Volcanoes 101 | National Geographic
Portals into the heart of the Earth, they burn bottomless cauldrons fueled by an ancient rat, bubbling and boiling thousands of miles beneath the surface and just waiting to burst through. Volcanoes are scattered across the globe; volcanoes can be found across the world, though countless others are on the ocean floor. Most volcanoes, whether on land or underwater, are located where tectonic plates meet.
In fact, the Ring of Fire, a path that traces the boundaries between several tectonic plates around the Pacific Ocean, contains about 75 percent of the planet's volcanoes. There are several types of volcanoes, primarily classified by shape and size. Major types include stratovolcanoes, which often appear as tall, steep mountains; shield volcanoes, which are flatter and dome-shaped; calderas, which are large depressions in the ground; and mid-ocean ridges, which are underwater chains of volcanic mountains.
No matter their shape or size, all volcanoes emit gas and molten rock. The journey of these emissions begins deep underground in the Earth's core, which can burn as hot as the surface of the Sun. It transfers its heat to the surrounding rocky mantle. In doing so, the heat melts some of the rock. This molten rock, or magma, is lighter than the surrounding solid rock layer, so it rises through the mantle.
The magma then escapes through vents in the Earth's crust, causing volcanic eruptions. Now, above ground, this magma is referred to as lava, and it can reach temperatures of over 2000 degrees Fahrenheit. In addition to lava, volcanoes may erupt with searing hot gas formed in the mantle in a phenomenon called pyroclastic flow. This gas, combined with hot ash, can race down the sides of a volcano as fast as 100 miles per hour, burning everything in its path.
To measure and classify eruptions, scientists develop the volcanic explosivity index, or VEI. It considers various factors such as the volume of lava, gas, and other emissions from the volcano, along with the height of the eruption cloud above the volcano summit. The VEI scale begins at zero; excessive measurement increases logarithmically, meaning that each magnitude is 10 times more powerful than the one before it.
The VEI scale does not have an upper limit, but the most catastrophic eruptions measured thus far are categorized as VEI 8. These eruptions occurred thousands and millions of years ago. The most destructive volcanic eruption ever witnessed occurred in Indonesia in 1815. Mount Tambora, a large strata volcano, erupted with a VEI measurement of seven. The blast caused earthquakes, tsunamis, and pyroclastic flows that decimated the land and took tens of thousands of Indonesian lives.
The eruption even destroyed the top of Mount Tambora itself, turning the 13,000-foot tall mountain into a 3,640-foot deep caldera. While volcanoes are some of the most destructive forces of nature, they have also helped make life on Earth possible. Volcanic ash provides nutrients to nearby soil, making the land fertile, and lava, when it cools, hardens into rock and creates new landforms.
With heat from the heart of the Earth, volcanoes have helped terraform the planet, making it the rich, dynamic landscape we see today.