Thursday, November 23rd, 2017

* Lagoon-A protected body of water separated from the sea by a reef or barrier island.
* Lake Effect Snowfall- Snow showers that are created when cold dry air passes over a large warmer lake, such as one of the Great Lakes, and picks up moisture and heat.
* Lake-A large, inland body of standing water that occupies a depression in the land surface.
* Land breeze- a cool, local wind blowing out to sea that occurs (usually at night) when air pressure over land becomes higher than the air pressure over the water. Land cools faster than water at night. Warm air rising over the water creates the lower air pressure at sea.
* Landslide-A general term for the downslope movement of rock and regolith under the influence of gravity.
* Latent Heat- The energy released or absorbed during a change of state. For example, the change of water vapor to liquid water releases the latent heat of condensation.
* Latent heat-The heat released or absorbed by a substance during a change in state, i.e., melting, freezing, vaporization, condensation, or sublimation.
* Lateral erosion-The action of a stream as it cuts into and erodes its banks. A meandering stream commonly swings from one side of its channel to the other, causing lateral erosion.
* Lateral moraine-A ridge-like moraine that forms on or adjacent to the sides of a mountain glacier.
* Laterite-A highly weathered re soil rich in oxides of iron and aluminum that usually develops in warm, moist tropical or temperate regions.
* Latitude- distance north or south of the equator in degrees.
* Lava -A general term for molten rock that is extruded onto the surface.
* Lava plateau-A sequence of horizontal basalt lava flows that were extruded rapidly to cover a large region of the Earth's surface. (syns: flood basalt, basalt plateau)
* Lava-Fluid magma that flows onto the Earth's surface from a volcano or fissure. Also, the rock formed by solidification of the same material.
* Layers 1, 2, 3-The three layers of oceanic crust. The uppermost layer, layer 1, consists of mud. Layer 2 consists of pillow basalt. Layer 3 directly overlies the mantle and consists of basalt dikes and gabbro.
* Leaching-The dissolution and downward movement of soluble components of rock and soil by percolating water.
* Lee Side or Leeward- The direction or side of an object (building, mountain, etc.) That faces away from the wind.
* Levee -A natural or artificial bank confining a river or stream channel.
* Light -Electromagnetic radiation that is visible to the eye.
* Light year-The distance traveled by light in one year, approximately 9.5 x 1012 kilometers.
* Lightning- All forms of visible electrical discharges moving through the atmosphere, usually emanating from tall cumulus and cumulonimbus clouds during thunderstorms. Lightning is often categorized for the manner in which it is visible to the observer- streak lightning, forked lightning, sheet lightning, heat lightning.
* Light-year (LY) - The distance light travels in a year, at the rate of 300,000 kilometers per second (671 million miles per hour); 1 light-year is equivalent to 9,460,530,000,000 km, 5,880,000,000,000 miles or 63,240 AU.
* Limb-The side of a fold in rock.
* Limestone-A sedimentary rock consisting chiefly of calcium carbonate.
* Liquefaction-A process in which a soil loses its shear strength during an earthquake and becomes a fluid.
* Lithic- Of or pertaining to stone.
* Lithification - The process that turns sediment into sedimentary rock.
* Lithification-The conversion of loose sediment to solid rock.
* Lithologic correlation-Correlation based on the lithologic continuity of a rock unit.
* Lithosphere- The rigid crust and uppermost mantle of the earth. Thickness is on the order of 60 miles (100 km). Stronger than the underlying asthenosphere.
* Lithosphere-The cool, rigid, outer layer of the Earth, about 100 kilometers thick, which includes the crust and part of the upper mantle.
* Litter-Leaves, twigs, and other plant or animal material that have fallen to the surface of the soil but has not decomposed.
* Loam-Soil that contains a mixture of sand, clay, and silt and a generous amount of organic matter.
* Loess- Very fine-grained sediments deposited by wind action. Usually associated with the margins of continental ice sheets. Large expanses of loess are currently responsible for much of the corn and wheat in Nebraska and Kansas.
