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How Do Volcanoes Erupt?
Deep within the Earth it is so hot that some rocks slowly melt and become a thick flowing substance called magma. Since it is lighter than the solid rock around it, magma rises and collects in magma chambers. Eventually, some of the magma pushes through vents and fissures to the Earth's surface. Magma that has erupted is called lava.
Some volcanic eruptions are explosive and others are not. The explosivity of an eruption depends on the composition of the magma. If magma is thin and runny, gases can escape easily from it. When this type of magma erupts, it flows out of the volcano. A good example is the eruptions at Hawaii’s volcanoes. Lava flows rarely kill people because they move slowly enough for people to get out of their way. If magma is thick and sticky, gases cannot escape easily. Pressure builds up until the gases escape violently and explode. A good example is the eruption of Washington’s Mount St. Helens. In this type of eruption, the magma blasts into the air and breaks apart into pieces called tephra. Tephra can range in size from tiny particles of ash to house-size boulders.
Explosive volcanic eruptions can be dangerous and deadly. They can blast out clouds of hot tephra from the side or top of a volcano. These fiery clouds race down mountainsides destroying almost everything in their path. Ash erupted into the sky falls back to Earth like powdery snow. If thick enough, blankets of ash can suffocate plants, animals, and humans. When hot volcanic materials mix with water from streams or melted snow and ice, mudflows form. Mudflows have buried entire communities located near erupting volcanoes.
A volcano is a vent in the Earth’s crust from which eruptions occur. There are about 1500 potentially active volcanoes worldwide. When volcanoes erupt they can spew hot, dangerous gases, ash, lava and rock that can cause disastrous loss of life and property, especially in heavily populated areas. Volcanic activities and wildfires affected 6.2 million people and caused nearly 2400 deaths between 1998-2017.
Every geological formation is unique. Their composition and construction depend on so many factors, that it would be impossible for two formations to be exactly alike. In the same way, each volcano and its eruptions are unique. However, we tend to see two major kinds of eruptions. We talked about eruption to mean both a violent explosion or a sort of silent spreading. These are the two types of volcanic eruptions that we see–explosive and non-explosive eruptions. When we think of volcanic eruptions, we often think of huge clouds of volcanic ash ejected high into the atmosphere and then thick rivers of red lava snaking down the mountainside. In reality, these two phenomena rarely occur in the same volcano. Volcanic eruptions tend to be one or the other.
There are different types of volcanic eruptive events, including:
- pyroclastic explosions, with is fast-moving hot gas and volcanic matter
- hot ash releases
- lava flows
- gas emissions
- glowing avalanches, when gas and ashes release.
Magma and Lava
Volcanoes wouldn’t be nearly as interesting without the great explosions they create and the glowing red rivers of lava. All igneous rock comes from magma or lava. The next time you go hiking near a volcanic zone, you might try to identify the types of lava that the volcano erupted, based on the types of igneous rocks you find.
Magma
Deep beneath the Earth, magma forms as the first stage in creating a volcano. This occurs because rock below the surface is subjected to great amounts of pressure from gravity. The decay of radioactive materials generates additional heat. The substantial heat and pressure melt the rock below the surface to form a taffy-like substance. You may have seen a candle that has been left out in the hot sun too long. It becomes softer and more like a liquid. As the molecules absorb heat, they begin to slide past one another becoming more fluid. A similar process occurs with magma. However, different substances melt at different temperatures. For that reason, the temperature at which rocks melt depends on the specific types of rocks. The Earth’s crust and mantle are made of many substances so the temperature required to create magma varies. Most magmas are formed between 600°C and 1300°C
A highly viscous lava is one that doesn’t tend to flow easily. It tends to stay in place. Lavas with high silica contents tend to be more viscous. Since it is so resistant to moving, it clogs the vents in a volcano. The pressure becomes greater and greater until the volcano finally explodes. This type of lava is found in explosive eruptions. It also tends to trap a lot of gas. When the gas is released, it makes the eruption more explosive. Most of this lava is shot up into the air where it hardens and becomes solid rock. This molten rock that solidifies in the air is known as pyroclastic material. In an igneous rock like pumice, small holes in the solid rock show where gas bubbles were when the rock was still liquid lava.
Low-viscosity lava slides or flows down mountainsides. There is more than one type of low-viscosity lava. The differences between them come from the lavas’ different composition and different spots where they come to the surface. The type of igneous formations formed depends on which type of lava it is. The three major categories are a’a, pahoehoe, and pillow lava.
A’a Lava
A’a lava is the more viscous of the non-explosive lavas (Figure 8.15). This lava forms a thick and brittle crust which is torn into rough and jagged pieces. The solidified surface is jagged and sharp. It can spread over large areas as the lava continues to flow underneath.
