46: Rising From the Ashes
For the past three episodes, we’ve been exploring the Isua region of SW Greenland, a lonely, remote tundra near the Arctic Circle. These rocks are the most well-preserved of Season 2, the crown jewel of this era. How old, you ask? These rocks are between 3.8 to 3.7 billion years old. On the great Earth Calendar, consider this March 3-11, over a week.
We started with the oldest rocks in the region, which are also the oldest surviving pieces of Earth’s mantle, the hidden world below our feet. In this episode, we move forward in time and return back to Earth’s ancient surface: the realm of oceans, volcanos, and life. What did Greenland look like back then?
Modern-day Isua is a cold tundra peppered with lakes, sitting next to a massive ice sheet. But Isua 3.8 billion years ago was a very different story. Traveling back in time, we see a vast blue ocean peppered with islands. This seascape looks far more like the modern Earth than Season 1, but there are some unnerving differences. The noonday sun, high in the sky, is noticeably dimmer: only as bright as the early dawn. The moon looms twice as large in the sky, with angry red craters cooling into the Man on the Moon. And let’s not forget that this is a sky we cannot breathe. The air has virtually no oxygen. Earth is growing more normal, but is still a very alien world.
As we gaze over this calm prehistoric sea, we hear an explosion on the horizon. In the distance, we see a pillar of gray ash and a fountain of red lava. It seems these islands are volcanic, building upward and outward with each new eruption. Today, we’ll explore these volcanos and I’ll introduce a new type of lava rock to the show. Finally, we’ll see if any place on Earth today resembles Greenland 3.8 billion years ago.
Part 1: The Blast Furnace
Fuego volcano, Guatemala. A “classic” volcano.
Close your eyes (if it’s safe) and imagine a volcano. A classic volcano from a movie or a cartoon. You’re probably imagining a dark conical mountain, like an upside-down ice cream cone. It might be sitting on a tropical island, it might be sitting in a mountain range, or glowering over the black swamps of Mordor. Your imaginary volcano probably has a hole at the top, with glowing red lava oozing from the edges. It might also be belching ash high into the sky. That sure sounds like a volcano to me. And it is, but this is just one type of volcano, one flavor amongst a wide buffet.
Some volcanos truly look like tall cones with holes: think of Mt. Fuji in Japan or Mt. St. Helens in the USA. Other volcanos are much wider and less steep, they spread out over many miles of ground. They’re still tall, but they look more like a broad dome than a pointy cone. Think of Mauna Loa in Hawaii, or Olympus Mons on Mars. Yet another type is just a huge crater sunk into the ground: no mountain, just a hole from a huge eruption. Think of Yellowstone National Park, or the isle of Santorini in Greece. These are just a few examples.
So what volcanos were present in ancient Isua? The answer is: many different types. We’ve seen some of these volcanoes in previous episodes, while others are brand new to the show. Today, I’ll briefly review our old friends, then introduce some new faces.
Sadly, if you visit Isua today, there are no giant cones or domes sticking out of the ancient landscape, no craters you can peek over. The only traces of the ancient volcanoes are the cooled lava flows and ash beds they’ve left behind. But compared with earlier locations, these stones are still a treasure trove, miles of rock to investigate and explore. In fact, most of Isua is volcanic rock. You’ll see green rocks, black rocks, gray rocks, and even pinkish rocks. Each color was formed by different lavas which made different volcanoes.
Before we roam across the Greenland tundra, let’s review how these different lavas form, and what they tell us about the ancient landscape. In this part, I’m going to cover two rocks we’ve seen before: basalt and tonalite, and their importance to the Greenland story. Think of them like links in a chain to reach today’s new friend. If you feel you’re solid on these old rocks and want to skip straight to the new kid, you can head to 13:49.
Lava is made by melting other types of rock. Any type of rock you can imagine can be melted down with enough heat. But! There’s a catch. Rocks usually don’t melt all at once. I’ll say that again, it’s critical to the rest of this episode: rocks don’t melt all at once. They’re made from many types of crystal, and each crystal has a unique melting point.
