47: The Limestone Family

We’re continuing our exploration of the Isua region of southwest Greenland, the crown jewel of Season 2. The Isua rocks are 3.8 to 3.7 billion years old, March 3 to 11 on the Earth Calendar. We started with the oldest rocks in the region, closer to March 3. These early rocks are mostly dark basalts, forming as volcanos erupted beneath the ocean waves. But we’ve also met new faces: slices of glittering green mantle pulled up from the underworld, and ghostly pale ash beds that stretch for miles, the last remains of violent island-forming eruptions.

Today, we’re shifting gears. First, we’re moving forward in time, from March 3 to March 7. For those keeping score, the real age is now 3.75 billion years old. I don’t usually get nitpick about that second decimal place, the 3.75, but there’s a reason we’re splitting hairs. This new layer forms a clear dividing line in the Isua region. All the rocks to the south are much older, 3.8 billion or March 3. These are the rocks we’ve explored since Episode 44. All the rocks to the north are much younger, around 3.7 billion or March 11 on the Calendar  . The thin strip of rock we’re standing on today is the only layer deposited in between. Geologists give this thin layer a special name: The Dividing Sedimentary Unit.

An Isua map. The Dividing Sedimentary Unit separates the lower blue/orange layers from the upper green layers. From Nutman et al 2022, Precam. Research

There’s another reason why we’re shifting gears today. All the Isua rocks we’ve met so far, the familiar black basalts, the deep green peridotites, or the pale ashy rhyolites, are all igneous rocks, forged from molten magma or lava. This new dividing layer is sedimentary rock, usually forged in water, like sandstone or mudstone. It’s right there in the name: this is The Dividing Sedimentary Unit. If you can’t tell, I’m excited for this one: I’m a sedimentary geologist at heart. Consider me a sedimental man.

Until now, we’ve only seen one sedimentary rock on the show: banded iron formation, or BIF for short. We saw BIFs in Canada, BIFs in other corners of Greenland, and today’s Dividing Sedimentary Unit is mostly BIF, but! There’s another rock as well- a new rock for the show. This new rock is just a small part of the dividing line, but it’s worth mentioning, and it will become much more important in future episodes. I’m very pleased to introduce a rock named dolomite to the program.

You may not have heard of dolomite, but it has a much more famous sibling: limestone. Today, I’m going to describe and compare limestone and dolomite. Why should we care? Because these rocks are the best places to find fossils as we move forward in time. For this reason, limestone and dolomite are my favorite rocks on Earth. But before we go looking for life in future sessions, we need to know more about these rocks themselves. Let’s learn about limestone.

Part 1: Calcium and Carbon

I’m going to start by describing limestone since it’s far more well-known and common than dolomite. You can’t describe ancient dolomites without a good grasp of modern limestone.

A classic piece of limestone, fizzing under acid. Credit: P. Cikovac

Let’s start with the basics. Limestone is one of the most well-known rocks out there. If I asked you to name five rocks off the top of your head, limestone is probably on that list. Limestone comes in a variety of colors and forms, but most hand samples look like a stereotypical cartoon rock. It’s usually dull gray or white throughout, no spotted speckles, no tiger stripes, just a pale chalky lump. If you see a building with cream-colored stone, it’s likely limestone, especially if you find fossils inside.

But don’t let this humble exterior fool you. Limestone has some tricks up its’ sleeve, tricks that help us identify it. The best way to tell if a rock is limestone is to drop acid on it. In the field, we use drops of diluted hydrochloric acid, but in your home, strong vinegar will do the trick. If you’re using vinegar, I’d recommend filling a glass and dunking your rock inside. Whatever acid you use, it should make limestone fizz and bubble like a fresh can of soda. You can even hear it if you listen carefully. This fizzing is a unique property of limestone and its’ siblings- it’s literally dissolving away.

That’s what limestone looks like, now let’s learn what it is. Limestone is a sedimentary rock. Sedimentary rocks form when loose particles of sediment solidify into stone, usually in water. Sediments can be any loose natural material: sand, dust, snow, shells, even leaves. The most common sediments are broken pieces of other rocks: such as boulders, pebbles, sand, and mud. Sand turns into sandstone, mud turns into mudstone, etc. 

