Hotspots

The major part of the Earth's volcanic activity takes place at plate margins. However, a significant fraction occurs within the interiors of plates. In oceans the intra-plate volcanic activity gives rise to linear island and seamount chains such as the Hawaiian-Emperor and Line Islands chains in the Pacific (Fig. 5.7). Moreover, several of these Pacific island chains appear to be mutually parallel. Where the volcanic centers in the chains are closely spaced, aseismic ridges are constructed, such as the Ninety-East Ridge in the Indian Ocean, the Greenland-Scotland Ridge in the North Atlantic, and the Rio Grande and Walvis ridges in the South Atlantic. These island chains and ridges are associated with broad crustal swells which currently occupy about 10% of the surface of the Earth, making them a major cause of uplift of the Earth's surface (Crough, 1979).

The island chains are invariably younger than the ocean crust on which they stand. The lower parts of these volcanic edifices are believed to be formed predominantly of tholeiitic basalt, while the upper parts are alkali basalts (Karl et al., 1988) enriched in Na and K and, compared to mid-ocean ridge basalts, have higher concentrations of Fe, Ti, Ba, Zr, and rare earth elements (REE) (Bonatti et al., 1977). Their composition is compatible with the mixing ofjuvenile mantle material and depleted asthenosphere (Schilling et al., 1976) (Section 6.8). They are underlain by a thickened crust but thinned lithosphere, and represent a type of anom

Hotspot Morgan 1971

Figure 5.7 Hotspot tracks on the Pacific plate. HE, Hawaiian-Emperor chain; A-C, Austral-Cook islands; L, Line islands; LS, Louisville chain; OP, Ontong-Java Plateau. Numbers on chains indicate the predicted age of seamounts in Ma (redrawn from Gaina et al., 2000, by permission of the American Geophysical Union. Copyright © 2000 American Geophysical Union).

Figure 5.7 Hotspot tracks on the Pacific plate. HE, Hawaiian-Emperor chain; A-C, Austral-Cook islands; L, Line islands; LS, Louisville chain; OP, Ontong-Java Plateau. Numbers on chains indicate the predicted age of seamounts in Ma (redrawn from Gaina et al., 2000, by permission of the American Geophysical Union. Copyright © 2000 American Geophysical Union).

alous feature that will eventually become welded to a continental margin as a suspect terrane (Section 10.6.1).

An example of an oceanic island chain is the Hawaiian-Emperor chain in the north-central Pacific Ocean (Fig. 5.7). This chain is some 6000 km long and shows a trend from active volcanoes at Hawaii in the southeast to extinct, subsided guyots (flat-topped sea-mounts) in the northwest. Dating of the various parts of the chain confirmed this trend, and revealed that the change in direction of the chain occurred at 43 Ma (Clague & Dalrymple, 1989). The Hawaiian-Emperor chain parallels other chains on the Pacific Plate, along which volcanism has progressed at a similar rate (Fig. 5.7).

As indicated above, a possible explanation of the origin of island chains was proposed by Wilson (1963). It was suggested that the islands formed as the lithosphere passed over a hotspot. These hotspots are now thought to originate from mantle plumes rising from the lower mantle that thin the overlying lithosphere (Section 12.10). The volcanic rocks are then derived from pressure-release melting and differentiation within the plume. Such plumes represent material of low seismic velocity and can be detected by seismic tomography (Section 2.1.8; Montelli et al., 2004a). Although the mantle plume mechanism has been widely adopted, some workers (e.g. Turcotte & Oxburgh, 1978; Pilger, 1982) have questioned the necessity for mantle hotspots and suggest that magmas simply flow to the surface from the asthenosphere through fractures in the lithosphere resulting from intra-plate tensional stresses. This mechanism obviates the problem of maintaining a mantle heat source for long periods. It does not, however, explain why fractures in the same plate should trend in the same direction and develop at similar rates (Condie, 1982a).

Morgan (1971, 1972a) proposed that mantle plumes remain stationary with respect to each other and the lower mantle, and are of long duration. If so, the hotspots represent a fixed frame of reference by which absolute motions of plates can be determined (Section 5.4).

Between 40 and 50 present day hotspots have been suggested (Fig. 5.8) (Duncan & Richards, 1991; Cour-tillot et al., 2003). It seems unlikely, however, that all of these centers of intra-plate volcanism, or enhanced igneous activity at or near ridge crests, are of the same type or origin. Many are short-lived, and consequently have no tracks reflecting the motion of the plate on which they occur. By contrast, others have persisted for tens of millions of years, in some cases over 100 million years, and can be traced back to a major episode of igneous activity giving rise to flood basalts on land or

Intraplate Igneous Activity
Figure 5.8 World-wide distribution of hotspots (modified from Duncan & Richards, 1991, by permission of the American Geophysical Union. Copyright © 1991 American Geophysical Union).

an oceanic plateau under the sea. These remarkable episodes of localized enhanced partial melting in the mantle punctuate the geologic record and are collectively termed Large Igneous Provinces (LIPs) (Section 7.4.1). It seems probable therefore that there are at least two types of hotspot and that those originating as LIPs are the most likely to be a result of plume heads rising from deep within the mantle, probably from the thermal boundary layer at the core-mantle boundary (Section 12.10).

