EARLY THAT AFTERNOON the whole family gathered for a Fourth of July dinner. There were fifteen of us in the dining room around a big table. I sat near Tom and George, who were still talking about what it must be like deep down in the Earth. George was telling Tom more about his family’s visit to the volcano during their trip to Hawaii, and how they had hiked across a crater where steam was still escaping from cracks in the ground.

“It was so hard to believe there was all that hot liquid down underneath us, even though I saw some of it come up on top of the ground,” I heard him say to Tom.

“I’m really glad we don’t have volcanoes around here like they do in Hawaii,” was Tom’s reaction.

George disagreed. “It was really cool to watch the red-hot lava flowing down the side of the volcano and sizzling into the sea,” he answered, shoveling down some potato salad.

NOW IT’S TIME FOR DESSERT. The cherry pies are brought in and put on the table near where Julie and Haley are sitting. They have cooled down, so the insides aren’t bubbling like they were in the oven. But the crusts softened while the pies were cooking, and settled down around the fruit, creating a bumpy surface. “Look, Julie,” Haley exclaims, pointing at one of the pies. “It’s like Gramps said. The crust of the pie is like the crust of the Earth, with hills and valleys.”

“Right,” Julie answers. “That makes me wonder how the real mountains and valleys got that way in the first place.” She glances across the table at me.

“That’s the next part of my story about the Earth’s crust,” I explain. She and Haley gave me the perfect opening. “Later this afternoon I’ll show you an amazing video that’ll help answer your question.”

After the meal is cleared away, the children dash outside and start throwing a Frisbee around on the lawn. The big meal has made me sleepy, so I stretch out on the couch for a nap. Sometime later, I awaken to the sound of quiet laughter. Across the room, Julie, Haley, and George are gathered around Tom, who’s in an armchair with my laptop, showing them a funny animal video. Seeing me stretch and yawn, Julie skips across the room. “Can you show us how the mountains and valleys and things are formed on the Earth’s crust?” she asks.

“Sure.” I answer, sitting up. I wave Tom over with the computer and bring up an image I’d stored on the desktop. “Check this out,” I say, as they find places where they can see the screen. “I want to show you something amazing about this land we’re standing on right now.”

The Plates of the Earth’s Crust
The Earth’s crust is made up of seven big plates and lots of little ones. These plates are moving around very slowly.

The image on the screen looks like our world, but on the surface the crust is broken into chunks. “See,” I say, “the crust of the Earth is actually broken up into pieces, like the pieces of a puzzle. Those pieces are called plates. The whole crust is broken into just sixteen plates. Seven of the pieces are large, and eight are smaller.”

Haley leans in to look closely. “It looks like some of the plates have land and water on them,” she says, pointing at one of the big pieces. “How come the water doesn’t just run right off the edge?”

What a great question. “You’re right, Haley—some ocean water does run into the cracks between the plates. But then it hits the hot mantle deep down. I can show you a cool video of what happens there.”

Tom breaks in, suddenly remembering something his science teacher had told them. “Those pieces of the earth’s crust—aren’t they actually moving around?”

“That’s right!” I’m excited. “The different plates are moving around and banging up against one another. That’s the first step in making a lot of our highest mountains. Here, let me show you.”

“What was happening there?” I ask when the video ends.

“We saw how the land on Earth has moved around over the past 500 million years,” Tom answers. “Millions of years ago, all the land in the whole world was one giant island … one continent. Then it broke up into plates, and the plates began to drift apart—the video shows how they’ve moved around as time passed.”

“That’s right,” I confirm. “Scientists have figured out that 250 million years ago all the land in the world was one supercontinent. They call it Pangea, which means entire earth. Here’s a picture that shows how the plates fit together long, long ago.”

Pangea, the Super Continent
This is what all the land on Earth looked like 250 million years ago.

The children take a close look at Pangea. They can see the names of today’s continents, and how South America was tucked in right against Africa back then.

Next I pull up a map that shows what the continents look like now, millions of years later.

How the Continents Look Today
This is the way the lands on Earth look today.

Julie wiggles in to take a close look at the map of today’s world. “That’s pretty amazing,” she says softly. “When I looked at maps, I always thought North and South America could snuggle right up to Europe and Africa, because the curves in their coasts kind of match up. Now I can see I was right! It really is like pieces of a puzzle.”

“You were right, Julie,” I answer. “Those continents used to be stuck together and gradually pulled apart. That video we just watched shows where scientists think the plates have moved over millions of years.