For example, let’s imagine a piece of green mantle rock from last episode, our new friend peridotite. Peridotite is mostly green, but there are a few black crystals scattered around. Now let’s throw that poor peridotite into a blast furnace and slowly crank up the temperature. I’m going to use Celsius here, but no matter which scale you use, it’s going to get really hot. At 100 C, nothing, just a hot rock. At 500 C, still nothing, just an even hotter rock. But at 900 C, we finally see a change. All the black crystals begin to melt and bleed away from their former home. The green crystals are perfectly fine, still solid. They can take the heat. Eventually we have two separate piles: one of olive-green crystals, and the other of molten, liquid lava.
A diagram of the partial melting process. “Wimpier” crystals melt first, forming separate lavas that form different rocks. Credit: Dexter Perkins, Open Petrology Book
Now let’s cool everything back down. The liquid lava turns into a solid black rock, which is not surprising. We melted black crystals to make the lava, it makes sense that it cools down into black crystals once again. Congratulations! We’ve made a brand-new lava rock: its name is basalt. Basalt is a very old friend of the show. It was one of the very first rocks we met way back in Episode 2, but we haven’t really talked about it since Episode 35. If you’re new, here are the basalt basics.
Basalt is dark black with scattered flecks of gray. If you’re watching a movie that features dark, volcanic rock, it’s usually basalt. If you’re a villain looking for the perfect foreboding lair, basalt is an essential building material. Fortunately, you can get it cheap: basalt is the most common volcanic rock on Earth. You can find it almost anywhere, even the Man in the Moon, but when I say basalt, I want you to imagine the dark, cold seafloor. Nearly all the Earth’s seafloor is paved with basalt, like giant slabs of asphalt.
Let’s bring these ideas back to ancient Greenland. Just like today, black basalt is the most common volcanic rock in Isua, 3.8 billion years ago. In fact, it’s the most common surface rock in Isua, period. If I were to list the Isua rocks layer by layer, it would sound something like this: peridotite, basalt, basalt, basalt, banded iron formation, basalt, basalt, basalt, sandstone, basalt, basalt, basalt, etc. We’ve talked about basalt a lot on the show already, and I want to focus on new rocks, new players on the scene.
So here’s a one-sentence summary about all this basalt, and then we’re moving forward. Basalt tells us that volcanoes were erupting on the ancient seafloor, melting and refining bright green mantle into dark black stone. If you want more info, check out Episode 31, which is dedicated to basalts.
Making today’s new rock requires a few extra steps. First, we need to throw basalt back into the blast furnace. Just like last time, let’s crank up the temperature slowly. I’ve mentioned many times that basalt is mostly black. But don’t forget there are little flecks of gray crystal inside. As we crank up the heat, those gray crystals are the first to wimp out and melt away into a puddle of lava. When we cool the furnace down, we’ve made another old friend: tonalite.
Tonalite is the star of Season 2. Every location we’ve seen has had lots of tonalite. If you held a piece in your hand, you would see a dull gray stone with flecks of white and even pink. Tonalites tell us that there was likely dry land around, but the details are vague and there are still many mysteries to solve. Tonalites cool deep underground, so they don’t tell us as much about volcanos on the surface. Were these small islands or larger landmasses? Did they form by normal plate tectonics, or from stranger processes we see on other planets? Furthermore, tonalite was super common in the ancient world, but is very rare on the modern Earth- what gives?
We discussed these conundrums in Episode 39, and we will discuss them again at the season’s end. Back in Greenland, tonalites are extremely common around the Isua area. They tell us there were islands around, but for now, not much more info than that. We have oceans, we have islands. Is there any more detail we can get? For that answer, let’s throw our old friend tonalite back into the blast furnace, and finally meet today’s new rock.
2: Pretty In Pink
If you jumped ahead, here’s the rundown. We started by partially melting a piece of green peridotite, the king of the mantle underworld. A few black crystals separated and cooled into our old friend basalt, the dark king of the seafloor. We then partially melted this black slab of basalt. A few gray crystals separated and cooled into another familiar face: tonalite, the dull king of the islands.
To make today’s new rock, we’re throwing tonalite back into the blast furnace to see what happens. Just like the previous experiments, we’re going to raise the temperature slowly and see which minerals melt first.
Tonalite is mostly dull gray, but there are a few flecks of white and pink scattered around. These are the wimpiest crystals of the bunch, the first to melt and bleed away into lava. When we cool the furnace back down, this lava transforms into a pale pink stone. Again, this makes sense: white and pink crystals melted, then re-froze back into white and pink.
