Hotspots and their mechanism of emplacement

There are over 100 'recognised hotspots', the location and types of which will be discussed later. Hotspots occur beneath both oceans and continents. In the latter situation, the hotspots tend to give rise to domal uplift and areas of high heat-flow associated with igneous activity and/or geothermal activity (e.g. Yellowstone Park, USA). In a marine environment, the hotspots can give rise to individual islands, or seamounts (usually accompanied by an area of domal uplift) as well as chains of such extrusive bodies.

Wilson (1963) suggested that such chains of volcanic islands formed as the lithosphere passed over a relatively stationary hotspot. Perhaps the most famous of these is the Hawaiian-Emperor chain (Figure 4.2), which extends for 6000 km, with a kink approximately halfway along, and took approximately 70-90 Ma to develop. Moreover, as will be seen in this figure, there are other island and seamount chains in the Pacific which show similar trends to the Hawaiian-Emperor chain. Other hotspots and ridges, such as Walvis/ Tristan/ Rio Grande and continental hotspots in the Sahara, are shown in Figure 4.3.

It is now recognised that hotspots are not 'immutably fixed for all time'. Burke et al. (1978) and Molnar and Atwater (1973) established that there has been relative movement between hotspots, within the same plate, over the last 120 Ma, of at least 180 km (i.e. a drift of about 0.15 mm a-1), while Molnar and Stock (1987) have determined relative motions between some hotspots which attain a rate of 0.20 mm a-1. Thus, to assume hotspots remain absolutely stationary over this period can result in a small error. Nevertheless, the hotspots do provide a most useful frame of reference.

We have noted that plumes, in continents, give rise to domal uplift. Erosion and sedimentation tend to affect the topography of these subaerial features. Away from spreading-ridges, the topography of the ocean floor is usually relatively simple compared with some continental areas; hence the best examples of the surface manifestations of a hotspot are to be found in the oceans.

A number of locations where relatively large quantities of eruptive rock occur on or near rifted margins and which exhibit circular or elliptical domes, are listed in Table 4.2. Two

Figure 4.2 Linear chains of volcanic islands, seamounts and ridges in the Pacific which are related to hotspots. The younger islands in the chains are always to the east.

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