Geology Bites

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.

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.

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.

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.

We have learned a great deal about the geology of the Moon from remote sensing instruments aboard lunar orbiters, from robot landers, from the Apollo landings, and from samples returned to the Earth by Apollo and robot landings. But in 2025, when NASA plans to land humans on the Moon for the first time since 1972, a new phase of lunar exploration is expected to begin. What will this mean for our understanding of the origin, evolution, and present structure of the Moon? A lot, according to Mahesh Anand. For example, as he explains in the podcast, satellite imagery suggests that volcanism continued for much longer than was previously thought, perhaps until as recently as 100 million years ago. In-situ inspection and sample return should help us explain this surprising finding.

Mahesh Anand is Professor of Planetary Science and Exploration at the Open University, UK.

At the core of Earth’s geological thermostat is the dissolution of silicate minerals in the presence of atmospheric carbon dioxide and liquid water. But at large scales, the effectiveness and temperature sensitivity of this reaction depends on geomorphological, climatic, and tectonic factors that vary greatly from place to place. As described in the podcast, to predict watershed-scale or global temperature sensitivity, Susan Brantley characterizes these factors using the standard formula for the temperature dependence of chemical reaction rates using an empirically-determined activation energy for each process. Overall, her results suggest a doubling of the weathering rate for each 10-degree rise in temperature, but this value changes with the spatial scale of the analysis.

Susan Brantley is a Professor in the Department of Geosciences at Pennsylvania State University.

Banded Iron Formations (BIFs) are a visually striking group of sedimentary rocks that are iron rich and almost exclusively deposited in the Precambrian. Their existence points to a major marine iron cycle that does not operate today. Several theories have been proposed to explain how the BIFs formed. While they all involve the precipitation of ferric (Fe3+) iron hydroxides from the seawater via oxidation of dissolved ferrous (Fe2+) iron that was abundant when the oceans contained very low levels of free oxygen, they disagree as to how this oxidation occurred. In the podcast, Clark Johnson describes how oxidation could have occurred without the presence of abundant free oxygen in the oceans.

Clark Johnson is a Professor Emeritus in the Department of Geoscience at the University of Wisconsin-Madison.

The geological history of most regions is shaped by a whole range of processes that occur at temperatures ranging from above 800°C to as low as 100°C. The timing of events occurring over a particular temperature range can be recorded by a mineral which crystallizes over that range. The mineral calcite is suitable for recording low-temperature processes such as fossilization, sedimentation, and fluid flow, and it is especially useful as it is virtually ubiquitous. But using uranium-lead radiometric dating in calcite is very challenging as it often contains very little uranium and the ragiogenically-produced lead isotopes can be swamped by common lead within a calcite crystal. In the podcast, Catherine Mottram explains how these challenges are being overcome and shares some of her findings based on radiometric dating of calcite. Mottram is an Associate Professor of Geology at the University of Portsmouth. 

In this episode, Martin Van Kranendonk lays out a convincing case for life on Earth going back to at least 3.48 billion years ago.

To find evidence for very ancient life, we need to look at rocks that have been largely undisturbed over billions of years of Earth history. Such rocks have been found in the Pilbara region of northwest Australia. As explained in the podcast, the 3.48-billion-year-old (Ga) rocks of the Pilbara's Dresser Formation contain exceptionally well-preserved features that show unmistakeable physical and chemical signatures of life. While older 3.7 Ga rocks in west Greenland may also prove to have harbored life, the Dresser Formation rocks represent the oldest widely accepted evidence for life on Earth.

Martin Van Kranendonk has devoted his long and prolific research career to the study of the early Earth. One major theme of his work has been to use detailed mapping and lab research to develop geological models for the environments of Earth’s oldest fossils. This has helped establish the biological origin of many ancient fossils. His recent work on a newly discovered find of exceptionally well-preserved 3.5-billion-year-old sedimentary rocks in the Pilbara Craton of Western Australia has provided the strongest evidence to date that structures of this great age were produced by the earliest forms of life.

Martin Van Kranendonk is a Professor in the School of Biological, Earth, & Environmental Sciences at the University of New South Wales in Sydney.

The Alps are the most intensively studied of all mountain chains, being readily accessed from the geological research centers of Europe. But despite this, there remains considerable uncertainty as to how they formed, especially in the Eocene (about 40 million years ago) when the events that led directly to Alpine mountain-building started. In the podcast, Rob Butler explains how much of this uncertainty stems from our fragmentary knowledge of the locations and structures of sedimentary basins and small continental blocks that lay between Europe and Africa at that time. In his research, he combines detailed studies of the sedimentary rocks flanking the Alps with the large body of structural and petrological knowledge amassed over the past two centuries to try to unravel the sequence of events leading up to the formation of the Alps.

Rob Butler is Professor of Tectonics at the University of Aberdeen, Scotland, UK.

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.

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.

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.

Web: geologybites.com

Twitter: @geology_bites

Insta: geologybites

email: geologybitespodcast@gmail.com

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.

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

Twitter: @geology_bites
Insta: geologybites
email: geologybitespodcast@gmail.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.

If you like Geology Bites, please rate and review the podcast.  It helps others find it.

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.

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. 

Website: geologybites.com with illustrations supporting each podcast

Instagram: @oliverstrimpel

Twitter: @geology_bites

Email: geologybitespodcast@gmail.com

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 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.

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

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.

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.   

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.