DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

of the low-density, low-melting-point materials
which floated to the top during differentiation
of the body. The crusts of the terrestrial planets
are composed primarily of silicate-rich rocks,
such as granite (Earth’s continental crust), basalt
(Earth’soceaniccrust; andvolcanicplainsonthe
moon, Mars, and Venus), and anorthosite (lunar
highlands).
crustal deformation Crustal movement.
crustal movement In geophysics, crustal
movementreferstopresent-daycrustaldisplacements
and deformation identified by geodetic
measurement. Changes in components
such as vertical movement, horizontal movement,
tilt, extension, and contraction can be obtained
from repeated measurement and continuous
observations of leveling and trilateration
surveys, electro-optical distance measurement,
GPS (Global Positioning System), VLBI (Very
Long Baseline Interferometry), SLR (Satellite
Laser Ranging), SAR (Synthetic Aperture
Radar), tiltmeter, extensometer, volumestrainmeter,
and tide gage. Periodic changes due
to the Earth’s tide and seasonal factors, deformation
accompanied by plate motion, and deformation
associated with large earthquakes and volcanic
eruptions are considered causes of crustal
movement.
crystal A solid formed by the systematic
arrangement or packing of atoms, ions, or
molecules. The repetitive nature of this packing
may cause smooth surfaces called crystal faces
to develop on the crystal during the course of
its unobstructed growth. A crystal is called anhedral,
subhedral, or euhedral if it, respectively,
lacks such faces, is only partly bounded by them,
or is completely bounded.
crystallization age The crystallization age
of a rock tells when it solidified from its parent
magma. It is usually obtained from the radioactive
decay of elements within the rock’s minerals.
The concentrations of the parent and daughter
elements are obtained from several minerals
within the rock and plotted on an isochron
diagram. The slope of the line, together with
knowledge of the half-life of the radioisotope,
allow the crystallization age to be determined.
© 2001 by CRC Press LLC
cumulus
CTD A standard oceanographic or limnic instrument
(profiler) to cast profiles of conductivity
(C) and temperature (T) as a function of depth
(D). Most instruments carry additional sensors
such as O2, transmissivity, pH, etc.
CTRS “Conventional Terrestrial Reference
System,” a geographic reference frame, fixed
in the mean crust of the Earth, and defined
by international agreement under supervision
of the IUGG (The International Union
of Geodesy and Geophysics). The definition
is based on agreed coordinates and velocities
(due to continental drift) of numerous
observing stations and accepted methods
for interpolating between them. See
http://hpiers.obspm.fr/webiers/general/syframes/convent/UGGI91.html
cumulative size-frequency curves Distribution
curves in which the logarithm of the cumulative
number of impact craters (above a certain
diameter) is plotted against the logarithm of the
diameter. It shows the number of craters on a
planetary surface generally increases with decreasing
crater size. Size-frequency slopes may
be affected if secondary craters have been included
in the counts; by slopes on the planetary
surface (surface craters are preserved more readily
on level terrain); and if crater saturation has
been reached.
Incremental plots are also used, for which the
number of craters within a specific size increment
is plotted against the diameter. See crater
saturation.
cumulus A sharply outlined cloud with vertical
development. The summit of the cloud,
in general dome-shaped, shows rounded bulges,
and its base is usually horizontal. The mean
lower level of the cumulus is 600 to 2000 m.
Cumulus clouds consist of water droplets and
are the product of water vapor condensation in
convective activity. Labeled with four subtypes,
based on the shape of the cloud: (1) cumulus
fractus — Cu fra; (2) cumulus humilis — Cu
hum; (3) cumulus congestus — Cu con; (4) an
intermediate type between Cu hum and Cu con
— Cu med.