Finally, after all that lead-up, it’s my pleasure to introduce this new rock to the show.
Meet rhyolite.
Credit: Michael C. Rygel via Wikimedia Commons
Rhyolite can be white, pale gray, or pink in color. The crystals inside are too small to see with the human eye, so instead of having a glittering or speckled appearance, rhyolites often just look like dull lumps of hardened play-doh, clay, or chalk.
One well-known variety of rhyolite is pumice. Pumice is famous because it floats on water, thanks to its light weight and a Swiss cheese texture of air holes. One of my favorite tricks to play on students is to pretend a piece of pumice is extremely heavy, then gently toss the rock over to them. It never gets old to see their faces change from temporary panic to confusion to wonder at holding such a light rock. You can also find pumice as a plant fertilizer or an abrasive. For example, my dad loves Lava brand soap, which has ground pumice inside. That stuff will get just about anything off your hands, including your top skin if you’re not careful. If anyone represents Lava soap out there, I’m happy to talk.
Interacting with rhyolite, it seems cute and quirky: the fairy godmother of the rock world. It’s pink, it floats, it helps clean your skin. Nothing like that dark gloomy basalt hanging around the villain’s castle lair. But let’s not forget that pretty pink rhyolite was also forged from molten lava and spewed out of a volcano, hundreds of degrees hot. And if you asked me which eruption I would be around: basalt or rhyolite, I would pick a basalt volcano every single time. To learn why, let’s revisit our blast furnace from earlier this episode.
Just like dark basalt and pink rhyolite are very different rocks, they came from very different lavas. The lavas’ color is not important here, it’s their textures. Basalt behaves like warm honey, flowing thickly but relatively fast. If you’ve seen videos of Hawaii or Iceland, you can see these runny lava flows in action. You’ll also see these dark lavas happily bubbling away, popping and spraying fountains of lava into the air.
A runny basalt lava flow, Hawaii
You might be scratching your heads right now. “Dylan, you said this is the volcano you’d want to be around. Fast flows and lava fountains don’t sound very appealing to me.” And you’re right- given a choice, I wouldn’t be around any active volcano. And basalt volcanos can cause lots of expensive property damage. In the news, such devastation has happened recently in Hawaii and Iceland. But think back to those news reports. Do you remember hearing about many, or any deaths associated with those eruptions in Hawaii or Iceland?
Basalt eruptions are dramatic, but rarely result in the loss of human life. Typical flow speeds are only one mile an hour, rarely breaking 5 miles an hour- pretty easy for most folks to escape. For example, Iceland went 50 years without any loss of life from an eruption until 2023, and many recent Hawaiian eruptions had no recorded deaths I could see. In fact, these active volcanoes are often tourist traps, you’ll see images of people walking fairly close to lava flows, though obviously you should still use common sense.
A quick personal story before moving on to rhyolite eruptions. In 2010, I was a freshman in college, and I had signed up for my first international geology field trip: a trip to Iceland. The lecture started in January, and we were slated to visit in the summer. But in between, our plans suddenly changed. An Icelandic volcano erupted in April 2010, filling the northern skies with ash. Over 100,000 flights were canceled that week, half of the world’s flights, the largest shut-down since World War 2. My school’s flight was fortunately months later, after the blow-up. As we watched the volcano calm down on the news, we knew we had to visit.
The volcano is called Eyjafjallajokull (ayy-a-fyat-la-yo-kull), and if you think that’s difficult to pronounce, so did many reporters, who shortened it to E15. We ascended the volcano about three months after it erupted, long after the ash and lava cleared. But as we ascended, we could still feel the heat from the ground pump through our boots. If you left a backpack too long on the ground, the hard plastic buckles would melt and stretch out like a Salvador Dali painting.
Finally, we made it to the very top, looking down into the crater. Sadly, there was no boiling pool of lava, just a steaming pile of gray and red rocks. But there was a small crack close by, about the size of a hand. Looking through that crack, I saw an orange glow brighter than anything I’ve seen since. It looked like a melted traffic cone. It was lava, and the heat belched so strongly you could only stand 30 feet away. The acrid stench of sulfur, of brimstone, was enough to cover your mouth, forming yellowish-green crystals thick as moss around the opening. It was both beautiful and terrifying to behold.
