Every year, there are about 500,000 earthquakes around the world. Of those, only about 100 are strong enough to cause damage. Most are so mild that no one even feels any shaking.

All earthquakes—big and small—happen because of sudden movements in Earth’s crust. This is our planet’s outermost layer. It’s the ground that holds up your school, your town, our oceans, and our mountains. Many miles thick, the crust is broken up into about 17 plates. These rest atop a layer of molten rock—Earth’s mantle.

Earth’s plates are always moving, in very slow motion, over the mantle. (See Where Earth’s Plates Meet) Sometimes two plates get stuck together. They push and grind against each other. Over years and years, pressure builds. Then, with sudden violence, the plates break free. Shock waves explode in all directions. Up top, on Earth’s surface, everything shakes. That’s an earthquake.

Earthquake scientists know all of this today. But at the time of the Alaska earthquake, in 1964, seismology was still a new science. Some important ideas had not been proved. Scientists knew that Earth’s crust was broken up into giant plates. They understood that earthquakes were caused by movements deep inside Earth’s crust. They knew there were giant cracks in the crust—these are called faults. It was even clear that most earthquakes happened because of movements at those faults.

But like a puzzle with missing pieces, scientists’ understanding of earthquakes had gaps. And there was one very big gap: Scientists hadn’t proved that Earth’s plates were always moving in slow motion.

Today scientists know this idea as the theory of plate tectonics. It is one of the most important theories in geology. But in the 1960s, only some scientists supported the idea. They needed more evidence to back it up.

In the shattered land of Alaska, George Plafker’s hunt for that evidence began.

About 500,000 earthquakes happen around the world every year. Of those, only about 100 are strong enough to cause damage. Most are so mild that no one even feels any shaking.

All earthquakes happen for the same reason. They’re caused by sudden movements in Earth’s crust. This is our planet’s outer layer. It’s the ground that holds up your school, your town, oceans, and mountains. The crust is many miles thick. It’s broken up into about 17 pieces. They’re called plates. These rest atop a layer of molten rock. It’s called Earth’s mantle.

Earth’s plates are always moving. They shift in very slow motion over the mantle. (See Where Earth’s Plates Meet.) Sometimes two plates get stuck together. They push and grind against each other. Pressure builds over years and years. Then the stuck plates violently break free. Shock waves ripple in all directions. Everything shakes on Earth’s surface above. That’s an earthquake.

Earthquake scientists know all of this today. But not in 1964, at the time of the Alaska earthquake. Seismology was still a new science back then. Some big ideas had not been proved. Scientists knew that giant plates made up Earth’s crust. They knew that motion deep inside Earth’s crust caused earthquakes. They knew there were giant cracks in the crust. These cracks are called faults. Scientists even knew about shifts along those faults. They led to most earthquakes.

But what scientists knew about earthquakes had gaps. It was like a puzzle with missing pieces. One of the biggest gaps had to do with Earth’s plates. Scientists hadn’t proved that they were always slowly moving.

Today this is known as plate tectonics. It is one of the most important ideas in geology. But only some scientists believed in the idea in the 1960s. They needed more evidence to back it up.

George Plafker’s hunt for that evidence began in Alaska.

As Plafker flew over Alaska in the days after the earthquake, his job was to study the effects of the disaster. He traveled over thousands of square miles. He spoke to survivors. After two weeks, he returned home. Plafker and his team wrote a report of their findings. Their project, it seemed, was finished.

But a huge question still bothered Plafker: What had caused this earthquake?

According to scientists’ understanding at the time, there must be a fault under the disaster zone. When Plafker was flying around, he had been looking for a fault—a miles-long crack in the ground. But he hadn’t seen one! The cause of the most powerful earthquake in U.S. history was a mystery.

Plafker couldn’t let it go. So he returned to Alaska later that spring. He was determined to solve it.

Plafker had some tools to do his work but nothing like what scientists use today. Now geologists rely on powerful computers and sensitive instruments to learn about rocks miles below Earth’s surface. Such tools didn’t exist in 1964. Plafker would have to rely on his eyes and his brain—his powers of observation.

Plafker spent two months traveling across areas torn apart by the earthquake. Like a detective trying to solve a crime, he searched for clues. One clue was that the land just off the coastline had been lifted by as much as 30 feet. It was as though an underground giant had hoisted the land up over its head. In other places, the land had dropped, as though the same muscled giant had yanked it down.

Plafker discovered the changes in the seafloor in a surprising way: by studying sea creatures called barnacles that turned up in a place he didn’t expect. These tiny creatures spend their lives attached to hard surfaces in the sea and along shorelines. To survive, they must be covered with water at least part of the time. On many cliffsides along Alaska’s coast, Plafker observed, the barnacles were dead.

Before the earthquake, the cliffs had been covered with water. Now there was a row of dried-up barnacles many feet above the water. To Plafker, this was evidence that parts of the seafloor had shifted up because of the earthquake.

