PlanetGeo: The Geology Podcast podcast

Intro to Sediments - Geology 101: Sediments Part 1

0:00
23:29
15 Sekunden vorwärts
15 Sekunden vorwärts

First, some news. The Roadside Geology of Colorado is now an audiobook on the Camp Geo mobile app, produced in partnership with the Geological Society of America. If you've ever driven across the country with a stack of Roadside Geology books riding shotgun (Chris and Jesse have, many times), this is that experience in your ears, with the images that go with it. It's the first title in what we hope becomes a whole series, so try it out, tell your friends, and let us know which state you'd like us to do next. Find us @PlanetGeoCast, through the contact link at planetgeocast.com, or through the contact link in the Camp Geo app menu.

Then we open Chapter 2 of the Camp Geo audio textbook: sedimentary rocks. They make up only about 7% of Earth's crust, but they cover roughly 75% of its surface, which means wherever you're sitting right now, odds are good there's a sedimentary rock under you, and odds are even better that it's a shale or a sandstone. Those thin layers hold nearly every fossil ever found, the record of ancient climates, oceans, and landscapes, the order of events in Earth's history, and almost all of our petroleum and natural gas. They're also the first thing a Mars rover goes looking for, because sediment averages a huge area of a planet into one sample.

Chris and Jesse lay out the three categories, in order of abundance. Clastic (or detrital) rocks are compacted and cemented fragments of other rocks, carried in by streams, glaciers, and wind, which leads to a brief detour into the very online geology debate over whether wind can actually lift sand. Biochemical (Chris prefers bioclastic) rocks are built from organisms that pulled ions out of seawater to make shells, from whole shells welded together down to microscopic calcium carbonate. Chemical rocks precipitate straight out of evaporating salty water, like the Bonneville Salt Flats, with no life required. They're small in volume but often big economically.

The episode closes with the question underneath all of it: where does sediment come from in the first place? That's weathering, and there are two kinds. Mechanical weathering breaks rock into smaller pieces, with ice wedging (water expands about 9% when it freezes) doing the heavy lifting in cold mountain country and transport downstream doing the rounding. Chemical weathering dissolves rock, because all rainwater is slightly acidic from dissolved carbon dioxide, and Jesse explains hydrolysis and why it matters that feldspar, the most abundant mineral in the crust, breaks down into clay minerals, which become shale, the most abundant sedimentary rock on the surface. Then the quiz, which Chris gets right and immediately declares himself unstumpable.

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