On our way down, our professors stopped us for a lunch break. They found a strong heat vent on the rocky slope: no lava, no brimstone, just heat. Our profs dug through their packs, and brought out sliced bread and cheese. Using two flat stones as a griddle, we made grilled cheese sandwiches using the volcano’s heat. That day is one of the best in my life, a top five moment if not higher. Big thanks to Drs. Barry Cameron and Tom Hooyer at UW-Milwaukee for making that trip possible.
Let’s bring that story back into our narrative about basalt, rhyolite, and volcanic eruptions. Most eruptions on Iceland and Hawaii barf up dark basalt. Basalt lava is runny like honey, but relatively safe- it’s more of a tourist trap than a death trap. But rhyolite lava is a different story. It’s more like peanut butter: stodgier, stickier, much less flowy. On one hand, that doesn’t sound too bad! Slowly-flowing lava is easy to avoid. But it’s not the speed of the lava you should be worried about. The danger is hiding within.
The slow, sticky nature of rhyolite lava means that gas bubbles are trapped inside- it’s hard to escape. Remember the merrily bubbling basalts of Hawaii, popping and fizzing along? Well, gas can’t really do that in rhyolite lava. The gas just stays inside, gathering more and more and more pressure, like a hellish piece of bubble gum. Instead of many small bursts, the gas suddenly blows up in a much larger, much more violent explosion. Think of those old videos of folks with Diet Coke and Mentos, with bottles exploding all over the place. That’s a rhyolite eruption.
The sheer eruptive force of rhyolite volcanoes automatically makes them deadlier, but wait! There’s more. When all those gassy bubbles explode in rhyolite lava, they instantly cool and shatter into billions of tiny, even microscopic shards. Each individual shard might be invisible, but together, they form a dense cloud of volcanic ash. That’s right: volcanic ash is the remains of shattered, frozen lava bubbles. If you look at this ash under a microscope, you can see these frozen bubbles for yourself- they look like broken pieces of those bubbly chocolate bars. I’ve put some pictures up on bedrockpodcast.com.
A piece of volcanic ash under the microscope, scale is 30 micrometers. Source: NASA
As whimsical as these tiny frozen lava bubbles are, volcanic ash is the deadliest feature of rhyolite volcanic eruptions. Lava doesn’t travel fast, but ash certainly does. Ash clouds can scream down a volcano at 100 km/h or 60 mph, the speed of a fast car. And this fresh ash is still very hot, up to 1000 C or 2000 F. And let’s not forget that it’s extremely bad to breathe ash in, no matter what temperature it is.
Mount Mayon eruption, Philippines, 1984
In short, volcanos that barf out rhyolite and its cousins are bad news. Think of Mt. Vesuvius, burying ancient Roman Pompeii in ash. Think of Mt. St. Helens in 1980, the worst eruption in US history. Think of Mt. Tambora in Indonesia which caused the Year Without a Summer in 1816. Before I continue, I can hear volcanologists typing in the comments, telling me that these volcanos aren’t technically rhyolite, and they’re right. There are many types of explosive lava with many other names. But for today, rhyolite is a useful shorthand for lavas that make deadly, explosive, ashy volcanos. I promise we’ll split hairs in later episodes.
You know what is literally rhyolite? Some ancient volcanic rocks in Greenland, 3.8 billion years old. This is the first time we’ve seen rhyolite or any rock like it on the show. Now that we’ve met our new, fiery friend on the modern Earth, what stories can it tell us about the ancient world?
Part 3: A Tale of Two Volcanoes
It’s high time to clear our heads and take a walk around the Isua tundra, near the Arctic Circle. Last episode, we saw the oldest stones in the region: glittering green rocks called peridotites. Peridotites tell us tales of the ancient mantle, the deep underworld. Let’s pack up camp and hike towards our new rhyolite friends.
As we walk northward, the rocks broadly get younger, and we begin to see a familiar pattern from earlier this episode. Just like our imaginary blast furnace, the green peridotites are followed by layers of black basalt, and finally pale rhyolites. That sequence of rocks we melted, green to black to pale, is a progression we’ll see over and over again on the show. If you melt the green mantle, you’ll make black seafloor. If you melt black seafloor, you’ll make pale islands. It’s a process of constant refinement over time.