But there are other ways to make sediments, ones that don’t need to break down any rocks. Some sediments crystallize straight out of water, just like rock candy in a glass of sugar water. Give these crystals enough time, and they will turn into a solid pavement on the seafloor. These special water-grown stones are called chemical sedimentary rocks, since they form when the chemistry of water changes.

One example of a chemical sedimentary rock is our old friend banded iron formation, or BIF for short. A BIF is made when particles of iron and silica settle on the seafloor into beautiful tiger-striped layers. If you change water chemistry, you can make rust crystallize in water. For more details, check out Episode 32: Pumping Iron. Our new friend limestone is also a chemical sedimentary rock grown from water, but it’s made from very different ingredients.

Limestone has three elements: calcium, carbon, and oxygen. Let’s examine them one by one. We can ignore the oxygen for now. The oxygen isn’t telling us anything about breathable air, it’s just here to balance the other two players. The element calcium is also not the star of today’s show, but it’s worth mentioning for a minute. Calcium is the fifth most abundant element in Earth’s crust. By itself, calcium is a silvery metal, but we usually don’t see it in this purest form. It’s usually found bonding with other elements- it’s very friendly. The most common place you’ll find calcium in nature is in chalky gray limestone.

Calcium is also a key ingredient in your body, especially inside your bones. But! Your bones aren’t made of limestone. They’re made from a different mineral we met in Episode 42. Do you remember that bone mineral? It has a funny name… it’s apatite: which is calcium plus phosphorus. Even if your bones are apatite and not limestone, you still need calcium to grow. For example, you might know that drinking calcium-rich milk helps keep your bones grow nice and strong. I know I heard that growing up in Wisconsin, America’s Dairyland. But even critters without bones need calcium: it’s critical for muscle and nerve health among others. In short, calcium isn’t alive, but it’s a common and important ingredient for living things.

Limestone outcrops, Ireland

Back to limestone. We’ve talked about oxygen, we’ve talked about calcium, but the most important element inside limestone is carbon. When we talk about carbon, we usually think about living things. But it’s important to note that carbon itself is not alive. For example, on this podcast we’ve seen carbon inside diamonds and graphite, and neither of those objects are living. The same thing is true for limestone: it has lots of carbon inside, and sometimes even fossils, but limestone is not a living thing. When I say lots of carbon, I mean it. Let’s crunch some numbers.

Let’s imagine you gathered every living thing on Earth, from bacteria to Bruce Springsteen, and vaporized them into pure carbon atoms. I know it’s a scary image, but just go with me here. Now let’s vaporize all the limestone on Earth into pure carbon. If we compare the carbon pile from life and the carbon pile from limestone, the limestone is the clear winner. Limestone’s pile of carbon would be thousands of times larger than all of life’s carbon combined. In short, there is far, far more carbon inside limestone than living things.  

That’s a humbling statistic: the vast majority of the world’s carbon, the essential building block for life on Earth, is locked away in dumb old limestone? If that makes you feel small or insignificant, here’s a helpful boost. Living things have less carbon than limestone, but we take that carbon and tweak it in fascinating ways. Some tweaks are obvious, like the beautiful diversity of animals, plants, bacteria, etc. Some tweaks are more invisible, like how life hoards away light carbon isotopes for itself. We won’t go into isotopes today, but it will be important soon. All you need to know for now is that carbon is constantly shifting between living creatures and non-living rocks, and we can follow that carbon conversation over billions of years.

Now that we know the ingredients of limestone, let’s find out how it forms.

Part 2: A Spoonful of Crystal

There are many different recipes to make limestone, but they can be clumped into two broad categories: recipes that need life, and recipes that don’t need life. We saw a similar situation with our old sedimentary friend: banded iron formation or BIF. There are a few recipes to make BIFs: some recipes involve life, and other recipes don’t. With limestone, we’re going to start with the non-living recipe, to keep things simple.

Let’s review the ingredients of limestone: calcium, carbon, and oxygen. Without these three elements, you’re out of luck. But making limestone is not as simple as dumping these elements in water and mixing them together. Just like any good recipe, you need to know the proper amounts of your ingredients.

