moderate earthquakes forming a doughnut-like
pattern. Named after its discoverer.
Mohr-Coulomb Equation. An equation in which the
shear strength of a soil is related to soil cohesion, a
force applied to the soil at right angles to the slope, and
the angle at which particles move relative to each other
within the soil.
Monsoon. Defines any region characterized by a distinct
180° change in wind direction between summer and
winter, resulting in seasonal alternation of copious rain
Montmorillonite. A group of clay minerals with an
aluminum layer sandwiched between two silica layers
bonded with magnesium or calcium cations. Such soils
expand easily because they can absorb water into the
layered structure when wetted. Chemical formula
[(Mg,Ca)O.Al 2 O 3 5SiO 2 .nH 2 O].
Mulching. Any process that causes soils not to compact,
but to become more friable towards the surface.
Magnitude-Frequency. The intensity or magnitude of all
geophysical events such as floods occur at discrete time
intervals in such a way that the lower the magnitude of
the event, the more frequent its occurrence. The
magnitude of an event can be plotted against how often
that event occurs, to produce a magnitude-frequency
Mantle. That part of the Earth from the crust to depths of
3000 km, which consists of molten, dense rock made up
of silicates of magnesium, iron, calcium, and aluminum.
Marigrams. Records of tsunami wave height on tide
Mass Extinctions. The dying-out or disappearance of
large numbers of plant and animal species in short
periods of time, supposedly because of a catastrophic
event that affected the climate of the Earth in the
Maunder Minimum. Refers to the lack of sunspots between
1650 and 1700 AD at the height of the Little Ice Age.
Maximum Probable Rainfalls. The amount of rain that
can theoretically fall within a given time interval over
a catchment under the most favorable conditions of
humidity, convection, and condensation.
Meandering. The process whereby a flow of air or water
tends to become unstable and travel in a winding path.
Mediterranean Climate. A seasonally dry climate having
rainfall in winter months. Its average temperature for
at least four months exceeds 10°C, with no monthly
average falling below -3°C. Rainfall of the driest
summer month is less than 40 mm and less than
one-third that of the wettest winter month.
Meridional Air Flow. Movement of air in a north-south
Microseisms. Very small earthquakes that can be
detected only by sensitive instruments.
M n Lunar Tide. A periodicity of 18.6 years that exists in
the orbit of the moon about the Earth because of the
orientation of the moon's orbit to the equatorial plane
of the sun.
Mobile Polar High. Pools of cold, shallow air that move
out from particular locations around the poles into the
mid-latitudes. They tend to lose momentum such that
successive highs stack up or agglutinate in a single
location over the oceans. When their pressure is
averaged over time, these locations appear as areas of
constant high pressure, which have been labelled
Mogi Doughnut. In some seismic areas, the epicenter of
a large earthquake is ringed in the prior decade by
Normal Stress. Any force applied at right angles to a soil
surface ( see Figure 12.3).
Normalize. A statistical process whereby the magnitude
of values in a time series are made independent of
their unit of measurement. This is usually done by sub-
tracting the mean of the data set from each value, and
dividing the result by the standard deviation. In most
climate time series, such as the Southern Oscillation
index, the resulting value is multiplied by 10.
North Atlantic Oscillation. The tendency for air pressure
to oscillate in intensity between the Icelandic Low and
the Azores High. The oscillation reflects inter-annual
variability in the strength of Rossby waves in the
northern hemisphere. See also Southern Oscillation.
North Pacific Oscillation. A statistical measure of the
strength and position of the Aleutian low-pressure
system, mainly in winter. The oscillation is linked to
short-term climate change in North America. See also
Nuée Ardente. A cloud of hot ash that, blown upwards
during an eruption, collapses under the weight of
gravity and begins to flow downslope at great velocity
suspended by the gases expelled in the eruption
(see Figure 11.3). Also called pyroclastic flow.
Orographic. Any aspect of physical geography dealing