THE EARTH’S CRUST AND WHAT LIES BELOW
IT’S NOW EARLY JULY, and summer is in full swing on Cape Cod. Lots of families travel to the Cape for their summer vacations, so the roads and beaches are crowded. The shorebirds of summer have also returned. We see them feeding on the sand flats in the bay at low tide and scampering along the ocean beach just ahead of the breaking waves.
Two weeks have passed since Tom and Julie first joined us for the summer. Today is July Fourth, Independence Day, and we’re all in the kitchen preparing a big meal to celebrate the holiday.
Tom and Julie’s cousins George and Haley are visiting for the long weekend. George is a very curious eleven-year-old with a good sense of humor and a strong interest in science. He’s a bit clumsy and likes to do his own thing at times. Nine-year-old Haley is small for her age, but she’s a wonderful athlete, very nimble and quick on her feet. She loves to spend time with Julie, and her shy smile comes easily.
When I stayed overnight in Boston with Julie and Tom two months ago, we talked about learning more about the living Earth this summer. Julie wanted to understand the oceans better, and Tom decided he would study the atmosphere. I promised to learn as much as I could about the Earth’s crust—the land we live on and the land beneath the oceans.
So far Tom, aka the Gas Guy, has taught us a whole lot about the atmosphere, and about how plants breathe in carbon dioxide and breathe out oxygen. From Julie —Water Woman—we’ve come to understand how the oceans suck up lots of that carbon dioxide gas. She’s also told us about the water cycle, and discovered a few of the thousands of creatures and plants that live in the sea.
Now it’s my turn. I can’t wait to share some really exciting things I’ve discovered about the Earth’s crust.
THIS MORNING, the children and I are in charge of making two cherry pies for the holiday feast. It’s hot in the kitchen, so they’re wearing their bathing suits. Haley and Julie have made the filling by mixing tart cherries with some sugar and cornstarch. I’ve taught Tom and George how to roll out store-bought pie crusts until they’re nice and thin. They’ve placed the crusts carefully across the bottoms of two glass pie plates.
After the girls poured their cherry filling on the bottom crusts, George and Tom took turns rolling out top crusts and laying them across the filling. I sealed both crusts tightly around the edges with a fork, and put the pies in the oven.
Tom and George roll out the pie crusts until they’re nice and thin.
The pies have been baking for forty-five minutes, and Julie is eager to see whether they’re ready. “You said they should cook for fifty minutes, right, Gramps?” she asks impatiently.
“I did,” I respond, “and the time is almost up. Do you want to open the oven and take a look?”
“Yes!” say all four kids. I signal to Tom, who opens the oven door. The kids gather around and peer in. The pie crusts are a light brown. Through the glass sides of the plates we can see the juices bubbling, and a sweet cherry smell fills the kitchen.
“Mmmm, smells so good!” chirps Haley.
Julie asks hopefully, “Should we take them out now?”
“Let’s give them five more minutes,” I recommend, and Tom closes the oven door.
While we’re waiting, Julie has a thought. “Those pies in there have crusts.” she says, pointing at the oven. “I get that. But, Gramps, you say the Earth also has a crust. Is it like the piecrust somehow?”
I give her a big smile. “What a good question! You just saw the juices in our pies bubbling away inside the glass plate, right?”
Julie nods, still puzzled.
“Well, it turns out that our Earth is kind of like that pie,” I explain, pulling my right earlobe—something I often do when I’m excited. “The middle of the earth is full of liquid metals, like the juice in our pies. But wrapped around all that hot stuff is a crust—kind of like the ones that Tom and George put on top of the pies.”
The Earth is kind of like a pie. It has a crust with layers underneath.
“Really?” Julie wrinkles her forehead, looking doubtful.
I continue. “But the Earth’s crust is really thick. In most places you’d have to drill down twenty to thirty miles before you’d get to where the rock softens. Once the pies are done, I’ll show you a picture.”
The stove timer buzzes, signaling that the pies are ready. I take them out of the oven—using potholders and being really careful because they’re so hot—and set them on metal racks on the kitchen counter. The cherry juices have bubbled up through the crust in several places, and over the edge of the pie plate. Through the glass plates, we can see that the juices are still cooking in the intense heat.
George bends down to pie level and watches the action intently. “If we could see under the Earth’s crust, would we see stuff bubbling like this?” he asks.
I cover the pies with the dishcloth and respond, “Come into the living room, and I’ll show you what’s down there.”
We all troop into the living room. I fire up my laptop while the kids gather around to see a picture of the Earth’s interior that I’ve saved on my desktop.
The Earth has four layers. Under the crust is the soft mantle. Moving inward, next come the liquid outer core and the solid inner core.
“This is what you’d see if we could lift a big piece right out of the Earth,” I explain. “It’s kind of like when you’ve eaten a few sections of an orange, and you can look right down into the center of what’s left.”
Crowding around the screen, the children see that there are layers inside the Earth, and each has a name. Julie focuses on the top layer, the crust.
“Okay, the crust is right on the surface,” she says. “It looks really thin in the picture, but didn’t you say it’s twenty or thirty miles thick, Gramps?”