Some crystals have a special balance with water. If you put a very small amount in water, they will dissolve and become invisible. If you keep adding in more and more crystal powder, eventually you’ll see it with the naked eye- it will stick around and grow into solid, visible crystals. This idea of balance can apply to household objects as well, not just natural minerals. I like to show this in my geology lectures, and you can follow along at home, especially if you’re in the kitchen. I use kitchen analogies a lot, but this is one you can actually follow along with, I highly recommend it. Here are the ingredients you’ll need.

Credit: Pexels

I want you to grab, or imagine, three tall glasses of water. In front of each, we have three different materials: flour, baking soda, and salt. We’re going to work through one at a time.

Let’s start with flour. Take a tablespoon of flour and mix it around in some water. You can stir that sucker as long as you want, the flour will never dissolve away, it will never turn invisible. You’ll always see the white powder. Flour behaves like most rocks and most materials- if you dump them into water, they will never dissolve. Thank goodness- it makes bathtime much more enjoyable when you’re not getting eaten alive.

But there are a few materials that aren’t as tough. Let’s move on to baking soda. Dump a tablespoon in a different glass of water and mix it around. Eventually you’ll see the white powder turn invisible, it dissolves completely away. The baking soda hasn’t teleported to a different dimension, it’s just separated into microscopic pieces by the water. If you don’t believe me, take a small sip. You can taste the baking soda inside, but it’s not very pleasant.

Now, put another tablespoon or two of baking soda and stir. Eventually, you’ll see some of the white powder sticking around- it won’t dissolve no matter how much you stir. In other words, water can only dissolve a little bit of baking soda. For example, a cup of water will dissolve about 20 grams of baking soda. Any less powder, and the baking soda will remain invisible, dissolved. Any more, and it will stick around, very visible.

This magic weight is called the saturation point. Think of the word “saturated” as meaning full. If a sponge is saturated with water, it can’t hold any more. If you’re saturated with chocolate, you couldn’t possibly have another bite. If water is saturated with baking soda, no more will dissolve- it has to stick around.

Different materials have different saturation points. A great example is salt. Take a tablespoon of salt and mix it around in another glass. Salt will dissolve almost immediately in water, it’s a very wimpy mineral. Again, if you’re unsure where that salt went, just take a small sip. It’s there. Now take another spoonful and mix it around. The salt should still dissolve away after some stirring. If you feel like it, try this experiment at home and see many spoonfuls of salt you can dissolve in a glass of water. When you reach the saturation point where salt keeps hanging around, get in the comments and tell me how many spoonfuls you needed. You’re going to need a lot. The point is this. Some minerals like salt dissolve easily in water, others like baking soda, not so much, and most materials don’t dissolve at all.

So where does limestone fall on this saturation scale? Is it tough like flour, or wimpy like salt? On our kitchen scale, limestone falls between flour and baking soda. It can dissolve in small amounts. This makes limestone much tougher than salt or baking soda, but! The fact that limestone dissolves at all in water makes it a fairly wimpy rock, behaving in strange, unique ways. For example: most caves are made as water slowly eats away at limestone underground. That doesn’t usually happen inside granite, or gneiss, or sandstone.

There’s more to the saturation story- just like any recipe, things like temperature and acidity can change how limestone dissolves or sticks around. It’s why limestone reacts so strongly to acids like vinegar. But in short- if you want to make a limestone, you need a decent amount of calcium, carbon, and oxygen in your water. We’ll get more into the chemistry on a later episode. For this last portion, let’s bring our discussion back to Greenland. As much as we love limestone, there isn’t any in the ancient Isua rocks. Instead, it’s time to meet limestone’s lesser known sibling: dolomite.

Interlude: The Name Game

Before we describe dolomite, here’s a necessary disclaimer for the hardcore geologists out there. I fully admit this is a way to protect my butt from future comments. It has to do with the word “dolomite”. It’s not a bad word, it doesn’t have a history loaded with human suffering, it’s just a headache. If you’re not interested, just zone out for the next few minutes until you hear the music for Part 3, or just skip to 25:14. But I’d recommend sticking around, this is an interesting slice of how rocks get their names.

