The Story of Rocks | The Three Major Types of Rock

Why should you care about rocks?

Without rocks, there simply would be no Earth. We either have molten rocks or solid rocks. But we can never have no rocks, as our planet is "rocky", terrestrial.⁠

I'm taking you on a virtual hike through the mountains and chat with you about

  • the three major rock types as defined by geologists
  • the difference between rocks and minerals
  • what minerals are made of
  • how minerals and rocks are held together

After listening to this episode you'll

  • want to scream out your window "I LOVE YOU ROCKS"
  • start to wonder what rocks are in your backyard
  • comprehend better why rocks are fascinating as heck to geologists
  • and why they should be interesting to everyone.

 

After listening or reading, I'd love to hear from you: Which types fascinates you the most? For me, I think it's metamorphic rocks. Or is it igneous rocks? Or rather the sedimentary rocks? 

It's hard to pick. I love them all.

Transcript

Hello you and welcome to the Science of Travel Podcast – the podcast that combines all things Earth, landscapes and travel destinations with Earth Science!

This episode is called: The Story of Rocks and we'll talk about the rock cycle. 

Why should you care about rocks?

Sounds boring...The Story of Rocks. That doesn't sound exciting. 

Let me ask you a few Qs:⁠

When you're on vacation, what do you find the most fascinating? Why do you travel anywhere?⁠ Why did you choose exactly this travel destination?

Most likely it's because of the landscape, right?⁠

And what is the landscape made of?⁠

Exactly: rock!⁠

And do you enjoy beaches? Where's that material, the sand, from?😊⁠

Yes, rock!⁠ Beaches only exists because somewhere there is or was rock which was eroded and deposited exactly at your favorite beach.

Do you think mountains are impressive? Have you seen the Grand Canyon or have you hiked in Arches Nationalpark? ⁠

What ARE all these things? They're ROCK FORMATIONS!⁠

Rocks are freaking beautiful.⁠

Oh and waterfalls wouldn't exist if there was no rock to fall down from, allowing gravity to do its job. Rivers wouldn't be there if there was no rock to flow on and cut into.

And what's the house that you live in built on?⁠

Yes, rock!⁠

So let me ask again: Why should you care about rocks?

Because they're everywhere. Because we enjoy them even if we're not aware of it (most people think rocks are boring).

They're grey, plain old, boring.

And most importantly: because we live on them.

Without rocks, we wouldn't exist. There would be no Earth.

Let’s start this episode off with a question: Have you seen rocks before in your life? Silly question!! Of course you have! But I bet you never thought about what type of rock it is.

Wait, she just said there are different types of rocks…YES – there are differents types of rocks. You probably know that not all rocks are equal. Perhaps you know that there is granite, or marble, or limestone, or sandstone or basalt. But all these terms have no order in your head, there’s no structure, it's just chaos.

Before I started studying geo science, this is exactly how it was for me. I didn’t really have a clue about rocks, seriously. It seemed so chaotic, without order and structure. Frankly, I didn't really care. But now that I’ve learned a few things about geology, I know that there is an order in nature, that there is a structure and that rocks are pretty great.

All the rocks on Earth can be put into three large categories.

Imagine three huge piles of rocks. We’ll label the first pile to the left igneous rocks, the pile in the center are all metamorphic rocks and the pile to the right shall be called sedimentary rocks.

Certain rocks have a certain composition. They all originate from the same source, but go very different paths during their lifetime. This is why we can categorize them into those three major rock type categories, again that’s: igneous rocks, metamorphic rocks and sedimentary rocks.

But before I dive deeper, let me set the stage:

Imagine we’re on an adventure together, just you and me. Anywhere, doesn’t matter. It’s just imagination.

We’re hiking on a narrow trail in a mountain valley, by a blue glacial lake, a remnant of the last glacial. The lake is surrounded by mountain peaks, high as the sky. Birds are chirping, the air is fresh and a slight scent of pine trees lingers in the air, it's just warm enough to wear shorts and a T-shirt. It’s breathtaking and so relaxing.  And while we’re hiking quietly, we pass rocks and we walk on rocks. Some bigger boulders, some smaller pebbles.

Everything looks chaotic right? The rocks are everywhere, everyting seems out of order. The rocks seem just to be placed there randomly.

You pick up a rock and you think: "Pretty rock." You look at it. You think: “Truly a pretty rock.” again. You throw it away.

Then I ask you: "Oh dear friend, why would you throw that precious rock away?"

You reply: "Well, it’s just a rock, Daniela!"

And I say: "But it has been on this planet longer than you can imagine. It’s endured strain and stress and weathering, until it met you here."

Rocks vs. Minerals

And I go on: "Do you know what rocks are made out of? This rock isn't just a rock. It's composed of minerals. Certain combinations of minerals make up certain types of rocks. And minerals are composed of elements.

Just like you, and me and your cat and everything else on the planet and in the universe.

