Geology Bites

The Franciscan Complex is a large accretionary prism that has been accreted onto the western margin of the North American continent. Unlike most such prisms, which are submarine, it is exposed on land, making it a magnet for researchers such as John Wakabayashi. In the podcast, he describes this remarkable complex and explains the mechanisms that may have operated over its 150-million-year history.

John Wakabayashi is a Professor in the Department of Earth and Environmental Sciences at California State University, Fresno. He has devoted much of his 40-year research career to the Franciscan Complex.

How can we tell if the sedimentary record is good enough to make solid inferences about the geological past? After all, it can be difficult, or even impossible, to infer what is missing, or indeed whether anything is missing at all.

As he explains in the podcast, Bruce Levell tackles this question by combining fieldwork with systematic analysis based on what we know about contemporary deposition and erosion. Armed with an understanding of preservational bias, he questions the confidence with which some widely held interpretations of the sedimentary record have been made. For example, by analyzing sequences of glacially-deposited rocks in southwest Scotland, he has shown with others that, contrary to the “Hard Snowball Earth” hypothesis, parts of the Earth probably experienced a persistently active hydrological cycle and were not simply fully-frozen, at least during the earlier of the two postulated snowball glaciations.

Bruce Levell is a Visiting professor in the Department of Earth Sciences at the University of Oxford. Previously, he was Chief Scientist for Geology at Royal Dutch Shell.

In a recent episode, Nadja Drabon spoke about newly discovered zircon crystals that formed during the late Hadean and early Archean, when the Earth was between 500 million and a billion years old.  The zircons revealed information about processes occurring in the Earth’s nascent crust, casting light on when and how modern-day plate tectonics may have started.  In this episode, we talk about a very different source of information about the early Earth, namely the abundances of noble gases occurring within present-day basalts.  It turns out that these can probe the Earth’s mantle and atmosphere even further back in time – to the first 100 million years of Earth history.

Sujoy Mukhopadhyay leads a team of researchers who have developed new techniques for measuring the abundances of noble gas isotopes in a variety of Earth materials.  By combining the results of these measurements with geochemical models, he has shed light on questions about the very early Earth and planet formation that have challenged researchers for decades.  Here we focus on one of these: “Do any structures originating from the very early Earth survive in today’s mantle?” Amazingly, the answer is "yes."

Sujoy Mukhopadhyay is Professor of Geochemistry at the University of California, Davis.

In 2011, a massive earthquake struck off the eastern coast of Japan. The destructive power of the earthquake was amplified by a giant tsunami that swept ashore, killing over 15,000 people. A major cause of the tsunami was the 50-m slip along the plate boundary fault between the subducting Pacific plate and the overriding North American plate. Patrick Fulton and his team set out to find out why there was so much movement along the fault by installing a temperature observatory in a borehole drilled right through the fault zone.

Patrick Fulton uses observation, quantitative analysis, and numerical modeling to study heat and fluid in fault zones. He applies his research to the physics of earthquakes, tectonic processes, and the transport of subsurface heat and fluids. In the podcast, he describes how he and his team installed a borehole temperature observatory below 7 km of ocean. The observatory detected the remnants of frictional heating generated by the slip that caused the 2011 Tōhoku Earthquake and the devastating tsunami that led to the Fukushima nuclear disaster.

Patrick Fulton is an Assistant Professor in the Department of Earth and Atmospheric Sciences at Cornell University.

For more about Geology Bites and illustrations that support the podcast, go to geologybites.com.

Romain Jolivet studies active faults and the relative motion of tectonic plates.  His research focuses on the relationship between slow, aseismic slip that occurs “silently” between earthquakes and the rapid slip accompanying earthquakes.  As he describes in the podcast, he uses interferometric synthetic aperture radar (InSAR) images from radar satellites to examine surface deformation over wide areas at meter-scale resolution.  InSAR images of the 2023 Turkey-Syria earthquakes reveal complicated slip patterns occurring on well-recognized plate boundary faults as well as on hitherto ignored faults.

Romain Jolivet is a Professor of Geoscience at the École normale supérieure in Paris.

For illustrations that support this episode and to learn more about Geology Bites, go to geologybites.com.