Granite, made of smaller crystals of pink feldspar, gray quartz, and black biotite

If I show a piece of dolomite to most geologists, they’ll say “Yep, sure is dolomite.” But there would be some that say “Um, actually, that’s dolostone, thank you very much.” This has happened to me in the real world. The conflict boils down to this. The word “dolomite” means two different things in geology, one big thing and one small thing. The Big Definition: Dolomite is a rock, a large solid chunk made of smaller crystals. The Small Definition: Dolomite is also the name for those tiny crystals inside the rock. This might seem like splitting hairs, but most rocks aren’t named like this.

For example: Limestone is made from smaller crystals called calcite. Different names. Granite is made from smaller crystals named quartz, feldspar, and biotite. You would never call the small crystals granite, and you would never call the entire rock quartz. It would be like saying a single brick was a house, or that your house was just a brick. Back to dolomite, both the smaller crystals and the larger rock are named dolomite. It’s like saying your house is made of house.

How did we get here? The word “dolomite” was invented in 1792, named after a French geologist. As time went on, there could be some confusion when using this name- were you talking about the big rock or the small crystal? In 1948, the word “dolostone” was proposed for the larger rocks. I’ll admit- it makes sense. Limestone is made of calcite crystals. Dolostone would be made of dolomite crystals.

And yet, this great name reform never quite caught on. Perhaps it was the momentum of history- the old name had been used for 150 years, why change now? Perhaps it’s that the confusion between big and small dolomite is not a huge issue- you can usually tell through context clues what you’re talking about.

Maybe your head is spinning with all these names, so here’s the long story short. On this podcast, we’ll just call everything dolomite, the big rock and the small crystals. It’s honestly how most geologists I know talk about this stuff. I just knew if I didn’t bring this up, someone would get in the comments. Heck, they probably still will. Deal with it. Finally, let’s meet the rocks themselves. If you need to take a brief ear break so that the word “dolomite” means something again, I won’t blame you.

Part 3: The Problem Child

I don’t like it when people compare rocks with moods. It can lead to some slippery slopes regarding emotional vs physical well-being. A crystal can make you happy, but not healthy. And yet, that tangential rant aside, whenever I think of dolomite, I can’t help but give it a personality based on its’ physical properties. Wi th all the love in the world, dolomite is a little brat. The word brat can be applied both in its’ original term of a spoiled child, or its’ new, more positive meaning of confident rebellion.

Dolomite, identical to limestone without further testing. California, USA.

Let’s compare dolomite with its’ more famous sibling limestone. If I held a piece of each in my hands, they would look exactly the same. Both would be gray, cream-colored or white, both would be fairly dull with no spots or stripes. The best way to tell a dolomite from its’ sister limestone is the acid test. Limestone will fizz like crazy in acid, like a new Alka-Seltzer tablet or a freshly-opened soda. Dolomite is much less cooperative. It will fizz, but you have to really look for it. You’ll only see a few small bubbles, like an old flat soda or a piece of Alka-Seltzer that’s been sitting around too long. I’m not even sure if dolomite will fizz in vinegar- I’ve never tried that. Sure, dolomite will fizz, but it makes you look for it. Dolomite is a brat.

Dolomite is also physically tougher than limestone. On the Moh’s hardness scale, where diamond is a 10, limestone is only a 3, pretty wimpy. Dolomite has a hardness of 4, not too much harder, but noticeable. I have a personal history with tough dolomites. I grew up near Milwaukee, Wisconsin, on the shores of Lake Michigan. On some of the beaches, you’ll find large boulders of cream-colored dolomite. These rocks have fossils on their surface, seashells and trilobites that look like giant pill bugs. The dolomite boulders were too large to break open with hammers, so we would throw smaller granite boulders to smash them apart caveman-style. Granite is a very tough rock, it should be much stronger than dolomite. But one time, our piece of granite cracked in two after hitting its’ dolomite target, splitting like a nut. We fell to the ground laughing, it was like hitting a piece of wood with a hammer and having the hammer fall apart. Dolomite is a brat.

Bradford Beach, Milwaukee, where I spend my early dolomite-smashing days as an undergrad student. Credit: Joy Powers, WUWM.

