Glossary - Discovering Physical Geography - page 579

Mesopause The upper boundary of the mesosphere where
temperature reaches its lowest point. 88
Mesosphere A layer of decreasing temperature in the atmo-
sphere that occurs from about 50 km to 80 km (~30 mi to 50
mi) in altitude. 88
Metamorphic rocks Rocks that form when igneous, sedi-
mentary, or other metamorphic rocks are subjected to intense
heat and pressure. 318
Microclimate Average temperature and precipitation
characteristics of a small area within a larger climate
region. 252
Mid-oceanic ridge A ridge-like feature that develops
along a rift zone in the ocean due to magma upwelling. 339
Midlatitude cyclone A well-organized low-pressure system
in the midlatitudes that contains warm and cold fronts. 173
Midlatitudes The zone of latitude that lies between about
35° and 55° in both hemispheres. 17
Milankovitch theory The theory that best explains Quater-
nary glacial/interglacial cycles through long-term variations in
Earth's orbital eccentricity, tilt, and axial precession. 224
Minerals Naturally occurring substances with distinctive
chemical configurations that usually manifest themselves in
some kind of crystalline form. 310
Mohorovicic discontinuity The boundary between the
Earth's crust and the upper part of the asthenosphere; seismic
waves change speed at this boundary. 308
Mollisols Soils that form through calcification and have a
mollic epipedon that overlies mineral matter, which is more
than 50% saturated with base ions. 292
Monocline A geologic landform in which rock beds are
inclined in a single direction over a large distance. 341
Monsoon The seasonal change in wind direction that occurs
in subtropical locations due to the migration of the Intertropi-
cal Convergence Zone (ITCZ) and Subtropical High (STH)
Pressure System. 124
Moraine A winding ridge-like feature that forms at the front
or side of a glacier or between two glaciers. 462
Mountain breeze Downslope airflow that develops when
mountain slopes cool off at night and relatively low pressure
exists in valleys. 129
Mudflow A well-saturated and highly fluid mass of
ine-textured sediment. 385
Mudpot A bubbling mixture of gaseous mud and water
at the Earth's surface that is associated with geothermal
activity. 368
Multipath error Error that results when a satellite signal
reaches the GPS receiver directly from the satellite as well as
others reflected from nearby buildings or other surfaces.

N
Natural gas Naturally occurring mixture of hydrocarbons
that often occurs in association with petroleum and is found in
porous geologic formations. 318
Natural hazard An actual or potentially occurring natural
event, such as an earthquake or volcanic eruption, that has a
negative effect on people or the environment. 353
Natural levee A small ridge that develops along the channel
of a stream through the deposition of relatively coarse sedi-
ment when flooding occurs. 431
Net radiation The difference between incoming and outgo-
ing flows of radiation. 76
Normal fault A steeply inclined fault in which the hanging
rock block moves relatively downward. 350

The half of Earth that lies north of Northern Hemisphere

the Equator. 15

o
o horizon The uppermost soil horizon that consists of unde-
composed plant litter. 281
occluded front The area where a cold front begins
to overtake a warm front and thus lift warm surface air
aloft. 178
ocean The entire body of salt water that covers about 71%
of Earth. 506
oceanic crust Basaltic part of the Earth's crust that makes
up the ocean basins. Oceanic crust is about 8 km (5 mi) thick
and is also called sima because it consists largely of silica and
magnesium. 308
offshore The nearshore zone that is permanently submerged
and where waves break. 519
offshore bar A small ridge on the bottom of the ocean that
separates the offshore and foreshore. 519
orogeny A period of mountain building, such as the
Allegheny Orogeny. 344
orographic uplift Uplift that occurs when a flowing body of
air encounters a mountain range. 157
outwash plain A broad landscape of limited relief created
by the deposition of glacial outwash. 463
overthrust fold A structural feature where one part of the
rock mass is shoved up and over the other. 342
overturned fold A structural feature in which the fold limb
is tilted beyond vertical, which results in both limbs inclined
in the same direction, but not at the same angle. 342
oxbow lake A portion of an abandoned stream channel
that is cut off from the rest of the stream by the meandering
process and filled with stagnant water. 428
oxic horizon A diagnostic soil horizon in tropical and sub-
tropical environments rich in iron oxides. 39 287