Everything responds to pressure, even rocks.
Deformation studies involve measuring and interpreting
the changes in elevations and horizontal positions of the
land surface or sea floor. These studies are variously referred
to as geodetic changes or ground-surface deformations and
are sometimes indexed under the general heading of geodesy.
Deformation studies have been particularly useful on active
volcanoes and in active tectonic areas.
A great amount of time and energy has been spent on
measuring geodetic changes on Kilauea and Mauna Loa
Volcanoes in Hawai`i. These changes include the build-up of
the surface by the piling up and ponding of lava flows, the
changes in the surface caused by erosion, and the uplift, subsidence,
and horizontal displacements of the surface caused by
internal processes acting beneath the surface. It is these latter
changes that are the principal concern of this review.
A complete and objective review of deformation studies
on active Hawaiian volcanoes would take many volumes.
Instead, we attempt to follow the evolution of the most significant
observations and interpretations in a roughly chronological
way. It is correct to say that this is a subjective review.
We have spent years measuring and recording deformation
changes on these great volcanoes and more years trying to
understand what makes these changes occur. We attempt to
make this a balanced as well as a subjective review; the references
are also selective rather than exhaustive.
Geodetic changes caused by internal geologic processes
vary in magnitude from the nearly infinitesimal - one micron
or less, to the very large - hundreds of meters. Their apparent
causes also are varied and include changes in material
properties and composition, atmospheric pressure, tidal stress,
thermal stress, subsurface-fluid pressure (including magma
pressure, magma intrusion, or magma removal), gravity, and
Deformation is measured in units of strain or displacement.
For example, tilt of the ground surface on the rim of
Kilauea Caldera is measured in microradians, a strain unit that
gives the change in angle from some reference. The direction
in which the tilt is measured must be defined - north or
south, or some direction normal to the maximum changes.
For displacements related to surface faulting, the changes are
normally given in linear measures of offset. Changes in the
diameter of a caldera can be given in either displacements
or strain units. In the later case, the displacement divided
by the 'original' diameter gives the strain ratio. Strains are
dimensionless numbers; displacements have the dimensions of
length. Vectors commonly are used to show the direction and
amount of displacements in plan view.
Strain results from stress. It can be elastic strain, when
the strain is linearly related to stress and is recoverable; it can
be viscous strain, where the rate of strain is proportional to the
stress and is not recoverable; or it can be plastic strain that is
often some complex stress-strain relationship, for example,
elastic up to some yield strength and viscous beyond. Volcanic
rocks are brittle when cold and under near-surface pressures
but plastic to viscous under higher temperature and pressure
regimes. It is important in deformation studies to try to define
the nature of the strain and the rheology of the rocks being
deformed. A good text on rheology is 'The Structure and Rheology
of Complex Fluids' by R.G. Larson, 1999.
Under changing tensional or compressional stresses, tiny
cracks in brittle rocks may open or close, causing a quasielastic
strain response. If the stresses exceed the breaking
strength of the rock, brittle failure occurs, and the stress-strain
relationship breaks down. This is generally the situation with
near-field deformation related to earthquakes. Stresses change
in complex patterns in both the near- and far-fields of the
fracture, and the near-fiel
Additional Publication Details
USGS Numbered Series
Evolution of Deformation Studies on Active Hawaiian Volcanoes