What is the rock cycle?

0:46 Welcome to Science is Not Boring by Kidopoly.com. I'm Mira. And I'm Finn. Before we start, we have a very special shout-out for Maeve, who is seven years old and listening all the way from Galway Island. She asked us to explain the different types of rocks and how they are formed.

1:06 And a huge belated fifth birthday shout-out to Max in Singapore, who loves the show, and also asked all about how rocks form. Happy birthday for September, Max! If you haven't already, tap follow wherever you're listening so you never miss an episode. So, since both Maeve and Max want to know, what is the deal with rocks?

1:29 They are just hard grey lumps on the ground, right?

1:32 Nope. Did you know that the ground beneath your feet is part of a giant, endless recycling machine? Every single rock on Earth is constantly changing.

1:43 Wait, rocks can change. I thought they just sat there being rocky for millions of years.

1:51 They do sit there for millions of years. But eventually, the earth breaks them down, melts them, or squashes them into completely new shapes.

2:01 So the Earth is just one giant recycling bin for rocks.

2:05 Exactly! The entire planet is basically a giant rock factory, using intense heat, crushing pressure, and powerful weather to keep making new stones out of old ones. To understand how the factory works, we have to talk to petrologists. Those are the special geologists who study rocks, because petro means rock in Greek.

2:28 How do they even study them? Just hit them with a hammer.

2:31 Sometimes, but they also cut rocks into slices thinner than a human hair. When they put these tiny slices under a special microscope, the minerals light up in rainbow colours. That helps them see exactly what ingredients make up the rock, and if it was melted, squashed, or glued together.

2:51 That is so cool. So what did the petrologist figure out?

2:55 They discovered that all rocks fit into three main types igneous, sedimentary, and metamorphic.

3:03 Igneous, sedimentary, and metamorphic? That sounds like a complicated magic spell.

3:09 It is science magic! And the scale is mind-boggling. The Earth's crust is the hard outer shell we live on. Under the continents, it can be up to 100 kilometres or 62 miles thick.

3:24 Whoa, that is a lot of rock under our feet. What kind of rock is down there?

3:29 If you look at the total volume of the Earth's crust, 95% of it is made of igneous and metamorphic rocks. They are the true heavyweights of our planet.

3:40 95%? That is almost the whole crust. So where are the sedimentary rocks?

3:46 Here is the wild part. Even though sedimentary rocks are only 5% of the crust's total volume, they are spread out like a thin blanket over the top. So if you just look at the surface of the Earth, 75% of the rocks you see are sedimentary.

4:03 Wait, so the top layer where we walk is mostly sedimentary, but the deep, deep crust is mostly igneous and metamorphic. Why is it split up like that?

4:13 It is all because of how and where they form. And to understand that, we have to travel deep underground, where things get dangerously hot. Let's start with igneous rocks. The word igneous actually comes from Latin meaning fire.

4:29 Fire rocks? That sounds like something from a video game.

4:33 It is real life! Deep inside the earth, rock melts into a glowing liquid called magma. When it erupts out of a volcano onto the surface, we call it lava.

4:44 How hot are we talking? Like a pizza oven?

4:47 Hotter! Basalt's lava erupts at temperatures between 1100 and 1250 degrees Celsius. That can reach over 2,200 degrees Fahrenheit. That would melt a frying pan instantly! It definitely would! When this liquid rock cools down, it freezes into solid igneous rock. Sometimes it cools instantly to form shiny black volcanic glass called obsidian.

5:17 Other times it traps gas bubbles and forms pumice. A rock so light it can actually float on water. A floating rock? That is insane! And sometimes it creates perfect geometry. Maeve in Ireland will love this next part. On the coast of Northern Ireland, there is an incredible igneous rock formation called the Giant's Causeway.

5:40 The Giant's Causeway? Where exactly is it? And what does it look like? It is in County Antrim, and imagine 40,000 massive interlocking stone pillars standing right on the edge of the Atlantic Ocean. Some are 12 metres or 39 feet tall.

5:59 40,000 pillars? They sound like they were carved by humans or giants.

6:05 People used to think an ancient giant built them, but science tells a better story. 60 million years ago, a massive lava flow cooled down. As the basalt rock shrank, it fractured at perfect 120-degree angles, creating perfect hexagonal six-sided columns. That is wild!

6:26 Nature just cracked the lava into perfect six-sided shapes.

6:30 But igneous rocks don't last forever. Over millions of years, wind, rain, and ice slowly break those solid rocks into tiny pieces of sand and mud. This is called weathering. So the giant pillars get worn away into dust? Yes. Rivers wash all that sand, mud and pebbles down into lakes and oceans. Over millions of years, those layers stack up, getting deeper and heavier.

