Mount Fuji in Japan
Credit: Credit: Photo is from Wikipedia by Morio, CC BY-SA 3.0.

Japan's famous Mount Fuji stratovolcano last erupted in 1707. The eruption was several times the size of the famous 1980 eruption of Mount St. Helens in Washington, USA and covered Tokyo in several centimeters of ash, even though Tokyo is about 60 miles (100 km) away.[6]

Defining terms used in discussing volcanoes

Volcanology is the study of volcanoes and related processes, and an important part of the science of geology, the study of rocks and solid earth.[2] Below are explanations about the several types of volcanoes and other basic volcanology terms. The intent here is to make the basics understandable for anyone.

Volcano type #1: Stratovolcano

Mount Tambora in Indonesia
Credit: Credit: Photo is from Wikipedia by Jialiang Gao, CC BY-SA 3.0.

This is the caldera of Mount Tambora in Indonesia. This volcano is responsible for the largest volcanic eruption of the past 22,000 years. It occurred in 1815 and killed 71,000 people, and it caused North America and Europe to essentially have no summer in 1816.[3] Livestock died and crops failed en masse, creating a famine that killed hundreds of thousands more.[4] Minor eruptions of Tambora have occurred since, most recently in 1967.[3]

Stratovolcanoes are generally tall and conical or triangular in shape.[5] A classic example as seen at the top of this article is Japan’s Mount Fuji. Mount Tambora, pictured above, is also a stratovolcano.

These volcanoes may have eruptions of various types and sizes, although they are known for having very explosive eruptions, like the sudden explosion of a bomb that can dwarf modern nuclear weapons.[5] Famous recent examples of stratovolcanic eruptions are Mount St. Helens in Washington, USA in 1980, and Mount Pinatubo in the Philippines in 1991.

Stratovolcanoes often have a caldera or crater on top, two terms which are defined further below. They are also called composite volcanoes.[5]

Volcano type #2: Shield volcano

Big Island of Hawaii
Credit: Public domain image courtesy of NOAA.

This is a true color image of the island of Hawaii, often called the Big Island. The large dark area in the southern half of the island is Mauna Loa, which makes up a majority of the island and is the largest active volcano on Earth. It is a shield volcano that rises from the ocean floor more than 30,000 feet (9,000 meters) to its summit.[10]

Shield volcanoes, in contrast with stratovolcanoes, are usually much more broad, with gentler slopes. Although some are tall, they do not reach the heights above sea level of the tallest stratovolcanoes.[7]

The eruptions from shield volcanoes are typically less explosive, with lava spilling, oozing, or bubbling out of the Earth and flowing down the side of the mountain.[7] The best example on Earth of a shield volcano is Mauna Loa, pictured above, which comprises most of the Big Island of Hawaii. The island is actually five shield volcanoes, although most of it is Mauna Loa, which is the largest active volcano, in physical size, on Earth.[8]

Shield volcanoes may have calderas or craters. Many have associated cinder cones,[7] and some have stratovolcanoes on them, such as Mount Teide in the Canary Islands, which is the world's second-largest active volcano after Mauna Loa.[9]

Volcano type #3: Cinder cone

Paricutin cinder cone
Credit: Public domain photo courtesy of USGS.

Paricutin is a cinder cone in Mexico, located 200 miles (320 km) west of Mexico City. In 1943 it suddenly rose out of a corn field and buried two villages in lava and ash, and kept erupting until it stopped in 1952. It is considered extinct and not expected to erupt again. Some volcanoes, after they erupt once, are done and never erupt ever again. The volcanic field from which this cinder cone arose, however, is still active and will erupt again.[1]

Cinder cones often have a shape similar to stratovolcanoes, although on a smaller scale. These volcanoes are most often associated with, or part of, a larger stratovolcano or shield volcano. They may form on the flanks or somewhere nearby, connected to the same magma source. They are composed of loose, fragmented material.[11]

Eruptions from cinder cones may splatter and sputter, or may ooze and flow, although on a smaller scale than what the larger volcanoes are capable of.[11] Good examples of cinder cones are Mexico’s Paricutin (first photo above) and California’s Pisgah Crater.

