Scientists say they can measure distances between continents by space geodesy with a precision of a few centimeters. If this is so, have they proven or measured continental drift, such as that across the Atlantic Ocean?

Goddard Space Flight Center was a pioneer in space geodesy, having discovered the "pear-shaped earth" in 1959. Since then, space geodesy has blossomed, with techniques such as satellite laser ranging, radio telescope measurements, and the Global Positioning System. Measurements of the distance between, for example, Hawaii and Japan, and between Massachusetts and Germany, are routinely made with precisions of close to one centimeter. What these measurements have achieved is shown in Fig. 1 (below), baselines in the Pacific Ocean. These show that plate motion is, for example, carrying the island of Maui toward Asia at about 7 centimeters per year.

Click on image to enlarge. (Fig 1)

Measurements across the Atlantic, shown in Fig. 2 (below), seem to show that space geodesy sites with radio telescopes are moving to the northeast at about 2 cm per year. Is this continental drift? It may be, but there are problems. The most important is that if sea-floor spreading drives continent-bearing plates, the European sites should be moving to the southeast, away from Iceland. Why is there this discrepancy? It probably comes from choice of terrestrial reference frames. The motions shown in Fig. 2 are actually motions calculated relative to the Pacific Plate. If the same measurements are calculated relative to North America, the site motions are to the southeast, in agreement with sea-floor spreading from the Mid-Atlantic Ridge.

Click on image to enlarge. (Fig 2)

To answer the question: Space geodesists have measured plate motion in the Pacific beyond reasonable doubt. But measurements across the Atlantic are questionable proof of continental drift, because they are sensitive to choice of terrestrial reference frame. Years of further measurements, and careful calculations with difference reference frames will be necessary to prove continental drift geodetically.

This week's question comes from Dr. Paul Lowman. Paul Lowman is a geophysicist, originally a geologist, in the Geodynamics Branch of the Laboratory for Terrestrial Physics. He joined GSFC in December, 1959, in the Theoretical Division led by Dr. Robert Jastrow. He initially worked on the origin of tektites, argued by Dr. John O'Keefe to come from the Moon. He helped plan the Apollo geophysical experiments, and trained Mercury, Gemini, Apollo, and Skylab astronauts in geologic terrain photography. His most recent work has been compilation of a global tectonic activity map, appearing in his 2002 book "Exploring Space, Exploring Earth." AND he fearlessly (or foolishingly, if you ask him) rides his bike to work along Greenbelt Road (a very high trafficked road) every day!! WOW!