So what makes dolomite different from limestone? What are the ingredients? Like limestone, dolomite has calcium, and carbon, and oxygen. But there’s one extra element: magnesium. Magnesium is similar to calcium, but it’s slightly less common in Earth’s crust This checks out: dolomite is more rare today than limestone. It needs more ingredients, and ones that are harder to find. Magnesium is also a small but important part of your body. It’s not as common as calcium in your bones, but it is inside all your cells, interacting with DNA. In your everyday life, magnesium is found in antacids and laxatives. Inside dolomite, magnesium changes the recipe just enough to make her tougher.

Old dolomite crystals, Spain

Speaking of the recipe, let’s see how dolomite is made. Remember, its’ more well-behaved sister limestone needs some calcium, carbon, and oxygen in just the right concentrations. There’s a lot more to it than that, but essentially, the more ingredients you put into normal water, the more likely you’ll make limestone. We also saw this with baking soda and salt: enough ingredients in, and you’ll start to make crystals in water. Each of these materials has a known saturation point, where crystals begin to form and become visible. Geochemists can calculate these saturation points using math and confirm them with experiments in the lab or the kitchen.

But once again, dolomite does not play by the rules. Chemists have calculated an official saturation point for dolomite, using the same rules as well-behaved salt or baking soda or sugar or limestone. But if you make this dolomite recipe in the lab, it won’t form or takes much longer than expected. One famous experiment made water concentrated in dissolved dolomite 1000 times over its saturation point. 1000 times more dolomite! If you did that with salt, your water would be popping out crystals left and right. But not with dolomite. The concentrated dolomite experiment sat around for 32 years and never made a crystal. Dolomite is a brat. The same pattern is true in nature. A glass of seawater has 10 times more dissolved dolomite than its saturation point. In other words, the oceans should be cloudy with white dolomite crystals, Dolomite should be everywhere today, just like limestone, but it isn’t.

When we look in Earth’s ancient past, the story gets even weirder. If dolomite is so hard to make today, you’d expect it to be an incredibly rare rock. This is true for more recent rocks a few million years old, December on the Earth Calendar. In these younger days of dinosaurs and mammals, well-behaved limestone is the queen of the sea. But the farther back you go, you begin to see more dolomite, even more than good old limestone. Eventually you see layers upon layers of dolomite in stacks hundreds or even thousands of feet thick. In the next few seasons, dolomite will be a far more common guest than limestone.

What the heck? It’s like dolomite is playing a huge practical joke, laughing at scientists over billions of years. It’s incredibly hard to make today, in nature or in the laboratory, and yet it is one of the most abundant rocks in Earth’s past. In geology, this paradox is simply called “The Dolomite Problem”. Very few rocks are ornery enough to get an entire problem named after them, but dolomite certainly a problem child. Dolomite is a big brat.

Science has been chipping away at the dolomite problem. A few major strides have been made in the last few decades, with some very surprising results. When chemistry and physics fail us, when there’s a rogue mineral that won’t play by the rules, you need a solution that’s also outside the box. You need a solution that challenges nature itself, that can shape and change the world around it, that fights for survival no matter the cost. You might just need life. Such a solution has been proposed for the Greenland dolomites, 3.75 billion years old. To quote Jurassic Park: “Life, uh, finds a way”. But that’s a story for next episode.

Summary:

Limestone and dolomite are two twins, similar in many ways, different in many more. Both are sedimentary rocks, forming in seas and lakes around the world. Both are incredible places to find fossils, traces of ancient life. Limestone is the queen of the modern ocean, far more common and well-known. The chemistry of limestone can be strange, but it’s well-understood. In contrast, dolomite is a problem child. Dolomite has just a slightly different recipe, but that makes it physically tougher, and apparently much tougher to make. Dolomite should be very common on Earth today, but is quite rare. On the other hand, it’s extremely common on the ancient Earth, and we’re still debating how it got there.

Next episode, we discuss one possible solution to the Dolomite Problem, one proposed for some of the oldest dolomites on Earth, 3.75 billion years ago. If this idea is correct, these rocks might be another clue in the search for Earth’s oldest fossils.