13: Message in a Bottle

For the past two episodes, we’ve taken a grand detour to learn about the Earth’s modern crust. To recap: The crust is divided into dozens of tectonic plates, slices of thin ocean crust and thick continents which drift on top of the mantle. The borders of these plates are mid-ocean ridges, where new crust is born, and ocean trenches, where old crust dies or morphs into volcanic islands. 

Last episode, I ended on a cliffhanger- how do these islands turn into larger landmasses? In other words, how were the continents born?

Today, we start to answer those questions by returning to the main plot of Earth’s history, 4.5 billion years ago. When last we left our story, the Hadean Earth had been struck by a rogue planet, a collision which formed the Moon. If there was any crust on Earth before this impact, it melted back into the magma ocean. But things are cooling down now, and Earth’s wounds are starting to heal. A new crust is being forged. 

The only fragments of this original crust are tiny zircon crystals in Western Australia, the oldest things on Earth, 4.4 billion years old- January 14th on the Earth Calendar. At first glance, trying to recreate Earth’s earliest crust from these zircons, smaller than a grain of sand, seems like taking a single ancient brick and recreating an entire Egyptian city. But there’s far more to a zircon than meets the eye. This episode, we’ll start gazing into these crystal balls and discover the origins of Earth’s crust.

Part 1: The Dark Crystal

Before we learn about Earth’s past through zircons, we need to know how to read them first, and that involves… chemistry.

Now if talking about atoms and elements starts making you sweat, don’t worry. I promise we’re not going to jump in the deep end. You don’t need to memorize the periodic table or atomic symbols here. In fact, chemistry was not my strong suit in college, and it took me a long time to get comfortable with it. I was always intimidated by clean labs, body suits, and strong acids. I was happier wandering the countryside, measuring and collecting rocks in the field.

Eventually, I learned that combining physical and chemical data from rocks produced the strongest science and the best stories. So when we take side-trips into chemistry, my goal is always to give you a story, not just flood you with random information. So, back to the story:

Every mineral, including a Jack Hills zircon, is defined by its atomic ingredients and the shape they form. Take table salt, for example. If you look at table salt with a magnifying glass, you’ll see tiny cubes. If you break them apart, you’ll make smaller cubes. Eventually, you would have a microscopic cube of sodium and chlorine atoms- it’s cubes all the way down. Think of sodium and chlorine as the mineral’s framework, like a jungle gym on a playground. Each atom is like an intersection of the jungle gym, connected through powerful bonds. 

If you change all the framework atoms, you change the mineral. In our table salt, let’s swap all the sodium and chlorine for silicon and oxygen. We now have quartz, which we met last episode, and our jungle gym shifts from a cube to a hexagon. 

But is there any way to just tweak a mineral without changing the entire recipe? Like say, adding a pinch of ginger to your pancakes, or some pepper flakes to your spaghetti? These dishes are still pancakes and spaghetti, but now with a bit more pizzazz. 

Turns out, minerals do this all the time. Sometimes, stray atoms get stuck in the mainframe like a basketball inside the jungle gym. Sometimes, just a few framework atoms become replaced. The atoms don’t completely change the mineral, they just give it a little flavor and color. 

Quartz is a great example, one you can easily see in your local gem store. Pure quartz is crystal clear, like glass. But when quartz traps other atoms, it changes into beautiful colors. A pinch of aluminum makes smoky quartz, a dash of titanium makes rose quartz, and a spritz of iron makes purple amethyst. All these minerals have the same framework of silicon and oxygen- they just have tiny amounts of other elements mixed in. 

Clear quartz, amethyst, and rose quartz: all SiO2, with traces of different elements. University Museum, Bergen, Norway.

 Let’s take this information and return to the Jack Hills zircons. We already know that zircon traps uranium- this simple fact provides nearly every date in this podcast, and if you want a refresher, check out Episode 3: The Dating Game. In that episode, I described zircons as the hourglasses of deep time, but a more accurate analogy would be a Swiss Army Knife- each element trapped inside is a tool that can answer a question. The uranium hourglass is just one of those tools.

Let’s start looking at the other tools one by one, figuring out what stories they can tell us about the world 4.5 billion years ago. We’ll begin with an element that was first discovered inside a zircon crystal, an atom that will give us a glimpse of Earth’s earliest crust. 

Part 2: Element 72

The year is 1922, one century ago. The place: the University of Copenhagen in Denmark. A familiar face crosses the campus- a young woman who has just returned home from a master’s degree in Cambridge. This is our old friend Inge Lehmann from Episode 6, the woman who will discover Earth’s inner core. But that’s still 14 years away- for now, she’s off to study calculus.

On her way, she walks past a laboratory where two young men are shooting zircon crystals with x-rays- a mustachioed Hungarian named Gyorgy de Hevesy, and a clean-shaven Dutchman named Dirk Coster. Coster and de Hevesy are just about to discover one of the final missing pieces of the periodic table of elements- the mysterious Element 72. 