Let me give you an example: You’ve probably heard of quartz, dear friend. Quartz is a mineral, some refer to it as crystal – which is true because almost all minerals have a crystalline structure and are therefore called crystals.

Back to the quartz: 1 quartz molecule is composed of 1 silicon atom and 2 oxygen atoms (SiO2). Quartz exists in numerous varieties. Quartz can be found everywhere, for example in granite. Granite is made up of quartz, feldspar and mica. Feldspar and mica are two other mineral groups that are very common in the Earth’s crust. But let’s not get caught up in the mineral terminology, that’s not my point.

The take away message is that rocks are accumulations of minerals and minerals are composed of elements.

Chemical Bonds in Minerals

One of many cool things about minerals is that they are held together by electrostatic forces. So what does that mean? Let’s take another mineral you most likely know: salt.

Salt, in mineralogy called halite, is made up of 1 sodium atom and 1 chlorine atom (NaCl).

Before I go on, just a quick side note and maybe you still know this from chemistry class in school:

Atoms are made up of the nucleus, which consists of protons and neutrons, as well as electrons which surround the nucleus on shells. The inner shell can hold only two electrons, all the other shells can hold up to eight electrons.

The very outer shell holds the valence electrons. Only those valence electrons take part in reactions and chemical bonds. An atom has exactly the same number of protons as it has electrons, because it has to be charged neutrally. Otherwise it is charged and called an ion. But more on that later. 

So back to the salt, the halite. Inside the crystal structure, the sodium exists as Na+ and the chlorine as Cl-, the latter is then called chloride. 

Electronegativity or: Transferring Books Between Penny and Sheldon

That is because both atoms have specific chemical properties, in this case it's electronegativity, that makes them want to get rid of or collect electrons. Think of it like this:

There are two friends, let’s call them Sheldon and Penny. Penny represents the sodium, Sheldon the chlorine. Both friends have a book shelf with space for eight books.

In this analogy the books represent the outer electrons and the book shelves the atoms' outer shells.

Penny's bookshelf has only one book. Sheldon's shelf is filled with seven books.

Penny doesn’t like to read at all and therefore she wants to get rid of aaaaall the books she possesses and gladly gives them away to Sheldon, who wants to receive aaaaall the books he can get his hands on and wants to fill his shelf with. He has room for eight books on his shelf. When Penny is close, she hands over, she transfers, the one book she has to Sheldon. However, as long as Sheldon has Penny's book, they will be forming an incredibly strong bond, an ionic bond.

(I totally missed the chance for a pun there: "The name is bond. Ionic bond." - only thought of this when editing the audio file :D)

This is the principle of electronegativity.

Ions and Ionic Bonds

Back to the atoms: Sodium (Penny) gets rid of one electron (book). And as electrons are charged negatively, and the sodium now has one more proton than electron, because it gave away an electron to the chlorine, the sodium will become more positive: it becomes Na+.

The chlorine (Sheldon) however now has an extra electron (book), it wants to be super negative, and therefore its charge becomes more negative. It now possesses more electrons than protons. The chlorine turns from chlorine to chloride (Cl-).

When atoms possess more or less electrons than they usually do in their neutral state, they’re called ions.

Some ions are charged positively, some negatively, just as the sodium ion and chloride. They can now form an ionic bond, a super strong bond which is held together by electrostatic forces, the opposite charge of those two ions, plus attracts minus, creates this strong bond.

I conclude: "My dear friend, isn’t that unbelievable? Minerals make up rocks. Therefore all rocks in the world are held together by this electrostatic force. I think it’s mind-boggling, isn't it?"

The Story of Rocks

I pause and am screening the scenerey for a comfortable place to sit down. I spot a perfect location in the shade of a big tree. Perfect to sit down for a short while. I walk over there and ask you to follow, squat down, tap my hand on the spot next to me and say: "Sit down. Now that you know what minerals are, let me tell you the story of rocks."

And I begin.

Igneous Rocks

"There once was an Earth that was immensely hot. It was so hot, that all rock was molten. The surface of Earth was a magma ocean. This is the Hadean Eon and Earth was just born, 4.56 billion years ago.

It took about 500 Million years for the molten magma to cool down just enough for the first minerals to crystallize out of this magma ocean. The oldest mineral found on Earth is a zircon. It is 4.4 billion years old (incredibly old!) and was found in Western Australia. This is evidence that the magma ocean had cooled down and the formation of crust was possible.

The first continents were born. More magma cools down and more land is created.

These rocks are called igneous rocks and are the first category of rocks on Earth. All rocks that form out of magma or lava are magmatic, which is another word for igneous, rocks. We can find these rocks anywhere there was or currently is volcanism, such as in Hawai’i, Iceland or the Outer Antilles, only to name a few. Igenous rocks can be categorized further, but we’ll not do this today.

Some igneous rocks you may know are basalt and granite."

Metamorphic Rocks

You look at me with big eyes and take a bite of your banana, expecting me to continue.