7:00 The crushing weight squashes the mud and sand together to become sedimentary rocks, like sandstone and shale.

7:07 Wait, is that why the Grand Canyon in Arizona has all those different coloured stripes? Exactly!

7:13 The Colorado River carved through that sedimentary rock like a knife through a giant layer cake. Some of the rock at the very bottom of the canyon is nearly two billion years old. And guess what is often trapped inside these layers? What?

7:29 Dinosaur bones?

7:30 Sometimes, but mostly the shells and skeletons of ancient sea creatures. Over time, trillions of tiny calcium carbonate shells get pressed together to form a sedimentary rock called limestone?

7:45 So limestone is partly made of squashed ancient sea shells. That is both slightly gross and completely awesome.

7:53 It gets even better. What happens if you take that limestone, bury it deep underground, and squeeze it with the weight of an entire mountain range? I have no idea. Does it just turn to dust? No, it bakes. The extreme heat and pressure from tectonic plates crashing together changes the rock entirely. We are talking about pressures thousands of times stronger than the air pressure you feel right now.

8:22 It is like balancing several elephants on a postage stamp. So what does it turn into? Metamorphic rock. Exactly. Metamorphic literally means change of form. Under intense pressure, the minerals in limestone recrystallise and interlock tightly. The limestone actually transforms into beautiful sparkling marble.

8:46 Wait, so fancy white marble statues used to be squashed seashells at the bottom of the ocean. Yep.

8:53 In fact, the famous Taj Mahal in India is built almost entirely of shimmering white marble. That means the whole building, which took 20,000 workers over 20 years to build, is made of metamorphosed ancient seabeds.

9:10 No way. That is mind-blowing. So any rock can turn into any other rock?

9:16 That is the endless rock cycle! Igneous gets crushed into sedimentary, sedimentary gets baked into metamorphic, and metamorphic rocks can get dragged back down into the Earth's fiery mantle at subduction zones, melting back into magma.

9:33 How long does it take for a rock to go through the whole cycle?

9:37 It is incredibly slow. The Earth recycles nearly all of its oceanic crust every 200 million years.

9:46 200 million years? So a rock that melted when the very first dinosaurs were walking around might just be erupting out of a volcano again today.

9:56 You got it. And the rock cycle doesn't just use materials from Earth. We are constantly getting deliveries from outer space.

10:04 Wait, what do you mean deliveries? Are there alien rocks falling from the sky?

10:10 Yes. Every single day, about 44,000 kilograms, or nearly a hundred thousand pounds, of meteoritic material falls from space and hits the Earth.

10:22 That is like dropping 10 elephants made of space rocks onto our planet every single day.

10:28 Exactly. Most of that space rock burns up in the atmosphere as shooting stars, but the pieces that do land are called meteorites. Our planet is a giant churning rock factory, always adding new materials to the mix. So the next time you pick up a pebble, remember, it has a wild history. Alright Finn, it's time for the quiz.

10:51 Are you ready? Okay, I'm ready. I hope my brain doesn't turn into squashed sedimentary rock. First question.

11:00 What percentage of the Earth's crust is made of igneous and metamorphic rocks? Next one. How hot can basalt lava get in degrees Celsius? Question three. What is the name of the famous rock formation in Northern Ireland with 40,000 basalt columns? Here is number four. What kind of rock does limestone turn into when it gets squeezed and baked deep underground?

11:39 Last question. How many kilograms of meteoritic material fall to Earth every single day? Great job, let's see how you did. The first answer is 95%. For question two, it can reach over 1100 degrees Celsius. Number three, the answer is the giant's causeway. Question 4. Limestone transforms into marble.

12:12 And the final answer is about 44,000 kilograms of space rocks.

12:18 I am never going to look at a pebble the same way again. It could have been magma, a seashell, or even part of a mountain. Science really is everywhere. Even right under our shoes. If you enjoyed exploring the rock cycle with us, please leave us a five-star review. You can just scroll down and tap the stars.

12:38 Tell us if you would rather visit a fiery volcano or the giant's causeway. And definitely tap follow right now so our next adventure pops up automatically.

12:49 Oh, and we have sister shows too. History and philosophy is not boring, and maths is not boring. Go give those a listen and a follow as well.

12:57 And if you want a shout out on the show or just want to say hi, email us at hello at kidopoly.com.

13:04 See you next time on Science Is Not Boring.

13:08 Bye!