A dike is a barrier used to regulate or hold back water from a river, lake, or even the ocean. In geology, a dike is a large slab of rock that cuts through another type of rock.


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Earth Science, Geology, Engineering, Geography, Physical Geography

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A dike is a barrier used to regulate or hold back water from a river, lake, or even the ocean. In geology, a dike is a large slab of rock that cuts through another type of rock.

Geologic Dike
A geologic dike is a flat body of rock that cuts through another type of rock. Dikes cut across the other type of rock at a different angle than the rest of the structure. Dikes are usually visible because they are at a different angle, and usually have different color and texture than the rock surrounding them.

Dikes are made of igneous rock or sedimentary rock. Igneous rock is formed after magma, the hot, semiliquid substance that spews from volcanoes, cools and eventually becomes solid. Magmatic dikes are formed from igneous rock.

Sedimentary rock is made of minerals and sediments that build up over time. Sedimentary dikes, also called clastic dikes, are formed from sedimentary rock.

Dikes frequently intrude on open spaces between rocks, called fissures. A dike will either flow or build up in a fissure, pushing the surrounding rock to the side. A dike is, therefore, younger than the rocks surrounding it. Dikes are often vertical, or straight up and down. But since Earth is constantly moving and shifting, the dike can end up horizontal after enough time goes by.

Dikes sometimes show up in swarms of several hundred dikes. A dike swarm is usually created by the same geologic event, such as a volcano.

Water Dikes
Dikes used to hold back water are usually made of earth. Sometimes, dikes occur naturally. More often, people construct dikes to prevent flooding. When constructed along river banks, dikes control the flow of water. By preventing flooding, dikes force the river to flow more quickly and with greater force.

The most familiar material used to build or augment dikes is the sandbag. People will fill cloth bags with sand and pile the sandbags along a river bank or lake shore. The cloth and sand absorb the water, letting very little pass through. Sandbags are very heavy and usually stay in place. Dikes made of sandbags can be many meters tall and twice as wide. They can be built quickly, which is why people living near rivers will start sandbagging as soon as heavy rains start to fall.

Enormous construction equipment can also help build dikes. Bulldozers and dredging machines haul in sand and soil from different areas to a specific line along a body of water. This isolates one part of a river, lake, or ocean from the larger body of water. Once the new dike is established, water from the isolated part is drained out of the area. The land on the drained side of the dike is no longer a body of water.

These dikes, which can be hundreds of miles long, are usually used to create farmland or residential space from a lakebed or even the ocean. The nation of the Netherlands has reclaimed more than a thousand hectares of land from the North Sea by constructing dikes along many tidal basins. The Dutch, people from the Netherlands, use the reclaimed land, called polders, for agriculture, residential, and industrial use. The first dikes in the Netherlands were constructed in the 1200s, and the country continues to maintain and expand the dike system today. In fact, dike is a Dutch word that originally meant the bank of a body of water.

Fast Fact

Mackenzie Dike Swarm
The world's largest dike swarm is the Mackenzie dike swarm in the Northwest Territories of Canada. It's more than 311 miles (500 kilometers) wide and 3,000 kilometers (1,864 miles) long!

Fast Fact

"The Hero of Haarlem"
A popular story concerns a young boy from the town of Haarlem, Netherlands, who notices a leak in the town's dike. The Spaarne River is flowing through a tiny hole in the barrier, threatening to flood the town. The young boy plugs the leak with his finger, and stays there all night. Adults find him the next morning and permanently repair the leak. Although first written about by an American (Mary Mapes Dodge, in her book Hans Brinker, or the Silver Skates), the story is from the Netherlands.

The story has been changed and retold many times. In most versions, the dike is holding back the North Sea, not a river. In some versions of the story, the young boy freezes to death during his all-night stay at the dike.

Media Credits

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Hilary Costa
Erin Sprout
Santani Teng
Melissa McDaniel
Jeff Hunt
Diane Boudreau
Tara Ramroop
Kim Rutledge
Hilary Hall
Mary Crooks, National Geographic Society
Tim Gunther
Jeannie Evers, Emdash Editing, Emdash Editing
Kara West
Educator Reviewer
Nancy Wynne
National Geographic Society
Last Updated

April 29, 2024

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