It’s easy to look at the periodic table today and think of it as set in stone, an immovable grid of 118 squares that has to be just so. And today, it basically is. But it wasn’t always that way. When the periodic table was first made in the 1860s, only half those elements were known, and the race to fill the gaps was on. By 1922, there were only a few holes left, and Number 72 was part of a heavyweight battle between chemists and physicists. 

According to the periodic table, Element 72 should be related to the element zirconium, but a French chemist, Georges Urbain claimed to have found it on the other side of the table. This was a bold claim, like saying that carbon should be next to iron, but he wasn’t backing down without a fight. 

On the physics side are our new friends Coster and de Hevesy. They figure if Element 72 was related to zirconium, the best place to look was zircon crystals. The pair borrowed samples from the local geology museum and used a new technique: shooting crystals with x-rays. Long story short, different atoms react to x-rays in different ways. The signatures Coster and de Hevesy found were completely new but made sense with the Periodic Table. Eventually, the pair separated Element 72 from zircon crystals and physically proved that it existed. 

The element they discovered was a silvery metal resembling aluminum or titanium, but much heavier. Coster and de Hevesy named it hafnium, after the old Roman name for Copenhagen, Hafnia. Don’t worry if you’ve never heard of hafnium before- it’s very rare. The most common use for hafnium is in nuclear reactors as stabilizing control rods. 

In geology, the story of hafnium is tightly linked with zircons, but before we return to the Jack Hills, there’s something else you need to know about hafnium’s discoverers. 

Dirk Coster

In the decades after hafnium’s discovery, much of Europe became increasingly hostile for people with Jewish backgrounds, including scientists, and these events would impact both Coster and de Hevesy. In 1938, Coster would play a crucial role in helping people escape the Nazis, including Lise Meitner, the woman who discovered nuclear fission. The escape reads like a movie plot. Meitner and Coster pretended to meet each other by chance at a railway station to avoid suspicion, and fooled border guards along the way. Coster is as famous for rescuing Meitner as for discovering hafnium. 

Gyorgy de Hevesy

De Hevesy himself came from a Hungarian Jewish family, and eventually he was forced to flee. Before he escaped, de Hevesy played one of the greatest games of hide-and-seek in science history. De Hevesy’s German friends had mailed him their gold Nobel Prizes for safekeeping. If the Nazis found that gold, everyone would be in trouble. So de Hevesy took those Nobel Prizes, dissolved them in acid, and left the bottle sitting on his lab shelf, hiding in plain sight. When de Hevesy returned after the war, the bottle was untouched. He recovered all the gold from the acid and sent it to the Nobel Society, who remade the medals good as new. 

 I don’t have a witty transition from these stories back to the Jack Hills zircons. They were clever solutions to problems that no one should ever have to face. The cases of Coster and de Hevesy are extraordinary but show that scientists can be real heroes outside the walls of the ivory tower.

Part 3: Incompatible

So now we know that hafnium was discovered in zircon crystals, and the two go together like peanut butter and jelly. Now, let’s put our new friend to work. What can hafnium tell us about the earliest crust?

Last episode, we followed the life, death, and rebirth of a piece of basalt across the ocean floor. Let’s see what happens to hafnium along the way. 

First, we pull the mantle up from deep within the Earth, transforming it from hot solid crystals to hot liquid magma. This is a huge change. Deep in the mantle, elements are imprisoned within solid minerals. But these cages start to melt away as the mantle rises, and now each element has a choice: would it rather stay inside its old mineral home, or break out into the new magma frontier? What path does hafnium choose?

Hafnium is a wild child and wants to break free. Given the choice between staying in a crystal or staying in magma, hafnium prefers magma nearly every time. Scientists call these rebellious spirits incompatible elements, they are always the first to leave a melting mineral, and the last to enter a newborn crystal. 

Scientists like to compare hafnium with another incompatible element, lutetium. We don’t have enough time for another history lesson, but I will note that lutetium was discovered by the Frenchman we met earlier- Georges Urbain. Just goes to show that everyone has good and bad days, even scientists.

All we need to know is that lutetium is more of a homebody than hafnium- it spends a bit more time inside crystals than molten rock. So inside a new magma rushing up towards a mid-ocean rift, there will be more hafnium that lutetium. This slightly hafnium-rich signal will be recorded in oceanic crust, in dark basalts. 

Now let’s fast-forward to an oceanic trench, where that crust is being pulled back down and melted into the mantle. As we saw last episode, some of the melting crust escapes back into the surface as magma, creating volcanic islands. If there’s magma, you know hafnium is going to be joining that party, while lutetium remains behind. 

Therefore, if you line up a piece of Earth’s mantle, a piece of dark oceanic basalt, and a pale volcanic rock from an island arc or continent, you will see higher and higher ratios of hafnium to lutetium. Scientists can measure these ratios in zircons and tell you what happened to the early crust.

