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band of the Hertzsprung-Russell diagram
depicting such stars. (p. 626)
mantle convection A process by which heat
from Earth’s inner and outer cores is
transferred through the mantle. (p. 180)
mantle The thickest of Earth’s layers, located
between the outer core and Earth’s crust,
composed mostly of compounds rich in iron,
silicon, and magnesium. (p. 72)
map A flat, two-dimensional representation of
Earth’s surface and features. (p. 44)
Glossary
Glossary
map scale On a map, the comparison of distance
units used with actual distances on Earth’s
surface; may be expressed as a ratio, a
fraction, or in a scale bar. (p. 46)
maria Extensive dark areas on the moon that
represent great basins and level plains. (p. 558)
mascons Areas of higher gravity that exist over
lunar maria; short for “mass concentrations.”
(p. 558)
mass movement The downslope transportation
of large masses of earth materials by gravity.
(p. 268)
mass number The sum of the numbers of
protons and neutrons in an atom. (p. 92)
matter Anything that has mass and volume.
meanders Broad, looping bends in a river.
(p. 290)
mechanical weathering The breakdown of rock
that takes place when a rock is broken into
smaller pieces of the same material without
changing its composition. (p. 258)
meridian An imaginary half-circle that runs in a
north-south direction from the North Pole to
the South Pole; a longitude line.
mesosphere The layer of the Earth’s atmosphere
that extends from the stratosphere to the
thermosphere, characterized by decreasing
temperatures. (p. 371)
metal An element that loses electrons easily to
form positive ions. (p. 94)
metamorphic rock Rock that has undergone
chemical or structural change due to the
effects of heat and pressure; one of three types
of rock in the rock cycle. (p. 118)
metamorphism The process by which a rock’s
structure or composition is changed by
pressure, heat, and moisture. (p. 133)
meteor The light made by a meteoroid as it
passes through Earth’s atmosphere. (p. 604)
meteor shower A large number of meteors
entering and burning up in the atmosphere;
meteor showers take place when Earth passes
through a debris field. (p. 604)
meteorite The part of a large meteoroid that
survives its trip through the atmosphere and
strikes Earth’s surface. (p. 604)
meteoroid A rocky or icy fragment that travels
through space. (p. 556)
meteorology The study of processes that govern
Earth’s atmosphere. (p. 436)
mica Soft silicate minerals with flat, shiny flakes
that are found in many rocks, including
granite and gneiss.
micrometeoroids Tiny rock fragments no larger
than sand grains that travel through space.
(p. 557)
middle latitudes The area between about 30
degrees and 60 degrees latitude, where surface
winds are determined by the locations of
transient high- and low-pressure systems.
(p. 424)
mid-ocean ridge A long chain of mountains with
a central rift valley that is located along a
divergent boundary on the ocean floor.
(p. 174)
millibar A unit used to measure air pressure;
34 millibars equals 1 inch of mercury.
mineral A naturally occurring inorganic solid
with a distinct chemical composition and
crystalline structure. (p. 96)
mineral deposit A deposit that is left behind
when groundwater that contains minerals
cools or evaporates. (p. 309)
mineralogy The study of minerals and their
properties. (p. 104)
mixed layer The surface layer of ocean water.
(p. 497)
model A simplified representation of an object,
process, or phenomenon, used as the basis for
further study or investigation. (p. 5)
Mohorovičić discontinuity (Moho) The
boundary between Earth’s crust and mantle.
moist-adiabatic lapse rate The rate at which
saturated air cools as it rises; varies from
about 5°C per kilometer to about 9°C per
kilometer. (p. 398)
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