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Why is heat flow not high at hotspots?

Carol A. Stein1 & Seth Stein2

1Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago IL 60607-7059 USA
cstein@uic.edu

2Department of Geological Sciences, Northwestern University, Evanston, IL 60208, USA
seth@earth.northwestern.edu

A peculiar aspect of hotspots is that heat flow data provide no evidence for their being hotter than lithosphere of the same age elsewhere. Originally, the uplift at Hawaii and similar midplate hotspots was thought to reflect a hot plume causing heating to about 50 km of the surface [Crough, 1983; McNutt and Judge, 1990]. Such heating predicts heat flow significantly higher than from the usual cooling of oceanic lithosphere as it spreads away from the mid-ocean ridges where it formed. Although anomalously high heat flow was initially reported, subsequent analysis showed that most, if not all, of the apparent anomalies resulted from comparing data to thermal models that underestimated heat flow elsewhere.

Figure 1 illustrates this for the Hawaiian swell, the largest and best-studied hot spot swell. Heat flow on the swell was originally thought to be anomalously high relative to the predictions of the a thermal model by Parsons & Sclater [1977] (PSM). This was consistent with the elevated heat flow expected for the lithosphere being thinned and heated [von Herzen et al., 1982]. A subsequent transect across the swell showed that the heat flow differs at most slightly from that for lithosphere of comparable ages [von Herzen et al., 1989]. Thus much of the apparent anomaly resulted from comparing the heat flow to PSM, which systematically underpredicts the heat flow and overpredicts the depths for old lithosphere such as that near Hawaii, where the crust is 100 Myr old.

Figure 1. Heat flow along (lower left) and across (lower right) the Hawaiian Swell. Heat flow, though anomalously high with respect to the PSM model, is at most slightly above that expected for GDH1 [Stein and Stein, 1993]. Bathymetry across the swell along the heat flow line compared to GDH1 and PSM model predictions (upper right).

A different picture emerges from comparison of the data to model GDH1, which fits the depth and heat flow data significantly better, especially for older lithosphere [Stein and Stein, 1992]. The swell heat flow is at most slightly above that expected for GDH1, leaving no significant anomaly. The situation is similar for the Bermuda, Cape Verde, and Crozet hot spots.

Similarly, heat flow is not unusually high for the Superswell region of the Pacific, which is substantially shallower than expected for its age. Although the shallow bathymetry is consistent with the plate being thermally thickened [McNutt & Judge, 1990], heat flow (Figure 2) does not differ from that for lithosphere of the same age elsewhere in the Pacific [Stein & Abbott, 1991] or a global average [Stein et al., 1995].

Figure 2. Depth (top), and heat flow (bottom) for the Superswell and lithosphere of the same age elsewhere in the Pacific, averaged in 20-Myr bins. Closed & open symbols: means & medians. Although the Superswell is shallow, consistent with lithospheric heating model predictions (dashed), heat flow is similar to the global average & the rest of the Pacific [updated from Stein and Stein, 1993].

Subsequent plume models have generally assumed that the uplift results from the dynamic effects of rising plumes [Liu and Chase, 1989; Sleep, 1994] and the associated compositional buoyancy. The thermal effects of these are postulated to be concentrated at the base of the lithosphere and thus to raise surface heat flow at most slightly, because tens of millions of years are required for heat conduction to the surface. An alternative model is that the uplift results from excess magma production rather than high temperatures, as proposed by Foulger [2002] for Iceland. In the case of that model, no heat flow anomaly is expected.

References

  • Crough, S.T., Hotspot swells, Annual Review of Earth and Planetary Sciences, 11, 165-193, 1983.
  • Liu, M., and C.G. Chase, Evolution of midplate hotspot swells - numerical solutions, J. geophys. Res., 94, 5571-5584, 1989.
  • McNutt, M.K., and A.V. Judge, The superswell and mantle dynamics beneath the south Pacific, Science, 248, 969-975, 1990.
  • Parsons, B., and J.G. Sclater, An analysis of the variation of ocean floor bathymetry and heat flow with age J. geophys. Res., 82, 803-827, 1977.
  • Sleep, N.H., Lithospheric thinning by midplate mantle plumes and the thermal history of hot plume material ponded at sublithospheric depths, J. geophys. Res., 99, 9327-9343, 1994.
  • Stein, C., and D. Abbott, Heat-flow constraints on the south-Pacific superswell, J. geophys. Res., 96, 16,083-16,100, 1991.
  • Stein, C.A., S. Stein, and A. Pelayo, Heat flow and hydrothermal circulation, in Physical, chemical, biological and geological interactions within hydrothermal systems, Am. Geophys. Un., Washington, D.C., 1995.
  • Stein, C.A., and S. Stein, A model for the global variation in oceanic depth and heat-flow with lithospheric age, Nature, 359, 123-129, 1992.
  • Stein, C.A., and S. Stein, Constraints on Pacific midplate swells from global depth-age and heat flow-age models, in The Mesozoic Pacific: Geology, Tectonics, and Volcanism, pp. 53-76, American Geophysical Union, Washington, D.C., 1993.
  • von Herzen, R.P., M.J. Cordery, R.S. Detrick, and C. Fang, Heat-flow and the thermal origin of hot spot swells - the hawaiian swell revisited, J. geophys. Res., 94, 13,783-13,799, 1989.
  • von Herzen, R.P., R.S. Detrick, S.T. Crough, D. Epp, and U. Fehn, Thermal origin of the Hawaiin swell - heat-flow evidence and thermal models, J. geophys. Res., 87, 6711-6723, 1982.

Additional key references on heatflow

  • Kaula, W.M., Minimal upper mantle temperature variations consistent with observed heat flow and plate velocities, J. Geophys. Res., 88, 10,323-10,332, 1983.
  • Kaula, W.M., Material properties for mantle convection consistent with observed surface fields, J. Geophys. Res., 85, 7031-7044, 1980.

last updated February, 2005

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