PlanetGeo: The Geology Podcast

Show Notes
Quick news up top: alongside the Roadside Geology of Colorado audiobook, the Camp Geo mobile app now also has Colorado Rocks, a shorter, lower-jargon intro to the same landscapes. If Roadside Geology is the deep dive, Colorado Rocks is the one to start with. Both are live in the app now, and since this is a trial run for a whole series of state titles, tell us how it lands. Find us @PlanetGeoCast, through the contact link at planetgeocast.com, or through the contact link in the Camp Geo app menu.
Then it's back to Chapter 2 of the Camp Geo audio textbook. Last episode ended with weathering, the breakdown of rock. This episode is everything that happens next: the sediment gets moved, it gets dumped somewhere, and it gets turned into rock. That's erosion, deposition, and lithification, and it's the heart of why sedimentary rocks are so useful, because every step leaves a fingerprint you can read back out of the rock millions of years later.
Chris starts by drawing a line he admits he blurs himself after all these years of teaching: weathering breaks the rock, erosion moves it. The movers are wind, streams, glaciers, and longshore currents (which, yes, have ended up in court over who stole whose beach). Chris's Michigan students always vote for glaciers as the most important agent of erosion, and you can see why when the whole skin of the state is glacial. But it isn't close. Streams move more sediment than everything else combined.
Water is also picky in a way glaciers aren't. A glacier will carry a house-sized boulder as easily as a sand grain, but moving water can only lift what its energy allows. That's why high mountain streams in Pennsylvania are full of boulders and the Mississippi is full of mud, and it's why one glance at the clast size in a sedimentary rock tells you the energy of the environment it was deposited in. Big pieces mean fast water. Dust-sized pieces mean water so still that nothing kept them in suspension, which usually means the deep ocean.
Jesse then explains sediment maturity, which Chris reframes as a measure of how far the stuff traveled. Near the source, clasts are angular and all mixed sizes, a poorly sorted sediment. Far downstream they're rounded, uniform in size, and increasingly just one mineral, quartz, because everything else has broken down along the way. Chris's version: hand anyone a jar of Skittles and say "sort it," and they know exactly what to do. A well-sorted rock is one pile of Skittles. So if you see sharp-edged boulders sitting next to tiny grains in a road cut, that sediment did not go far.
Finally, lithification, the two-step process that turns beach sand into sandstone. Compaction happens as more sediment piles on top, squeezing out water and air and shrinking the pore space. For some rocks that's the whole story: shale, the most abundant clastic rock, is only compacted, which is why it's soft, crumbly, and forms slopes instead of cliffs. Chris calls it a wimpy rock, and it's the easiest one to spot doing "roadside geology at 70 miles an hour" out a bus window, a phrase Jesse first heard as a high schooler on Chris's field course, delivered over the walkie-talkie while Chris drove one-handed. The second step is cementation: quartz can dissolve and re-bond to itself under pressure, but more often fluid moving through the pore spaces precipitates silica, calcium carbonate, or iron oxide between the grains and glues them together. That last one is why the sandstones of the Painted Desert are so red, and it takes surprisingly little iron to stain an entire rock. (It's also the brown streak on your sidewalk, unless, as Jesse points out, you live in Pennsylvania.) Chris wraps it with a Dixie cup demonstration you can do at home: two cups of packed sand, one flushed with fresh water, one with salt water. A week later, one is a handful of loose sand and the other is a cup-shaped rock.
Then the quiz, which Chris talks himself through like a seasoned test-taker before landing the answer. Next time: the clastic sedimentary rocks themselves.
In this episode
New on Camp Geo: Colorado Rocks, the shorter intro companion to Roadside Geology of Colorado
Where Episode 1 left off: weathering, mechanical and chemical
Weathering vs. erosion, and why even Chris has to be deliberate about the difference
The agents of erosion: wind, streams, glaciers, longshore currents
Longshore current lawsuits over stolen sand
Why Michigan students always say glaciers, and why streams win anyway
Water discriminates by energy; glaciers don't
Mountain boulders vs. Mississippi mud: reading depositional energy from clast size
Why the finest particles only settle in very still water
Sediment maturity: rounding and sorting as a record of transport distance
Mineral sorting: how a mixed sand becomes pure quartz downstream
The Skittles test for sorting
Lithification step one: compaction
Shale, the "wimpy rock" that's only compacted
Roadside geology at 70 miles an hour (and one-handed bus driving)
Lithification step two: cementation by silica, calcite, or iron oxide
Why the Southwest is red, and the brown streaks on your sidewalk
The Dixie cup rock: fresh water vs. salt water
The quiz: what best describes what happens to particles as they're transported?
Up next: clastic sedimentary rocks
Download the CampGeo app now at this link.

