Episode 2: Three Recipes for Rocks
Last episode, we highlighted the subject of this podcast, the Precambrian Era, the first 90% of Earth’s history. I gave you a teaser trailer for the Precambrian’s greatest hits: planetary collisions, the earliest life, the rise of oxygen, global glaciations, and eventually the origin of animals. All these events are recorded in ancient rocks- geologists literally call this history “the rock record”, just like a Led Zeppelin album.
But how can we tell, just by looking at a rock, that the earth was once covered by glaciers, or that there was an ocean in the Australian Outback?
What stories do these rocks tell us?
My PhD adviser had a great analogy in his Geology 101 course. When you meet someone new, you start a conversation by asking them basic questions: “What’s your name?” “Where are you from?” before you get to difficult questions like “Is a hot dog a sandwich?”. Whenever a geologist picks up a rock, they must ask basic questions before getting to more complicated information.
Today, we will look at several common rock types and find out how they got here. Just like a real relationship, we’ll tactfully save questions about age for a later time. By the end of this episode, when you see a piece of basalt or sandstone, you’ll know how and where it was formed- not a specific place like Acapulco, but a general environment like a lava flow or an ancient beach. When you look at a rock on a hiking trail, it was likely formed by one of three processes. As I mentioned last time, consider these “three recipes for making a rock”.
Let’s go through the recipes one by one, noting what rocks you can make with them, and how changes between environments can produce very different rocks.
Recipe 1: Igneous rocks: Start with magma, then chill.
Rocks that are formed from molten material such as magma or lava are called igneous rocks, from the same root word as ignite. The two igneous rocks we’ll cover today are granite and basalt. To start an igneous recipe, we need to dig deep down, several kilometers below Earth’s surface. Here, we will find magma chambers, pockets of molten material as large as cities. The magma itself is around 1000 C, or 2000 F. That’s twice as hot as a tandoori oven, hot enough to melt most minerals. Magma is mainly made of silica and aluminum oxide, with pinches of iron, calcium, sodium, and many other elements. Slight changes in these elements will produce rocks with different properties and colors. For example, igneous rocks with less silica like basalt are black, dark gray, or even green while rocks with more silica like granite can be white, light gray, pink and red.
The scene I have just painted for you of a giant underground hot pocket can only be observed in our imaginations. No one has ever visited a magma chamber in person, even though several mines and caves reach appropriate depths. In fact, despite decades of searching, the first documented case of a drill hitting a magma chamber wasn’t until 2005 in Hawaii, and only a handful of other cases have occurred since then.
And yet, geologists knew that magma chambers existed well before then, not just through subsurface imaging, but through rocks currently sitting on Earth’s surface. So how did those rocks get there?
Torres del Paine, Chile: once a pool of magma under Earth’s surface. 12 million years old (Dec 31 on the Earth Calendar)
This question brings us to the second stage of an igneous recipe- cooling. As a magma chamber cools down over time, liquid magma becomes solid rock, just like liquid water becomes solid ice. One major difference is that the ice cubes in your freezer are 100% water- hydrogen and oxygen. In contrast, the diversity of elements inside a magma chamber- silica, iron, potassium- produces many different minerals. You can see this diversity when you look closely at a granite countertop- the speckled patterns are formed from several different minerals- pink feldspars, white plagioclase, clear quartz, and black amphiboles. You can imagine these crystals slowly forming like technicolor snowflakes as the magma cools.
And slow is the key word here. Nestled deep within Earth’s crust, magma chambers take thousands of years to finally cool down. It takes even longer for these lithified blobs to become exposed through uplift and erosion, forming incredible granite outcrops seen around the world such as Half Dome or Tis-a-ack in Yosemite and Torres del Paine in Chile. Every piece of granite was once molten magma cooling deep below the surface.
Lava dramatically cooling, Pu’u Kahaluea, Hawaii
So what happens if we speed up the cooling process? The best way to cool molten material quickly is to bring it to Earth’s surface through volcanoes, where magma becomes lava. Hopefully this episode should solve any future trivia debate- magma is molten rock beneath Earth’s surface, lava has been exposed onto the surface itself. Rocks like basalt that form from lava are called extrusive, like extruding toothpaste out of a tube, while rocks like granite forming inside the earth are intrusive, just like an introvert who wants to stay inside and chill.
In open air or water, lava cools down within days to months- you can see red-hot lava turning to black basalt before your very eyes in Hawaii and Iceland. Minerals that form in cooling lava have less time to grow than in magma, and crystals in extrusive rocks are much smaller than intrusive rocks. Looking at a basalt sometimes requires a magnifying glass to see individual crystals. If cooling is instantaneous, crystals don’t have any time to form, and instead lava turns into obsidian, or volcanic glass.
Granite and basalt are by far the most common igneous rocks, and both will appear many times as we travel forward in Earth history. We will also see strange volcanic rocks that were common billions of years ago but have only erupted a few times since June on our Earth calendar- these are the old dinosaurs of the igneous world. For now, whenever you see snow-capped granite peaks, remember that they were once deep hot lakes of magma, and any dark basalt you find was once a lava flow oozing out of an ancient volcano.
