Hotspots and Mantle plumes

a brief idea about worlds various hotspots, hotspot tracks , their distribution and occurrences

1. DEPARTMENT OF APPLIED GEOLOGY A SEMINAR PRESENTATION ON world’s mantle plumes and hotspots Presented by- Barun Bedanta Sahoo M.Tech 4 th sem Reg. No- Y16251021

2. Contents 1.Introduction What is mantle plume ? What is hotspot ? How mantle plumes are related to hotspots ? 2.Characteristics of hotspots 3. Hotspot tracks 4. Distribution of hotspots and hotspot tracks around the world 5.The plume model 6.Hotspots Swells 7.Distinct geochemical signature 8.Association with flood basalt 9.The fixity of the hotspots 10.Conclusion

3. 1.Introduction What is mantle plume ? Mantle plumes appear to be long, nearly vertical columns of hot, upwelling materials that buoyantly rise from deep in the mantle, first proposed by J Tuzo Wilson in 1963 . A mantle plume is posited to exist where hot rock nucleates at the core-mantle boundary and rises through the Earth's mantle becoming a diapir in the Earth's crust. What is hotspot ? These are the volcanic regions thought to be fed by the underlying mantle . Eg . Hawaiian island Their position on the Earth’s surface is independent of tectonic plate boundary.

4. How mantle plumes are related to hotspots ? Mantle plumes are areas where heat and/or rocks in the mantle are rising towards the surface. A hotspot is the surfacial expression for mantle plumes . About 95% of the world’s volcanoes are located near the boundaries of tectonic plates. The other 5% are thought to be associated with mantle plumes and hot spots. (Generation of hotspots and mantle plumes , www.google.com/wiki )

5. 2.Chacateristic features of hotspots In ocean basins, hotspots form topographic highs of 500-1200 m with typical widths of 1000-1500 km. These highs are probably indirect manifestations of ascending mantle plumes. Many hotspots are capped by active or recently active volvcanoes . Examples are Hawaii and Yellowstone Park in the western United States. Most oceanic hotspots are characterized by gravity highs reflecting the rise of more dense material from the mantle. Some, however, have gravity lows. One or two aseismic ridges of mostly extinct volcanic chains lead away from many oceanic hotspots. Most hotspots have high heat flow , probably reflecting a mantle plume at depth.

6. 3.Hotspot tracks In continental areas, the age of magmatism and deformation may increase with distance from a hotspot. These features are known as hotspot tracks. Chains of seamounts and volcanic islands are common in the pacific basin, and include such well-known island chains as Hawaiian- Emperor Line, Society and Austral islands, all of which are subparallel to either the Emperor or Hawaiian chains and approximately perpendicular to the axis of East Pacific Rise. The life span of hotspots vary and depend on such parameters as plume size and tectonic environment into which plume is emplaced. In the pacific ocean, three volcanic chains were generated by hotspots between 70 and 25 Ma, whereas twelve chains have been generated in the last 25 My.

7. (Major hotspot tracks in the world ,www.gooogle.com /wiki )

8. Isotopic dates demonstrate that the focus of volcanism in the Hawaiian chain has migrated to the southeast at a linear rate about 10 cm/y for the last 30 My. (Linear increase of ages with distance along the Hawaii- Emperor chain , hilo.hawaii.edu/~ kenhon /GEOL205 )

9. Similar linear decrease in the age of the volcanism occur towards the south-east in the Marquesas, Society, and Austral island in the south pacific, with rates of migration of the order of 11cm/y , and in the Pratt-Welker seamount chain in the Gulf of Alaska at a rate about 4 cm/y. (Gradual decrease in elevation with increasing distance from the active volcano , www.google.com /wiki )

10. 4.Distribution of hotspots and hotspot tracks around the world Hotspot tracks hotspots (www.googe.com/wiki)

11. Table . Hot Spot Locations Hot Spot Overlying Plate Latitude Longitude ( degree ) ( degree ) Hawaii Pacific 20 − 157 Samoa Pacific − 13 −173 St. Helena Africa −14 −6 Bermuda N. America 33 −67 Cape Verde Africa 14 −20 Pitcairn Pacific −26 − 132 MacDonald Pacific − 30 − 140 Marquesas Pacific − 10 −138 Tahiti Pacific −17 − 151 Easter Pac- Naz − 27 − 110 Reunion Indian − 20 55 Yellowstone N . America 43 − 111 Galapagos Nazca − 92 Juan Fernandez Nazca −34 −83 Ethiopia Africa 8 37 Ascencion S . Am- Afr − 8 −14 Afar Africa 10 43 Azores Eurasia 39 − 28 Iceland N. Am-Eur 65 −20 Madeira Africa 32 − 18 (After Crough and Jurdy (1980 )., www.google.com/wiki/ blogspot )

12. Somewhere between 40 and 150 active hotspots have been described on the Earth. The best documented hotspots have rather a irregular distribution occurring in both oceanic and continental areas. Some occur on or near the ocean ridges, such as Iceland. St.Halena and Tristan in the Atlantic basin while others occur near the centres of plates such as, such as Hawaii.

