The condition of equilibrium, comparable to floating, of the units of the
lithosphere above the asthenosphere. Crustal loading, as by ice, water,
sediments, or volcanic flows, leads to isostatic depression or
downwarping; removal of load leads to isostatic uplift or upwarping. Two
differing concepts of the mechanism of isostasy are the Airy hypothesis of
constant density and the Pratt hypothesis of constant thickness.
See also:isostatic compensation
Subjected to equal pressure from every side; being in hydrostatic
equilibrium; relating to or characterized by isostasy.
See:stress trajectory
a. The difference between the observed value of gravity at a point after
applying to it the isostatic correction and the normal value of gravity at
the point. AGI
b. Anomaly on a map of observed gravity anomalies after applying the
isostatic correction. Negative isostatic anomalies indicate
undercompensation, implying a tendency to rise; positive isostatic
anomalies connote overcompensation and a tendency to sink. AGI
c. A gravity anomaly calculated on a hypothesis that the gravitational
effect of masses extending above sea level is approx. compensated by a
deficiency of density of the material beneath those masses; the effect of
deficiency of density in ocean waters is compensated by an excess of
density in the material under the oceans. See also:anomaly
The adjustment of the lithosphere of the Earth to maintain equilibrium
among units of varying mass and density; excess mass above is balanced by
a deficit of density below, and vice versa. See also:isostasy
Syn:isostatic adjustment; isostatic equilibrium. AGI
The adjustment made to values of gravity, or to deflections of the
vertical, observed at a point, to take account of the assumed mass
deficiency under topographic features for which a topographic correction
is also made. AGI
The shifting of the rock beneath the Earth's crust in response to the
shifting in the weight above the Earth's crust.
Syn:isostatic compensation
a. Refers to minerals that are closely similar in crystallographic,
physical, and chemical properties but have little tendency for isomorphous
substitution; same as isotypic. AGI
b. Said of minerals that have the same ionic or molecular crystal
structure. Isostructure is more rigorous than isomorphism; the latter
requires similar crystal forms, the former a one-to-one correspondence of
atomic particles. Isostructural minerals may differ markedly in chemical
composition and physical properties, e.g., fluorite and uraninite, or may
be closely similar, e.g., the calcite group of carbonates.
A line connecting points of equal temperature. Isotherm maps are often
used to portray surface temperature patterns of water bodies. AGI
a. A change taking place at a constant temperature. Strock, 2
b. Pertaining to the process of changing the thermodynamic state of a
substance, such as its pressure and volume, while maintaining the
temperature constant. AGI
a. Reduction in the volume of a fluid without any change in its
temperature. Standard, 2
b. Compression in which there is no change in the temperature of the air;
used as a standard against which the conditions of actual compression may
be checked. Lewis
The expansion of air under constant temperature. Since the air does work
on expanding, it loses heat; consequently, heat must be added to the air
to maintain it at constant temperature. Lewis
A water column through which a constant temperature exists. Hy
A descriptive term applied to igneous rocks with an orbicular texture in
which the nuclei of the orbicules are composed of the same rock as the
groundmass. CF:crystallothrausmatic; homeothrausmatic;
heterothrausmatic. AGI
Said of a medium with properties the same in all directions; in crystal
optics, said of a crystal with refractive index that does not vary with
crystallographic direction. Isometric crystals and amorphous substances,
such as glass, are generally isotropic. Noun, isotropy. CF:uniaxial;
anisotropic.
A mass having the same property (or properties) in all directions.
ASCE
a. The condition of having properties that are uniform in all directions.
Adj. isotropic. CF:uniaxial
b. Refers to matter with properties the same in all directions; in optics,
a crystal with an index of refraction that does not vary with
crystallographic direction. Isometric crystals and amorphous substances,
e.g., glass, are generally isotropic. CF:anisotropy
Having analogous composition and closely similar crystal structure, but
not capable of intercrystallizing to form solid solutions. Examples are
calcite and soda niter; galena and NaBr. CF:isomorphous
One-dimensional polymorphism; e.g., alternate stacking of identical layers
of micas or clays. Also called isotypism. CF:polymorphism
The phenomenon of sound being the same in all parts of a given water
column. Hy
Lines constructed on a map of a coalbed connecting points of equal
volatile matter, delineating the distribution of volatile matter of the
coal. AGI