During the last ~100 years, tectonic geodesy has evolved from sparse field-based measurements of crustal deformation to the use of space geodetic techniques involving observations of satellites and from satellites orbiting Earth, which reveal a variety of tectonic processes acting over a wide range of spatial and temporal scales. Early terrestrial measurements using triangulation and leveling techniques characterized large displacements associated with great earthquakes and led to the recognition of the fundamental mechanics of seismic faulting and the earthquake cycle. More precise measurements using ground-based laser ranging allowed for the characterization and modeling of interseismic strain buildup and determination of slip rates on major faults. Continuous and highly accurate point measurements of strain, tilt, and fault creep have captured intriguing deformation transients associated with slow slip events on active faults. The greatly improved precision, spatial and temporal resolution, global coverage, and relatively low cost of space geodetic measurements led to a revolution in crustal deformation measurements of a range of tectonic processes. Very Long Baseline Interferometry, the Global Positioning System, Interferometric Synthetic Aperture Radar, and space-based image geodesy complement each other to comprehensively capture tectonics in action at scales ranging from meters to global and seconds to decades. Space geodetic measurements allow for the precise measurement of global plate motions, the determination of strain rate fields and fault slip rates in distributed plate-boundary deformation zones, and characterization of subtle intra-plate deformation. These measurements provide increasingly important constraints for earthquake hazard studies. Space geodesy also allows for the recognition and detailed model exploration of a number of transient deformation processes during the post-earthquake deformation phase of the earthquake cycle. Measurements of postseismic deformation transients provide important insights into the mechanisms, rheological properties, and dynamics of crustal deformation. Increasingly, seafloor geodetic measurements provide information about deformation on the 70% of the Earth's surface that were previously inaccessible. Future improvements of modern geodetic techniques promise to further illuminate details of crustal deformation at all spatial and temporal scales, leading to an improved understanding of the dynamics of active tectonics.

Abstract Volcanoes are not randomly distributed over the Earth's surface. Most are concentrated on the edges of continents, along island chains, or beneath the sea where they form long mountain ranges. More than half of the world's active volcanoes above sea level encircle the Pacific Ocean (see Fig. 1 ). The concept of plate tectonics explains the locations of volcanoes and their relationship to other large-scale geologic features. The Earth's surface is made up of a patchwork of about a dozen large plates and a number of smaller ones that move relative to one another at <1 cm to ~10 cm/yr (about the speed at which fingernails grow). These rigid plates, with average thickness of ~80 km, are separating, sliding past each other, or colliding on top of the Earth's hot, viscous interior. Volcanoes tend to form where plates collide or spread apart ( Fig. 2 ) but can also grow in the middle of a plate, like the Hawaiian volcanoes ( Fig. 3 ). Of the more than 1,500 volcanoes worldwide believed to have been active in the past 10,000 years, 169 are in the United States and its territories ( Ewert et al., 2005 ) (see Fig. 4 ). As of spring 2007, two of these volcanoes, Kilauea and Mount St. Helens, are erupting, while several others, including Mauna Loa, Fourpeaked, Korovin, Veniaminof, and Anatahan, exhibit one or more signs of restlessness, such as anomalous earthquakes, deformation of the volcano's surface, or changes in volume and composition

Early flow paths can be traced along structural segments (single fractures, fracture intercepts, or zones of closely spaced fractures) through hundreds of kilometers of branchwork caves in structurally complex settings along the eastern Allegheny Plateau of West Virginia, USA. Identification of the early fracture conduits presupposes the following primary conditions: (1) prominent fracture traces are retained on bedrock perimeters; (2) fracturing associated with later cave enlargement is minimal and distinguishable from transmissive fractures; (3) conduit enlargement and modification are not so extensive or so directed as to have totally destroyed the fractures; and (4) there is minimal covering of relevant fracture traces by clastic or chemical sedimentation. Criteria used to infer structural segments include: (1) the presence of anastomoses or other dissolutional features along fractures; (2) the presence of tubes, half tubes, or segments of passage concordant to fractures; (3) the existence of anastomoses and other tubes that grow upward from initially transmissive fractures; (4) the presence of features of entrenchment that are lower than remnant tubes, half tubes, and most early parts of joint fissures; and (5) the continuity of flow along fractures, except at locations of dissolutional mining to create mined segments. If structural segments are identified and mapped with high-precision leveling surveys, a framework can be provided to decipher many details of flow-path integration and enlargement. It is then possible to reconstruct cave history using evidence provided by analysis of passage morphology, sediments, and the relationships of cave features to local and regional surface features.