DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

the matter stress-energy tensor, or to zero for an
empty (vacuum) solution. Those four equations,
the constraint equations, contain no time derivative
of either gij or of Kij ; hence, they must be
satisfied at the particular time in question. They
can be written as one equation, which is a nonlinear
generalization of Newton’s gravitational
equation including the contribution of the gravitational
field as a source; and three components
of a tensor equation which is a transversality
condition on the momentum. It is easy to show
that these four equations put real restrictions on
the gij and the Kij that can be chosen for initial
data (hence constraints). However, it can
further be shown that the evolution via the hyperbolic
equations preserves the solution of the
constraint equations after their solution at the
initial time. These equations are elliptic, and a
standard procedure has been developed to solve
this. See ADM form of the Einstein–Hilbert action.
contact binary A binary star in which the
two components are touching. Main-sequence
contact binaries are also called W Ursa Majoris
(abbreviated as W UMa) and β Lyrae binaries,
depending on the mass of the primary.
continent Regions of substantial area on the
surface of the Earth that rise above sea level.
The continents are made up of thick, light continental
crust. The granitic continental crust has a
typical thickness of 35 km, whereas the basaltic
oceanic crust has a typical thickness of 6 km.
The continents do not participate in the plate tectonic
cycle but move and float on the surface of
the mantle so that the continents are much older
than the oceanic crust, about 2 billion years vs.
100 million years.
continental climate The climatic type describing
the climate character within a great
land. Comparing to the marine climate, the main
features of continental climate are strong annual
temperature variations, which cause colder winters
and hotter summers, and strong diurnal variations.
continental collision When subduction in an
ocean basin dominates over sea-floor spreading,
the ocean closes and the continents on the two
© 2001 by CRC Press LLC
continental margin
sides of the ocean collide. This is happening
today in the Himalayas where the Indian subcontinent
has collided with Eurasia.
continental drift Refers to the movement of
continents with respect to each other. The remarkable
similarity between the east coast of
the Americas and the west coast of Europe and
Africa has been recognized for several centuries
and led to the hypothesis that these continents
were once attached into a single supercontinent,
which then broke apart and created the distribution
of land masses we see today, called continental
drift. The idea was popularized by German
meteorologist Alfred Wegener in 1922, although
many other people from 1620 onward
contributed to the idea. The remarkable similarity
extends beyond the similarity of continent
boundaries to the distribution of geologic features,
such as mountain ranges and glacial remains
and fossil types, and the divergence in
the evolution of flora and fauna after a certain
point in time. It was initially rejected by geophysicists
who could not envision that the solid
mantle of the earth could flow.
All the continents were combined into a single
supercontinent called Pangea about 300 million
years ago. Pangea broke apart into a northern
continent (Laurasia, composed of North
America, Europe, Asia, and Greenland) and a
southern continent (Gondwana, composed of
South America, Africa, Australia, Antarctica,
and India) about 250 million years ago, and
those two continents subsequently broke apart
into the continents we see today. The original
idea of continental drift suggested the continents
moved over the ocean floors. In the 1960s, studies
of the magnetic polarity of ocean crust revealed
that the seafloor was also moving. Combining
the ideas of continental drift and seafloor
spreading led to the development of the theory of
plate tectonics to explain the geologic activity on
the Earth’s surface. Direct ranging to geodetic
satellites accurately measures the rate of continental
drift, with speeds of up to 15 cm/yr.
continental margin The boundary between
the continents, including continental shelves,
and the ocean basins. There is no unique definition
of the margin, it is usually taken as an
arbitrary water depth, say 1 km. There are two