Recipe 2: Sedimentary rocks: Take another rock, break it into bits, then let them settle.
Sedimentary rocks usually form from the pieces of other rocks, but also from shells, organic matter or crystals growing in water. Sedimentary geology is my specialty, but I’ll do my best to give these rocks equal time to their counterparts. Today we’ll cover sandstone, shale, and limestone- which is a bit different from the other two. Starting a sedimentary recipe is much less extreme than an igneous one- for this section, you can imagine yourself on a beach. For bonus points, you can listen to this episode on a real beach- make a geology-themed sandcastle and send a picture to bedrock.mailbox@gmail.com.
As soon as a rock is exposed on Earth’s surface, it begins to break down. Water is the main culprit here- it carves rocks down physically through waves, rainfall, and ice crystals. Water also chemically alters rocks, reacting with certain minerals to form new crystals and weakening the overall rock structure. The rock doesn’t just disappear- these little pieces must go somewhere. Again, water usually provides the transport, picking up small pieces and moving them down streams to rivers and eventually to lakes and the ocean. Wind can also break down and transport dust, but for nearly every sedimentary rock you’ll find, it’s telling you a story of water, just like igneous rocks tell stories of magma or lava.
On our imaginary beach, we see the waves pound against large boulders. If you look around, you’ll find smaller portions of the same rocks- cobbles and pebbles. Given many years, these too will be ground into individual grains of sand. If you pick up a handful, the sand runs through your fingers, and unfortunately your sandcastle probably won’t survive the day. So how does loose sand turn into sandstone? The answer is burial and once again- water.
Water never stops breaking down rocks all over Earth’s surface, and the sand you see on today’s beach will be covered by new material within a few years. As one layer of sand gets buried, it experiences greater and greater pressures. You can feel this yourself when you dig down for that good sandcastle material- eventually the sand becomes packed and is much harder to scoop up. This is the first step to turn sand into sandstone.
You’ll also notice that this packed sand is wet. Water becomes more important the deeper we go, below the range of shovels. Here, under even greater pressures, minerals start to precipitate between the sand grains. These tiny crystals bind the sand together like cement, helped by increasing pressure and temperatures. The Precambrian sandstones we’ll investigate have been pressurized for billions of years and require a sledgehammer to break apart, but they were once loose sand just like a modern beach.
Leaving our sandcastle behind, let’s walk into the sea. The farther out we go, and the deeper the water gets, we notice that the sand beneath our feet becomes finer, first silt and then mud. This mud is the same material as the beach sand but ground down even smaller. Particles of mud are much lighter than sand and are easily disturbed by any passing wave. Therefore, they only settle down on very calm seafloors far from the active beach. You can test this idea yourself at home- fill a bottle with water, then mix in sand and mud and shake it. The heavier sand will instantly fall to the bottom, but the mud will float around for hours, even days. Eventually, this mud gets buried and cemented, and turns into shale. Therefore, when geologists find shales, we’re instantly transported to the quiet bottom of an ancient ocean.
Any good cookbook has secret family recipes scribbled on the margins- for sedimentary rocks, the best example is limestone. Like sandstone and shale, limestone usually forms underwater, but limestone is not made from the pieces of other rocks. Instead, limestone is made of the mineral calcium carbonate. When water contains enough individual ingredients of calcium and carbonate, minerals start to crystallize, usually in warm shallow waters like the Bahamas or your own bathroom, as lime builds up around your tub if you don’t clean it often. In modern oceans, many animals have evolved to make limestone skeletons, like coral or sea urchins. But these won’t appear until the very end of this podcast. Instead, Precambrian limestones simply form on the seafloor, or on microbial colonies, producing some of the earliest fossils.
Rocks that form by mineral precipitation like limestone are called chemical sedimentary rocks, while those that form like sandstone are clastic, from the ancient Greek word for “fragments”. Chemical sediments were abundant in the Precambrian, including oddball examples made from iron, sulfur, and even barium. There will even be some clastic deposits made from sulfur and uranium that disappear with rising oxygen levels.
For now, just remember when you go to the beach, that the sand beneath your feet will one day be a sandstone for future geologists.
Recipe 3: Metamorphic rocks: Take another rock and pressure cook it.
Metamorphic rocks require igneous or sedimentary material as a first ingredient, but instead of breaking down the original rock, we’ll be transforming it into something new. For this final section we’ll cover schist (rhymes with list) and gneiss (pronounced: nice). If igneous rocks tell stories of magma, and sedimentary rocks tell us about water, the tales of metamorphic rocks are about heat and pressure during burial.
Let’s start where we left off, at the bottom of ocean. We saw that buried mud becomes cemented into shale. But what happens if we keep burying our poor shale bed?
