Geoscience Reference
In-Depth Information
melted ice or water, and ash - or years later by the failure
of volcanic ash deposits in the presence of heavy rain.
Landslide. The general term given to movement of
material downslope in a mass.
Lapilli. Volcanic fragments about 2-60 mm in diameter.
Latent Heat. (1) of evaporation: each kilogram of water
converted to water vapor at 26°C requires 2425 kilo-
joules of heat energy. Because this heat energy is not
felt, it is termed 'latent'. (2) Of condensation: if this air
is cooled and moisture condenses, the latent heat
energy is released back to the atmosphere, warming
the air.
Lava. Molten rock. Different terms are used to describe
the nature of the lava, mainly as determined by viscosity.
Levee. When a river overflows its banks, there is an
immediate decrease in velocity. This results in deposi-
tion of suspended mud, and eventually the build-up of
an embankment that can contain the river above the
elevation of its adjacent floodplain.
Liquefaction. If the weight of a sediment deposit or flow
is not supported by the touching of individual grains
against each other, but by the pressure of water or
air entrapped in voids between particles, then such
sediment will liquefy - that is, behave as a fluid.
When liquefaction occurs, the sediment loses its
ability to support objects and is able to travel at high
velocities.
Lithology. Refers to the physical composition of any rock.
Little Ice Age. The period between 1650 and 1850 AD,
when sunspot activity was minimal, global tempera-
tures were 1°C cooler, and temperate mountain
glaciers advanced.
Loess. Dust picked up from barren ground in front of
icecaps by dense, cold winds blowing off the icecap
and deposited hundreds of kilometers downwind.
Longitudinal Or Meridional. Any movement that occurs
in a north-south direction.
Longshore Drift. The movement of water and sediment
within surf zones due to waves approaching shore at an
angle.
Lunar Cycle. See M n Lunar Tide.
Isobars. Lines plotted on a map joining points of equal
pressure, usually at a fixed spacing of 4 hectopascals
(hPa).
Isostatic. Describing local changes in sea level induced by
regional tectonic variation.
Isothermal. Having equal degrees of heat.
J
Jet Stream. A fast-moving flow of air at the top of the
troposphere, usually linked to large-scale uplift or
subsidence of air. The two most common jet streams
are the polar jet stream, attached to uplift along the
polar front, and the subtropical jet centered over
the poleward and trailing side of Hadley cells.
Jökulhlaups. See Glacier Burst.
Jurassic. The geological period 180-135 million years BP,
dominated by the dinosaurs.
K
Kaolinite. A common, clay mineral, which is resistant to
further weathering, containing only metals of aluminum
and silica. Chemical formula [Al 2 O 3 .2SiO 2 .2H 2 O].
Karst. The process in which solution is a major mechanism
of erosion, or landscape developed by that process.
Karst most commonly occurs in limestone, but in some
instances can involve sandstone and volcanic rocks.
Katabatic Winds. Winds formed by the flow under
gravity down topographic lines of denser air cooled
by long-wave emission under clear skies and calm
conditions. The resultant wind can reach speeds in
excess of 100 km hr -1 and is prevalent in winter,
mountainous terrain or adjacent to icecaps.
Kelvin Wave. A water wave trapped along the boundary
of a coast, channel, embayment, or two bodies of water
differing in some physical characteristic. The wave
moves slowly parallel to the boundary and has a height
that decreases rapidly away from the boundary. The
movement of water across the Pacific Ocean during an
ENSO event occurs as a 500-1000 km long, 10-50 cm
high Kelvin wave, trapped within 5° of the equator and
moving eastward at a speed of 1-3 m s -1 .
M
Magma. Liquid rock derived from the mantle.
Magnetic Reversal. The Earth has a magnetic north-
south polarity centered within 1000 km of its present
axis of rotation. The intensity of this magnetic field can
vary and, at intervals of hundreds of thousands of years,
weaken and reverse polarity very rapidly.
L
La Niña. The opposite of an ENSO event, during which
waters in the west Pacific are warmer than normal;
trade winds or Walker circulation is stronger; and
consequently rainfalls heavier in South-East Asia.
Lahar. A mudslide induced by volcanic eruption either at
the time of the eruption - by the mixing of hot gases,
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