“And the plates are still moving, floating around on that soft mantle that begins twenty or thirty miles below us. Right here on Cape Cod, on the east coast of America, we’re still creeping away from Europe and Africa very slowly.”

“Then how come we don’t feel the ground moving underneath us?” Haley asks.

“The movement is so slow that we can’t feel it at all,” I explain. “The land we’re standing on is moving about as fast as your fingernails grow. You can’t actually see your nails grow, right? But over a month or so, you can see they’ve gotten longer. That’s what I mean by slowly.”

Tom holds out a hand and looks at his nails. “It’s true,” he murmurs, then looks up. “How much farther apart do America and Europe get each year?”

“About three inches,” I answer. “Satellites circling the Earth can measure the change.”

Now I bring up another world map on the screen, this one also showing the sixteen plates that make up the Earth’s crust. “Each plate is a different color,” I point out. “The brown one is the one we’re on. You can see North America where it’s a tan color, and the ocean part of our plate is darker brown.”

Plates Move in Different Directions
The arrows show whether plates are moving toward or away from each other.

George peers at the picture. “Over in California, our brown plate is touching the yellow Pacific Plate.” He points at the screen. “What do those arrows mean?”

Tom remembers something else his science teacher told the class. “Do the arrows show us where the plates are rubbing up against each other?” he asks.

“That’s partly right.” I point to a spot where the top of the Pacific Plate touches the North American Plate. “See the two arrows pointing at each other here? That means that the plates are bumping directly against one another.”

Julie turns to me with a worried look. “What happens when the plates bump like that? Can you feel it?” she asks.

I turn to face her and put my hands flat on the table, palms down, with one thumb overlapping the other. When I push my hands hard against one another, the fingers pop up upward so my hands make an upside-down V.

“See? That’s what happens with the edges of the plates when they bump one another. Their edges pop up and form mountains. But the mountains grow up really slowly, over millions of years, so you wouldn’t feel anything happening.”

Julie looks surprised. “You mean it takes millions of years for mountains to grow up on the land?”

“That’s right. Sometimes it happens faster, when a volcano pushes up from underground, but usually it takes a really long time. Look here.” I click over to another image. “This picture shows some mountains formed by two plates coming together.”

How Mountain Ranges Are Formed
When two plates push against each other, the edges rise up to form mountains. This takes millions of years.

Julie and Haley lean closer to the screen. “I get it,” Haley says after a minute. “The two plates come together, and the pressing with all that weight behind it forces the land right up in the air to make mountains!”

George asks me to go back to the picture showing all the plates and the arrows. Then he points to a different place where the Pacific and North American plates touch. “Here the arrows don’t point right at one another,” he says. “Instead, one points to the left and one points to the right. What does that mean?”

“Do you have a guess?” I ask.

“Maybe instead of bumping right straight into each other, the plates are rubbing together sideways, like this.” He puts his hands flat on the table, brings them together so they touch, and slides them back and forth against one another, instead of pushing them toward each other like I did.

“Exactly!” I exclaim. “And what would happen if pieces of the Earth’s crust slide past one another like that?”

I can see that Tom knows the answer, but he lets George think about it. “Wouldn’t there be a big crack in the ground right there?” George finally says.

“You bet.” I nod. “And the land would shake. That’s how an earthquake happens. That place you pointed to on the map, where the North American Plate rubs against the Pacific Plate … that’s California. They get earthquakes out there quite often.”

Julie tugs on my t-shirt sleeve. “Show me where we are right now on that map,” she commands. I point to a spot on the brown North American Plate, where the east coast of the United States meets the Atlantic Ocean. “See that tiny little bump poking out into the ocean? That’s Cape Cod.”

Earthquake Cracks in the Earth’s Crust

The picture of this long crack was taken from an airplane.

This little boy is standing in a crack that crosses the road.

“That’s what I thought.” Julie looks relieved. “No plates come together here. So that means we won’t have earthquakes. Good news!”

Haley notices another set of arrows farther out in the Atlantic, where the brown North American Plate and the green Eurasian Plate touch. “Look,” she says, “these arrows are pointing in opposite directions. So that must mean those plates are moving away from each other.”

“Correct, Haley.” I give her arm an encouraging squeeze. “In some places the plates are moving together, and in other places they’re moving apart. Sometimes when they move together the earth does shake, but when they’re moving apart it’s so slow that we don’t feel it at all.”

