regions of plate convergence (New Zealand–New Guinea–Japan–Kamchatka–Aleutians and western South America) indicate mass variations associated with mantle convection and the variation of density and strength in Earth’s interior. For example, Hager (1984) demonstrated that these geoid highs over subduction zones require a substantial increase in viscosity with depth between the upper mantle and lower mantle, resulting in an impediment to convective mass transport across the boundary between these two layers. The gravity low over Hudson Bay (Figure 11.2) is due, in part, to a remaining depression in the surface caused by the weight of the great Laurentide ice sheet that melted at the end of the last ice age. The estimate that almost half of this gravity low is the result of ongoing post-glacial rebound again requires a substantial increase in the viscosity of the mantle with depth, otherwise the surface depression would have relaxed more by now (Simons and Hager 1997). The recent observation by GRACE of the rate at which this gravity low is decreasing in amplitude confirms that almost half of this gravity low is the remnant of the former ice sheet (Tamisiea et al. 2007).

THE GLOBAL POSITIONING SYSTEM

NASA missions provided major contributions to the development of the global reference frame through the GPS, Satellite Laser Ranging, and Very Long Baseline Interferometry technology. GPS and Interferometric synthetic aperture radar (InSAR) methods have provided precise measurements of Earth’s shape and surface positioning (Box 11.1), thus providing detailed local and global topographic and deformation information. Current InSAR satellites include the European Remote Sensing Satellite (ERS), the European Environmental Satellite (ENVISAT), the Japanese Advanced Land Observation Satellite (ALOS), and the Canadian Radarsat program. These satellites and the constellation of GPS satellites track current motions of Earth’s surface at centimeter precision over time and reveal many geophysical processes occurring on the surface and at depth, where they are generally inaccessible to surface observation. Ironically, the use of gravity and deformation data obtained from space has greatly improved our understanding of structure and change deep within the Earth (see below).

PLATE TECTONICS, TOPOGRAPHY, SEISMOLOGY, AND VOLCANOLOGY

The theory of plate tectonics was driven largely by observations in the 1950s from ocean vessels mapping the magnetic field and the seafloor shape, which can now be obtained more easily from satellite observations (Figure 11.3). Several decades later satellite observations enabled a scientific revolution in advancing the theory of plate tectonics by providing highly detailed, quantifiable measurements of Earth’s surface. GPS has enabled measurement of plate positioning and velocities, thus resolving geologic

BOX 11.1

Earth Reference Frame

Few scientific accomplishments are as “transformative” as the advances in space geodesy over the past five decades, particularly with the ubiquitous introduction of GPS devices. This breakthrough not only has transformed the field of geodesy but also provides vital information for studying global sea-level change, earthquakes, and volcanoes, as well as providing precise position information for all Earth science research.

At the time of the International Geophysical Year, the geolocation of most points at the surface of the Earth entailed errors that reached hundreds of meters in remote areas, even after much effort. Today, scientists rely on an International Earth Reference Frame from which geographical positions can be accurately described relative to the geocenter, in three-dimensional Cartesian coordinates to centimeter accuracy or better—a 2 to 3 orders-of-magnitude improvement compared to 50 years ago. This is true anywhere, on an active planet where every piece of real estate moves relative to every other. Geodesy observations from space have enabled modern measurements of Earth’s rotation. The change in position of the rotation axis (the poles) is determined daily to centimeter accuracy, and changes in the length of a day are determined to millisecond accuracy within a few hours. Improvements in GPS measurements over the past few decades have enabled instantaneous geodetic positioning (Genrich and Bock 2006)—a real-time GPS. GPS receivers are now available inexpensively to consumers, who are rapidly becoming accustomed to GPS navigation on the road, on the water, and in the air without realizing the enormous body of science behind this technological achievement: accurate ephemerides of the satellites, very stable clocks, well-calibrated atmospheric corrections, and even relativistic corrections.

and contemporary velocities. For example, Iaffaldano et al. (2006) found that the Nazca Plate moves at a velocity of 6.9 cm per year, compared to its geologic velocity of 10.1 cm per year 10 million years ago. Geologic timescale velocities typically disagree with present rates, with implications for crust-mantle interaction. Factors such as friction or time-dependent processes can be modeled if we understand how the rates vary with time.



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