The geological record shows that the Earth’s carbon cycle suffered over 30 major disruptions during the Phanerozoic.  Some of the biggest ones were accompanied by mass extinctions.  Dan Rothman analyzed these disruptions to find a pattern governing their magnitude and duration.  As he explains in the podcast, this pattern is suggestive of a non-linear dynamical system that, once excited, undergoes a large excursion before returning to where it was.  Could we be exciting such a disruption now?

He shows that the mass of anthropogenic carbon emissions forecast by the end of the century is about the same as the mass of carbon dioxide outgassed by the massive volcanism that generated the portion of the Deccan Traps deposited just before the end-Cretaceous extinction.  This leads him to hypothesize that, while the Chixclub meteor impact may have been the direct cause of the extinction, the disruption of the carbon cycle caused by the outgassing of CO₂ during this prolific series of eruptions contributed to the environmental change associated with mass extinction. 

Go to https://www.geologybites.com/ for illustrations that support this episode and to learn more about the Geology Bites.

Vanishingly few traces of the early Earth are known, so when a new source of zircon crystals of Hadean age is discovered, it makes a big difference to what we can infer about that eon.  In the podcast, Nadja Drabon describes how she analyzed the new zircons she and her colleagues discovered and what they reveal about the Earth’s crust between about 4 and 3.6 billion years ago. Nadja Drabon is Assistant Professor of Earth and Planetary Sciences at Harvard University. For podcast illustrations and more about Geology Bites, go to geologybites.com.

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.

Go to geologybites.com for illustrations that support the podcast and to learn more about Geology Bites.

This episode is the second of two of my conversation with Martin Gibling.  In the first episode, we discuss fluvial deposits in the geological record and we trace the effect that the break-up of Pangea around 200 million years ago had on river systems.  In this episode, we address the history of the rivers of Europe and the Americas, as well as the impact of the recent ice ages on today’s rivers.  We end by considering how humans have changed rivers and their deposits throughout mankind’s history. Martin Gibling has spent a lifetime studying rivers and river sediments around the world.  He is Emeritus Professor in the Department of Earth and Environmental Sciences at Dalhousie University in Halifax, Nova Scotia in Canada. 

For pictures and figures that support this podcast, go to geologybites.com.

Rivers can seem very ephemeral, often changing course or drying up entirely.  Yet some rivers have persisted for tens or even hundreds of millions of years, even testifying to the breakup of Pangea, the most recent supercontinent, about 200 million years ago.  On the one hand, their courses may be determined by tectonic processes such as the formation of mountain belts.  And on the other, they themselves can affect tectonic processes by creating continent-scale features, such as giant submarine fans. Martin Gibling has spent a lifetime studying rivers and river sediments around the world.  He is Emeritus Professor in the Department of Earth and Environmental Sciences at Dalhousie University in Halifax, Nova Scotia in Canada. This episode is the first of two of our conversation about rivers.  In this episode we talk about fluvial deposits in the geological record and the impact of the break-up of Pangea on river systems.  In the second episode we talk specifically about the history of the rivers of Europe and the Americas, as well as the impact of recent ice ages.  We end by considering how humans have changed rivers and their deposits throughout human history. 

For pictures and figures that support this podcast, go to geologybites.com.

This episode is a bit of a departure from the objective approach to geology of past episodes in that here we address the subjective nature of various rocks as experienced by a rock climber with a literary bent.

A rock climber’s very survival can depend on the properties of a rock encountered along a climbing route.  This engenders a uniquely intense relationship between climber and rock.  Anna Fleming has written perceptively about this intense relationship gained from climbing in Britain and the Mediterranean.  In a book entitled Time on Rock, she writes about her experiences climbing gritstone in England’s Peak District, slate in the disused slate quarries of North Wales, gabbro and granite on the Isle of Skye, sandstone on the northeast coast of Scotland, and limestone cliffs on the Greek island of Kalymnos, among others.

For pictures and figures that support this podcast, go to geologybites.com.

Between 1.3 and 1.1 billion years ago, magma from the Earth's mantle intruded into a continent during the assembly of the supercontinent called Nuna. Through good fortune, the dykes and central complexes that resulted have been preserved in near-pristine condition in what is now the south of Greenland. The dykes are extraordinarily thick, and the central complexes contain an order of magnitude more exotic minerals than otherwise similar complexes around the world. In the podcast, Brian Upton describes what he found during over 20 seasons of field work there and explains how extreme fractionation of the magma might be responsible for the one-of-a-kind central complexes.