* Loess-A homogenous, unlayered deposit of windblown silt, usually of glacial origin.
* Longitude- the distance in degrees east or west of the Prime Meridian.
* Longitudinal dune-A long symmetrical dune oriented parallel with the direction of the prevailing wind.
* Longshore current- a current that flows parallel to the shoreline, caused by waves moving towards the beach at an angle.
* Longshore current-A current flowing parallel and close to the coast that is generated when waves strike a shore at an angle.
* Longshore drift-Sediment carried by longshore currents.
* Losing stream-A stream that lies above the water table. Water percolates from the stream channel downward into the saturated zone. (syn: influent stream)
* Low velocity layer-A region of the upper mantle where seismic waves travel relatively slowly, approximately the same as the asthenosphere.
* Low-pressure area (Low) - an area that has lower pressure than the surrounding air
* Luminosity- the actual, or true, brightness of a star.
* Lunar eclipse- an eclipse that occurs when the full moon passes into Earth's total shadow (umbra).
* Lunar eclipse-See "eclipse."
* Luster - The way that minerals reflect light. There are two types of luster- metallic and nonmetallic.
* Luster-The quality and intensity of light reflected from the surface of a mineral.
* L-wave (surface wave) - an earthquake wave that travels along earth's surface.

* Mafic- An igneous composed chiefly of one or more dark-colored minerals.
* Mafic rock-Dark-colored igneous rock with high magnesium andiron content, and composed chiefly of iron- and magnesium-rich minerals.
* Magma- Molten rock beneath the surface of the earth.
* Magma-Molten rock generated within the Earth.
* Magmatic arc-A narrow elongate band of intrusive and volcanic activity associated with subduction.
* Magnetic reversal-A change in the Earth's magnetic field in which the north magnetic pole becomes the south magnetic pole, and vice versa.
* Magnetometer-An instrument that measures the Earth's magnetic field.
* Magnitude (of an earthquake)-A measure of the strength of an earthquake determined from seismic recordings. (See also "Richter scale.")
* Magnitude -A) The degree of brightness of a celestial body designated on a numerical scale, on which the brightest star has magnitude -1.4 and the faintest visible star has magnitude 6, with the scale rule such that a decrease of one unit represents an increase in apparent brightness by a factor of 2.512; also called apparent magnitude. B) A numerical expression of the amount of energy released by an earthquake, determined by measuring earthquake waves on standardized recording instruments (seismographs.) The number scale for magnitudes is logarithmic rather than arithmetic; therefore, deflections on a seismograph for a magnitude 5 earthquake, for example, are 10 times greater than those for a magnitude 4 earthquake, 100 times greater than for a magnitude 3 earthquake, and so on.
* Main sequence-A band running across a Hertzsprung-Russell diagram that contains most of the stars. Hydrogen fusion generates the energy in a main sequence star.
* Manganese nodule-A manganese-rich, potato-shaped rock found on the ocean floor.
* Mantle convection-The convective flow of solid rock in the mantle.
* Mantle plume-A rising vertical column of mantle rock.
* Mantle- The zone of the earth below the crust and above the core.
* Mantle-A mostly solid layer of the Earth lying beneath the crust and above the core. The mantle extends from the base of the crust to a depth of about 2900 kilometers.
* Marble-A metamorphic rock consisting of fine- to coarse-grained recrystallized calcite and/or dolomite.
* Mare -Latin word for "sea." Galileo thought the dark featureless areas on the Moon were bodies of water, even though the Moon is essentially devoid of liquid water. The term is still applied to the basalt-filled impact basins common on the face of the Moon visible from Earth.
* Maria-Dry, barren, flat expanses of volcanic rock on the Moon first thought to be seas.
* Marine climate- the climate of areas near oceans, with a small yearly temperature range.