Courtillot et al. (2003) proposed five criteria for distinguishing such primary hotspots (Section 12.10). They suggest that, on the basis of existing knowledge, only seven present day hotspots satisfy these criteria, although ultimately 10-12 may be recognized. The seven are Iceland, Tristan da Cunha, Afar, Reunion, Hawaii, Louisville, and Easter (Fig. 5.8). The first four of these hotspots are within the "continental hemisphere," which consists of the Indian and Atlantic Oceans and the continents that surround them. All four were initially LIPs characterized by continental flood basalts, and associ ated with the rifting of continental areas, followed by the initiation of sea floor spreading (Sections 7.7, 7.8). The Parana flood basalts of Uruguay and Brazil, and the Etendeka igneous province of Namibia, emplaced 130 Ma ago, were the first expression of the Tristan da Cunha hotspot, and precursors of the opening of the South Atlantic. The Deccan Traps of western India were extruded 65 Ma ago coinciding with the creation of a new spreading center in the northwest Indian Ocean. This hotspot would appear to be located at the present position of Reunion Island (Fig. 5.9). The first igneous activity associated with the Iceland hotspot would appear to have occurred 60 Ma ago giving rise to the North Atlantic igneous province of Greenland and northwest Scotland, and heralding the initiation of sea floor spreading in this area. The Afar hotspot first appeared approximately 40 Ma ago with the outpouring of flood basalts in the Ethiopian highlands, and igneous activity in the Yemen, precursors of rifting and spreading in the Red Sea and Gulf of Aden. The remaining three primary hotspots of Courtillot et al. (2003) occur

Reunion Hotspot

Figure 5.9 Hotspot tracks in the Atlantic and Indian Oceans. Large filled circles are present day hotspots. Small filled circles define the modeled paths of hotspots at 5 Ma intervals. Triangles on hotspot tracks indicate radiometric ages. WM, White Mountains; PB, Parana flood basalts; EB, Etendeka flood basalts; DT, Deccan Traps (modified and redrawn from Müller et al., 1993, courtesy of the Geological Society of America).

Figure 5.9 Hotspot tracks in the Atlantic and Indian Oceans. Large filled circles are present day hotspots. Small filled circles define the modeled paths of hotspots at 5 Ma intervals. Triangles on hotspot tracks indicate radiometric ages. WM, White Mountains; PB, Parana flood basalts; EB, Etendeka flood basalts; DT, Deccan Traps (modified and redrawn from Müller et al., 1993, courtesy of the Geological Society of America).

within the "oceanic hemisphere," i.e. the Pacific ocean, and have produced distinctive traces across the Pacific plate (Fig. 5.7). The Louisville Ridge originates at the OntongJava Plateau of the western Pacific. This formed approximately 120 Ma ago and is the largest LIP in terms of the volume of mafic igneous material emplaced. The Hawaiian-Emperor seamount chain may well have had a similar origin but the earlier part of this track has been subducted, the oldest seamounts in the chain dating at approximately 80 Ma. The Easter Island-Line Islands track originated about 100 Ma ago, not as an LIP, but in an area with an unusually high density of submarine volcanoes known as the mid-Pacific mountains.

The relative positions of the continents around the Atlantic and Indian oceans, for the past 200 Ma, are well constrained by the detailed spreading history contained within these oceans (Section 4.1.7). If one or more hotspot tracks within this Indo-Atlantic hemisphere are used to determine the absolute motions of the relevant plates in the past, tracks for the remaining hotspots in this hemisphere can be predicted. Comparison of the observed and predicted tracks provides a test of the fixed hotspots hypothesis, and a measure of the relative motion between the hotspots. Such an analysis by Müller et al. (1993) suggests that the relative motion between hotspots in the Indo-Atlantic reference frame is less than 5 mm a-1, i.e. an order of magnitude less than average plate velocities. A similar analysis for Pacific hotspots by Clouard & Bonneville (2001) yields a similar result for the Pacific reference frame. However, there are problems in linking together the two reference frames; in other words, in predicting Pacific hotspot traces using the Indo-Atlantic reference frame or viceversa. This is because, for most of the Mesozoic and Cenozoic, the oceanic plates of the Pacific hemisphere are surrounded by outward dipping subduction zones, except in the south. This means that in order to determine the motion of the Pacific Ocean plates relative to the Indo-Atlantic hemisphere one must have a detailed knowledge of the nature and evolution of the plate boundaries around and within the Antarctic plate in the South Pacific area. Unfortunately there are still uncertainties about this, but an analysis based on the model of Cande et al. (1999) for the evolution of these boundaries suggests that the two reference frames or domains are not compatible, despite the compatibility of hotspot tracks within each domain (Fig. 5.10). The discrepancy is greatest before 40-50 Ma, when the relative motion between the two hotspot frames is approximately