On the app you can get tons of free content, exclusive images, and access to our Geology of National Parks series.
You can also learn the basics of geology at the college level in our FREE CampGeo content series - get learning now!

Like, Subscribe, and leave us a Rating!

——————————————————
Instagram: @planetgeocast
Twitter: @planetgeocast
Facebook: @planetgeocast
Support us: https://planetgeocast.com/support-us
Email: planetgeocast@gmail.com
Website: https://planetgeocast.com/

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.
Download the CampGeo app now at this link.

On the app you can get tons of free content, exclusive images, and access to our Geology of National Parks series.
You can also learn the basics of geology at the college level in our FREE CampGeo content series - get learning now!

Like, Subscribe, and leave us a Rating!

——————————————————
Instagram: @planetgeocast
Twitter: @planetgeocast
Facebook: @planetgeocast
Support us: https://planetgeocast.com/support-us
Email: planetgeocast@gmail.com
Website: https://planetgeocast.com/

Big Announcement: Roadside Geology Comes to Audio — Plus an Introduction to Sedimentary Rocks
The glove-compartment classic is now in your headphones. In this episode, we announce a partnership between Camp Courses (the team behind the CampGeo app) and the Geological Society of America to produce audio-visual editions of Roadside Geology of Colorado and Colorado Rocks! — available right now in the CampGeo mobile app.
These aren’t ordinary audiobooks. Geology books live and die by their maps, cross-sections, and outcrop photos, and CampGeo’s patented approach syncs the full narration with those images on your phone — so you can see the geology while you hear it. For us, this one is personal: the Roadside Geology series is how we learned to read a landscape, riding shotgun on rock-collecting trips from Colorado to New England.
GSA now owns the entire Mountain Press geology library, and these two titles are the first brought to audio. If listeners find them useful, more will follow — so check them out and tell us what you think. This launch is timed with the GSA Connects conference in Denver.
Download the CampGeo app now at this link.

On the app you can get tons of free content, exclusive images, and access to our Geology of National Parks series.
You can also learn the basics of geology at the college level in our FREE CampGeo content series - get learning now!

Like, Subscribe, and leave us a Rating!

——————————————————
Instagram: @planetgeocast
Twitter: @planetgeocast
Facebook: @planetgeocast
Support us: https://planetgeocast.com/support-us
Email: planetgeocast@gmail.com
Website: https://planetgeocast.com/

Chris's college mineralogy lab final was 20 specimens, and one of them was a quartz crystal his professor had dipped in graphite. The luster was off, the color was off, and Chris got it wrong even though every other property said quartz. That one exam question shows why mineral ID is hard and why you need a system. This episode lays out that system.
We finish the minerals chapter of the Camp Geo audio textbook with polymorphs, which are minerals with the same chemical composition but different crystal structures. The classic example is graphite and diamond. Both are pure carbon, but in diamond each carbon atom bonds to four others instead of three. That denser arrangement is stable at high pressure, which makes polymorphs useful clues to where a rock formed. Along the way we get into synthetic diamonds, companies that turn ashes into gems (Jesse will pass on that one), and a white dwarf star that Chris describes as a 2,500-mile-wide diamond, nicknamed "Lucy" after the Beatles song.
Then Chris walks through the mineral ID flowchart he's refined over decades of teaching. Start with luster, because metallic or non-metallic decides your next step. Metallic minerals go straight to streak, and non-metallic minerals go to hardness. Chris brackets hardness on the Mohs scale with things you already carry: a fingernail (2.5), a copper penny (3.5), a knife or glass plate (5.5), and feldspar (6) and quartz (7) if you can find them. From there it's cleavage vs. fracture (the key test for telling feldspar from quartz) and why chemical bonding controls both. Then streak, where shiny silver specular hematite from Michigan's Upper Peninsula leaves a reddish-brown powder. Last come specific gravity, color, crystal form, and a few specialty properties like magnetism and fluorescence.
Color gets a public trial, because amethyst, rose quartz, and milky quartz are all the same mineral. That leads to a rescue story: Chris hauled a rose quartz specimen down a scree slope and everyone else thought it was junk. He still has it and says he was right. Then the quiz, which Chris aces.
In this episode
The graphite-dipped quartz that cost Chris a question on his mineralogy final
Polymorphs: same composition, different crystal structure
Graphite vs. diamond, and why four bonds beat three at high pressure
Synthetic diamonds, memorial diamonds, and a diamond-core white dwarf
Why mineral ID needs a structured workflow
Luster first: metallic vs. non-metallic decides your next test
The Mohs hardness scale and the fingernail–penny–knife–glass bracket
What controls hardness: the weakest link in the chemical-bond chain
Cleavage vs. fracture, and how to tell feldspar from quartz in the field
Streak: silver hematite, reddish-brown powder
Specific gravity, the "heft test," and how density separates zircons in the lab
Why color is the least reliable property (amethyst, rose, and milky quartz)
Crystal form: diagnostic when crystals have room to grow, which is rare
Magnetism, fluorescence, and willemite under a black light
The quiz: Chris goes 2 for 2
Up next: sedimentary rocks
Download the CampGeo app now at this link.

