Oh, hey, Sam.
Have you finished sorting your rock collection yet?
Eh, I'm taking a break from sorting, but I don't want to put my rocks away yet.
They're just so pretty.
They sure are.
Hi, everyone.
I'm Jesse's friend, Savannah, and I've been working with Sam on his rock collection.
And it's so much work.
Well, if you're feeling tired, Sam, maybe we can let some of these rocks tell us a story.
A story?
Yeah, because if you know how to listen, a rock can tell you all about where it came from.
But how can a rock tell a story?
Well, take this rock, for example.
It's called nice.
Yeah, it is pretty nice.
Yeah, that too.
But the name nice is actually spelled differently from nice, the word you might use to describe someone kind.
Can you describe this bit of nice for me?
Well, it's got a bunch of wavy stripes in it.
That reminds me of sedimentary rocks, which can have different colored layers.
But I haven't seen any sedimentary rocks in my collection that have wavy layers.
That's a great observation, Sam.
And these wavy lines might be telling us the story that this gneiss used to be a sedimentary rock, but over time it changed into a completely different kind of rock.
Rocks can change from one kind to another?
Yes, they can.
We've already covered this a little bit when we talked about sedimentary and igneous rocks.
What do you remember about sedimentary rocks?
They're made of sediment, which can be tiny pieces of rock that are broken down from bigger rocks.
They get compressed into a new rock.
Exactly.
And what about igneous rocks?
Igneous means fire.
So those rocks form when liquid rock cools down into a solid rock.
But where did that liquid rock come from?
Hmm, well, I guess it could have been hiding underground the whole time.
But if an old rock gets super duper hot, maybe it would get so hot that it would melt into liquid.
You got it.
But we haven't learned about the third main kind of rock out there.
Rocks like this piece of gneiss.
We've got igneous rocks, which can be made by melting old rocks, sedimentary rocks, which can be made by breaking down old rocks into teeny, tiny pieces, and finally metamorphic rocks.
And how do metamorphic rocks form?
Well, just like the names igneous and sedimentary, the name metamorphic gives us a bit of a hint.
It sounds like metamorphosis, which I know is what happens when caterpillars turn into butterflies or moths.
So is a metamorphic rock just a rock that used to be another rock?
Exactly.
It doesn't need to be melted or broken down.
It just goes from being one type of rock to another.
And it turns out the same type of metamorphic rock might have different stories to tell.
For example, some nice rocks start off as sedimentary rocks, like shale, but others start off as an igneous rock like granite.
And then what happened?
What makes them change?
Well, the rock would have been underground.
And for some reason the ground around it either got really hot, but not hot enough to melt the rock, or there was a bunch of extra pressure.
Something started pushing on the rock really, really hard.
Or maybe it was a combination of both extra heat and extra pressure.
That sounds intense. is intense.
Some metamorphic rocks are made.
When an old rock touches hot magma that's coming up through the Earth.
You know how heat can cook your food, changing how it looks or feels?
Well, the magma's heat basically cooks a rock into looking different, too.
But that's not all.
Some metamorphic rocks are made from the movement of the Earth's tectonic plates.
Plates?
Like we have in the kitchen?
No, not quite.
Tectonic plates are kind of like giant puzzle pieces that make up the Earth's crust.
But unlike a puzzle you can put together on a table, tectonic plates are floating on layers of magma and water.
So they're constantly moving around and pushing up against one another.
Wait, I remember now.
When tectonic plates try to move past each other, they can make earthquakes.
Right.
But all that pushing also means a lot of pressure.
So if you're looking for metamorphic rocks, you might want to start with a map of the Earth's tectonic plates and then go to places where two plates are touching.
So what exactly is changing inside a rock with all that heat and pressure?
Like, how did this piece of ice get those funky stripes?
Well, rocks are generally made up of smaller building blocks called minerals.
And if you apply enough heat and pressure, you can rearrange those building blocks or even create different minerals altogether.
In either case, enough has changed that you've got yourself a completely different rock.
You're rocking my world, Savannah.
Sometimes a metamorphic rock can even show us how the heat and pressure twisted and folded those minerals.
Because nice isn't the only metamorphic rock with foliation, which is the word that geologists use instead of stripes.
And foliation happens when pressure squeezes a rock's minerals so much they flatten out into layers, whether the original rock had layers or not.
Check out this granite. which is an igneous rock.
The minerals have grown into fairly large crystals, but they aren't lined up very neatly.
But when granite undergoes a lot of heat and pressure, like where two tectonic plates touch, it turns into nice with a bunch of visible foliation.
That's neat.
But not all metamorphic rocks look like this.
For example, the sedimentary rock limestone can turn into the metamorphic rock marble.
The heat and pressure is high enough to rearrange the limestone's minerals, growing brand new crystals out of the same ingredients.
But it isn't high enough to squeeze the crystals into a bunch of different layers when the marble is done forming.
I've heard of marble.
People use it to make statues.
Yes, they do.
I wonder if anyone has ever combined limestone and marble to make a statue of a caterpillar turning into a butterfly.
That would be an extra great example of metamorphosis.
So, both igneous and sedimentary rocks can turn into metamorphic rocks.
Can metamorphic rocks turn into other metamorphic rocks?
They can.
For example, with the right amount of heat and pressure, the sedimentary rock shale can become the metamorphic rock slate.
But if you take a piece of slate and expose it to even more heat and pressure, you can turn it into a piece of schist.
And then that piece of schist can be melted down to make an igneous rock or get broken down to make a sedimentary rock.
Yep, basically every kind of rock can turn into any other kind of rock.
Geologists call it the rock cycle.
And they show how it works using pictures like this.
Let's see.
An igneous rock gets worn down and carried away as sediment.
That becomes sedimentary rock.
That sedimentary rock can go through metamorphosis to become metamorphic rock.
And when a metamorphic rock becomes so hot it melts into magma.
It'll eventually become an igneous rock all over again.
Or maybe that metamorphic rock gets worn down to make a sedimentary rock which melts to form an igneous rock.
That sounds exhausting.
It kinda does.
It's a good thing rocks can't get tired like we can.
Tell that to my pet rock, Gloria.
Dr. Turtleman gave her to me.
She's my favorite.
Wow, Gloria looks amazing.
But is she nice?
Wait, which nice are you talking about?
The rock nice or the kind nice?
How about both?
Thanks for learning about rocks with Sam and me and Gloria.
It's been so fun visiting the fort.
Do you have a favorite kind of rock?
Does anyone in your family or do any of your friends like rocks as much as Sam and I do?
Why don't you ask them?
And we'll see you next time here at the fort.
Bye.
