DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

neath. In a compensated mountain belt, the total
mass in a vertical column of rock is equal to the
total mass in the adjacent lowlands. A mountain
belt behaves like a block of wood floating
on water.
In cosmological scenarios where topological
defects are suddenly formed, both the geometry
of spacetime and ordinary matter and radiation
fluids are perturbed in such a way as to satisfy
theoretical conservation laws imposed by general
relativity theory. Matching conditions between
times before and after the relevant phase
transition during which defects are generated require
that cosmic fluid perturbations compensate
for the energy density inhomogeneity of
space in the presence of the defect. See cosmic
phase transition, cosmic topological defect,
Kibble mechanism.
compensation depth The depth of the Earth
atwhichtheoverlyingrocksareassumedtoexert
a constant pressure. Below this depth lateral
variations in density are assumed to be small.
composite volcanos Composite volcanos,
also called stratovolcanos, are steep-sided volcanic
cones consisting of alternating layers of
lava flows and ash deposits. Eruptions from
composite volcanos are often very explosive and
deadly, as evidenced by the eruptions of volcanos
such as Vesuvius in Italy, Mt. St. Helens
in the United States, and Mt. Pinatubo in the
Philippines. Composite volcanos are found near
subduction zones on Earth and are produced by
silica-rich magmas moving upwards from the
subducted plate. The high silica content of the
magmas allows them to be very viscous and retain
much gas, which leads to the explosive nature
of the eruptions.
compound channel A channel or river in
which the equation for flow area vs. depth exhibits
a discontinuity.
compressibility The ratio of the fractional
change in volume dV/V (volumetric strain) in
response to a change in pressure dp, that is,
© 2001 by CRC Press LLC
β= 1 dV
V dp =−1
dρ
ρ dp
Compton scattering
where ρ is density. Isothermal or adiabatic
changes of V and p yield slightly different values
of β. β is a modulus of elasticity, and its
reciprocal is the bulk modulus.
Compton, Arthur H. Physicist (1892–
1962). His studies in X-rays led him to discover
the Compton effect, that is the change in wavelength
of a photon when it is scattered by a free
electron. The discovery of the Compton effect
confirmed that electromagnetic waves had both
wave and particle properties.
Compton cooling The reduction of energy
of a free electron, due to its interaction with a
photon. If the kinetic energy of the electron
is sufficiently high compared to the incoming
photon, the energy of the incoming photon plus
part of the kinetic energy of the electron is redirected
as a photon of higher energy. Named after
Arthur H. Compton (1892–1962).
Compton reflection The Compton scattering
of hard X-ray radiation by a layer of dense and
thick matter, such as the surface of a star or of
an accretion disk. Hard X-ray radiation is scattered
off the surface of the layer after having lost
part of its energy. Compton reflection creates a
distinguishing spectral feature, an enhancement
in the spectral energy distribution between 10
and 50 keV. Such features have been detected in
the spectra of several Seyfert-1 galaxies and of a
galactic object, the black hole candidate Cyg X-
1. See Seyfert galaxies, Cygnus X1.
Compton scattering The inelastic scattering
of high energy photons by charged particles,
typically electrons, where energy is lost by the
photon because of the particle recoil. A photon
carries momentum, part of which is exchanged
between the photon and the particle. Conservation
of energy and momentum yields an increase
in the photon wavelength (and hence a decrease
in photon energy) as measured in the initial rest
frame of the electron equal to
λ − λ0 = λC(1 − cos θ) ,
where λ0 is the wavelength of the incident photon,
θ is the angle between the initial and final
direction of propagation of the photon, and λC is
a constant, called the Compton wavelength, and