On the app you can get tons of free content, exclusive images, and access to our Geology of National Parks series.
You can also learn the basics of geology at the college level in our FREE CampGeo content series - get learning now!

Like, Subscribe, and leave us a Rating!

——————————————————
Instagram: @planetgeocast
Twitter: @planetgeocast
Facebook: @planetgeocast
Support us: https://planetgeocast.com/support-us
Email: planetgeocast@gmail.com
Website: https://planetgeocast.com/

There are roughly 7,000 known minerals on Earth. You need to know about ten of them. In the third installment of the Camp Geo Intro to Minerals series, Chris and Jesse finish the minerals chapter by working through the rock-forming minerals — the short list that makes up the overwhelming majority of the rocks in Earth's crust — and by explaining where minerals actually come from in the first place.
Before the top-ten list, they step outside the silicates. Just as the silica tetrahedron defines the silicate family, other mineral groups are defined by their own dominant anion: the carbonates (CO₃²⁻, home to calcite, arguably the second most important mineral group on the planet), the sulfates (gypsum, which Jesse grew up seeing mined in Michigan), and the phosphates (apatite — the stuff your teeth are made of, and a mineral Jesse's lab dates routinely). Along the way, Jesse clarifies a point from Part 1: an anion can be a single charged atom or a charged molecule like CO₃ or SiO₄.
Then, the four environments in which minerals grow — igneous (crystallizing from cooling magma or lava), metamorphic (solid-state recrystallization under heat and pressure), hydrothermal (precipitating from hot circulating water, as at Yellowstone and, far more commonly, at mid-ocean ridges), and sedimentary (salts left behind as cool saline water evaporates, think Bonneville Salt Flats). Chris flags the one that trips students up most: hydrothermal and sedimentary both precipitate minerals from water, but only one of them involves heat. This detour also produces the episode's best story: the day the two of them drove up a sketchy dirt road to the Gore Mountain garnet quarry in upstate New York without permission, watched a black Chrysler 300 come roaring up behind them, and left with about two tons of volleyball-sized garnets in the back of Chris's F-250.
Finally, the countdown: feldspar, quartz, the micas, the olivines, the pyroxenes, the amphiboles (hornblende, for intro purposes), the clay minerals, calcite, dolomite, and halite plus gypsum. Seven of the ten are silicates, and Chris and Jesse tie each one back to its tetrahedral structure from Part 2 — framework, sheet, single chain, double chain, single tetrahedron. They cover the tricks for telling calcite from dolomite (a drop of HCl fizzes like Alka-Seltzer on one and like flat pop on the other), why mica rarely survives in sedimentary rocks, why clay minerals are basically what feldspar turns into when it hits a stream, and Chris's field-tested (and decidedly not medically endorsed) use of crushed limestone as heartburn relief.
The episode closes with Jesse's quiz question for Chris, which Chris promptly declares "has potential" but needs reworking.
Download the CampGeo app now at this link.

On the app you can get tons of free content, exclusive images, and access to our Geology of National Parks series.
You can also learn the basics of geology at the college level in our FREE CampGeo content series - get learning now!

Like, Subscribe, and leave us a Rating!

——————————————————
Instagram: @planetgeocast
Twitter: @planetgeocast
Facebook: @planetgeocast
Support us: https://planetgeocast.com/support-us
Email: planetgeocast@gmail.com
Website: https://planetgeocast.com/