By Jonny Lupsha, News Editor
Volcanoes are divided into broad categories: ash cones, shield volcanoes and stratovolcanoes. 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, he has created a lava flow that has destroyed homes, schools and the crops most crucial to the local economy: bananas. Farms and their adjacent fields are succumb to the burning lava, which could also pollute the local water supply if it is not contained quickly. The loss of home, livelihood and clean water would be devastating for the community.
When volcanoes erupt, they have one of three cone shapes: ash cones, shield cones and composite cones (stratovolcano). In his video series Nature of the Earth: Introduction to Geology, Dr John J. Renton, professor of geology at the University of West Virginia, explained the differences between them.
Cinder cones are the simplest type of volcano and are mainly the result of basaltic eruptions. Basalt is an igneous rock that makes up 90% of the volcanic rock on Earth.
“Imagine, for example, a basalt eruption, and at most you are talking about an amount of gas that can throw molten rock into the air maybe 1,000 feet to 2,000 feet at most,” he said. Dr Renton. “As the molten rock rises, it sort of spreads out and shatters into little fragments that solidify on the way down and fall around a vent and form a cone. The size of the materials is the size of the ash, so we call it an ash cone.
Dr Renton said the ash cones are very characteristic of any geographic area that experiences basalt eruptions. One example he gave is the Rio Grande Rift in New Mexico.
Volcanoes’ protective cones are found in volcanic hotspots, which are unique in that they are not found along the boundaries of Earth’s tectonic plates. Hot spots, according to Dr. Renton, are spots of basaltic magma beneath the ocean basin that form at the top of the asthenosphere and enter the ocean lithosphere to form another magma chamber.
“Then he pierces the surface […] from the bottom of the ocean, ”he said. “So now imagine an eruption of basaltic lava on the ocean floor, not accumulating very high because it’s very, very fluid, so it’s spreading. [as] a diaper. The next rash does the same, forming another layer that spreads out; and as we build we build a cone, but notice that the cone is not very high relative to its width – in other words, it is much wider than it is tall.
This is because liquids usually don’t stack high. They spread out, like putting pancake batter in a griddle. Many shield cones never accumulate high enough to pierce the surface of the ocean and are therefore called “seamounts”. If the shield volcano builds up high enough to shatter 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, one with a denser oceanic crust often slides under the other – it subdues the upper plate. Subduction causes volcanoes to form in an arc along the subduction known as the volcanic arc, and stratovolcanoes are built from magma that erupts on the surface. According to Dr. Renton, they are still explosive.
“The first eruption is going to be an explosive eruption, a lot of pyroclastic material being created, pieces of rock: boulders, bombs, ash, all, and it’s going to build up around the vent,” he said. -he declares. “If you think of the pile of debris around the vent, there’s a certain angle this thing makes: it’s called the angle of repose.”
“It doesn’t matter the material or the size, as long as it’s solid, oddly shaped, and you pour it into a pile, the pile will have 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. The whole process is repeated a number of times, creating layers of strata, resulting in [the volcano] his [stratovolcano] Name. And unlike [volcanoes with shield cones], stratovolcanoes have a very impressive aspect ratio.
La Palma is a volcanic oceanic island and Cumbre Vieja already erupted in 1949 and 1971.
Edited by Angela Shoemaker, The Great Courses Daily