Crust and Lithospheric Structure – Hot Spots and Hot-Spot Swells

Hot spots, defined as volcanic regions unrelated to standard plate boundary processes such as plate separation or subduction, are widespread on the surface of the Earth in both ocean basins and on continents. Early studies of oceanic islands, atolls, and seamounts recognized that these volcanic features frequently occur in linear chains, that these volcanoes subside as they age, and that the volcano ages increase monotonically along the chains. With the advent of plate tectonic theory came the ability to model oceanic volcanic chains as the result of plate movement over stationary, subplate magmatic sources and the recognition that similar time-transgressive volcanic features occur on continents. The plume hypothesis, in which discrete plumes of hot buoyant material ascend from deep within the mantle, each producing a thermal anomaly and partial melting at the base of the overlying moving plate, became the most promising hypothesis to explain hot-spot volcanism. However, three decades of observations and modeling have failed to unequivocally establish a plume origin for any hot spot, and many instances of intraplate volcanism are clearly not related to a deep-mantle plume. This chapter reviews the types, geophysical characteristics, and current understanding of hot spots. Hot-spot volcanism ranges in scale from individual volcanic chains, clusters, and ridges to larger areas of copious volcanism called large igneous provinces (LIPs) and uplifted regions containing multiple hot-spot chains known as superswells. The majority of the long hot-spot chains are located on the Pacific Plate, and here most of the similar-aged chains have been shown to have orientations consistent with a single rotation pole. However, detailed age dating of individual volcanoes has revealed that most of the hot-spot chains do not have simple monotonic age progressions as predicted by the plume hypothesis. Many active hot spots exhibit a swell, or region of relative uplift, surrounding the active volcanism. Basic geophysical observations such as depth, subsidence, gravity and geoid anomalies, and heat flow are used to probe the crust and lithospheric structure of hot spots and their swells. The depth anomalies of mid-plate swells require density reduction at depth, and the geoid height requires the low-density material to be below the mid-lithosphere in most cases. Heat flow evidence suggests that the low-density material is at least partially thermal in origin, although the effects of near-surface fluid circulation effectively mask much of the spatial and temporal pattern of any thermal disturbance. In order to use teleseismic data to constrain global mantle structure and potentially to relate mantle heterogeneities to hot spots, the shallow crustal and lithospheric structure must first be understood. Marine seismic reflection and refraction techniques are used to image the crustal and upper-mantle structure of hot spot features. Seismic reflection profiles image the sediments deposited around seamounts and islands, revealing the flexure of the preexisting crust due to the volcanic loading. In many cases, the volume of sediments eroded from the volcanic edifices equals or exceeds that of the remaining volcanoes, greatly increasing estimates of hot-spot magmatic flux. Seismic refraction experiments show that magmatic underplating is a common, but not universal, feature of many types of hot-spot volcanism. Global seismic tomography is beginning to resolve upper-mantle features at a resolution sufficient to relate surface and deep-Earth features, but studies to date have identified different sets of candidate plumes. Plume theory remains viable, but only a small number of hot spots appear associated to deep-mantle plumes, and the nature of the plumes remains very much in question.