Philosophical Society of Washington

Minutes of the 2100th Meeting

Speaker: Paul G. Silver, the Carnegie Institution of Washington
Topic: “Probing the Depths of Geology”

President Garavelli called the 2100th meeting to order at 8:18 p.m. on February 5, 1999. The Recording Secretary read the minutes of the 2099th meeting and they were approved.

The speaker for the 2100th meeting was Paul G. Silver of the Carnegie Institution of Washington. The title of his presentation was, “Probing the Depths of Geology.”

There have been two major revolutions in the field of geological sciences. The first, about 200 years ago, was the discovery of deep time: That the Earth is very old.

The second major revolution began about 30 years ago with plate tectonics. Plate tectonics demonstrate that what we call geology is embedded within constantly moving tectonic plates that are driven by convective motion in the underlying Earth's mantle. Recent research shows that forces deep within the Earth's interior, even down to its core, control many geological processes. With seismological techniques, plate tectonics link geological processes that are separated by great distances on the Earth's surface. It allows remote sensing of otherwise inaccessible regions of our planet's interior. Measurement of seismic wave velocity and characteristics such as the extent and orientation of polarization provides a wonderful probe of the Earth's interior. The seismic velocity can be used to describe the composition of the Earth's crust, the underlying mantle and the still deeper core. It can also be used to describe minerals in the mantle, the temperature profiles of the interior regions, deformation of the plates and convective flow of the mantle.

Plate tectonics provides a coherent model of the characteristics of continental drift and sea-floor spreading. It describes the Earth's lithosphere as broken into fairly rigid plates. These plates are considered rigid for all intents and purposes, with all deformation occurring at the boundaries. Further, all plates are under stress with cracks occurring at weak areas of plates. Convection currents within the underlying, less rigid asthenosphere causes the overlying plates to move relative to each other. This movement is manifested in continental drift and sea-floor spreading.

Interestingly, flow-coupled plate interactions link the Andean and the Alpine deformations that occurred 30 million years ago. Initially, the African Plate moved eastward away from the Atlantic basin while the South American Plate moved westward. The eastward movement of Africa finally stopped due to its collision with the fixed Eurasian plate. This collision formed the Alps. The westward movement of the South American Plate away from the Atlantic basin continued resulting in the formation of the Andes. Although the Andes looks like a collisional boundary, it really isn't. The descending slab and subslab mantle resisted the South American westward motion and it was this interaction formed the Andes. Thus, the European Alps and the South American Andes are related.

Mr. Silver also described how Southern Africa's very old geology containing only a very high plateau came to be. The origins were described using a plate tectonic model with a low velocity “upwelling” deep in the mantle directly beneath Southern Africa that raised the surface as a plateau.

A very important aspect of the ongoing geology work effort is the prediction of earthquakes. There are between 5 and 6 earthquakes everyday worldwide, and plate tectonics cause 90% of them. Attempts for the last 25 years to predict earthquakes using empirical data have not worked and offer no promise. However, within the next 20 years, the use of measured data and modeling of transient plate deformations that occur before earthquakes will allow earthquakes to be realistically predicted. Stay tuned.

Mr. Silver then closed his presentation and kindly answered questions from the floor. President Garavelli thereupon thanked Mr. Silver for the society, announced the next meeting and made the usual parking announcement. He then adjourned the 2100th meeting to the Social Hour at 9:35 p.m.


Respectfully submitted,
Bill Spargo
Recording Secretary

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