Brian Upton is Emeritus Professor of Geology at the University of Edinburgh.  During his long and prolific research career, he has conducted field studies in many parts of the world, concentrating especially on the Arctic.  But throughout his career he has continued to investigate the unique alkaline rocks of South Greenland.  As he explains in the podcast, these rocks contain an unrivalled number of exotic minerals, many of them not known to occur anywhere else.

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Subduction zones are places where a slab of oceanic lithosphere plunges into the mantle below.  The slab consists of the sediments on top, crustal rocks in the middle, and the lithospheric mantle on the bottom, all plunging down together as a kind of sandwich.  In each of these layers is an ingredient that plays a key role in shaping the evolution of the Earth over geological time – and that is water.

Geoff Abers has conducted extensive research on water in subduction zones.  In this episode, he explains how he uses seismic observations to map the distribution of water in subducting plates and in the overriding mantle.  He then couples these observations with computer-based models of the physics and chemistry of the subducting plates to predict the fate of the water in the downgoing plate.  The results are surprising — over geological time, the amount of water in all of today’s oceans may have been mixed into the deep mantle by subducting plates.  

Geoff Abers is Professor of Geological Sciences at Cornell University.

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Popular reconstructions of ancient environments, whether they be in natural history museum dioramas, in movies, or in books, present a world of color. But are those colors just fanciful renderings, perhaps based on the colors we see around us today?  Or is there evidence in the fossil record that we can use to determine the actual color of plants and animals that lived in the geological past?

Maria McNamara tries to answer these questions by studying the fossil preservation of soft tissues, such as skin, muscle, and internal organs. She does this by analyzing fossils that come from sedimentary deposits that contain extraordinarily well-preserved fossils. She also does lab experiments to investigate the processes of soft tissue degradation and preservation. She is Professor of Paleontology at University College Cork in Ireland.

For illustrations supporting this podcast, go to geologybites.com.  

For many years, efforts to limit climate change have focused on curtailing anthropogenic emissions of greenhouse gases.  But it is increasingly clear that such curtailment will not, on its own, be able to prevent the damaging effects of global warming.  Therefore, more attention is now directed to mitigating climate change by enhancing the removal or sequestration of greenhouse gases from the atmosphere.  As a result, our climate change goals are now often specified in terms of when we plan to reach net zero emissions rather than on when we can just reach emission reduction targets.

Phil Renforth is an expert on carbon sequestration.  He is especially interested in enhancing the weathering of rocks and has performed in-depth investigations of geochemical techniques of removing atmospheric carbon dioxide.  He is an Associate Professor in the School of Engineering and Physical Sciences at Herriot-Watt University.

There are illustrations supporting this episode at geologybites.com.

When plate tectonics was adopted in the 1960s and early '70s, researchers quickly mapped out plate movements.  It seemed that plates moved as rigid caps about a pole on the Earth's surface.  But since then, a lot of evidence has accumulated suggesting that plates are not, in fact, totally rigid.  In fact, we can see them flex in response to stresses that are imposed on them.  Such stresses can arise on plate boundaries, such as when two plates collide and one plate flexes down to subduct under the other.  For example, we see a flexural bulge in Northern India where the Indian plate bends down under the Eurasian plate.  Similar bulges are seen at subduction zones where the oceanic lithosphere flexes up before it bends down into a trench, such as off the eastern coast of Japan.  Stresses can also be imposed in plate interiors when the plate is subjected to a load, such as a volcano or a sedimentary basin.  An example of sediment loading occurs in river deltas, such as that of the Ganges in the Bay of Bengal.

Our guest today pioneered an ingenious method of determining the flexural strength of oceanic plates.  The method uses the flexural sag of plates in response to the weight of seamounts, most of which were emplaced on their surfaces by mid-ocean eruptions.  His results suggest that less than half of an oceanic plate actually contributes to its elastic strength.  The rest is brittle (top layer) or ductile on the relevant time scales (bottom layer).Tony Watts is Professor of Marine Geology and Geophysics at the University of Oxford and a Fellow of the Royal Society.

