Earth Materials. John O'Brien

others suggested that hotspots were the surface expression of fixed, long‐lived mantle plumes. Mantle plumes were hypothesized to be columns of warm material which rose from near the core–mantle boundary. Some plumes appear to develop plume heads, as they spread outward near the base of the lithosphere (Griffiths and Campbell 1990). These evolving plume heads may be the cause of the apparent drift of hot spots, depending on how they spread out beneath the lithosphere. Later workers hypothesized that deep mantle plumes originate in the ultra‐low velocity zone (LVZ) of the D″ layer at the base of the mantle and may represent the dregs of subducted slabs warmed sufficiently by contact with the outer core to become buoyant enough to rise. Huge superplumes (Larson 1991) were hypothesized to be significant players in extinction events, the initiation, and location (Arndt and Davaille 2013; Condie 2015) of continental rifting, and in the supercontinent cycle (Sheridan 1987) of rifting and collision that has caused supercontinents to form and rift apart numerous times during Earth's history. Eventually most intraplate volcanism and magmatism was linked to hotspots and mantle plumes.

      The picture has become considerably muddled in the twenty‐first century. Many Earth scientists have offered significant evidence that mantle plumes do not exist (Foulger et al. 2005). Others have suggested that mantle plumes exist, but are not fixed (Nataf 2000; Koppers et al. 2001; Tarduno et al. 2009). Still others (Nolet et al. 2006) suggest on the basis of fine‐scale thermal tomography that some of these plumes originate near the core–mantle boundary, others at the base of the transition zone (660 km) and others at around 1400 km in the mesosphere. They suggest that the rise of some plumes from the deep mantle is interrupted by the 660 km discontinuity, whereas other plumes seem to cross this discontinuity. This is reminiscent of the behavior of subducted slabs, some of which spread out above the 660 km discontinuity, whereas others penetrate it and apparently sink to the core–mantle boundary. Recent advances in new imaging methods that use powerful supercomputers have suggested that plumes originating near the base of the mantle do exist beneath many hotspots (French and Romanowicz 2015; Nelson and Grand 2018; Sanni et al. 2019) including Yellowstone, Hawaii, and Iceland, even though they are not always vertical. Wang et al. (2017) demonstrated that most groups of hotspots migrate very slowly, if at all, over time. It is very likely that hot spots are generated by a variety of processes related to mantle convection patterns, but these are still not well understood. Deep Earth tomography will continue to be an exciting area of Earth research over the coming decade.

      In this chapter, we have attempted to provide a spatial and tectonic context for the processes which form Earth materials. One part of this context involves the location of compositional and mechanical layers within the geosphere where Earth materials form. Ultimately, however, the geosphere cannot be viewed as a group of static layers. Plate tectonics implies significant horizontal and vertical movement of the lithosphere with compensating motion of the underlying asthenosphere and deeper mantle. Global tectonics suggests significant lateral heterogeneity within layers and significant vertical exchange of material between layers caused by processes such as convection, subduction and mantle plumes.

      Helping students to understand how variations in composition, position within the geosphere and tectonic processes interact on many scales to generate distinctive Earth materials is the fundamental task of this book. We hope you will find what follows is both exciting and meaningful.

CONTENT ASSESSMENT

      1 What properties distinguish the following zones of Earth's interior? Elaborate.continental crust, oceanic crust, and mantlelithosphere, asthenosphere, and mesospherelow velocity zone (LVZ), transition zone, and D″ layercore, outer core, and inner core

      2 Detail the processes by which oceanic crust is created and grows through time and contrast these with the processes by which it shrinks and is “destroyed.”

      3 Explain why the age of oceanic crust generally increases systematically away from the ridge system axis in both directions and the major reasons why there are so many local exceptions to this rule.

      4 Describe the three major types of plate boundaries and the features that are associated with and produced by each.

      5 Explain how transform faults between two ridge segments form and how, over time, they can generate long fracture zones in oceanic crust. In addition, contrast the earthquake activity on transforms with that on (the external portions of) fracture zones and explain the major reason for this contrast.

      6 What is the major process are involved in “collisional tectonics”? Detail the features are produced by and that record such collisional events.

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      3 Arndt, N. and Davaille, A. (2013). Episodic earth evolution. Tectonophysics 609: 661–674.

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      24 Nataf,