Geology Bites

Where tectonic plates collide, the result is clear: the edge of a plate crumples and throws up a mountain belt. But where a plate is stretched rather than squeezed, the evidence is usually lost, because the thinned lithosphere subsides and the basins that form fill with sediment.The Basin and Range Province of the western United States and northwestern Mexico is the great exception. Between the Colorado Plateau and the Sierra Nevada, dozens of mountain ranges march north in near-parallel array, in a crust that has been pulled out to about twice its original width — and it is still stretching today.In this episode we look at how we know that: from GPS antennas bolted into bedrock that measure the province widening in real time, to the record of ancient earthquakes, to the dismembered thrust sheets that let the whole region be reassembled as it was 20 million years ago. We also discuss the low-angle detachment faults that have brought rock from fifteen kilometres down up to the surface and why a stretched province still stands a mile high.Brian Wernicke is an Emeritus Professor in the Division of Geological and Planetary Sciences at Caltech.For photos, figures, and further reading, see https://www.geologybites.com.

In this episode, Jiří Žák describes the two main orogenies whose remnants figure prominently in central European geology: the Cadomian orogeny that lasted from the late Neoproterozoic to the early Cambrian (c. 700 Ma to c. 425 Ma) and the Variscan orogeny that occurred in the late Paleozoic (c. 380 Ma to 280 Ma). The Cadomian took place on the northern margins of Gondwana, only later to rift and travel north to form what was to become Europe. The Variscan was caused by the collision of Gondwana with Laurussia in the final stages of the assembly of the supercontinent Pangea. Both orogenies have been heavily eroded, and we see their imprint in the form of metamorphic rocks, volcanic rocks, granites, and deformation structures. These are scattered across Europe, from southern Britain to eastern Europe.Žák has been studying the geology of central Europe for over 25 years using methods ranging from structural studies in the field to detrital zircon geochronology. He is a Professor in the Institute of Geology and Paleontology at Charles University in Prague.

In the podcast, Cees Van Staal tells us about the Paleozoic tectonic events that led to the formation of the Appalachians. The events are closely related to those involved in the Caledonian orogeny and the mountains it created in what is now Ireland, Scotland, east Greenland, and Norway, as discussed in the episode with Rob Strachan. However, the Appalachians that we see today are not the worn-down remnants of the Paleozoic mountains. Instead, they reflect much more a topography that was created during processes associated with rifting and magmatism that accompanied the opening of the Atlantic Ocean as well as the effects of the ice ages as recently as about 10,000 years ago.Van Staal has been studying the Appalachians for over 35 years, focusing especially on the large-scale tectonics of their formation. He is Emeritus scientist at the Geological Survey of Canada and an Adjunct/Research Professor in the Department of Earth and Environmental Sciences at the University of Waterloo in Ontario.

When the Earth formed, it was covered by a hot magma ocean. So when and how did thick, silica-rich continental lithosphere form? Were the first, ancient continents similar to the present-day continents? And did the continents form in a burst of activity at a certain point, or was it a gradual build-up over Earth history?In the podcast, Renée Tamblyn addresses these questions, as well as how early geological processes created molecular hydrogen that may have powered the first forms of life. In her own research, she has focused on the critical role played by water released from hydrous minerals that formed within oceanic lithosphere on the sea floor. Tamblyn is a Postdoctoral Researcher at the University of Bern.

Most of Earth’s salt is dissolved in the oceans. But there is also a significant amount of solid salt among continental rocks. And because of their mechanical properties, salt formations can have a dramatic effect on the structure and evolution of the rocks that surround them. This gives rise to what we call salt tectonics – at first sight, a rather surprising juxtaposition of a soft, powdery substance with a word that connotes the larger scale structure of the crust.In the podcast, Mike Hudec explains the origin of salt in the Earth’s crust and describes the structures it forms when subjected to stresses. He also discusses how salt can play in important role in the formation of oil and gas reservoirs.Hudec is a research professor at the Bureau of Economic Geology at the University of Texas at Austin.

The planets formed out of a cloud of gas and dust around the nascent Sun. Within the so-called snow line, it was too hot for liquid water to exist. Since the Earth lies well within this line, why does it have water? Did it somehow manage to retain water from the outset or did it acquire its water later? In the podcast, Lindy Elkins-Tanton explains how these two scenarios might have played out but she says the evidence strongly favors one of these theories. Elkins-Tanton has concentrated much of her research career on the formation and evolution of planets, and especially the role of water. She is a Professor in the School of Earth and Space Exploration at Arizona State University and Principal Investigator of the NASA Psyche mission.

The late Paleozoic ice age began in the Late Devonian and ended in the Late Permian, occurring from 360 to 255 million years ago. It was similar to the present day in two key respects: rising atmospheric CO2 and recurrent major ice sheets. In the podcast, Isabel Montañez explains how we can use proxies to learn about the climate and ocean conditions that prevailed then. And with the help of a model, she says that we can also learn about sensitivities and feedbacks of Earth systems to rising CO2. Among other things, the model suggests that when the atmosphere reaches the present day level of CO2, significant parts of the ocean may become anoxic and ocean circulation patterns alter.Montañez is a Distinguished Professor in the Department of Earth and Planetary Sciences at the University of California, Davis.