Geology Reference
In-Depth Information
Table 14.4 Causes of eustatic change
Seat of change
Type of change
Approximate magnitude
of change (m)
Causative process
Ocean basin volume
Tectono-eustatic 50-250
Orogeny, mid-ocean ridge growth, plate
tectonics, sea-floor subsidence, other
Earth movements
Ocean water volume
Glacio-eustatic
100-200
Climatic change
Hydro-eustatic Minor
Changes in liquid water stored in
sediment, lakes, and clouds; additions
of juvenile water; loss of connate water
Ocean mass distribution
and surface
Geoidal eustatic Up to 18
A few metres
Tides
Obliquity of the ecliptic
'topography'
1 m per millisecond
of rotation
Rotation rate
Up to 5
Differential rotation
2 (during Holocene)
Deformation of geoid relief
Climo-eustatic
Up to 5 for major
ocean currents
Short-term meteorological, hydrological,
and oceanographic changes
Source:
From Huggett (1997b, 151), partly adapted from Mörner (1987, 1994)
normally balance losses from evaporation. (Gains
from juvenile water probably balance losses in buried
connate water.) However, when the climate system
switches to an icehouse state, a substantial portion
the world's water supply is locked up in ice sheets
and glaciers. Sea level drops during glacial stages, and
rises during interglacial stages. Additions and subtrac-
tions of water from the oceans, other than that con-
verted to ice, may cause small changes in ocean volume.
This minor process might be termed hydro-eustasy.
caused by a reduced rate or cessation of mid-ocean ridge
production.
Other eustatic effects
Geoidal eustasy results from processes that alter the
Earth's equipotential surface, or
geoid
. The ocean
geoid is also called the geodetic sea level. The relief
of the geoid is considerable: there is a 180-m sea-level
difference between the rise at New Guinea and the
depression centred on the Maldives, which places lie
a mere 50-60 degrees of longitude from one another.
There is also a geoid beneath the continents. The con-
figuration of the geoid depends on the interaction
of the Earth's gravitational and rotational potentials.
Changes in geoid relief are often rapid and lead to swift
changes in sea level.
On a short time-scale, local changes in weather,
hydrology, and oceanography produce relatively tame
fluctuations of sea level. These fluctuations might be
called climo-eustasy. They may involve up to5mof
sea-level change for major ocean currents, but less than
half that for meteorological and hydrological changes.
Tectono-eustatic change
Geological processes drive tectono-eustatic change.
Even when the water cycle is in a steady state, so that
additions from precipitation balance losses through
evaporation, sea level may change owing to volumet-
ric changes in the ocean basins. An increasing volume
of ocean basin would lead to a fall of sea level and
a decreasing volume to a rise of sea level. Decreasing
volumes of ocean basin are caused by sedimentation,
the growth of mid-ocean ridges, and Earth expansion
(if it should have occurred); increasing volumes are