“That’s right. Here’s a picture of the crust taken at the Grand Canyon—a big deep valley out west. Maybe you’ve heard of it?” I click on the picture I’ve saved. “See all the different layers? And those are just in the crust.”
The Grand Canyon
The Grand Canyon is in Arizona. The exposed layers in the Earth’s crust there tell a story of how the crust was formed over millions of years.
The children lean in to look. “Wow,” George breathes. “How deep is that canyon, anyway?”
“It’s about a mile deep and more than fifteen miles wide. When I was Tom’s age, I hiked down to the Colorado River at the bottom and back up again.”
Julie’s eyes widen. “Really?” she asks. “Was it cool?”
“No, actually it was really hot down there,” I answer, grinning.
“You know what I mean! Like, was it fun?” She shoots me a look of pretend annoyance.
“It was fun, but it was a really hard hike back up to the top.”
Julie’s ready to move on. “Go back to that other picture,” she instructs. “So, right under the crust comes the mantle.”
“What exactly is the mantle?” Haley asks.
“The mantle is the layer under the crust—it’s soft but not liquid,” I explain. “It’s kind of like clay, or the cherries in our pies. The cherries get soft when they cook, but they aren’t liquid like the juice.”
“The mantle is the layer of Earth just under the crust. It’s soft but not liquid.”
Julie is beginning to understand. “Okay,” she says, leaning casually against the table. “I get the idea of the crust. It’s the land we’re standing on, and the land under the oceans.”
“That’s correct. Actually the crust at the bottom of the oceans is thinner than the crust on dry land. Under the water it’s only five or six miles thick.”
Julie’s eager to get deeper into the Earth. “So after the crust comes the mantle,” she goes on. “It’s kind of squishy, like the cherries, but it isn’t liquid. Down below the mantle is the liquid part. The picture says that’s called the outer core. Then right in the middle is the inner core, and that’s hard again. Core means middle, like the core of an apple, right?”
“Right,” I say. “The inner core is solid because it’s really, really hot right in the center of the Earth. Some scientists think it’s almost as hot as the sun down there. For millions of years the rest of the Earth has pressed inward on the center, squeezing it really hard. All that squeezing turned the very center of Earth back into solid metal.”
Tom and George are looking at the picture of the Earth with the slice cut out. Tom wants some hard numbers. “How far is it to the center of the Earth, anyway?”
I bring up another picture.
The Thickness of Earth’s Layers
The thickest layer of the Earth is the mantle, which is 1,800 miles deep.
“Check this out,” I say. The picture shows each layer of the Earth, with the thickness in miles of each layer. Tom reaches for a pen and some paper, and does the math:
Crust: 20–30 miles thick
Mantle: 1,800 miles thick
Outer Core: 1,400 miles thick
Inner Core: 800 miles thick
TOTAL: 4,030 MILES THICK
“More than four thousand miles from where we’re standing to the center of the Earth,” he announces.
“Grandpa, how far is that—you know, as if it was on the surface of the Earth?” Julie wants to know. “Like, from Boston to where?”
I think for a minute, then ask the search engine how far it is to Alaska from Boston. The answer comes back: about 4,000 miles. “If you wanted to dig to the middle of the Earth, it would be like digging a tunnel from here to Alaska,” I say.
“Wow!” George whispers, running his hands through his sandy-colored hair. “But even if you had a machine that could tunnel that far, you couldn’t actually do it. Because of the liquid in the outer core and the heat being so intense.”
George persists. “If we can’t dig a tunnel way down into the middle of the Earth, how do we know that it’s really hot in there?”
“Because sometimes the hot melted rock deep down in the Earth pushes up through the crust and spills out onto the surface. Here, let me show you some pictures.” I search for photos of volcanoes in Hawaii, and lots of pictures fill the screen.
“Hey! We’ve been there!” George exclaims, turning to his sister. “Remember, Haley, Mom and Dad took us to the Big Island last winter!” Haley smiles and nods.
The kids examine the pictures of the hot liquid rock—called magma—down inside the Earth, and what it looks like when it spills out onto the Earth’s surface.
“That’s pretty awesome,” Tom murmurs after a minute, “especially that picture where you can look down the hole in the crust right into the boiling magma.”
Volcanoes, Magma, and Lava Rock
Magma from this volcano flows in rivers down the sides of the mountain.
This river of magma is flowing into the ocean.
Through the lava you can see the hot magma underneath.
When magma cools, it turns into lava rock.
Inside the crater at the top of this volcanic mountain lies a pool of hot magma.
He pauses and then asks, “How come it’s liquid if it comes up from the mantle? You said the mantle is soft but not melted.”
“Good question. I checked, and learned that the liquid in a volcano is actually melted crust. Exactly why that crust melts is in the next part of my story, about how the land on Earth is formed.” I shut down my computer and close the lid.
“If you want to find out more about volcanoes and earthquakes and things like that, I’ll show you some amazing videos later this afternoon.”
“That would be cool,” George says, looking at Tom, who nods in agreement.
“After we eat our cherry pie, right?” Haley says, yawning and stretching.
“You mean after we eat our cherry mantle with the crust on top!” Julie giggles, poking her cousin and then dancing away from her. They chase each other out into the July sunshine while the boys head upstairs to put on shirts for the holiday meal.