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Life only emerged from water in the Ordovician.  By that time, life had been thriving in oceans and lakes for billions of years.  What did the colonization of the land look like, and how did it reshape the Earth’s surface?  Neil Davies describes how we can decipher the stratigraphic sedimentary record to address these questions.  Perhaps surprisingly, it’s easier to recognize small and fleeting events than to recognize large-scale features such as mountains, valleys, and floodplains.  He also describes his remarkable 2018 discovery of the largest known arthropod in Earth history — a 2.6-meter-long millipede.

Neil Davies is a Lecturer in Sedimentary Geology at the University of Cambridge.  He studies the interconnections and feedback loops between life and sedimentation.  His research aims to understand how such interactions manifest themselves in the rock record.  He does this by combining analyses of sedimentary structures and textures, stratigraphy, and trace fossils.

The asteroid Psyche is probably the most metal-rich body we have discovered.  There are two, quite different, theories as to how it may have formed: Either it formed that way, or it originally had a more typical composition, but its rocky outer portion was blasted off during a major collision.  To help determine which is most likely, NASA is sending a space probe there, to be launched on August 1, 2022.  And if we can unravel the history of Psyche, we will also learn how other planets may have formed, since both the asteroids and the planets are thought to have been assembled from the same population of planetesimals – the small bodies that first formed out of the solar nebula.  We might also learn about the Earth's own metallic core, since, according to the second theory, Psyche may be a naked core.

Ben Weiss is Deputy Principal Investigator and Magnetometry Investigation Lead on the Psyche mission.  He is a Professor of Planetary Sciences at the Massachusetts Institute of Technology.  His research focuses on the formation, evolution, and history of the terrestrial planets and small bodies.  He is especially interested in paleomagnetism and geomagnetism.  In the podcast, he explains the various formation scenarios for Psyche.  He then describes the various instruments on board the spacecraft and what we will be able to infer from the measurements they will make when orbiting the asteroid.

We hear about earthquakes in the Himalaya, especially when they claim lives and cause damage. And we understand that, broadly speaking, it is the continued northward movement of India ploughing into Tibet that causes these earthquakes. But where exactly do the earthquakes occur, how do they occur, and what determines how much damage they inflict?

Roger Bilham has conducted a detailed study of the historical record of earthquakes in the Himalaya over the past millennium. He tries to reconcile what we’ve observed with our current understanding of the physical mechanisms at play. This in turn helps us assess future hazard potential. He is Emeritus Professor of Geology at the University of Colorado, Boulder.

For podcast illustrations and more about Geology Bites, go to geologybites.com.  

The fossil record of complex life goes back far beyond the Cambrian explosion, to as far back as 1,600 million years ago in the late Paleoproterozoic with the first appearance of eukaryotes.  But these creatures only started to diversify much later, around 750 million years ago.  What enabled this evolutionary change has been a puzzle, but one idea is that it reflects the appearance of microscopic predators.  In the podcast, Susannah Porter tells us how she discovered incontrovertible signs of predation in vase-shaped microfossils dating from this period.

Susannah Porter is a professor in the Department of Earth Sciences at the University of California, Santa Barbara.  She studies microfossils of eukaryotic life forms that lived in the Neoproterozoic, about 750 million years ago. 

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Does the pull of a subducting slab drive plate motions?  Or is it the upwellings of convection cells in the mantle?  We now have a new way to shed light on this question.  It's called seismic anisotropy, which is the spreading out of seismic waves according to their direction of polarization.  This happens when the mantle through which the waves travel has crystals which are preferentially aligned, and that occurs when there is deformation or flow going on.  So we can work backwards to use the observed dispersion of seismic wave arrival times to infer flow patterns in the mantle.  In the podcast, Ana Ferreira explains how we do this, and describes our first views of an interaction between a mantle plume pushing up into and flowing around a subducting plate in the southwest Pacific.  But, despite the vigor of such plumes, she concludes that it is probably the pull of the sinking slabs that is still the primary driver of plate motions.

Ana Ferreira is Professor of Seismology at University College, London.   She collects and analyzes seismic data from around the world, focusing particularly on seismic anisotropy.

