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, semi-liquid 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 the 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.

dike
Many forms of agriculture, like these rice paddies, rely on dikes to control irrigation.

"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.

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!

Noun

the art and science of cultivating the land for growing crops (farming) or raising livestock (ranching).

angle
Noun

slanting space between two lines that ultimately meet in a point.

augment
Verb

to enlarge or add to.

bulldozer
Noun

vehicle used for moving large obstacles, such as boulders or trees.

clastic dike
Noun

band of sedimentary rock that fills in a crack in a rock.

construction equipment
Noun

tools and instruments used for constructing buildings, roads, or other projects.

Noun

a barrier, usually a natural or artificial wall used to regulate water levels.

dike swarm
Noun

series of rock slabs that cut into other types of rock.

dredge
Verb

to remove sand, silt, or other material from the bottom of a body of water.

earth
Noun

soil or dirt.

enormous
Adjective

very large.

establish
Verb

to form or officially organize.

farmland
Noun

area used for agriculture.

fissure
Noun

narrow opening or crack.

frequent
Adjective

often.

geologic dike
Noun

slab of rock that cuts into another type of rock.

geologic event
Noun

occurrence that changes the rock and earth formation of a region, such as a volcanic eruption.

geology
Noun

study of the physical history of the Earth, its composition, its structure, and the processes that form and change it.

horizontal
Adjective

left-right direction or parallel to the Earth and the horizon.

igneous rock
Noun

rock formed by the cooling of magma or lava.

isolate
Verb

to set one thing or organism apart from others.

Noun

body of water surrounded by land.

Mackenzie dike swarm
Noun

series of magmatic dikes in Northeast Canada more than 500 kilometers (311 miles) wide and 3,000 (1,864 miles) kilometers long.

Noun

molten, or partially melted, rock beneath the Earth's surface.

magmatic dike
Noun

slab of igneous rock that flows into fissures between another type of rock.

mineral
Noun

inorganic material that has a characteristic chemical composition and specific crystal structure.

Noun

large body of salt water that covers most of the Earth.

polder
Noun

land reclaimed from a body of water by dikes and dams, and used for agriculture, housing, or industry.

regulate
Verb

to determine and administer a set of rules for an activity.

Noun

large stream of flowing fresh water.

river bank
Noun

raised edges of land on the side of a river.

rock
Noun

natural substance composed of solid mineral matter.

sandbag
Noun

bag filled with sand or earth and placed near a river to prevent flooding.

Noun

solid material transported and deposited by water, ice, and wind.

sedimentary dike
Noun

slab of sedimentary rock that builds up in fissures within another type of rock. Also called a clastic dike.

sedimentary rock
Noun

rock formed from fragments of other rocks or the remains of plants or animals.

specific
Adjective

exact or precise.

spew
Verb

to eject or discharge violently.

tidal basin
Noun

depression in the earth that fills with water at high tide.

vertical
Noun

up-down direction, or at a right angle to Earth and the horizon.

Noun

an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.