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Some Problems with the Plume Hypothesis for the Galápagos Islands

Karen Harpp1 & Dennis Geist2

1Dept. Geology, Colgate University, 13 Oak Drive, Hamilton, NY 13346, U.S.A. kharpp@mail.colgate.edu
2Dept. Geological Sciences, University of Idaho, Moscow, ID 83844-3022, U.S.A. dgeist@uidaho.edu

Introduction

Many aspects of Galapagos geology may be inconsistent with a plume origin.  Most of the irregularities have been attributed to the proximity of the plate boundary, which results in abnormally hot upper mantle material (because of upwelling), abnormally thin lithosphere, and abnormal stresses, especially because of ridge-transform intersections (e.g. Geist et al., 1988; White et al., 1993; Kurz & Geist, 1999; Harpp & White, 2001; Harpp & Geist, 2002).  We believe that a plume rooted in the lower mantle best explains the major features of the archipelago and related seamounts, but despite that we also believe we are obligated to explore alternative hypotheses.

1.  Volcanic alignments seem to indicate control by lithospheric stresses

Figure 1

Instead of forming a single or dual line of volcanoes, the volcanoes are distributed in two alignments that were first noted by Charles Darwin (Figure 1).  The lineaments have no relation to either "absolute" plate motion (easterly red arrow) or plate spreading (southerly red arrow).  These  NE- and NW- oriented lineaments must be giant fractures or weak spots in the lithosphere. Of course, lithospheric fracturing and the presence of a deeply-rooted plume are not exclusive, and in fact might be expected.

2. Volcanism is not strictly time-transgressive

The line in Figure 2 indicates the predicted island age, on the basis of the hotspot-reference frame model of Gripp & Gordon (1990), and assuming that the hotspot is currently at the longitude of Fernandina.  On the whole, the exposed lavas are much younger than predicted by the hotspot reference frame.


Figure 2

3.  The "hotspot" magmas aren't very hot

Figure 3 shows our best estimates of the temperatures of the erupted magmas directly over the hotspot.  These temperatures are 50-100°C cooler than those of Hawaiian basalts. This could be due to cooling during ascent through the lithosphere or simply that the magmas never started out all that hot.

Figure 3

4.  Isotopic evidence is equivocal

Figure 4

Many of the Galápagos Island lavas are isotopically indistinguishable from those of the Galapagos Spreading Center. Those island lavas that are more extreme can be explained as low-degree melts of heterogeneities in the upper mantle with compositions similar to FLO, WD, and PLUME. FLO, WD, and PLUME are compositional endmembers, as identified by principal component analysis of isotopic and trace element data on all Galapagos Island and seamount data (Harpp & White, 2001).

5.  The aseismic ridges don't have hotspot-like subsidence trends

Figure 5

The Nazca and Carnegie ridges do not have  subsidence patterns that would result from normal contraction of reheated lithosphere.  Volcanism at Cocos Island, which supposedly passed the hotspot at 6 Ma, is dated at only 2 Ma.  The Cocos Ridge is effectively flat with some anomalous bumps on it, consistent with early growth along most of its length and some anomalous younger events.

Conclusion

Although there are features of the Galapagos archipelago that may be inconsistent with simple plume theory, most of them can be explained by perturbations attributable to the proximity of the Galapagos Spreading Center.  No-one, including us, has proposed a viable, testable alternative hypothesis that is consistent with the observations presented here.

... and for the alternative view ...

References

  • Geist, D. J., White, W. M. & McBirney, A. R. (1988). Plume asthenosphere mixing beneath the Galápagos Archipeligo. Nature, 333, 657-660.
  • Gripp, A.E. & Gordon, R.G., (1990), Current plate velocities relative to the hotspots incorporating the NUVEL-1 global plate motion model, Geophys. Res. Lett., 17, 1109-1112.
  • Harpp, K. S., White, W. M. (2001). Tracing a mantle plume; isotopic and trace element variations of Galápagos seamounts. Geochemistry, Geophysics, Geosystems,2, 46. (paper number 2000GC000137).
  • Harpp, K., Geist, D., Wolf-Darwin lineament and plume-ridge interaction in northern Galápagos, Geochemistry, Geophysics, Geosystems, 3, DOI number 1029/2002GC000370, 2002.
  • Kurz, M.D. & Geist, D. (1999), Dynamics and evolution of the Galapagos hotspot from helium isotope geochemistry, Geochim Cosmochim Acta,63, 4139-4156.
  • White, W.M., McBirney, A.R. & Duncan, R.A. (1993). Petrology and geochemistry of the Galápagos Islands: portrait of a pathological mantle plume. J. Geophys. Res., 98, 19,533-19,564.
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