* Marker horizon (or bed)- A distinctive horizon which is used for regional correlation of lithology. A good marker horizon is distinctive, widespread, and represents a relatively short period of geologic time. For example, ash from a volcanic eruption, debris from a meteorite impact, etc. Humans will represent one of the earth's finest marker horizons in several hundred million years. Our effect on the surface is certainly distinctive and widespread, and, at the rate we are going, it is possible that our species will have a relatively short lifespan (speaking in terms of geologic time, of course). see Key bed
* Mass wasting-The movement of earth material downslope primarily under the influence of gravity.
* Matrix- The solid matter in which a fossil or crystal is embedded. Also, a binding substance (e.g., cement in concrete).
* Meander- one of many bends in a stream
* Meander-One of a series of sinuous curves or loops in the course of a stream.
* Mechanical weathering (physical)-The disintegration of rock into smaller pieces by physical processes.
* Medial moraine-A moraine formed in or on the middle of a glacier by the merging of lateral moraines as two glaciers flow together.
* Mercalli scale - a numerical description of an earthquake's intensity (damage) using roman numerals from I to XII.
* Mercalli scale-An scale of earthquake intensity that measures the strength of an earthquake in a particular place by its effect on buildings and people. It has been replaced by the Richter scale.
* Mercury Barometer- The most common form of liquid barometer designed on the principle that the downward pressure of the air's weight will support a column of liquid mercury in an inverted, evacuated glass tube. The height of the column is the measure of atmospheric pressure and was originally reported as millimeters or inches of mercury (Hg). Today, the preferred unit is the Pascal, usually expressed as the kilopascal (kpa).
* Meridian-A circle on the celestial sphere that passes through the two poles and the zenith of your position.
* Mesa -A broad, flattop, erosional hill or mountain, commonly bounded by steep slopes.
* Mesa-A flat-topped mountain or a table-land that is smaller than a plateau and larger than a butte.
* Mesosphere- the layer of the atmosphere between the stratosphere and the thermosphere.
* Mesosphere-The layer of air that lies above the stratopause and extends from about 55 km upward to 80 km above the Earth's surface. Temperature decreases with elevation in the mesosphere.
* Mesozoic- an era of geologic time lasting from 245 to 66 million years ago.
* Mesozoic era-The part of geologic time roughly 245 to 65 million years ago. Dinosaurs rose to prominence and became extinct during this era.
* Metallic mineral resources-Ore; valuable concentrations of metals.
* Metamorphic facies-A set of all metamorphic rock types that formed under similar temperature and pressure conditions.
* Metamorphic- From the Greek "meta" (change) and "morph" (form). Commonly occurs to rocks which are subjected to increased heat and/or pressure. Also applies to the conversion of snow into glacial ice.
* Metamorphic grade-The intensity of metamorphism that formed a rock; the maximum temperature and pressure attained during metamorphism.
* Metamorphic rock-A rock formed when igneous, sedimentary, or other metamorphic rocks recrystallize in response to elevated temperature, increased pressure, chemical change, and/or deformation.
* Metamorphism-The process by which rocks and minerals change in response to changes in temperature, pressure, chemical conditions, and/or deformation.
* Meteor shower- a time when many meteoroids enter Earth's atmosphere.
* Meteor -The luminous phenomenon seen when a meteoroid enters the atmosphere, commonly known as a shooting star.
* Meteorite -A part of a meteoroid that survives through the Earth's atmosphere.
* Meteorite-A fallen meteoroid.
* Meteoroid -A small rock in space.
* Meteoroid-A small interplanetary body in an irregular orbit. Many meteoroids are fragments of asteroids formed during collisions.
* Meteorology- the study of Earth's atmosphere.
* Mica-A layer silicate mineral with a distinctive platy crystal habit and perfect cleavage. Muscovite and biotite are common micas.
* Microburst- The microburst is a downburst which is confined to a small area, less than 4 km (2.5 miles) in diameter from the initial point of downdraft impact and lasting for less than 10 minutes. An intense microburst can result in damaging winds near 280 km/hr (170 mph).
* Microcontinent-A small mass of continental crust, or an island arc, similar to Japan, New Zealand, and the modern islands of the southwest Pacific Ocean. The first supercontinent is thought to have formed when many microcontinents joined together about 2 billion years ago. Modern continents consist of many microcontinents that welded together over geologic time.
