Plates, planets, and phase changes: 50 years of petrology

Walker, David

Three advances of the previous half-century fundamentally altered petrology, along with the rest of the Earth sciences. Planetary exploration, plate tectonics, and a plethora of new tools all changed the way we understand, and the way we explore, our natural world. And yet the same large questions in petrology remain the same large questions. We now have more information and understanding, but we still wish to know the following. How do we account for the variety of rock types that are found? What does the variety and distribution of these materials in time and space tell us? Have there been secular changes to these patterns, and are there future implications? This review examines these bigger questions in the context of our new understandings and suggests the extent to which these questions have been answered. We now do know how the early evolution of planets can proceed from examples other than Earth, how the broad rock cycle of the present plate tectonic regime of Earth works, how the lithosphere atmosphere hydrosphere and biosphere have some connections to each other, and how our resources depend on all these things. We have learned that small planets, whose early histories have not been erased, go through a wholesale igneous processing essentially coeval with their formation. By inference, this also happened to Earth. The early differentiation on a small planet produces observable basaltic rock types—and produces little else besides a residue and a planetary core. In contrast, the larger Earth’s preservation of its original differentiation products has been eroded by continued activity which still involves extensive basaltic volcanism with further reprocessing through plate tectonic cycles to form continents and cratons. We also now have a good understanding of the pressure-induced phase changes that are responsible for the Earth’s mantle’s seismic layered structure. It is unclear the extent to which this layered seismic structure corresponds to chemical layering as well as to mineralogical layering. Earth’s transition zone, lower, and upper mantles may not have the same composition. It is possible that still larger exoplanets might be expected to develop additional modes of activity with emphasis on additional phase changes producing more internal layering and differentiation.


Also Published In

The Web of Geological Sciences: Advances, Impacts, and Interactions
The Geological Society of America

More About This Work

Academic Units
Earth and Environmental Sciences
Geological Society of America Special Papers, 500
Published Here
June 19, 2013