Isostasy and Gravity: A Geophysics Perspective, Study notes of Physics

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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× 20 and 3× 21 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). Further study 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.