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.
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.
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.