As we go deeper in the crust, heat and pressure change the physical and chemical properties of materials. Carbon is a great example. When placed under high temperatures and pressures, organic carbon will turn into graphite, the material in our pencils. On a microscopic scale, the chaotic jumble of carbon atoms in dead organic matter slowly become squeezed into more ordered parallel flat plates in graphite, like a deck of cards. Under even more extreme conditions, these sheets of graphite will shift into cubic patterns of diamond.
Coming back to our shale, a similar process happens to the entire rock as we slowly push it deeper into the earth’s pressure cooker. First, shale becomes slate, the classic material in school blackboards. Slate is harder than shale, but things don’t get really interesting until around 300 degrees Celsius and 2000 atm of pressure. For reference, that’s similar to the hottest ovens and twice as much pressure than the bottom of the Mariana Trench.
Graphite schist (~4 cm wide)
Here, the rock begins to feel like a piece of schist. You heard me- schist, from a Greek word meaning “stone that splits easily”. The word Schist has the same root as other words meaning split, such as schism and schizophrenia. As shale becomes schist, flat minerals are re-oriented into parallel sheets, just like carbon into graphite. For a similar effect, throw some uncooked spaghetti on a table, then use your hands to squeeze the pasta together. Most of the sticks will eventually line up with the palms of your hands.
In schists, this pressure often exposes the broad faces of flat minerals together like the pages of a book, which makes the rock easy to split. In this example, I used shale as the parent rock, but compression can turn most sedimentary and igneous rocks into schists, and sometimes a simple glance is not enough to tell what the first rock was. Identification becomes even more difficult as we turn up the pressure cooker, 700 C and beyond, at pressures between two and ten thousand atmospheres.
It’s here that things start to get gneiss. That’s gneiss spelled G-N-E-I-S-S, from an old German word for “spark”. Gneisses have distinct layers, divided into alternating light and dark bands like zebra stripes. These bands form when the rock is sheared in one particular direction, like pressing and shifting a deck of cards between your hands. This shearing force stretches and divides minerals into beautifully contorted bands of gneiss.
A piece of the Acasta Gneiss, the oldest accepted rock on Earth (4.0 billion years old, Feb 12 on the Earth Calendar)
While gneisses have layers like schists, they are not easily split apart. Higher temperatures and pressures have converted flat minerals into blocky shapes, welding the rock into an incredibly tough material. The oldest accepted rock on Earth is a gneiss- the Acasta Gneiss, 4 billion years old in the lands of the Tlicho people in Canada’s NW Territories. Other incredibly old gneisses are found around the world and will be some of our earliest acquaintances in the next episodes of this podcast. In fact, many of the rocks discussed in Bedrock are metamorphic, especially in the early months of the Earth calendar. The original stones were made billions of years ago, and have had plenty of time to stew under Earth’s surface. The fact that these rocks are exposed at all is incredible.
I hope you’ve enjoyed this brief tour of the rock cycle. Now, when I talk about the earliest gneisses, limestones with fossil pond scum, or granite domes, we’ll be on the same page, and can get on to the interesting questions from Earth’s past. More practically, if you run into any of these rocks in the wild or in your own house, you will know where they came from.
If this is all review already, thanks for hanging in- we’re almost to the actual history. But first we need to answer the question I get the most as a geologist:
“How do you know how old that rock is?” Next episode, put on your best outfit and check yourself in the mirror, because to answer that question, we need to play The Dating Game.
Thank you for listening to Bedrock, a part of Be Giants Media. As the show takes off, I would love to hear your input on style, topics, and people to interview- you can drop me a line at bedrock.mailbox@gmail.com. See you next time.
Images:
Lava fountain: J.D. Griggs
https://commons.wikimedia.org/wiki/File:Pahoeoe_fountain_edit2.jpg
Torres del Paine: Natalia Reyes Escobar
https://commons.wikimedia.org/wiki/File:Macizo_Paine_al_Amanecer_Torres_del_Paine.jpg
Pebbles: Kayah on Pixabay
https://pixabay.com/photos/stones-coast-sea-beach-pebble-2091889/
Sandcastle: Kirt Edblom
https://commons.wikimedia.org/wiki/File:Sandcastles_on_the_beach_(%5E304_explore_12-11-20)_-_Flickr_-_Kirt_Edblom.jpg
Seafloor: Dimitris Siskopoulos
https://upload.wikimedia.org/wikipedia/commons/1/16/Sea_floor_sand.jpg
Graphite schist: James St. John
https://commons.wikimedia.org/wiki/File:Graphite_schist_(16921730322).jpg
Acasta gneiss: Pedroalexandrade
https://commons.wikimedia.org/wiki/File:Acasta_gneiss.jpg
Music:
Taking it In by Michael Brandon
24 Caprices for Solo Violin by Niccolò Paganini, performed by Elias Goldstein and Christina Lalog
https://commons.wikimedia.org/wiki/File:Paganini_Caprice-24.ogg
TV Mambo by Daniel Belardinelli