See her maps of mantle flow at geologybites.com.  

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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.

Matt Jackson is a Professor of Earth Science at the University of California, Santa Barbara.  He probes the chemical composition of the mantle by analyzing trace elements and isotopes in hot-spot lavas from around the world.  In the podcast, he describes the intriguing heterogeneity among the hot-spots of the so-called “hot-spot highway” in the western Pacific.  The heterogeneity there, as well as on larger spatial scales is challenging our ideas about the motions of the mantle over the billions of years of Earth history.

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.

Go to geologybites.com for podcast illustrations and to learn more about Geology Bites.

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. 

For podcast illustrations, go to geologybites.com.

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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After months of high earthquake activity, a fissure opened up near the southwestern tip of Iceland on March 19, 2021. Over a period of about seven months, several other fissures opened up, generating lava flows several kilometers long that filled several valleys and created a new 150-meter high mountain, a sort of mini-shield volcano. The eruption has been intensively studied by geologists because it is the first eruption of its kind in Iceland in living memory, and also because it’s extremely accessible – just a short drive from Reykjavik.

Ed Marshall has made dozens of visits to the eruption site to sample the lava.  He then performs geochemical analyses in his lab which show that the magma originates from the very bottom of the crust where it meets the convecting mantle.  Such eruptions generally only occur at mid-ocean ridges, so this is a rare opportunity to study what is happening on the crust-mantle boundary.  It appears to be a place where heterogenous batches of mantle are mixing vigorously, injecting rapidly changing melts into the conduit carrying the magma to the surface. 

Ed Marshall is a Postdoctoral Research Fellow at the University of Iceland.

Long before radiometric dating appeared on the scene, the geological time scale was defined by the sedimentary record, and particularly by key fossils preserved within them.  Throughout the Cambrian, and to a lesser extent until the end-Permian extinction about 300 million years later, trilobite fossils served as some of the most useful of these key fossils.  Richard Fortey explains why.  Here he is holding a trilobite from the calymene genus.

Richard Fortey is formerly head of arthropod paleontology at the Natural History Museum in London and is visiting professor of paleobiology at Oxford University.  He has devoted much of his research career to the study of trilobites — their systematics, evolution, and modes of life, and has named numerous trilobite species.

Podcast illustrations and more about Geology Bites are on geologbites.com.

We’re all familiar with the idea of ice ages during which the polar ice caps advance to cover significant portions of their respective hemispheres, and then, after a period of tens to hundreds of thousands of years, retreat back to the polar regions.  But now we believe that twice during the Earth’s history, the ice advanced all the way to the equator, almost completely blanketing the Earth with a sheet of ice several kilometers thick.  This is the Snowball Earth hypothesis.  In the podcast Paul Hoffman explains the mechanisms that could have brought about a snowball Earth, and describes the extensive evidence that has now been amassed in support of two such periods in the Neoproterozoic, about 720 and  650 million years ago.

Paul Hoffman is Emeritus Professor of Geology at Harvard University.  His research on the sedimentary rocks of Namibia, and that of his students in North America, Arctic Europe, and Mongolia, uncovered compelling evidence of glaciation near the equator about 650 million years ago in the Neoproterozoic.  More than anyone else, he is responsible for synthesizing the evidence in the geological record to develop a persuasive case for a Snowball Earth.

For illustrations that support this podcast and to learn more about Geology Bites, go to geologybites.com.

The heat liberated during the formation of our planet created an ocean of magma.  As it began to cool, the Earth differentiated into a dense metallic core surrounded by a less dense rocky mantle.  At some point, we know that the surface of the Earth must have formed itself into the rigid blocks we call plates, and that these plates began to move and interact with each other as parts of the global process we call plate tectonics.  But did the plates form and did plate tectonics start soon after the Earth differentiated into the metallic core and rocky mantle over four billion years ago, or was there a long interim period before the initiation of plate tectonics?  There is an ongoing debate about this, with some finding evidence for plate tectonics as far back as the Hadean, i.e., over 4 billion years ago, and others suggesting that it was not until the Neoproterozoic, about 750 million years ago, that modern plate tectonics got going. 

