Wow, check this one out, Savannah.
Hmm, sometimes it's hard to pick my favorite.
Oh, hi, everyone.
Sam and I are sorting his rock collection.
Savannah is Jesse's friend who knows a ton about rocks.
Last time they were telling me all about igneous rocks, and now I want to know how many of them I have.
Hmm, what do you think about this one?
I got it from my parents' cave.
I think that is a piece of limestone.
Wait, limestone?
If I lick it, will it taste sour?
Well, you probably don't want to lick anything that's been on the ground, Sam.
But even if it were clean, no, it wouldn't taste like lime.
It's not named after the fruit.
It just happens to have a name that sounds the same.
Well, that's confusing.
Is limestone an igneous rock?
No, limestone belongs to another large group of rocks called sedimentary rocks.
And just like the name igneous helped us understand how igneous rocks form, the name sedimentary tells us how sedimentary rocks are made.
Sediments are little pieces of material that have settled onto the bottom of a liquid, but it doesn't matter how big that liquid is.
It could be as big as an ocean or as small as this glass.
So like when I finish my drink of hot cocoa and I find a bunch of brown clumps have built up in a ring around the bottom of the cup.
Yeah, those clumps are sediment.
But when we're talking about rocks, the sediment tends to be small pieces of dirt or pebbles, or even bits of broken seashells.
I wouldn't want to drink those.
But how do they get into the water in the first place?
Well, it can happen a bunch of ways, but one of the most common is called erosion, which is also sometimes called weathering, because it can be done by wind and running water.
Basically, over really long periods of time, wind and water will flow over the surfaces of rocks and slowly break off tiny pieces and carry them away.
The original rock maybe one of those igneous rocks we learned about last time will get smaller and its surface will get smoother over time.
But all the pieces that are carried away have to go somewhere.
They might wind up at the end of a river, spilling into a calmer body of water like a lake.
So over time, you can get a bunch of different sediment layers that grow on top of one another.
And after an even longer time, these layers get pressed together into a sedimentary rock.
But remember, sediment isn't just tiny bits of rock.
So sometimes the layers inside a sedimentary rock can be made from things that were once alive, like plants or shells.
So what you're saying is that sedimentary rocks are kind of like rock recycling?
Kind of, yeah.
And depending on what materials make up the sediment and how all the pieces stick together, scientists have come up with ways to sort sedimentary rocks.
Oh, I love sorting rocks.
For example, some sedimentary rocks are made by taking smaller rocks that were created by erosion and squeezing them together really tight until they fuse together.
The individual pieces are called clasts, so the resulting rocks are called clastic sediment.
Sandstone is one of the most common versions of that, and it's exactly what it sounds like a rock made from sand.
Can you see all the layers of sand that were built up on top of one another?
Whoa, what about my piece of limestone?
Some kinds of limestone can be called chemical sedimentary rocks.
They form when water does more than just break a rock into tiny pieces.
It actually dissolves the rock into particles that are too small to see.
If water ever evaporates like a big lake that slowly dries up and completely disappears, it will leave sediment behind that can turn into sedimentary rock.
You can test this out yourself by dissolving some table salt into a glass of water, setting the glass on the table and waiting for the water to evaporate.
The salt will be left behind at the bottom.
In fact, halite is basically table salt, but in sedimentary rock form.
Whoa, that's so cool.
But some limestone can also be sorted into a different group of sedimentary rocks called biological sedimentary rocks.
Can you guess what that group is for?
Well, biology means the study of living things, so biological sedimentary rocks are rocks made of living things.
Yes, well, they're made of pieces of living things, at least.
They form when living things die, get piled up, and squeeze together.
In some limestone you can see the actual chunks of seashells that were left behind by the animals that grew them.
But there are other examples of biological sedimentary rocks, like coal, because coal forms when swamp plants turn into rock after tens of millions of years.
That's a long time.
Yeah, some changes in nature happen very quickly, while others can take a long time.
Like that pumice we looked at last time.
That was created super fast after it got shot out of a volcano up into the air.
But it takes a lot longer for individual pieces of sediment to come together and get compressed into a new rock.
Look at this coal, Sam.
Can you see the layering?
It's a little hard because the rock is so black, but...
Yeah, coal sure looks like, well, coal.
But how do you know that my limestone is really limestone?
Well, I don't know for sure.
But lucky for us, there's an experiment we can do to find out.
Want to try it?
Yeah, absolutely.
Okay, if you want to do this at home as well, get a grown-up to help you out.
First, gather your materials before starting.
You'll need a clear glass cup, some vinegar, and any rocks you think might be limestone.
They should all be small enough to put inside of the cup.
First, place your rock in the bottom of the cup and then gently pour some vinegar in.
What do you see?
Whoa, it's bubbling.
That's because all types of limestone are mostly made of a material called calcium carbonate.
And when calcium carbonate interacts with the acid inside vinegar, a chemical reaction happens that breaks down a little bit of the rock and creates carbon dioxide gas.
That's the gas that's inside all those tiny bubbles.
An acid reacts with rock to make gas?
That's awesome.
Does this happen with all sedimentary rocks?
No, it only works with rocks made from the same material as limestone, calcium carbonate.
So this can be a fun way to narrow down what rock you have.
So I got this limestone from a cave.
Are there other places where we can find limestone and other sedimentary rocks?
Great question.
Sedimentary rocks exist all over the world.
For example, the pyramids of Egypt were partially constructed out of huge limestone blocks.
And Uluru in Australia is a giant piece of sandstone.
But one of my favorite examples of sedimentary rocks can be found near the town of Punakaiki on the southwestern coast of New Zealand the Pancake Rocks.
These rocks are made from alternating layers of limestone and siltstone, which is another kind of sedimentary rock that's made from clay and mud.
They formed about 30 million years ago and were slowly pushed above the water's surface by a series of earthquakes.
Then, over thousands of years, a combination of blowing wind, crashing waves and sprinkling rain eroded parts of each layer.
The siltstone was eroded more easily than the limestone.
So now the towers of rock look like stacks of limestone pancakes.
Man, looking at those rocks makes me hungry.
Me too.
I think it's time for a snack.
And when we're done eating, we can get back to sorting your rock collection.
Thanks for joining us here at the fort.
We've learned about igneous and sedimentary rock so far, but there's one more kind to go.
Stay tuned for more.
See ya.
Bye.
