This chapter provides a catalog and maps of those volcanic, tectonic, and geochemical features that have become known as "hotspots," including those that may have a shallow plate tectonic or asthenospheric origin. Many proposed hotspots, including isolated structures and the active portions, or inferred ends, of seamount chains, do not have significant swells, substantial magmatic output, or tomographic anomalies. A hotspot catalog, as opposed to a volcano catalog, is therefore subjective. Recent lists of those purported to be underlain by deep mantle plumes disagree strongly. A melting anomaly, or hotspot, may result from localized high absolute mantle temperature or from a localized fertile or fusible patch of the asthenosphere. Some have been called "wetspots," and some have been called "hotlines." The localization may be due to lithospheric stress or architecture. The common characteristics of features designated as hotspots suggest an underlying common cause. This chapter provides references and brief evaluations of individual features and mechanisms that can be used to evaluate the origins of hotspots. Discussions of individual hotspots, volcanic chains, and tomographic results are given in three appendixes.

Hydraulic fracture stimulation of low permeability reservoir rocks is an established and cross–cutting technology for enhancing hydrocarbon production in sedimentary formations and increasing heat exchange in crystalline geothermal systems. Whereas the primary measure of success is the ability to keep the newly generated fractures sufficiently open, long–term reservoir management requires a knowledge of the spatial extent, morphology, and distribution of the fractures — knowledge primarily informed by microseismic and ground deformation monitoring. To minimize the uncertainty associated with interpreting such data, we investigate through numerical simulation the usefulness of direct-current (DC) resistivity data for characterizing subsurface fractures with elevated electrical conductivity by considering a geophysical experiment consisting of a grounded current source deployed in a steel cased borehole. In doing so, the casing efficiently energizes the fractures with steady current. Finite element simulations of this experiment for a horizontal well intersecting a small set of vertical fractures indicate that the fractures manifest electrically in (at least) two ways: (1) a local perturbation in electric potential proximal to the fracture set, with limited farfield expression and (2) an overall reduction in the electric potential along the borehole casing due to enhanced current flow through the fractures into the surrounding formation. The change in casing potential results in a measurable effect that can be observed far from fractures themselves. Under these conditions, our results suggest that farfield, timelapse measurements of DC potentials can be interpreted by simple, linear inversion for a Coulomb charge distribution along the borehole path, including a local charge perturbation due to the fractures. This approach offers an inexpensive method for detecting and monitoring the time-evolution of electrically conducting fractures while ultimately providing an estimate of their effective conductivity — the latter providing an important measure independent of seismic methods on fracture shape, size, and hydraulic connectivity.

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