There’s a lot of debate about the idea that the global changes brought about by humans define a new geological epoch, dubbed the Anthropocene. Should such an epoch be added to the official geological time scale? If so, what aspect or aspects of anthropogenic change should be used, and exactly where do we place the golden spike that will define the base of the Anthropocene?

Such questions come under the purview of the International Commission on Stratigraphy, whose current secretary general is our guest in this episode.  Phil Gibbard is Emeritus Professor of Quaternary Palaeoenvironments at the University of Cambridge.  In 2009, he was a founding member of the Anthropocene Working Group tasked by the International Commission on Stratigraphy to examine the status, hierarchical level, and definition of the Anthropocene as a potential new formal division of the Geological Time Scale.  He explains that no consensus on the Anthropocene has been reached, and it remains controversial as to whether there is even a need for such an epoch at all.

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Subduction zones are a fundamental aspect of plate tectonics, yet we still don't really understand how subduction initiates.  It's a tough problem because as oceanic plates move away from a mid-ocean spreading center and cool, they get stiffer and should become more and more resistant to bending and sinking down into the mantle.  But recent work suggests that the clue to this puzzle lies in the physics of grains at the microscale.  

David Bercovici is one of the geologists who has pioneered the approach that looks at what happens in the rocks of the oceanic lithosphere on the scale of a few microns.  It turns out that stresses in the plate can generate bands of vertically aligned weakness that could make the lithosphere much weaker in precisely the direction in which it sinks into the mantle. 

David Bercovici is a professor of Earth and Planetary Science at Yale University.  In the podcast, he explains how the behavior of stressed mineral grains at the microscale can radically affect the strength of oceanic lithosphere on the plate scale.  The model is consistent with lab experiments and the contrasting seismic observations of the Pacific and Atlantic plates.

New rock types emerge during the history of the Earth.  For example, the silica-rich felsic rocks such as granite that characterize continental crust, accumulated during the course of Earth history.  Granite only forms in certain specific tectonic settings, such as above subduction zones and when lower crustal rocks melt in mountain belts.  But what about the minerals themselves?  Have they been around since the Earth formed, or did they too only appear on the scene later as a result of some geological process?

The question of how and when the minerals evolved is a relatively new subject, and was, and continues to be, pioneered by this episode's guest.  Bob Hazen is Senior Staff Scientist at the Earth and Planets Laboratory of the Carnegie Institution for Science and Professor of Earth Sciences at George Mason University.  At a Christmas party in 2006, a well-known biophysicist asked him the question: “Were there clay minerals in the Archean?”  Apparently, nobody had given this much thought prior to 2006.  The topic quickly became the focus of his research, rapidly blossoming into a whole new branch of mineralogy.

Throughout geological history, various points on the Earth’s surface have been lifted up to great elevations and worn down into low, flat-lying regions.  Determining surface elevation histories is difficult because rocks that were once on the surface are usually eroded away or buried.  Furthermore, most rock-forming processes are not directly affected by elevation.  But it turns out that we can overcome these challenges, as Carmie Garzione explains in the podcast. 

Carmie Garzione is Dean of the College of Science at the University of Arizona.  She has managed to pin down the history of elevation changes by analyzing stable isotopes of carbon and oxygen in carbonate rocks.  She describes how the method works, and presents her findings for the Tibetan plateau and the Andes.  They show pulses of very rapid (geologically speaking) uplift.  What might this be telling us about what has been going on in the lower crust and upper mantle in these regions?

The magnetic stripes frozen into the sea floor as it forms at mid-ocean ridges record the Earth’s magnetic field at the time of formation.  Reversals in the Earth’s magnetic field define the edges of these stripes, in effect time-stamping the sea floor position.

Chuck DeMets is Emeritus Professor of Geoscience at the University of Wisconsin, Madison.  He studies the magnetic anomalies in seafloor rocks to reconstruct plate motions at a temporal resolution five times better than has been done hitherto.  This has revealed unexpected speed-ups and slow-downs in plate motions that provide juicy puzzles for geodynamicists.  In the podcast he focuses on the detailed motions of the Indian plate that show, among other things, that its northward movement actually sped up for a period after the collision with Eurasia before it settled down to a steady slower speed.

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As the name implies, oceanic lithosphere underlies the oceans of the world.  Except when they are ophiolites, when oceanic lithosphere is thrust on top of a continental margin.  Are ophiolites a special kind of oceanic lithosphere?  Or are there peculiar tectonic circumstances that emplace denser oceanic rocks on top of lighter continental ones?  Mike Searle addresses these questions, and reveals the sequence of events that created the world's most extensive and best-preserved ophiolite - the Semail ophiolite in Oman.

Mike Searle is Emeritus Professor of Earth Sciences at the University of Oxford and at the Camborne School of Mines in Cornwall. 

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Some of the most extensive sandstone deposits in the world were deposited by wind.  How do such aeolian rocks differ from water or ice-deposited rocks?  And  what do they reveal about the environments in which they formed?  In the podcast she describes the dunes we see in the geological record on Earth, as well as on Mars and on a comet, and explains what we've learned from them.

Mackenzie Day is an Assistant Professor in the Department of Earth, Planetary, & Space Sciences at the University of California Los Angeles. She is an expert in aeolian processes.  In the podcast she describes the dunes we see in the geological record on Earth, as well as on Mars and on a comet and explains how they formed.

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The best maps we have of Venus were made by Magellan, a space probe that flew in the 1990s.  In the summer of 2021, NASA approved a new mapping mission that will produce radically improved maps of the topography, radar reflectivity, and gravity field, and the first ever global map of surface rock type.  Sue Smrekar, the mission Principal Investigator, explains why this will revolutionize our understanding of Venus and perhaps also throw light on the early history of Earth when processes analogous to those happening on Venus today may have occurred.

Sue Smrekar is a geophysicist who has played key scientific and managerial roles in multiple planetary exploration missions, include Magellan and the InSight robot lander on Mars.

Almost all the evidence about the nascent solar system has been erased by processes accompanying the formation of the Sun and the bodies that formed out of the circumsolar disk about 4.6 billion years ago.  But some meteorites and the tiny dust grains contained within them have anomalous compositions that can only be understood by invoking a history going back to the giant molecular cloud progenitor of the solar system, and to the stars that ejected the material that formed the cloud.  Rick Carlson explains what the anomalies are and how we can use them to learn about what might have being going on in the solar nebular as the solar system was forming.

Rick Carlson is an isotope geochemist who measures such anomalies and uses them as clues to the birth of the solar system.  He is Director of the Earth and Planets Lab at Carnegie Science in Washington DC.

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