50 mm a-1. Intriguingly, this corresponds with a period of major reorganization of global plate motions (Rona and Richardson, 1978), the age of the major bend in the Hawaiian-Emperor seamount chain, and a period in which the rate of true polar wander (Section 5.6) was much greater than during the period 10-50 Ma ago, when it was virtually at a standstill (Besse & Courtillot, 2002).

If hotspots remain fixed, and provide a framework for absolute plate motions, then paleomagnetic studies should be able to provide a test of their unchanging latitude. Paleomagnetic data for the oceanic plates of the Pacific are sparse, and subject to greater uncertainties than those obtained for continental areas. Nevertheless preliminary results (Tarduno & Cottrell, 1997) suggest that the Hawaiian hotspot may have migrated south through as much as 15-20° of latitude during the period 80-43 Ma. Paleomagnetic results obtained from Ocean Drilling Program drill core, from which any latitudinal change in of the Reunion hotspot could be deduced (Vandamme & Courtillot, 1990), suggest that this hotspot may have moved northwards through approximately 5° of latitude between 65 and 43 Ma. These latitudinal shifts are compatible with the discrepancy between the two hotspot reference frames prior to 43 Ma ago, and support the assumptions regarding Cenozoic - late Mesozoic plate boundaries within and around the Antarctic plate. These results also imply that the major bend in the Hawaiian-Emperor seamount chain at approximately 43 Ma does not reflect a major change in the absolute motion of the Pacific plate, as originally thought, but can be accounted for almost entirely by the southward motion of the Hawaiian hotspot (Norton, 1995).

Predicted hotspot traces in the Atlantic and Indian Ocean (Müller et al., 1993) are shown in Fig. 5.9, superimposed on volcanic structures on the sea floor and on land. The correlation between the two is excellent. For example, the Reunion hotspot began beneath western India and was responsible for the Deccan Traps flood basalts: India's northwards motion was then recorded by the Maldive-Chagos Plateau and the Mascarene Plateau. The gap between these two features results from the passage of the mid-ocean ridge over the hot spot approximately 33 Ma ago. The hotspot is currently beneath a seamount 150 km west of the volcanically active island of Réunion.

It will be noted that Iceland has not been included in Fig. 5.9. If one assumes that this hotspot was initiated 60 Ma ago beneath East Greenland then its track

150" 160" 170" 180" 190" 200" 210'

150" 160" 170" 180" 190" 200" 210'

Reunion Hotspot
Figure 5.10 Predicted Hawaiian hotspot track (solid line) from plate reconstructions assuming that the Indo-Atlantic hotspots are fixed. Ages in Ma (redrawn from Steinberger & O'Connell, 2000, by permission of the American Geophysical Union. Copyright © 2000 American Geophysical Union).

implies that its position is not fixed relative to the other major hotspots in the Indo-Atlantic domain. However one can use the absolute motions derived from the other hotspots (Müller et al., 1993) to predict the track of the Iceland hotspot on the assumption that it is fixed in relation to this frame of reference. Such an analysis has been conducted by Lawver & Müller (1994) with intriguing results (Fig. 5.11). The track can be projected back to 130 Ma, at which time the hotspot would have been beneath the northern margin of Ellesmere Island in the Canadian Arctic. Lawver & Müller (1994) suggest that such a track might explain the formation of the Mendeleyev and Alpha Ridges in the Canadian Basin of the Arctic Ocean and the mid-Cretaceous volcanic rocks of Axel Heiberg Island and northern Ellesmere Island. At 60 Ma the hotspot is predicted to have been beneath West Greenland where there are volcanics of this age, for example on Disko Island. At 40 Ma it would have been beneath East Greenland which may explain the anomalous post-drift uplift of this area. On this model the North Atlantic igneous province, initiated at approximately 60 Ma, was a result of rifting of lithosphere that had already been thinned by its proximity to a hotspot, rather than the arrival of a plume head. In contrast to this interpretation, however, there is considerable doubt, on the basis of geochemical and geophysical data, that the Iceland hotspot is fed by a deep mantle plume (Section 12.10). The Iceland hotspot is therefore something of an enigma.

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