Peter Cawood is a professor in the School of Earth, Atmosphere, and Environment at Monash University in Melbourne.  His research aims to discover the origin of the Earth’s continents by studying the ancient interiors of continents called cratons.   It is these cratons that preserve the clues as to the conditions and processes that prevailed in the early Earth, which in turn can tell us when and how plate tectonics started.   

Many processes in geology affect the temperature of rocks.  Erosion is one example — as a surface is eroded, the rocks below get closer to the surface, cooling as they go.  So if we know the temperature history of a rock, we can infer its erosion history.  Becky Flowers has a thermochronology lab in which she determines the cooling history of rocks as recorded in specific crystals they contain, such as zircon and apatite.  She explains how this works, and how she has used her results to unravel histories such as that of the Grand Canyon, the South African plateau, and even of that of the lunar surface.

Becky Flowers is a Professor of Geological Sciences at the University of Colorado, Boulder. She has a thermochronology lab in which she measures the amounts of the different radioactive decay products within crystals to figure out the cooling history of rocks. Knowing how rocks cooled gives us new insights on geological histories and processes of many parts of the Earth, and even of the Moon.

For podcast illustrations, and to learn more about Geology Bites, go to geologybites.com.

The geological history of Central Europe is quite complicated.  The region is composed of several continental blocks having quite distinct origins that came together over 300 million years ago in the Paleozoic Era.  Then, in the Mesozoic, many of the original rocks were overlaid, and continued plate movements caused mountain belts to form.  In a previous Geology Bites podcast, Douwe van Hinsbergen explained how he used an analysis of the geological structure of mountain belts to reconstruct tectonic plate motions.  In this episode, we hear about a totally different approach to reconstructing plate motions and paleogeography, and see how it was able to reveal the plate motions that assembled central Europe.  

Ulf Linnemann is the Head of the Geochronology Department at the Senckenberg Museum of Mineralogy and Geology in Dresden.  In his research he uses detrital zircon crystals, which are crystals that have survived the erosion of the magmatic rocks in which they formed, and that have been recycled into sedimentary rocks.  His team has determined the ages of large populations of detrital zircons as the basis for disentangling the events that led to the assembly of central Europe.

Ever since Alfred Wegener proposed the theory of continental drift in 1912, we have been aware that blocks of the Earth’s lithosphere are moving with respect to each other.  With the advent of plate tectonics in the 1960s, these moving blocks became identified with the tectonic plates that tile the Earth’s surface.  We now have accurate measurements of plate motion speeds, which range from about ½ a cm per year to 10 cm per year.  But there is still no general consensus as to what makes plates move.  Broadly speaking, there are two competing explanations.  In the first, the plates ride on top of convection cells of a vigorously convecting mantle.  In the second, it is the forces acting on plate boundaries, principally the pull of dense lithospheric slabs subducting into a less dense mantle that drive the plates.

Douwe van Hinsbergen is a Professor of Global Tectonics and Paleogeography at the University of Utrecht.  He has reconstructed the history of plate motions in various locations around the world with the primary goal of using this history to understand the dynamics of the mantle.  And his latest research is directed to shedding light on the long-standing question as to what drives tectonic plates.

Oceanic plates are continually manufactured at mid-ocean spreading ridges.  But exactly what processes go on at these ridges?  It turns out that it depends on what type of ridge it is—fast-spreading or slow-spreading.  And that our traditional view of vanishingly thin plate thickness at ridge axes is inaccurate.  Mathilde Cannat describes our modern understanding of mid-ocean ridges and the observations that led us there. Mathilde Cannat is a research director at the Institut de Physique du Globe of Paris. Her research on mid-ocean ridges has fundamentally changed our understanding of the geological processes that create new oceanic crust at these ridges. For podcast illustrations and more about Geology Bites, go to geologybites.com.

The lithium-ion battery was invented about 40 years ago, and is now commonplace in a range of products, from smartphones to electric cars. But if we are to meet the carbon emission goals that governments are setting, electrification, and with it the need for electricity storage, will increase dramatically. Although many new electricity storage methods are being developed, none are as mature as the lithium-ion battery, which will therefore need to be a major part of a carbon-free infrastructure.

