Over the course of Earth history, many parts of the crust have undergone multiple episodes of metamorphism.  Modern methods of dating and measuring trace-element abundances are now able to tease out the timing and conditions of the individual episodes.  But new techniques were needed before these methods could be scaled up to unravel regional tectonic events such as the formation of mountain belts and subduction zones and continental rifting.  In the podcast, John Cottle describes one such technique that he and his group developed and that ushered in a revolution in the study of metamorphic rocks.  He discusses how the technique was used to resolve the multiple phases of metamorphic history in the Himalaya, Antarctica, and New Zealand.  
John Cottle is a Professor in the Department of Earth Science at the University of California, Santa Barbara.
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John Cottle on the Petrochronology Revolution

What moves the continents, creates mountains, swallows up the sea floor, makes volcanoes erupt, triggers earthquakes, and imprints ancient climates into the rocks? Oliver Strimpel, a former astrophysicist and museum director asks leading researchers to divulge what they have discovered and how they did it. 

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Science

Earth Sciences

We know that most magma originates in the Earth’s mantle. As it pushes up through the many kilometers of lithosphere to the surface, it pauses in one or more magma chambers or partially melted mush zones for periods of up to a few millennia before erupting.  But while we have seismic evidence and models and support this picture, we have not hitherto been able to watch how magma actually moves in the upper mantle and crust. 
Bob White has set out to change that.  Using a dense array of seismometers, he has been able to pinpoint thousands of tiny earthquakes that reveal the detailed movement of melt through the thick crust of Iceland just before it erupted.  White combines this seismic data with geochemical analyses of the lava that can tell us about the depths at which the melt is formed. 
White is Emeritus Professor of Geophysics in the Department of Earth Sciences at the University of Cambridge.



Bob White on How Magma Moves in the Crust

At roughly 15-25-million-year intervals since the Archean, huge volumes of lava have spewed onto the Earth’s surface. These form the large igneous provinces, which are called flood basalts when they occur on continents. As Richard Ernst explains in the podcast, the eruption of a large igneous province can initiate the rifting of continents, disrupt the environment enough to cause a mass extinction, and promote mineralization that produces valuable mineral resources. 

Richard Ernst studies the huge volcanic events called Large Igneous Provinces (LIPs) — their structure, distribution, and origin as well as their connection with mineral, metal, and hydrocarbon resources; supercontinent breakup; and mass extinctions. He has also been studying LIP planetary analogues, especially on Venus and Mars. He has written the definitive textbook on the subject.
Ernst is Scientist in Residence in the Department of Earth Sciences, Carleton University, Ottawa, Canada, and Professor in the Faculty of Geology and Geography at Tomsk State University, Tomsk, Russia. 

Richard Ernst on Large Igneous Provinces

Perhaps as many as five times over the course of Earth history, most of the continents gathered together to form a supercontinent. The supercontinents lasted on the order of a hundred million years before breaking apart and dispersing the continents. For decades, we theorized that this cycle of amalgamation and breakup was caused by near-surface tectonic processes such as subduction that swallowed the oceans between the continents and upper mantle convection that triggered the rifting that split the supercontinents apart. As Damian Nance explains in the podcast, newly acquired evidence suggests a very different picture in which the supercontinent cycle is the surface manifestation of a process that involves the entire mantle all the way to the core-mantle boundary.


Damian Nance draws on a wide range of geological evidence to formulate theories about the large-scale dynamics of the lithosphere and mantle spanning a period going back to the Archean. A major focus of his research is the supercontinent cycle.  He is Distinguished Professor Emeritus of Geological Sciences at Ohio University.

Damian Nance on What Drives the Supercontinent Cycle

The Earth’s tectonic plates float on top of the ductile portion of the Earth’s mantle called the asthenosphere. The properties of the asthenosphere, in particular its viscosity, are thought to play a key role in determining how plates move, subduct, and how melt is produced and accumulates. We would like to know what the viscosity of the the asthenosphere is, and how it depends on temperature, pressure, and the proportion of melt and water it contains. Few mantle rocks ever reach the Earth’s surface, and those that do are altered by weathering. So, as he explains in the podcast, David Kohlstedt and his team have tried to replicate the rock compositions and physical conditions of the mantle in the lab.  Using specially-built apparatus, he has been able to determine the viscosity of the asthenosphere to within an order of magnitude, which is an enormous improvement on what was known before.

David Kohlstedt is Professor Emeritus at the School of Earth and Environmental Science at the University of Minnesota.

David Kohlstedt on Simulating the Mantle in the Lab

In many countries, nuclear power is a significant part of the energy mix being planned as part of the drive to achieve net-zero greenhouse-gas emissions. This means that we will be producing a lot more radioactive waste, some of it with half-lives that approach geological timescales, which are orders of magnitude greater than timescales associated with human civilizations. In the podcast, Claire Corkhill discusses the geology such storage sites require, some new materials that can confine radioactive isotopes over extremely long timescales, and the kind of hazards, including human, we need to guard against.

Claire Corkhill is Professor of Mineralogy and Radioactive Waste Management in the School of Earth Sciences at the University of Bristol, UK. 

John Cottle on the Petrochronology Revolution

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