Throughout the course of Earth’s history, there have been some pretty major geological events. From the Ice Age to earthquakes that helped to reshape the land, the Earth has been no stranger to major land events. Another aspect of geological events is volcanic eruptions. Whether we’re talking about Mount Vesuvius destroying Pompeii or the eruption of Mount St. Helens in Washington state, volcanoes can be dangerous when they’re active. But why do volcanoes form, and what causes them to erupt?

A geology crash course

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Before we explain how volcanoes erupt, it’s time to get a simplified geology lesson. Earth is composed of four main layers: the crust, mantle, outer core, and the inner core. For the purpose of explaining volcanic activity, the crust and mantle are the two most important layers. We live on the Earth’s crust, and in comparison to the other layers, it is incredibly thin with a maximum depth of five miles. The continents and plates exist on this layer.

The mantle is the thickest Earth layer at 1,800 miles deep. Because the mantle is so dense, temperatures can vary widely in this layer. Close to the crust, the temperature reaches “only” 1,600 degrees Fahrenheit, but closer to the core, that temperature can soar to 4,000 degrees! However, the mantle isn’t static. Even though it’s made of rock, the mantle moves slowly. Geologists believe this is because of convection currents that cause the hotter materials to rise within this layer and displace cooler rock that sinks to the bottom. As the mantle moves, so do the plates on the Earth’s crust.

So, what is a volcano anyway?

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The short answer is that a volcano is a vent in the earth’s crust that can occur above or below water. In the Earth’s mantle, molten rock or magma is constantly moving. Because the magma is lighter than the surrounding solid rock, it rises towards the Earth’s surface. When magma reaches a vent in the crust, this superheated material escapes and creates a volcano.

However, it’s important to note that volcanoes are not unique to the Earth. They have been found on other planets and moons. While Venus and Mars both have extinct volcanoes all over their surface, the moons of Jupiter, Saturn, and Neptune are all home to active volcanoes.

Magma versus lava

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You already know that magma is just melted or molten rock. So, what about lava? Lava is literally magma that has escaped the Earth’s surface.

Types of volcanic eruptions

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While explosive volcanoes get the most news attention, a volcanic eruption can also be a calm or steady stream of escaping magma. The explosiveness of an eruption is entirely dependent on the magma’s texture. Runny, thinner magma allows for the gases within it to easily escape, creating lava flows rather than eruptions. A perfect example of this type of volcanic eruption is in Hawaii. These eruptions don’t usually pose physical danger because the lava escapes slowly enough for people to easily avoid it.

In contrast, thick or sticky magma doesn’t allow for gas to easily escape. This means that pressure continues to build within magma tubes until it becomes unsustainable and an explosion occurs at a vent. These types of eruptions are the most dangerous because of the blast clouds known as tephra. Tephra is magma that is released into the air and falls as ash and other particles. Explosive volcanoes can trigger mudflows, destroy communities, and damage surrounding ecosystems.

Active, dormant, or extinct?

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Now you know what a volcano is and what causes them. But there are a few different classifications for them: active, dormant, and extinct. The biggest difference between these categories is their activity and when activity was last recorded.

  • An active volcano has had activity within the last 10,000 years or since the      last Ice Age. Technically, an active volcano could be in the process of erupting or be dormant.
  • An erupting volcano is—obviously—one that is in the process of experiencing volcanic activity and is actively releasing lava.
  • A dormant volcano is classified as active but isn’t currently erupting. These types of volcanoes have the potential to erupt within the next 10,000 years.
  • An extinct volcano is one that hasn’t recorded an eruption in at least 10,000    years with no predictions of potential activity within the next 10,000 years. However, this classification can be misleading as events have been recorded from volcanoes that were considered extinct.

Volcanoes are complex geological events that are dependent upon a number of factors that we couldn’t fully cover in this article. But even if you’re not a geologist, Oregon State University’s Volcano World website is a great tool for gaining understanding of the mechanisms behind volcanoes and their worldwide frequency.