 c.e.
local base level
A base level upstream from a drainage network’s mouth.
lodgment till
A flat layer of till smeared out over the ground when a glacier overrides an end moraine as it advances.
loess
Layers of fine-grained sediments deposited from the wind; large deposits of loess formed from fine-grained glacial sediment blown off outwash plains.
longitudinal (seif) dune
A dune formed when there is abundant sand and a strong, steady wind, and whose axis lies parallel to the wind direction.
longitudinal profile
A cross-sectional image showing the variation in elevation along the length of a river.
longshore current
A current that flows parallel to a beach.
lower mantle
The deepest section of the mantle, stretching from 670 km down to the core-mantle boundary.
low-grade metamorphic rocks
Rocks that metamorphose under relatively low temperatures.
low-velocity zone
The asthenosphere underlying oceanic lithosphere in which seismic waves travel more slowly, probably because rock has partially melted.
luster
The way a mineral surface scatters light.
L-waves
Surface seismic waves that cause the ground to ripple back and forth, creating a snake-like movement.