bly, new conditions are created by these natural movements. The original state can never be exactly regained. The new conditions are the result of what is referred to as secular change—change with time.

The most obvious secular change that has taken place during earth history is the early transformation of its surface from a landscape of naked rock, barren seas, and toxic atmosphere to a landscape seething with life, with organisms that exist on a variety of scales and in a medley of forms. As the cycles have churned away and new secular changes have occurred, sporadic catastrophic events have thrown the whole dynamic system into chaos.

Geoscientists use an assortment of techniques and instruments to investigate the complex interactive systems that have created the surface environment. A 200-year-old tradition of field mapping and detailed description offers a solid foundation—called ground truth—for new technologies like remote sensing and for new conceptual models such as ones that explain how biological evolution has altered the chemistry of the atmosphere.

Remote-sensing technology is not limited to satellite imagery and geodesic laser measurements; remotely sensed magnetic and gravitational anomalies help trace the vertical and horizontal movements of the crust.

Fine-tuned seismic reflection research has also provided a new way to "see" inside the Earth. Seismic reflection can now produce subsurface images that rival, in an areal extent, drilled cores for locating geological boundaries; the cores, however, are often needed to provide ground truth. Tomography combines sets of seismic reflectance data to create cross sections through various planes.

Mapping also reveals patterns of past environments, ranging from tropical seas to jungles and deserts. These environments are identified by a great variety of evidence, including fossil occurrences, key sedimentary rocks, and the isotopic composition of shells. Maps of fossil occurrences show how organisms were distributed in space and where they lived bears directly on how they evolved.

Once geologists have determined what the patterns are, they can study how those patterns changed; on a planetary scale, this dual effort is at the heart of studies of global change. Geochronology provides the framework for arranging temporal sequences in the geological record. It also provides estimates for rates of chemical, physical, and biological change. Paleontology and stratigraphy led to the first arrangement of geological time scales. Intervals of the Earth's past gained names like Precambrian, Devonian, Cretaceous, and Pleistocene. Today, new quantitative methods for analyzing fossil occurrences are improving our ability to compare ages of strata throughout the world.

Techniques that use naturally occurring radioactive isotopes can now provide dates within a 2-million-year range of accuracy for events that affected particular materials 2.5-billion-years ago. At the other extreme of recency and resolution, close documentation of tree-ring patterns yields chronologies accurate to less than 1 year.

Reconstruction of ancient seas, climates, and continental geography for key intervals of the past provides data for constructing and testing



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