By Jonny Lupsha, Wondrium Staff Writer
Volcanoes are split into major categories: cinder cones, shield volcanoes, and strato-volcanoes. These categories come from many basic characteristics of the volcanic event. An eruption in La Palma, Spain endangers life and land.
On September 19, the Cumbre Vieja volcano in La Palma, Spain, began to erupt. Since then, it has created a lava flow that has destroyed houses, schools, and the most crucial crops to the local economy: bananas. Farms and their adjacent fields are succumbing to the burning hot lava, which could also pollute the local water supply if not contained quickly. The loss of home, livelihood, and clean water would be unspeakably devastating to the community.
When volcanoes erupt, they have one of three cone shapes: cinder cones, shield cones, and composite cones (stratovolcano). In his video series Nature of Earth: An Introduction to Geology, Dr. John J. Renton, Professor of Geology at West Virginia University, explained the differences between them.
Cinder cones are the simplest kind of volcano and are primarily the result of basaltic eruptions. Basalt is an igneous rock that makes up 90% of the volcanic rock on Earth.
“Picture, for example, a basaltic eruption, and at the most you’re talking about an amount of gas that may throw the molten rock up into the air maybe 1,000 feet to 2,000 feet at the most,” Dr. Renton said. “As the molten rock goes up, it sort of spreads out and breaks into little fragments which solidify on the way down and fall around a vent and form a cone. The size of the materials is cinder size, so we call it a cinder cone.”
Dr. Renton said that cinder cones are very characteristic of any geographical area that is undergoing basaltic eruptions. One example he gave is the Rio Grande Rift in New Mexico.
Shield cones for volcanoes occur at volcanic hotspots, which are unique because they aren’t along the boundaries of Earth’s tectonic plates. According to Dr. Renton, hotspots are blobs of basaltic magma under the ocean basin that form at the top of the asthenosphere and break through into the oceanic lithosphere to form another magma chamber.
“Then it breaks through the surface […] of the ocean bottom,” he said. “So picture now an eruption of basaltic lava on the ocean bottom, not building up very high because because it’s very, very fluid, so it spreads out [as] one layer. The next eruption does the same thing, forming another layer that spreads out; and as we build, we build a cone, but note that the cone is not very high relative to its width—in other words, it is much wider than it is high.”
This is because liquids don’t generally pile high. They spread out, like putting pancake batter in a griddle. Many shield cones never pile up high enough to break through to the surface of the top of the ocean and are therefore called “sea mounts.” If the shield volcano does pile up high enough to break the surface, it becomes a volcanic island like Hawaii.
Composite cones of stratovolcanoes involve the geological phenomenon known as subduction. When two tectonic plates converge, the one with denser oceanic crust often slides under the other one—it subducts the higher plate. Subduction causes volcanoes to form in an arc along the subduction known as a volcanic arc, and stratovolcanoes are built from magma that erupts to the surface. According to Dr. Renton, they are always explosive.
“The first eruption is going to be an explosive eruption, lots of pyroclastic material being created, bits and pieces of rock: blocks, bombs, cinders, the whole bit, and it’s going to accumulate around the vent,” he said. “If you think about the pile of debris around the vent, there is a certain angle that stuff makes: It is called the angle of repose.
“It doesn’t make any difference what the material is or what the size is, as long as it’s solid, of odd shape, and you pour it into a pile, the pile will have just about the same angle of repose—it’s about 40 degrees.”
After the eruption, the magma comes out, forms lava, flows down the side of the cone, and seals it off. The entire process repeats a number of times, making layers of strata, which gives [the volcano] its [stratovolcano] name. And unlike [volcanoes with shield cones], stratovolcanoes have a very impressive height-to-width ratio.
La Palma is a volcanic ocean island and Cumbre Vieja has previously erupted in 1949 and 1971.