13. 5.The plume model Morgan’s plume model (Morgan, 1971): Volcanic islands are produced by plumes rising through the mantle . The plumes come from the lower mantle - and are therefore fixed . Plume flow derives the plates. (Schematic illustrations for the plume model., www.google.com /wiki )

14. Seismic topography- Seismic images suggest that some mantle plumes originate at lower mantle.

15. Contd … If all hotspots have been remained fixed with respect to each other, it should be possible to superimpose the same hotspots in their present position on their predicted positions at other times in the last 150-200 my. However, except for hotspots in the near proximity of each other or on adjacent plates, its not possible to do this, suggesting that hotspots move in the upper mantle (Duncan and Richards,1991) In comparing Atlantic with Pacific hotspots, there are significant differences between calculated and observed hotspot tracks (Molnar and Stock , 1987 ). Rates of interplate hotspot motion, however, are more than an order of magnitude less than plate velocities. For instance, using paleolatitudes deduced from seamounts, Tarduno and Gee (1995) show that Pacific hotspots have moved relative to Atlantic hotspots at a rate of only 30 mm/y.

16. 6.Hotspot swells Most hotspots are associated with topographic swells. Hot spot swells are regional topographic highs with widths of about 1,000km and up to 3 km of anomalous elevation. The swell associated with the Hawaiian hot spot is illustrated in Figure. The swell is roughly parabolic in planform and it extends upstream of the active hot spot, i.e., toward the spreading center of the East Pacific Rise. The excess elevation associated with the swell decays rather slowly down the track of the hot spot

17. There is considerable observational evidence that the topography of hot spot swells is directly associated with a geoid anomaly ( Haxby and Turcotte , 1978). This correspondence is strong evidence that the excess topography and mass of the swell are compensated at depth by anomalously light, possibly hot mantle rock. One model for isostatic compensation assumes horizontal variations in density over a prescribed depth ’ W’ , the so-called Pratt compensation. The variable density ρp is related to the elevation h above the adjacent ocean basins by ρ p = ρ W + ρ w h W + h where ρ = reference density corresponding to zero elevation, ρ w = is seawater density, W = depth of compensation

18. With the ocean basin as reference, the geoid anomaly and associated with the compensated topography is- The geoid anomaly is linearly dependent on the topography so that the local geoid to topography ratio should be a constant for each swell.

19. 7.Distinct geochemical signature The content of incompatible elements is by 1 to 2 orders of magnitude higher in Ocean Island basalt (OIB, e.g. Hawaii, EM-1 and HIMU) than it is in Mid-Oceanic Ridge Basalt (MORB). This implies different reservoirs for OIB and MORB . (Figure from H ofmann ,1977., hilo.hawaii.edu/~ kenhon /GEOL205 )

20. The position of the OIB between MORB and continental crust suggests that OIB source may be the result of back mixing of continental material into the mantle. How different chemical reservoirs may still exist if the mantle is undergoing global mixing is yet an open question. (Figure from H ofmann ,1977., hilo.hawaii.edu/~ kenhon /GEOL205 )

21. 8.Association with flood basalt Morgan, in 1981, pointed out that a number of hotspot tracks originate in flood basalt* provinces. He explained that flood basalt was produced from a plume head arriving at the base of the lithosphere. (Flood basalts of the world., www.google.com/wiki )

22. Flood basalt are the largest known volcanic eruptions in the geologic record, and typically comprise basalt of the order of 1 km thick over an area up to 2000 km across . The association of the Deccan trap in India with the Reunion hotspot track. The flood basalt eruption is due to the arrival of the plume head, and the hotspot track is formed by the plume tail. (Figure from White and McKenzie, 1989) (Figure from Dynamic Earth by G.F. Davies)

23. 9.The fixity of hotspots Paleo -magnetic data strongly suggests that all of the lava solidified at 19.5 degrees north latitude , precisely the latitude of the hotspot today. At least with respect to latitude it would seem that the Hawaiian hotspot has been nearly fixed for at least the past 65 million years . ( Paleomagnetic determination of hotspot location. ,/wiki/ blogspot )

24. That portions of island chains of similar age are parallel to each other suggests that the hotspots themselves remain mostly fixed with respect to each other, otherwise the chains might be expect to trend in different directions as the plumes generating them moved independently. Contd … ( A closer look at the Pacific hilo.hawaii.edu/~ kenhon /GEOL205 )

25. 10.Conclusion From the foregoing, it should be clear that, like the theory of plate tectonics, the model of a mantle plume is a simple but powerful concept It explains much of the geologic activity in the central parts of plates that never seemed to fit a simple interpretation of plate tectonics . Volcanic islands, rifts in continents, flood basalts , and continental calderas find explanations in the mantle plume model. Recently , mantle plumes have been used to explain another class of phenomena, including climate change, mass extinctions, and even changes in Earth’s magnetic field.

26. 1.Condie , K. C. 2001. Mantle Plumes and Their Record in Earth History. New York: Cambridge University Press. 2.Duncan , R. A., and M. A. Richards. 1991. Hotspots, mantle plumes, flood basalts, and true polar wander. Reviews of Geophysics 29: 31–50. 3.Hill , R. I., I. H. Campbell,G . F. Davies, and R.W. Griffiths. 1992. 4.Mantle plumes and continental tectonics. Science 256: 186–193. 5.Larson , R. L. 1995.The mid-Cretaceous superplume episode. Scientific American 272 (2):82–86. 6.Smith , R. L., and L.W. Braille. 1994.The Yellowstone hotspot. 7.Journal of Volcanology and Geothermal Research 61: 121–127 . 8.Earth’s Dynamic Systems Website- www.prenhall.com/hamblin References

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