tions made on ancient sedimentary rocks and improved models for the climate system that will eventually enable us to predict the magnitude and consequences of climate changes.

  1. How has life shaped Earth—and how has Earth shaped life? Earth scientists have a tendency to view Earth’s geological evolution as a fundamentally inorganic process. Life scientists, in the same spirit, tend to regard the evolution of life as a fundamentally biological issue. Yet the development of life has clearly been influenced by the conditions of Earth’s surface, while Earth’s surface has been influenced by the activities of life forms. The atmosphere would not contain oxygen if it were not for life, and the presence of oxygen has enabled other types of life to evolve. We know that geological events and meteoroid impacts have caused massive extinctions in the past and influenced the course of evolution. But the exact ties between geology and evolution are still elusive. On the modern Earth we are interested in the role of life in geological processes like weathering and erosion. And we seek to understand how life may have manifested itself and left traces preserved in the geological records of other planets.

  2. Can earthquakes, volcanic eruptions, and their consequences be predicted? Thanks largely to sensitive new instrumentation and better understanding of causes, geologists are moving toward predictive capabilities for volcanic eruptions. For earthquakes, progress has been made in long-term forecasts, but we may never be able to predict the exact time and place an earthquake will strike. Continuing challenges are to deepen our understanding of how fault ruptures start and stop, to improve our simulations of how much shaking can be expected near large earthquakes, and to increase the warning time once a dangerous earthquake begins. Studies of volcanic activity have entered a new era as a result of real-time seismic, geodetic, and electromagnetic probes of active subsurface processes. But it remains a challenge to integrate such real-time data with field studies of volcanoes and laboratory studies of volcanic materials. The ultimate objective is to develop a clear picture of the movement of magma, from its sources in the upper mantle to Earth’s crust, where it is temporarily stored, and ultimately to the surface where it erupts.

  3. How do fluid flow and transport affect the human environment? Good management of natural resources and the environment requires knowledge of the behavior of fluids, both below ground and at the surface. The major scientific objectives are to understand how fluids flow, how they transport materials and heat, and how they interact with and modify their surroundings. New experimental tools and field measurement techniques, plus airborne and spaceborne measurements, are offering an unprecedented view of processes that affect both the surface and the subsurface. But we still have difficulty determining how subsurface fluids are distributed in heterogeneous rock and soil formations, how fast they flow, how effectively they transport dissolved and suspended materials, and how they are affected by chemical and thermal exchange with the host formations. Much better models of streamflow and associated erosion and transport are needed if we are to accurately assess how human impacts and climate change affect landscape evolution and how these effects can be managed to sustain ecosystems and important watershed characteristics. The ultimate objective—to produce mathematical models that can predict the performance of natural systems far into the future—is still out of reach but critical to making informed decisions about the future of the land and resources that support us.

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