Unformatted text preview: PX266 Geophysics (2010/11)
Lecture 8 Handout – Isostasy
Dr. Gavin Bell
Isostatic compensation: Airy model Hudson Bay area – rate of change of geoid height measured by GRACE Image taken from Tamisiea et al., Science 316, p. 881 (2007). The paper models the
observed gravity patterns using both post-glacial uplift (relatively rapid, starting at the
end of the last Ice Age) and slower upper mantle convection processes. The viscosity
of the upper mantle comes out at between 8×1020 and 3×1021 Pa s.
Notes Also a possible meteorite impact site? (geological features on bay’s east coast) Hudson Bay may be source of oldest rock (“protocrust”) ever found on Earth
(4.3 billion years by Sm-Nd) – reported in Science 321 (September 2008).
Q. 10 on isostatic compensation and gravity anomalies.
Q. 11 on a simple isostasy model for glacial depression of the lithosphere.
Q. 12 researching a little bit about the post-glacial uplift of Sweden and Finland and
using its time dependence to estimate the viscosity of the asthenosphere. You should
find out a little bit about the topic of post-glacial uplift. Summary of Gravity topic – what you need to know
Gravitational field due to spherical shell (you don’t need to be able to derive it).
Gravitational field due to an infinite slab (ditto, formula would be given in exam).
Qualitative details of the Reference Gravity Formula, Reference Spheroid and Geoid.
How gravity surveys are made (qualitatively – satellites & gravimeters).
How to convert from gravity anomaly to geoid height anomaly.
How to apply free-air and Bouguer corrections to simple slab-like models (formulae
would be given in an exam.
The basic ideas of isostasy: what is isostatic equilibrium and how it relates to the
Earth’s mechanical / compositional layer structures, how to calculate equilibriums for
simple slab-like models, the Pratt and Airy models, what happens with rapid mass
changes such as melting ice sheets, and how Bouguer anomalies arise due to isostasy. ...
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This note was uploaded on 09/12/2011 for the course ECON 102 taught by Professor Gavinbell during the Spring '11 term at LSE.
- Spring '11