Harry W. Green II

Distinguished Professor of Geology and Geophysics

Ph.D. UCLA (1968)

Ph: 951.827.4505

hgreen@mail.ucr.edu

• Mechanisms of earthquakes at depth Brittle shear fracture and frictional sliding are strongly dependent on the normal stress across a fault or potential fault. Therefore, brittle failure is strongly inhibited by pressure. In contrast, ductile flow of materials is only weakly dependent on pressure but strongly inversely dependent on temperature. As a consequence, brittle failure becomes impossible at depths in Earth of not more than several 10s of km where both the pressure and temperature are elevated. But earthquakes occur in subduction zones to depths approaching 700km! In my laboratory we investigate alternative mechanisms of shear failure that can potentially explain this paradox. In 1989, a graduate student and I discovered phase-transformation-induced faulting that can occur during exothermic phase transformations involving a volume change [1]. Although the conditions under which this faulting mechanism can be triggered are very restricted, we showed that the appropriate conditions exist (indeed are unavoidable) for the olivine-spinel transformation during subduction of oceanic lithosphere into the mantle. We showed that this mechanism can explain the upturn in frequency of earthquakes at ~400km depth and the complete cessation of earthquakes at ~700 km [4]. See [ 3] for a layman's discussion of this discovery. Recent work in the laboratory has been investigating other potential mechanisms, especially that associated with breakdown of serpentine, the low-temperature, hydrous, equivalent of olivine. In particular, we are investigating the effect of the change of volume during this breakdown as it varies from a positive value to a negative value. Conventional theory predicts that failure should become impossible when the change in volume becomes negative but our experiments have shown that not to be the case. We are currently attempting to determine why this is so [9].
  
• Rheology of rocks and minerals at high pressure and temperature This has been a major theme of my research throughout my career; I have developed unique apparatus to measure rheology at these high pressures [2]. We have worked extensively on the basic flow mechanisms of olivine, the most abundant and "softest" mineral of Earth's upper mantle, and how those mechanisms can explain various aspects of mantle convection. It was those studies that led to discovery of the phase-transformation-induced faulting mechanism. We currently are investigating the flow mechanisms and properties of eclogite. Eclogite is the high-pressure equivalent of basalt and therefore, when the oceanic crust is subducted, it is changed into this rock. Questions have been raised as to whether the eclogitic oceanic crust can delaminate from subducting lithosphere at great depth, with significant implications for the plan form of mantle convection and the nature and composition of volcanic islands. The minerals of eclogite are only stable at high pressures and therefore the flow properties of this rock cannot be measured in conventional apparatus. Our current research is placing important constraints on these problems [8].
  
• Exhumation of rocks from great depth in continental collision zones Until less than 15 years ago, it was believed that continental rocks, because they are less dense than mantle rocks, could not be subducted into the mantle. In 1990, however, diamonds were reported from rocks in Kazakhstan. Although first disbelieved, it has been shown over the last 10 years that subduction of low-density rocks to such depths (>120 km) and return to the surface has happened multiple times. In 1996, we published first evidence [5] of exhumation from much greater depths, perhaps 300-400 km. Again, despite initial skepticism, we [6, 10] and others have presented additional evidence that such exhumation has occurred multiple times. Although the mechanism by which this process operates is still not known, all localities where it has been demonstrated are localities of continental collision. It remains a major gap in our understanding of plate tectonics. We are pursuing understanding of this problem both in the laboratory and in the field, as well as developing new theories of the growth of diamonds [7, 11].

1. Green, H. W., II and P. C. Burnley,A new self-organizing mechanism for deep-focus earthquakes, Nature, 341, 733-737 (1989).

2. Green, H. W., II and R. S. Borch,A new molten salt cell for precision stress measurement at high pressure, European J. Mineral, 1, 213-219 (1989).

3. Green, H.W., II, Solving the paradox of deep earthquakes, Sci. Amer. 271, 64-71 (1994).

4. Green, H.W., II and H. Houston, The mechanics of deep earthquakes. Ann. Rev. Earth and Planet. Sci., 23, 169-213 (1995).

5. Dobrzhinetskaya, L., H.W. Green, II, and S. Wang, Alpe Arami: A peridotite massif from depths of more than 300km, Science 271, 1841-1845 (1996).

6. Bozhilov, K.N., H.W. Green, II, L. Dobrzhinetskaya, Clinoenstatite in Alpe Arami peridotite: Additional evidence for very high pressure, Science, 284, 128-132 (1999).

7. Dobrzhinetskaya, L., H.W. Green, II, T. Mitchell, R.M. Dickerson, Metamorphic Diamonds: Mechanism of Growth and Inclusion of Oxides. Geology, 29, 263-266 (2001).

8. Jin, Z-M, J. Zhang, H.W. Green, II. and S. Jin, Eclogite rheology: implications for subducted lithosphere. Geology, 29, 667-670 (2001).

9. Green, H.W., C.J. Marone, Instability of deformation, in Plastic Deformation of Rocks, Wenk, H.R. and Karato S. eds., Mineral Soc. Amer., Washington, DC (2002).

10. Bozhilov, K.N., L.F. Dobrzhinetskaya, and H.W. Green, II, Quantitative 3D measurement of ilmenite abundance in Alpe Arami olivine: Confirmation of high pressure origin, Am. Mineral, (in press) (2003)

11. Dobrzhinetskaya LF, H.W. Green II, T. Mitchell, R. Dickerson, Nanometric solid inclusions in metamorphic diamonds from Kazakhstan: evidence of continental slab-mantle interaction, J. Metamorphic Geol., (in press) (2003).