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Facts about Volcanoes

How are volcanoes formed?

The Earth is divided into 4 main layers - the crust, the mantle, the outer core and the inner core. The crust rests on top of the mantle, which is much hotter and comprised of heavier elements than the crust. These heavier elements include oxygen, silicon, magnesium, aluminum and iron. The temperature of the mantle ranges from approximately 1000 - 1500 degrees Fahrenheit at its border with the crust to 6500 - 7000 degrees Fahrenheit at its border with the outer core. The elements that the mantle consists of are solid rock, but because the temperature of the mantle is so high, the mantle acts more like a plasma over long periods of time.

The crust, which is comprised of lighter elements than the mantle, is split into plates that 'float' on top of the hot mantle. These plates are referred to as tectonic plates. As gas pressure builds up within the mantle, the seams in the tectonic plates act as a pressure release valve for the Earth, and hot materials from the mantle are vented or ejected to the Earth's surface. As the material is ejected, it cools and can begin to make formations, such as the mountain-like shapes that are called stratovolcanoes.

How does magma form?

Most magma formation occurs in one of three ways. These include subduction, magma that originates in rift zones, or magma that originates in hot spots.

Subduction. Subduction occurs when one tectonic plate that is submerged under the ocean (its top layer is ocean), slides under a tectonic plate covered by land. This process, called subduction, allows water and minerals saturated with water to penetrate down into the mantle. As the water-rich plate continues to subduct down into the hot mantle, liquids are released, and the liquids lower the temperature at which the hot solid rocks in the mantle melt, forming magma. As the rocks melt into magma, the magma becomes less dense and filled with gas bubbles. The magma begins to form cracks and fissures in the surrounding rock, and it begins to rise towards the surface of the Earth, where it may collect into magma chambers.

Types of Volcanoes

Different resources may classify volcanoes into as little as two to three main types or as many as six types. The most common classification system groups them into three main types: stratovolcanoes or composite volcanoes, cinder volcanoes and shield volcanoes. In this classification system, they are defined by their shape and by how they are formed.

Composite Volcanoes. Composite volcanoes have the most well-defined shapes and are formed from past eruptions. They have steep slopes and are cone-like in appearance. The eruptions are generally violent, and spew lava, rock, ash and other debris. The lava is thick, and cools rapidly, forming the steep slopes. As the lava cools, all of the ejected materials combine and cool together on the slopes of the volcano, which is where the term composite comes from. Most famous volcanoes, including Mount Saint Helens and Mount Vesuvius fall into this category because their eruptions the most destructive and powerful, and therefore gain substantial attention.

Shield Volcanoes. These volcanoes are not nearly as violent as composite types and the lava streams out rather than explodes out. The lava is thin and cools over a widespread area, forming wider and shorter shield-like structures. The Hawaiian islands are actually shield volcanoes.

Diagram of a Shield Volcano

Cinder Volcanoes.

Where on Earth are volcanoes found?

As volcanoes are defined as any point where these hot materials are vented to the Earth's surface, the vast majority of volcanoes on Earth are located along the edges of tectonic plates. These are not always in the form of the large cones that are often associated with them.

In fact, over 80% of the volcanoes on Earth are located around an area that is famously called the Ring of Fire or Pacific Ring of Fire.

Landmark Volcanic Eruptions

Mount Vesuvius. In 79 A.D., Mount Vesuvius erupted and destroyed multiple villages including the city of Pompeii and another city called Herculaneum. Oplontis was a smaller village but was also very close to the eruption and destroyed.

Mount St. Helens. Mount St. Helens erupted in 1980 A.D. It was said to have 'blown its top off', and a massive landslide followed the eruption on the north side of the mountain. A large crater is now located at the top where the explosion ejected lava, rock and ash.

Krakatao Volcano. In 1883 a volcano located on the island of Krakatau, which is found between Java and Sumatra in Indonesia, erupted and killed over 36,000 people. Many of the deaths were not directly caused by volcanic activity but by tsunamis that formed as a result of the explosion and struck surrounding islands.

Volcanoes on other planetary bodies

These geological phenomena are not only found on Earth. Utilizing the definition of a volcano, it should be possible to locate them on any planet or moon that has an active core which builds up gas and pressure, and a solid or mostly solid crust through which these elements escape. Planetary scientists are now utilizing evidence of volanic activity, or lack of, to help them identify planetary bodies with or without active inner layers and cores. Some of the planetary bodies within our solar system that have or at one time had volcanic activity include:

  • Jupiter's fifth moon, Io. Io is the only planetary body aside from Earth that scientists have positively identified volcanic activity on, and a lot of it has been observed. Because of its close proximity to Jupiter, the core of Io most likely remains active due to Jupiter's strong gravitational field essentially 'wreaking havoc' on it, as Io orbits the massive planet.
  • Mars. While Mars no longer has volcanic activity, it certainly did in the past. Using satellite imagery, planetary scientists have been able to identify a number of ancient but inactive volcanoes on the red planet's surface. These include Olympus Mons - a massive volcano that is over two times the size of Mount Everest, Alba Mons - a volcano that is approximately one-third the height of Olympus Mons but covers about the same area as the United States, and the Tharsis Montes, a group of three enormous shield volcanoes.

Volcanoes in Literature, Film and Culture

  • In 1990, Tom Hanks and Meg Ryan starred in a romantic comedy called "Joe Versus the Volcano". In the film, Tom Hanks plays a dissatisfied employee who gets tasked to save a wealthy man's island by sacrificing himself to a volcano that the native islanders believe is a deity.
  • Pierce Brosnan starred in a 1997 movie titled "Dante's Peak", in which he plays a professor that believes a volcano is about to erupt but has difficulty convincing the people of a nearby town and evacuating it.
  • There are numerous paintings that have been created in regards to volcanic eruptions. One example titled "The Eruption of Mount Vesuvius", was painted by Pierre-Jacques Volaire in 1777, and is now located at the North Carolina Museum of Art.