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A fissure is a volcanic event in which a crack in the Earth’s crust results in magma emerging onto Earth’s surface. Fissures are usually a few meters wide, but they can be many kilometers long. Unlike cone-shaped volcanoes, fissures do not usually result in an explosion of lava. Instead, lava escapes the fissure slowly and can cover a wide area. An illustration of a deep hole in Earth's crust. This is a fissure.

How do fissures form?

Fissures have two major ways of forming. The first occurs when tectonic plates move away from one another. When this happens, the space between them allows any magma under the surface to erupt into lava. An example of this type of fissure is the global mid-ocean ridge. It is the largest volcanic landform on earth.
This ridge formed when tectonic plates moved away from each other. The magma that comes out of these fissures forms an underwater mountain range. Elsewhere on the planet, colliding tectonic plates send old crust back into the mantle, where it becomes magma again.

An image showing the structure of the ocean floor with fissures in several places.

This is an illustration of the ocean floor. The fissures are the deep cracks in the ground.

Fissures can also form in the areas surrounding large volcanic eruptions. These events disrupt the Earth’s surface, resulting in many fissures forming away from the central volcano. These fissures, just like the fissures under the ocean floor, allow magma to escape.
Examples of this type of fissure surround volcanoes in Hawaii, such as Kilauea. These fissures send large flames high into the air and are sometimes called “curtains of fire”.

Fissures create landforms

Over time, the lava flowing from a fissure will harden into igneous rock. As the fissure continues to add lava on top of hardened lava, a structure called a spatter cone will form. Spatter cones are much smaller than other cone-shaped volcanoes. They are usually 3-5 meters in height. Fissures can also cause flood basalts. A flood basalt is the result of a volcanic event covering large areas of land or ocean floor with basalt lava. These features can be massive! The largest known flood basalt is the Central Atlantic Magmatic Province. It is over 11,000 square kilometers in size. Scientists think that this flood basalt formed 201 million years ago. The eruptions that made this flood basalt may have caused the Triassic extinction.

Other Great Resources

Fissure Eruptions – SDSU Geology: http://www.geology.sdsu.edu/how_volcanoes_work/Fissure.html

Other Types of Eruptions – HowStuffWorks: https://science.howstuffworks.com/nature/natural-disasters/volcano5.htm

‘Fissure Vent Facts’ by Kiddle: https://kids.kiddle.co/Fissure_vent

Written by: Francis Aguisanda