Kathryn Goodenough is Principal Geologist at the British Geological Survey. She studies the geology of critical raw materials, and particularly of lithium. She is the Principal Investigator for the Lithium for Future Technology, an international consortium that investigates all types of lithium deposits and how they can be extracted sustainably.

For podcast illustrations and more about Geology Bites, go to geologybites.com.

The fossil record goes back through the Phanerozoic eon, about 540 million years, and even earlier, into the Ediacaran period.  But while the fossils provide incontrovertible evidence of ancient life, the fossils themselves are certainly not alive.  In fossils, the original organic matter belonging to the fossilized life form has been replaced by inorganic materials, cast into the shape formerly occupied by the life form.  However, in some situations, the original organic matter does survive.  For example, original spores as old as 350 million years have been identified using their original organic material.  And DNA can survive for as much as a million years.  But in 2019, bacteria that had been buried 100 million years with barely any access to nutrients were not only identified but shown to be alive.  Had they been in suspended animation for all that time?  Or were they managing to eke out a living using much less energy than was previously thought to be necessary?

Steve D’Hondt is a Professor of Oceanography at the University of Rhode Island.  He studies life beneath the sea floor and was on the team that discovered bacterial cells living in 100 million-year-old sediment.

To see podcast illustrations and learn more about Geology Bites, go to geologybites.com.

The subfield of geology called geodynamics most commonly refers to the motions associated with convection in the mantle.   These are slow by human standards and lead to phenomena such as plate motions, seafloor spreading, mountain building, and volcanos.  But the Earth’s interior actually undergoes motion on a whole range of timescales.  The shortest of these occurs within seismic waves – in which the vibrations triggered by earthquakes cause tiny elastic movements of the material as they pass through.  And the longest of these are the large-scale viscous motions of the convecting mantle driven by the temperature difference between the bottom and the top of the mantle.  But there are a whole range of intermediate timescale motions lasting, hours, days, years, and millennia.  

Harriet Lau is an Assistant Professor in Department of Earth and Planetary Sciences at the University of California at Berkeley.  In this episode the describes the motions of the solid Earth that take place over these intermediate timescales and explains what they can reveal about the Earth’s interior.

For podcast illustrations and to learn more about Geology Bites, go to geologybites.com.  Follow Geology Bites on Twitter @geology_bites, @oliverstrimpel, and on Instagram oliverstrimpel.

Many hundreds of films have been shot on location in the American West.  The rugged, inhospitable landscapes are an integral part of what gives so many American Westerns their distinctive character.  Although the region is vast, stretching from the Rockies in Wyoming and Colorado to Utah, New Mexico, Nevada, Arizona and California, a few locations have captured the lion’s share of the film industry’s attention.  What is it about these landscapes that makes them so photogenic?  And how did they get that way?

In this episode, Craig Jones casts light on the answers to these questions.  He is a Professor in the Department of Geological Sciences at Colorado University at Boulder.   His research aims to uncover the origin of mountains and elevated topography in continents,  primarily in the western United States.  His main tool for doing this is seismology, but he also incorporates observations from other disciplines, such as gravity measurements, paleoaltimetry, and petrology.  He has written a book on the Sierra Nevada and leads field trips to the rugged terrains we’re discussing here. 

For podcast illustrations and to learn more about Geology Bites, go to geologybites.com.  Follow Geology Bites on Twitter @geology_bites, @oliverstrimpel, and on Instagram oliverstrimpel.

We know the Earth was born as a baking inferno. So the presence of a relatively cool surface today suggests that the Earth has cooled a lot since it formed. But the very thin crust on which we subsist is not representative of the Earth as a whole, and although some heat must be escaping from the Earth, there are also ongoing sources of heat. So the answer to the question as to whether Earth is cooling down is not straightforward.

Claude Jaupart is Professor of Geophysics at the Université de Paris & Institut de Physique du Globe. His research aims to understand the physics of igneous processes in the Earth – such as those occurring in volcanic eruptions, magma chambers, and the mantle. He is especially interested in how heat flows out of the Earth, particularly in continents. In addition to advancing the geophysical theory of these processes, he has measured heat flow in the field, mostly in remotest Canada, where he discovered surprisingly low heat flow.

