prominent example of a transform boundary is the San Andreas Fault, which undergoes strike-slip motion on 100–1,000 year time scales, resulting in destructive earthquakes. Mountainous areas on the continents, such as the Himalayas and the Alps, are formed by convergent motion (collisions) between continental plates. However, where an oceanic plate collides with a continental plate, such as the North American Cascades, major earthquakes and volcanism can be expected.

Global-scale geodetic measurements of plate motions from Very Long Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR), based on less than a decade of data, show remarkable agreement (at the 3–5 millimeters per year level) with plate motions derived from the 1–3 million year average rates derived form the geological and geophysical data (Herring et al., 1986; Carter and Robertson, 1986). Some tectonic plate studies, however, drive a requirement for a higher level of accuracy. For example, the boundaries of the plates have narrow regions where shorter timescale plate-to-plate interactions are important. These include areas of high crustal strain (Figure 3.2), which result in destructive earthquakes and volcanoes. In addition, the plates do not behave exactly rigidly, and the measure of horizontal intraplate deformation could be associated with the thermal contraction of the cooling oceanic lithosphere. This plate shrinkage has recently been detected at the 3 millimeter per year level (Kumar and Gordon, 2009).

Post-Glacial Rebound

During the last Ice Age, vast ice sheets up to 4–5 kilometers thick lay over the Hudson Bay region in northern Canada and across much of Scandinavia. The amount of ice locked up in the ice sheets at the time was enough to cause global sea levels to lie 100–150 meters below their present levels (Peltier, 2004). The pressure from that ice load on Earth’s crust caused the underlying mantle to be depressed. When the ice melted, starting roughly 20,000 years ago and continuing until approximately 10,000 years ago for Canada and Scandinavia, Earth began to rebound. That rebound (also known as glacial isostatic adjustment) continues today because Earth is viscous, and it takes time for a viscous body to fully respond to the removal of a load. Observations of the rebound rate provide information about Earth’s viscosity profile, which plays a key role in determining the pattern and vigor of convection in Earth’s mantle that drives plate motion and causes earthquakes

FIGURE 3.2 Geologic, geodetic, and earthquake data help determine the zones on Earth where the crustal motion diverges from rigid plate motion. Areas of high strain, in red, experience increased earthquakes and volcanoes. SOURCE: Kreemer et al., 2000.

FIGURE 3.2 Geologic, geodetic, and earthquake data help determine the zones on Earth where the crustal motion diverges from rigid plate motion. Areas of high strain, in red, experience increased earthquakes and volcanoes. SOURCE: Kreemer et al., 2000.



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