Geology Bites: An Earth and Planetary Science Podcast

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.