But how had that happened?

Plafker’s job was to study the effects of the earthquake. He traveled over thousands of square miles in the days after the disaster. He spoke to survivors. He returned home after two weeks. Plafker and his team wrote a report of their findings. It seemed their project was finished.

But a huge question still bothered Plafker. What had caused this earthquake?

Scientists thought there must be a fault under the disaster zone. Plafker had looked for one as he flew overhead. But he hadn’t seen a miles-long crack in the ground! The cause of the most powerful earthquake in U.S. history was a mystery.

Plafker couldn’t let it go. So he returned to Alaska later that spring. He felt he had to figure out what had happened.

Plafker had some tools to do his work. But they were nothing like what scientists use today. Now geologists rely on powerful computers. They use sensitive instruments. This allows them to learn about rocks miles below Earth’s surface. Such tools didn’t exist in 1964. Plafker would have to rely on his eyes and brain. He also had his powers of observation.

Plafker spent two months traveling across areas torn apart by the earthquake. He was like a detective trying to solve a crime. He searched for clues. He found that the land just off the coastline had been lifted. It shifted by as much as 30 feet. It was like an underground giant had pushed the land up. The land had dropped in other places. It was like it had been yanked down.

Plafker made the discovery in a surprising way. He studied tiny sea creatures. They’re called barnacles. They spend their lives stuck to hard surfaces in the sea and along shorelines. They must be covered with water at least part of the time. Otherwise, they won’t survive. Plafker observed barnacles on cliffsides along Alaska’s coast. But many were dead.

That meant water must have covered the cliffsides before the earthquake. Now there were dried-up barnacles many feet above the water. To Plafker, this was evidence that parts of the seafloor had shifted up.

But how had that happened?

Plafker returned home to California. He spent weeks reviewing his data. He’d taken hundreds of measurements of where the land had been lifted and where it had fallen. He’d taken many notes and photographs. He spent hours thinking.

Plafker’s mind kept returning to the idea that Earth’s plates were moving—the theory that would become known as plate tectonics. He read all he could about it. He spoke to other geologists to hear their ideas.

Two of those giant plates were near the disaster zone—that much was certain. One plate held up the Pacific Ocean. The other held up the continent of North America. Based on all Plafker had learned, it was clear that those plates had to be moving—and pushing against each other. And if that was right, the fault he was looking for was actually the place where the two plates jammed together!

Plafker concluded the Pacific plate pushed into and under the North American plate. Pressure had been building for centuries. Until finally, at that terrifying moment on March 27, 1964, the plates finally broke free. That caused the Alaska earthquake. And it’s why parts of the land and seafloor had popped up and parts had dropped down. (See How Plates Move.)

The more Plafker considered this idea, the more certain he was. Not only did it explain the cause of the Great Alaska Earthquake but the evidence supported the idea of plate tectonics.

Would other scientists agree with him?

They did. Over time, Plafker’s work in Alaska transformed science. Today, at the age of 94, George Plafker is honored as one of the world’s most important geologists. His work has helped scientists learn more about where and when powerful earthquakes are likely to happen. This helps communities at risk keep people safer. Buildings like schools can be built to stay standing in an earthquake.

We can’t stop what’s happening miles under the earth’s surface. But when the ground starts to shake, this knowledge helps us be better prepared.

Plafker returned home to California. He spent weeks looking over his data. He’d taken hundreds of measurements. They showed where the land had been lifted. They also showed where it had fallen. He’d taken many notes and pictures. He spent hours thinking.

Plafker’s mind kept returning to one idea. Earth’s plates must be moving. This is the theory that would become known as plate tectonics. He read all he could about it. He spoke to other geologists to hear their ideas. 

Two of those giant plates were near the disaster zone. That much was certain. One plate held up the Pacific Ocean. The other held up the continent of North America. Plafker thought about all he’d learned. He felt it was clear that those plates had to be moving. They’d been pushing against each other. And suddenly he knew where to find the fault he’d been looking for! It was the place where the two plates jammed together.

Plafker realized the Pacific plate pushed into and under the North American plate. Pressure had been building for centuries. Then the plates finally broke free. That caused the terrifying earthquake on March 27, 1964. And it’s why parts of the land and seafloor had popped up and parts had dropped down. (See How Plates Move.)

Plafker kept thinking about this idea. He became more certain it was right. It explained the cause of the Great Alaska Earthquake. And it supported the idea of plate tectonics.

Would other scientists agree with him?

They did. Over time, Plafker’s work in Alaska changed science. Today Plafker is 94. He’s seen as one of the world’s most important geologists. His work has helped scientists learn more about earthquakes. That includes where and when powerful ones are likely to happen. This helps communities at risk keep people safer. Buildings like schools can be built to stay standing in an earthquake.

We can’t stop what’s happening miles under the earth’s surface. But this knowledge helps us be better prepared when the ground starts to shake.