When scientists look at the Jack Hills zircons, they have high hafnium to lutetium ratios, closer to island arcs or continental material. Not exactly, but closer. In fact, chemical clues tell us that many zircons came from a specific type of rock, one that many people know and love. Humans have used it as a building material since ancient Egypt. Even today, you can see this speckled white, gray, or pink rock in walls, floors, and kitchen countertops. It’s easy to take this rock for granted, because it is granite. 

Way back in Episode 2, we first met pale granite inside magma chambers, pockets of molten rock hidden in Earth’s crust. Last episode, we met the same magma chambers, belching out pale volcanic rocks like rhyolite. Turns out, these two rocks are very closely related. Both start with a magma chamber made from re-melted crust, slowly winnowing out the dark minerals of olivine and pyroxene. The remaining minerals are much lighter in color, pink, white, and clear like quartz. If they cool quickly, they form volcanic rocks like rhyolite with small crystals. But leave them to stew under the surface long enough, and those crystals will grow into pieces the size of your thumb, crystals you can see in buildings every day. 

Granite is not only a popular building material for humans, it’s popular amongst continents as well. If you stripped every continent down to its core, ripping off all of the soil and younger rocks sitting on top, you would mostly see granites underneath. If dark basalt provides the foundation of the oceans, then granite is the basic bedrock of the continents. 

So, if there were granites 4.4 billion years ago, does this mean that there were continents back then? Probably not, though it depends on who you ask. It turns out, that fundamental question remains one of the most hotly debated topics in modern geology, one we will return to for the next billion years. 

For now, here’s what we can say. The Jack Hills zircons most likely formed in granite rocks 4.4 billion years ago. Those rocks began their journey as dark basalts pulled out of Earth’s mantle. Eventually this dark crust melted back down into magma, surviving as a giant hot pocket nestled within the crust. This magma chamber cooled into a pale speckled granite, looking very different than its’ basaltic neighbors. This granite probably wasn’t part of a landmass, or even an island, but it was an early step from the world of magma oceans to the world of continents.

Next week, we’ll continue expanding our zircon toolbox, learning more about the ancient crust, and eventually, the first oceans.

 ***

Thank you for listening to Bedrock, a part of Be Giants Media.

If you like what you’ve heard today, please take a second to rate our show wherever you tune in- just a simple click of the stars, no words needed unless you feel like it. If just one person rates the show every week or tells a friend, that makes us more visible to other curious folks. It always makes my day, and that one person could be you. You can drop me a line at bedrock.mailbox@gmail.com. See you next time!

Images:

Halite: https://commons.wikimedia.org/wiki/File:Halite_2.jpg

Quartz, rose quartz, amethyst: personal photo at the University of Bergen museum, Bergen, Norway

https://commons.wikimedia.org/wiki/File:Hafnium_(72_Hf).jpg

Dirk Coster: https://commons.wikimedia.org/wiki/File:DirkCoster1930s.jpg

Gyorgy de Hevesy: https://commons.wikimedia.org/wiki/File:George_de_Hevesy.jpg

Subduction: https://commons.wikimedia.org/w/index.php?search=subduction&title=Special:MediaSearch&go=Go&type=image

Granite: https://commons.wikimedia.org/wiki/File:Muséum_de_Nantes_-_028_-_Granite_beige_(Guern,_Côtes-d%27Armor,_France).jpg

Music:

A Look Inside by Doo Dah Music

Aquarium from Carnival of the Animals, Camile Saint-Saens:

https://commons.wikimedia.org/wiki/File:Saint-Saens_-_The_Carnival_of_the_Animals_-_07_Aquarium.ogg

Sweet Georgia Brown by Ben Bernie Orchestra:

https://commons.wikimedia.org/wiki/File:1925_(USA)_Archives_1925_03_19_Ben_Bernie_Orchestra_-_Sweet_Georgia_Brown.mp3

Winter (1st Movement) from The Four Seasons by Antonio Vivaldi (PremiumBeat License)

Glockenspiel: https://commons.wikimedia.org/wiki/File:Haus-des-Glockenspiels-2.ogg

The Pines of the Appian Way from The Pines of Rome by Ottorino Respighi:

https://commons.wikimedia.org/wiki/File:The_Pines_of_Rome_-_IV._The_Pines_of_the_Appian_Way_-_United_States_Army_Band.mp3

The Goldberg Variations: No. 3 by Johann Sebastian Bach, performed by Kimiko Ishizaka https://commons.wikimedia.org/wiki/File:Goldberg_Variations_04_Variatio_3_a_1_Clav._Canone_all%27Unisuono.ogg

The Goldberg Variations: No. 5 by Johann Sebastian Bach, performed by Kimiko Ishizaka

https://commons.wikimedia.org/wiki/File:Goldberg_Variations_06_Variatio_5_a_1_ovvero_2_Clav.ogg

Seven Days of Flying by Remember the Future