Science Opportunities - Basic Research Opportunities in Earth Science - page 53

few millimeters over baselines of thousands of kilometers. Through the
combination of space geodesy and ground-based geological mapping,
modern plate boundaries can now be dissected with an unprecedented
level of precision ( Figure 2.11 ), paving the way for advances in
earthquake physics and crustal rheology.
• Construction of precise topographic data sets and digital elevation models
has reinvigorated the study of active tectonics and surface processes
though the analysis of landforms and river drainages. The quantification
of topographic relief has been joined with process-oriented field work and
simulations based in dynamical systems theory to explore the strong
interactions between tectonism, erosion, and climate.
• Isotope geochronologists have dated rocks as old as 4 billion years with an
uncertainty of less than 1%, making it possible to measure the rates at
which major, sometimes catastrophic, events occurred in the distant past.
These constraints have provided a much more precise temporal framework
for investigating the field relationships among geologic, climatic, and
biologic phenomena (e.g., volcanic eruptions, mass extinctions, and major
glacial events).
• Seismic tomography has provided three-dimensional images of the
continents that show the strong correlation between surface geology and
mantle structure. The ancient cratons are underlain by deep “keels”
extending to depths of several hundred kilometers, raising important
questions about the composition and evolution of the subcontinental
mantle that can be addressed by geochemical observations. Measuring the
directional dependence of seismic wave propagation (seismic anisotropy)
and comparing this with laboratory measurements of mineral deformation
and rock textures have made it possible to infer strains within the mantle
part of the lithosphere that can be related to crustal deformations mapped
at the surface.
• Advances in petrology and geochemistry have elucidated the pressure and
temperature variations with time in rocks metamorphosed over a wide
range of depths, including at subcrustal conditions. These techniques have
been applied to samples brought up from depths of hundreds of kilometers
in continental volcanic eruptions, allowing geochemists to constrain the
history of even the deepest parts of the continental lithosphere. Based on
comparisons with the results of high-pressure mineral physics
experiments, some of these rocks appear to have come from as deep as the
lower mantle, providing unique data on large-scale dynamical processes
of the Earth's interior.
Basic research on continental processes has been applied to many of the
practical problems discussed in Chapter 1 , ranging from the search for natural
resources to the mitigation of natural hazards. For example, field work on ancient
fault ruptures (paleoseismology) has been linked with laboratory