Which brings us to the rhyolites, the most refined rock in this Greenland volcanic suite. After walking over a thousand yards of black basalt, the rhyolite layer stands out like a sore thumb, a wide highway of pale gray stone. Look to the left, look to the right, the rhyolite stretches to the horizon in both directions, from one end of the region to another over many miles. Clearly, something major happened here. Let’s investigate the clues.
The first feature you’d notice is that the pale rhyolite is stacked in layers, like pages of a book. These layers are thicker in the west, about the width of a human palm. If you followed these layers east, they would thin out, thinner than a finger. What are these layers telling us?
Rhyolite layers, Isua, Greenland, 3.8 billion years ago. Source: Nutman et al., 2015: Precambrian Researcher
Let’s review what we know about rhyolite. It’s extremely sticky and slow-flowing, like peanut butter. It doesn’t flow across the landscape for miles, it stays put, piles up, and blows sky-high. And yet we’re seeing neat rhyolite layers stretching to the horizon. What we’re seeing in Greenland are not ancient lava flows, they’re ancient ash beds, now hardened into solid rock after billions of years.
Remember, ash travels much faster than lava, especially when it has the benefit of wind blowing it around. For example, when Mt. St. Helens blew up in 1980, the wind blew the ash over a thousand miles. The ash close to the volcano was thick, enveloping cars and buildings. Farther away, the ash was only a faint dusting, like gray snow that never melted.
This idea brings us to the second clue from the Greenland rhyolites. The beds of ash get thicker and thicker to the west. Just like Mt. St. Helens, the thicker layers tell us we’re getting closer to Ground Zero, the eruption center. And indeed, in 2021, when Dr. Allen Nutman and his team looked in the westernmost rocks, they found massive un-layered rhyolites up to 30 feet thick, riddled with empty holes like Swiss Cheese. These unlayered rhyolites were thick, chunky lava flows, and the holes within were former gas bubbles that never got the chance to erupt, a former time bomb frozen in stone.
One last note on these rocks, and then we’ll wrap up. In 2015, Dr. Nutman and crew examined the Isua ash layers and found chemical evidence that they had been sitting around on land, exposed to the wind and rain before burial. That might not sound like much, but on this show, seeing an actual piece of ancient dry land is very noteworthy. Until now, every surface rock we’ve talked about formed beneath the ocean waves, from undersea eruptions to chemical vents on the seafloor. On this show, far more rocks will be tied to the oceans than dry land. Even today, 70% of the world is covered in the sea, and it was even more extensive in Earth’s early days. But every once in a while, you find an ancient piece of the surface, something humans could have walked on like astronauts on an alien world. To my knowledge, this ash bed might be the oldest piece of dry land on Earth, but I could be way off base. In either case, it’s a cool fact to end this episode on.
Speaking of dry land, let’s summarize what we’ve learned by traveling back in time and walking along the shores of one of these ancient volcanic islands.
Summary:
The time is 3.8 billion years ago, March 3 on the Earth Calendar.
We’re standing on an island beach, looking out over the wide sea. As usual, the air is unbreathable, and we need spacesuits just to survive. The noonday sky is dim, dimmer than usual with roiling overcast clouds. A gentle shower of flakes falls from the sky to the beach, covering our small island with what looks like gray snow. It’s a beautiful sight, almost out of a Christmas card. But this is not snow, it’s volcanic ash, perhaps still warm to the touch. In the distance, we see the eruption from the intro belching even more ash into the air. We’re far enough away to be safe, but it’s still an unnerving sight.
This ash will one day turn into pale rhyolite on a Greenland tundra, covered by real snow. This rhyolite, like others ancient and modern, warn us which volcanos are death traps, time bombs full of gas just waiting to erupt. The rhyolite is a herald of death and destruction, but also of growth and change. Each eruption builds new islands in this ancient sea, islands which will merge and grow, becoming the cores of continents, massive expanses of dry land. The world we know and love is slowly emerging from the waves.
Next episode, we will jump forward in time, March 7 on the Calendar, and return to the sea. We’ve learned about new volcanic rocks today, now it’s time to meet some new sedimentary rocks, including my favorite type of rock on Earth.