"Fast forward to approximately 420 million years ago. During this time, the Caledonian Orogeny, which is just a fancy word for mountain building processes, occured. Mountain building processes are SUPER slow motion events. The Caledonian Orogeny took about 20 million years. At this point we’ll not talk about what happened during this specific orogeny. But this orogeny shall represent what happens to rocks during all orogenies ever.

Mountains form when two continental plates collide and are uplifted. What happens, too, is that the crust will thicken, as two plates merge. This results in a higher pressure for material in the lower parts of the newly formed mountain.

The rocks from the former continents are remodelled during this immense pressure and heat that occurs during massive orogenies. 

The heat always increases with depth. That means when rocks or sediments are buried, the heat in the lower parts heightens.

The former minerals can’t stand the high pressure environment. They prefer to be in a different state. The immense pressure causes new minerals to form under these high pressure and high temperature conditions. This also means that new rocks emerge, new rock types. We call these altered rocks metamorphic rocks. This is the second rock category.

A metamorphic rock that you probably know is marble.

The protolith, so the rock that existed before the marble was there, is limestone. Marble only exists because limestone was exposed to heat and pressure. We’ll talk about limestone in another episode. Just like igneous rocks, it is possible to classify metamorphic rocks further, but we’ll not do this today.

Not only during mountain building processes, but also during subduction tectonics and other geological settings do these metamorphic rocks form. We can also find metamorphic rocks in very, very old crust, where sediments were loaded upon it, creating high-pressure environments in the old crust. Generally said, high pressure, temperature (and sometimes fluids like water or gases) lead to the formation of metamorphic rocks.

Now, we’ve built crust and we’ve built a mountain. The Caledonian Mountains. Huge mountain chain, extremely high and long and broad. I'll talk about them in a podcast episode in the future.

However, if I recall correctly, there are no Caledonian Mountains anymore today. Right? Where are they? 

Sedimentary Rocks

They’re gone.

At one point, the crust and mountains will be eroded. By wind and water and glaciers. These forces have the power to destroy the rocks and minerals in a process called weathering. There is physical and chemical weathering, but we’ll not go into that distinction now.

Important to know is that as a result of weathering the rock of the mountains will be broken apart and/or change its chemical composition and form new minerals.

As a result, the mountains shrink and the now loose material is carried away and transported somewhere else, again by wind, water (rivers) or glaciers. This material consists of particles in the size of rocks or boulders, as well as sand, silt and clay.

An interesting thing to know is that in sedimentology, that part of the geo sciences that researches sediments and sedimentary rocks, the words boulders, gravel, pebbles, sand, silt and clay are merely the words for different grain sizes (the size of the particle) not for material with a different composition.

Clay is the tiniest grain size, boulder is the largest grain size. Sand is somewhere in between with a grain size usually given as 2 mm-0.065 mm diameter.

These sediment particles are being washed away by rivers and streams and later will be deposited in basins, in lakes, in the ocean, or at the coast as beaches.

Now, they're called sediment. This sediment will just rest there until the energy of the water is high enough to carry it away again. If the sediment will rest there for thousands and millions of years without being moved, more sediment will be loaded upon it, burying the older sediments below. This again increases the temperature, pressure and squeezes out the water that was caught in the holes and pores of the sediment. This is what we call compaction.

Ions in the circulating water (connect and) precipitate, forming minerals which glue the tiny sediment particles together in a process called cementation.

This entire process, the lithification, brings us to the third rock category: sedimentary rocks.

A common sedimentary rock that you most likely know is sandstone.

Either these sedimentary rocks will be uplifted and again eroded at some point. Or they’ll be further buried. If they’re buried further, they’ll eventually become metamorphic rocks.

Just like igneous rocks and metamorphic rocks can be classified further, sedimentary rocks can be distinguished within their own category. For example there are (gray)wacke stones and mudstones."

And I finish: "And that’s the story of rocks."

Already chewing on your second banana you reply: "Wow, that was cool. But I have so many questions now. How do erosion, transport and sedimentation work exactly? What is the role of rivers and why are sediments only deposited in basins? And what is a basin, how do basins form? And what about limestone? And where does salt form?"

Satisfied I say: "I know how you feel. When you start learning about a topic, suddendly a hundred questions pop up in your head. And that’s a great thing. And these are great, great questions my dear friend. But it's getting a little chilly and I think we better get going. We have to reach the cabin before dark. We’ll discuss these other questions on our next hikes together."

We leave our safe patch of grass below the large tree, put on a sweater and continue our hike.

After a few minutes I say: "It was so nice hiking with you today. Thank you so much for joining me. I enjoy talking about Earth so much, it makes me happy. And it makes me happy that you are interested enough and listen to my stories of Earth."

Okay that is all for today! I hope you liked this little short story! Subscribe to the newsletter to be notified of new podcast episodes and other interesting stories of Earth that I might not share anywhere else. 😊

Bye!

Daniela

 

[Cover image: Grand Canyon, AZ, United States (taken by Daniela Dägele)]

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