A Guide to Volcanoes

In 1991 Mt. Pinatubo, a formerly docile peak in the Philippine island of Luzon, erupted (see the PBS Nova article described below). The volcano spewed ash 16 kilometers into the air, caused over 500 deaths, and forced the evacuation of over 50,000 residents. Furthermore, the world temperature dropped by one degree Celsius thanks to the large amount of debris in the atmosphere. Although Pinatubo was one of the most powerful eruptions of the 20th century, it was by no means the largest or deadliest volcano to affect human civilization, and there is good reason to believe that far more catastrophic eruptions lay in wait. According to data taken from the Smithsonian Global Volcanism Program, there are between 50 to 70 volcanic eruptions each year and around 20 in the process of erupting right now.

Compiled here are some of the most informative websites about, and databases of, volcanoes. Although the sections below do provide some introductory information about the topic, including the classification, internal structure, seismic processes, and key examples of volcanoes, links to mostly university, K-12, and government sources are provided throughout. This information will be most helpful to students or any lay audience that is curious about the science behind volcanoes, although researchers can also access scientific data via the U.S. Geological Survey and the Smithsonian’s Global Volcanism Program.

The Structure of Volcanoes

The fuel of an eruption (see illustration) can be found beneath the mountain itself in the magma chamber (1), a pool of molten rock that lies under the crust. This magma rises through the central vent, also known as a “pipe” or “conduit” (3), although secondary branches (6) are common, which may come out through fissures (vents) or “parasitic cones” (11) that are somewhat removed from the main throat (10). The place where the eruption occurs is the crater (14) or caldera. Eruptions are often responsible for the creation of mountains themselves, which may be composed of alternating layers of ash (7) and hardened lava (9).

Besides the diagram above, provided by MesserWoland, the Australian Museum offers an annotated 3-D chart of a volcano’s interior in PDF format, and an accompanying article that goes into more detail. More in-depth information can be found in the K-12 website, Volcanoes Online, which is a part of ThinkQuest. This site provides a somewhat more simplified description of a volcano’s structure, but the site also goes into the detail about the different kinds of volcanoes, the three main ones being:

  • Composite volcanoes, which are alternately known as stratovolcanoes. These are seemingly inert mountains that are characterized by the high, pyramidal peak common to the tallest mountains and are comprised of recognizable strata of lava and rock.
  • Cinder cones, which are distinguished by their conical shape. Cinder cones are formed by modest eruptions, often in close proximity to composite volcanoes.
  • Shield volcanoes, which often have a gradual slope. They usually cover a wide area and built up from numerous lava flows.

More details can be found at the USGS’s Principal Types of Volcanoes page.

The Volcanic Process

Volcanoes and earthquakes are closely related, as they are both caused by plate tectonics. The Earth’s crust is made of continent-sized plates that rest on an ocean of viscous rock, the mantle. The mantle is prone to melting in certain places due to high pressue, forming magma that can reach up through weak spots in the crust called fissures, especially at the faults where two plates meet. With enough pressure and heat, this magma (and its accompanying gas and ash) can puncture the crust in a massive explosion: an eruption. Once magma reaches the surface of the Earth, it is called lava. This information comes from the San Diego State University’s page, How Volcanoes Work, which provides a far more exhaustive explanation of the process by using numerous examples. Also consulted was the simpler, more essential introduction to volcanology, Interactives: Volcanoes, from Learner.org.

Famous Active Volcanoes

The following are some of the deadliest and most powerful volcanoes on record that are still threatening to erupt today. A separate page is devoted to each example:

  • Vesusvius, located in Italy, has erupted several times over the centuries, the most infamous explosions occurring in 79 A.D. when it buried the Roman city of Pompeii under a layer of ash (leaving it largely preserved for archaeologists) and in 1631 when it killed around 4,000 residents of nearby communities. The preceding link leads to a page by the Wheeling Jesuit University and NASA.
  • Krakatoa’s eruption in 1883 (in Indonesia) was responsible for over 30,000 deaths and is largely considered one of the largest explosions on Earth. More information is available via the NPR news story about the volcano that is forming in its place, Krakatoa: The Son Also Rises.
  • St. Helens’s eruption in 1980 was caused by a 5.1 earthquake in Washington state. The resulting explosion threw debris 15 miles into the air and buried nearby towns under a layer of ash. This information comes from the USDA Forest Service’s Mount St. Helens Monument site.
  • Eyjafjallajokull, located underneath a glacier in Iceland, erupted in 2010 after a series of thousands of earthquakes. The ash spewed into the atmosphere required the closure of airports throughout Europe for nearly a week. Close monitoring of the volcano is conducted by the Institute of Earth Sciences’ Nordic Volcanological Center.
  • Pinatubo, which lay dormant for 600 years, erupted in 1991 and was several times more powerful than St. Helen’s. The PBS NOVA documentary about the event is available for download.

Time Magazine also maintains a list of the Top 10 Famous Volcanoes while the Smithsonian’s Global Volcanism Program tracks active volcanoes around the world, providing data for the Google Maps API Active Volcanoes site. This interactive tool allows visitors to filter the results by numerous criteria: type of volcano (cinder cone, shield, etc.), region, associated earthquakes, and more. Finally, the New York Times’s Volcanoes section provides recent news stories about volcanoes. The site also archives hundreds of articles about the subject from 1876 onward.

General Resources

Most information about the inner-workings of volcanoes, their structure, and the most dramatic examples is available on websites that address volcanoes in general. For example, Oregon State University’s Volcano World and National Geographic’s Nature’s Fury: Volcanoes provide some background about the phenomena for educational purposes. The U.S. Geological Survey’s Volcano Hazards Program and the National Geophysical Data Center: Volcano Data and Information provide up-to-date scientific information that may be of interest to scientists and researchers. Finally, Volcanoes.com specializes in striking photos and video presentations, and the University of California at Berkeley’s Plate Tectonics website delves into detail about the mechanics behind both volcanoes and earthquakes.

Image is from MesserWoland and found on Wikimedia Commons where more information about the labels is available.