DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

continental plate
types of continental margins, active and passive.
An active margin is also a plate boundary, usually
a subduction zone. An example is the west
coast of South America. A passive margin is not
an active plate boundary. An example is the east
coast of the United States.
continental plate In geophysics, the large
pieces of material comprising the outer surface
of the lithosphere. Plates are about 50 km
in thickness and support the continents, which
float on the denser viscous-fluid mantle below
them. Current knowledge defines 15 major
plates: Pacific, Philippine, Juan De Fuca, Cocos,
Nazca, Antarctic, Scotia, South American,
North American, Caribbean, Arabian, Indian,
African, Eurasian, and Australian.
continental shelf Shallow oceanic margins
underlain by continental crust. The water depth
is usually taken to be less than 1 km.
continental shelf waves The waves that are
generatedbytheslopingcontinentalshelftopography.
In the Northern Hemisphere, the continental
shelf wave propagates along a constant
topography line with the coast on the right.
continuum In mechanics, a description that
ignores the granular or quantum nature of matter.
In spectroscopy, the part of a spectrum without
apparent lines, arising from solid, liquid, or
gaseous sources, in which atomic lifetimes are
too short to produce specific line emission or
absorption.
contravariant vector See vector.
convection Large-scale flow of gases (or
other fluids) in stars (or elsewhere) that carries
heat energy from one place to another. In the
stellar context, hot currents rise, cool ones fall,
and the solar granulation is direct evidence for
the occurrence of convection near the surface of
the sun. Convection sets in whenever a stellar
temperature gradient is steeper than the adiabatic
one:
− dT
dr >−
1 − 1
T dp
γ p dr
© 2001 by CRC Press LLC
where γ is the ratio of specific heats. This can
happen either when gas is quite opaque to radiation
(as it is near the surfaces of cool stars,
including the sun, where hydrogen is in the process
of being ionized) or when a nuclear reaction
depends on a very high power of temperature, as
does the CNO cycle at the cores of massive stars.
Stars are fully convective during early phases
of their formation (guaranteeing initial chemical
homogeneity) and throughout their lives for
stars of less than 0.3 solar masses. Convection
is the primary way that material inside stars is
mixed from one zone to another. In relatively
dense regions of stars, convection will carry all
the available energy, and the temperature gradient
will be very nearly adiabatic. At lower densities,
convection becomes inefficient, radiation
carries much of the energy, and the temperature
gradient can be much steeper.
In the absence of any adequate theory of convection
(or other turbulent processes in fluids),
convection is often treated in the Mixing Length
Approximation, in which gas is assumed to rise
or fall through a fixed fraction of the pressure
scale height (half is typical) and then come into
temperature equilibrium with its surroundings,
depositing its extra heat or soaking up its deficit.
The approximation is more than 50 years old,
and modern numerical computations of gas flow
processes are just beginning to replace it in standard
computations of stellar structure and evolution.
The absence of an adequate theory of
convective energy transport is one of the major
remaining uncertainties in our understanding of
stellar physics. See CNO cycle, solar granulation.
convection zone A layer in a star in which
convection currents provide the main energy
transport mechanism. In the sun, a convection
zone extends from just below the photosphere
down to about 0.7 R⊙.
convective adjustment One way to parameterize
the physical process of convection in climate
modeling. For example, in a simple version,
one first examines the relative humidity
and lapse rate in each grid column at the end of
each time step of integration; if the lapse rate
is superadiabatic, the temperature profile is adjusted
to dry static neutrality in a manner that