I see that Haley’s thinking hard. I think she’s remembering her earlier question about the cracks between the plates in the ocean. “Like you said before, water runs into the crack when the plates move away from each other,” she finally says. “So what happens then?”

I remind Haley that there’s lots of heat coming up from the hot mantle and liquid core underneath the ocean. “Remember the bubbling-hot filling under the crust of our cherry pies? Well, once water gets down inside the Earth and hits the mantle, it heats up again and comes shooting back out as steam.”

I bring up a video that shows what it looks when steam shoots up out of the cracks. “Ocean scientists call these cracks vents. And they’re learning some amazing stuff about the living things that hang out around the vents.”

The children watch silently as steam pours out of a crack in the Earth’s crust, deep under water on the ocean floor. “Kind of like the volcano we saw last year in Hawaii,” George says after a minute. “Hey, Gramps—what about volcanoes anyway?”

I’m not George’s grandpa, but he calls me Gramps anyway. I kind of like it, and I really like the way he’s always asking questions. “Some volcanoes are actually born under the water,” I explain. “When the plates move away from each other, melted rock bubbles up out of the crack and then hardens when the water cools it. This makes a small mountain of lava rock under the water. Sometimes that mountain gets so big it rises up above the surface of the ocean, forming an island.”

George and Haley both light up. “Just like in Hawaii!” she cries, as he says, “That’s how the Hawaiian Islands were made, the guide told us when we there.”

“The Hawaiian Islands are actually the tops of old volcanoes that began under water, that’s true,” I answer. “But they’re in the middle of the Pacific Plate, so they didn’t come from plates rubbing together. The Hawaiian volcanoes you saw rose up through really hot places in the Pacific Plate.”

“But what about volcanoes along our West Coast? Wasn’t Mount Rainier, near Seattle, made by a volcano?” Tom wonders.

“Yes, that’s a good example of a volcano made by one plate sliding under another. Sometimes when the edges of two plates come together, they don’t push up to form mountains. Instead, one edge slides down under the other.” First I demonstrate with my hands edge to edge, like before, then bring up the plates-and-arrows picture again.

Mount Rainier is close to the city of Seattle, Washington. It was formed by a volcano millions of years ago.

“See where it says Juan de Fuca Plate—the small blue plate between the Pacific and the North American plates?” I go on. “Many years ago, when the Juan de Fuca Plate slid under the North American Plate, hot magma from the mantle bubbled up through the Earth’s crust. That boiling magma hardened when it hit the air above the ground, forming cone-shaped hills and mountains. Mount Rainier is one of those mountains. Take a look at this video.”

The children’s eyes are glued to the screen as the volcano erupts and lava pours down the mountainside. When the video is over, they ask to see it again.

Tom then has a question. “In the video they talked about a ‘ring of fire.’ Is that something around the edges of one of the plates we were looking at earlier?”

“It is,” I respond. “Let’s look at that map again, and see if you can find the plate with the ring of fire around it.”

Plates Move in Different Directions
The arrows show whether plates are moving toward or away from each other.

Tom looks carefully at the different colored plates on the map. The other three kids are pretty sure they know which one to pick, so when Tom points to the yellow Pacific Plate, they nod vigorously in agreement. “Why do you think the ring of fire goes around that plate?” I ask.

“In the video they said the ring of fire surrounds the Pacific Ocean,” Tom ventures. “So the volcanoes must be around the edges of the Pacific Plate.”

“Another thing,” George adds, pointing to the edges of the Pacific Plate. “All around the Pacific Plate, the arrows are pointing at each other. See? That means the plates are pushing against each other. All that pressing is what causes the volcanoes and earthquakes.”

Julie has her finger on the computer screen where the Pacific Plate and the North American Plate touch each other. Now she traces her way across the United States to where we are on the east coast. “No plates come together here,” she says, pointing at Cape Cod. “We don’t have any mountains on the Cape—nothing but sandy beaches and some woods. I wonder how Cape Cod was made?”

“I know how,” I say, giving her a smile. “But let’s save that for tomorrow. You guys are going to be up late tonight when we shoot off the fireworks, so you may want to rest up a little to get ready.”

Julie squeals and pokes Haley, who pokes her back. “Fourth of July fireworks!” Julie shouts as they dash out the door. “Almost like a volcano!”

Tom and George exchange a look and a shrug—those girls are a little much sometimes. George says, “She’s right about the fireworks, though. You should go see that volcano in Hawaii. It’s incredibly cool.”

Tom is not convinced.