For podcast illustrations and to learn more about Geology Bites, go to geologybites.com.  Follow Geology Bites on Twitter @geology_bites, @oliverstrimpel and on Instagram oliverstrimpel.

How can we tell what is happening at the bottom of the lithosphere, especially in one of the most remote places on the planet?  Peter Molnar describes how many diverse lines of evidence, from the fossil record to normal faulting point to abrupt elevation changes in Tibet, both before and well after India collided with it.  He thinks this tells us that the bottom of the thickened lithosphere there is gravitationally unstable and hot enough to literally drip off into the asthenosphere below, after which the remaining lithosphere becomes more buoyant.

Peter Molnar is Professor of Geological Sciences at the University of Colorado at Boulder.  He has worked on a great many subjects, but especially on how mountain ranges are built and how climate on geological timescales is affected by topography and crustal movements.  His research on mountain ranges has focused on the high terrain in Asia - the Himalaya, Tibet, and Tien Shan - all of which he has studied extensively in the field.

For podcast illustrations, and to learn more about Geology Bites, go to geologybites.com. 

Katie Stack is Deputy Project Scientist for the NASA Perseverance rover that landed in Jezero crater on Mars in February 2021.  A geologist by training and an expert on the Martian sedimentary rock record, she has been mapping the geology of Mars since the 2000s.  She leads a large team of scientists that will combine orbiter and rover image data to investigate processes that took place on the the ancient surface of Mars.

She describes what we are learning with the powerful instruments aboard Perseverance as it traverses an ancient Martian river delta deposit.

For podcast illustrations, and to learn more about Geology Bites, go to geologybites.com.

Jan Smit is a paleontologist who specializes in abrupt changes in the geological record.  After the discovery of an end-Cretaceous surge deposit in North Dakota, he was part of the team that pieced together the striking evidence it contained, particularly its perfectly preserved fossils and tiny glass spherules called tektites.  He describes how this led to a detailed picture of the dramatic events that unfolded within an hour or two following the asteroid impact.

Jan Smit is Emeritus Professor of Event Stratigraphy at the Free University of Amsterdam.  Initially a paleontologist studying planktonic foraminifera, he became intrigued by what appeared to be extremely sudden events in the fossil record, especially at the boundary between the Cretaceous and the Paleogene, known as the KT boundary.

To see podcast illustrations as well as to learn more about Geology Bites, go to geologybites.com.

Marie Edmonds is Professor of Volcanology and Petrology at Cambridge University.  She studies the cycling of volatile elements such as carbon between the atmosphere and the mantle and the role that volatiles play in melting, magma transport, and the style of  volcanic eruptions. She describes how all volcanos emit gas and how the gas can reveal a lot about the origin of the magma and also forewarn of eruptions.  Here is is monitoring gases emitted during the 2018 eruption of Kilauea on Hawaii.

For podcast illustrations and to learn more about Geology Bites, go to geologybites.com. 

Gillian Foulger is a leading proponent of the plate hypothesis of volcanism, which posits that volcanism away from plate boundaries can be explained by extensional deformation of the lithosphere with melting of the upper mantle.  The plate hypothesis uses plate tectonic theory to explain all volcanism without invoking plumes or hot-spots that originate in the lower mantle.  She hosts a lively debate on whether mantle plumes exist at mantleplumes.org.

Gillian Foulger is Emerita Professor of Geophysics at Durham University.  A key prediction of the plate hypothesis is that the lithosphere should be extending wherever we see intra-plate volcanism.  She explains that many of the observations we already have can be reinterpreted as consistent with the plate hypothesis, and suggests new experiments that could clearly distinguish the competing hypotheses.

For podcast illustrations go to geologybites.com.

Sarah Stewart uses computer-based dynamical simulations and lab experiments to create scenarios for the collision of a massive body with the Earth that can reproduce the composition, orbits, and spins of the Earth and Moon today.  

Sarah Stewart is a Professor in the Department of Earth and Planetary Sciences at the University of California Davis.  In the podcast, she explains how, following a massive impact with another body, the Earth formed a synestia - an inflated disk of gas in which the impacting body and Earth were thoroughly mixed, and out of which the Moon and a new Earth solidified.

For podcast illustrations, including videos of simulated Moon-forming impacts with synestias, and orbital simulations, go to geologybites.com.