* Microwave radar-Instruments and processes using reflections of electromagnetic radiation in the microwave range. The data measure subtle swells and depressions on the sea surface, which reflect sea-floor topography.
* Mid-channel bar-An elongate lobe of sand and gravel formed in a stream channel.
* Midnight Sun- A high latitude phenomenon observed around midsummer when the sun does not sink below the horizon throughout the 24 hour period; therefore the sun may be seen at midnight. At the poles, the midnight sun occurs from the Spring to Autumnal Equinoxes.
* Mid-oceanic ridge-A continuous submarine mountain chain that forms at the boundary between divergent tectonic plates within oceanic crust.
* Migmatite-A rock composed of both igneous and metamorphic-looking materials. It forms at very high metamorphic grades when rock begins to partially melt to form magma.
* Millibar -This is 1/1000 of a bar; the standard sea-level pressure is about 1,013 millibars. . One millibar is equal to one hectopascal.
* Mineral- A naturally occurring, inorganic, crystalline solid with a definite internal structure and chemical composition.
* Mineral deposit-A local enrichment of one or more minerals.
* Mineral reserve-The known supply of ore in the ground.
* Mineral resources-All useful rocks and minerals.
* Mineral-A naturally occurring inorganic solid with a definite chemical composition and a crystalline structure.
* Mineralization-A process of fossilization in which the organic components of an organism are replaced by minerals.
* Minor planets -Another term used for asteroids.
* Miocene- An epoch in Earth's history from about 24 to 5 million years ago. Also refers to the rocks that formed in that epoch.
* Moho- Also called the Mohorovicic discontinuity. The surface or discontinuity that separates the crust from the mantle. The Moho is at a depth of 5-10 km beneath the ocean floor and about 35 km below the continents (but down to 60 km below mountains). Named for Andrija Mohorovicic, a Croatian seismologist.
* Mohorovicic discontinuity (Moho)-The boundary between the crust and the mantle identified by a change in the velocity of seismic waves.
* Mohs hardness scale - A scale that measures and compares the hardness of minerals.
* Mohs hardness scale-A standard, numbered from 1 to 10, to measure and express the hardness of minerals based on a series of ten fairly common minerals, each of which is harder than those lower on the scale.
* Moment magnitude-An earthquake scale based on the amount of movement and the surface area of a fault. The moment magnitude scale closely reflects the total amount of energy released during an earthquake.
* Monocline-A fold with only one limb.
* Mons -A mountain.
* Monsoon- Very persistent winds which may blow for months at a time from one general wind direction during one season and then a quite different, often opposing direction for another season. Originally, the name for the seasonal winds that blow across the Arabian Sea. For six months the winds blow consistently from the northeast and for six months from the southwest. The name has been extended to other areas of the world. The most well-known are the Asian monsoons which blow across India. In the winter, these cold, dry winds blow from the northwest off the Asian Plateau. In winter, they come from the southeast and bring hot, humid weather often with torrential rains. For this reason, the name has often been applied by the layperson to heavy rainfalls.
* Monsoon- a continental wind system caused by uneven heating of land and sea. Monsoons generally blow from the sea to the land in the summer, when the continents are warmer than the ocean, and bring rain. In winter, when the ocean is warmer than the land, the monsoon winds reverse.
* Moraine- General term for material deposited beneath, along the sides, and/or at the terminus of a glacier. See also till.
* Moraine-A mound or ridge of till deposited directly by glacial ice.
* Mud cracks-Irregular usually polygonal fractures that develop when mud dries. The patterns may be preserved when the mud is lithified.
* Mudflow- A flowage of water-saturated earth material possessing a high degree of fluidity during movement. A less-saturated flowing mass is often called a debris flow. A mudflow originating on the flank of a volcano is properly called a lahar.
* Mudflow-Mass wasting of fine grained soil particles mixed with a large amount of water.
* Mudstone-A non-fissile rock composed of a mixture of clay and silt.
* Mud-Wet silt and clay.