La Niña
The movement of colder-than-normal surface waters across the Pacific towards Indonesia that sometimes follows in the year after an El Niño event.
Lahar
A fast-moving mudflow of unconsolidated volcanic ash, dust, breccia, and boulders mixed with rain, melting ice, or the water of a lake displaced by a lava flow.
Landslide
Any perceptible downslope movement of a mass of bedrock or unconsolidated rock, sand and dirt, or a mixture between the two.
Latitude
Imaginary lines that allow measurement of position north or south of the equator (“horizontal”). Latitude is measured in degrees (at the equator one degree = 60 nautical miles, or 111 kilometers). The equator is at a latitude of 0° and the poles lie at latitudes of 90° north (North Pole) or 90° south (South Pole).
Lava
Molten rock that erupts onto the Earth’s surface through a volcanic vent or fissure.
Lava Dome
A dome-shaped mass of sticky, gas-poor lava erupted from a volcanic vent, often following a major eruption.
Lehmann Discontinuity
The boundary between the Earth’s solid inner core and liquid outer core. Named for the Danish seismologist Inge Lehmann.
Limestone
A common sedimentary rock consisting mostly of calcium carbonate, predominantly formed from the skeletons of marine organisms, calcareous sand, microorganisms, shell fragments, and coral.
Lithosphere
The solid, outermost shell of the Earth (~100 km thick), where rocks are more rigid than those below. The lithosphere is made up of the uppermost mantle and the crust.
Lodestone
A piece of naturally occurring magnetic iron oxide (Fe3O4, the mineral magnetite).
Longitude
Imaginary lines that wrap around Earth intersecting at the north and south geographic poles. Lines of longitude are numbered from 0° (Greenwich Meridian, passing through London, England) to 180°. Longitudes are designated east if they fall east of the Greenwich Meridian, and west if they fall west of the Greenwich Meridian.
Lower Mantle
The part of the mantle that lies below a depth of 1,000 kilometers. In this layer, seismic velocity increases slowly with depth.
Lunar Highlands
Light-colored regions of the Moon that are mostly composed of the rock anorthosite.
Lunar Magma Ocean
A sea of molten rock that existed shortly after the Moon formed which covered the entire Moon and extended down to depths of several hundred kilometers.
Lunar Mare
Dark-colored, low-lying regions of the Moon comprised mostly of basalt.