Volcano type #4: Lava dome

Lassen Peak in Northern California
Credit: Credit: Photo is from Wikipedia by Daniel Schwen, CC BY-SA 2.5.

Northern California's Lassen Peak is the southernmost peak in the Cascade Mountain Range, and it is the world's largest lava dome.[14][13] Along with Washington's Mount St. Helens, it is one of two volcanoes to erupt during the 20th century within the contiguous 48 states. Lassen erupted in 1915, and today is closely watched. It is the main feature of Lassen Volcanic National Park.[14]

Lava domes are generally round in shape, and most often form on a stratovolcano, often within a crater or caldera. Sometimes however, they exist independently.[12]

Lava domes are unpredictable in how they will form, grow, and possibly break apart or erupt. Eruptions are often explosive, as seen with stratovolcanoes. The lava domes may grow internally, with new magma pushing the outer layers further outward, or may grow through magma seeping through and adding layers on the outside.[12]

Good examples of lava domes are that which now sits in the crater of Mount St. Helens in Washington, USA, and massive Lassen Peak in California, USA. Lassen Peak is the largest lava dome on Earth (pictured above).[13]

Volcano type #5: Supervolcano

Steamboat Geyser in Yellowstone National Park
Credit: Credit: Photo is from Wikipedia by Hsing-Mei Wu, CC BY-SA 3.0.

Yellowstone National Park sits over the largest magma chamber on Earth, which powers numerous hot springs and geysers. There is enough magma to fill the Grand Canyon at least eleven times![21] The park has about half of the world's 1,000 or so geysers, all of which are located within the Yellowstone Supervolcano's caldera. In the past this volcano covered much of North America in thick ash.[16] The photo is Steamboat Geyser.

Very different from the four volcano types described above, supervolcanoes can be considered a fifth type of volcano, but are so different that some geologists call them something fundamentally different from volcanoes.

The three largest supervolcanoes on Earth that could have supervolcanic eruptions again someday are Taupo in New Zealand, Toba in Indonesia, and Yellowstone in the USA. The last time any of them erupted was Taupo 26,500 years ago.[15]

Supervolcanic eruptions are one of the two largest possible natural disasters, the other being asteroid or comet impacts. They spew out far more lava and ash than regular volcanoes, with a minimum of about seven times as much as what Mount Tambora did in 1815.[15]

Supervolcanoes have massive magma chambers that slowly fill as magma rises from beneath, and as magma is less dense than the surrounding solid rock, these chambers are buoyant and push upward. Eventually the earth above just splits open and a massive eruption takes place.[15]

Bardarbunga Volcano in Iceland
Credit: Credit: From Wikipedia by peterhartree, CC BY-SA 2.0.

This is the Bardarbunga stratovolcano in Iceland, photo taken September 4th, 2014.

Other important volcanology terms

This page is meant to share basics about volcanoes, so many terms relating to volcanology won’t be here – such as the different types of lava, and some other more technical information. 

Active volcano – Any volcano that has erupted within the past 10,000 years and is expected to erupt again someday. This is a short amount of time, geologically speaking.[20]

Ash – Volcanic ash is tiny particles, less than 2 millimeters in size, that can erupt out of volcanoes when more viscous magma explodes. The particles are like tiny shards of glass.[18]

Crater vs caldera – The two can look similar, but form differently. Craters form when a portion of earth or of a mountain is literally blasted away by an eruption. Calderas form when a magma chamber beneath the ground, or beneath a mountain, empties and the earth above collapses.[18]

Dormant volcano – Any volcano that has not erupted within the past 10,000 years, but is thought to have potential for future eruptions.[20]

Ejecta – Material that has been explosively ejected (erupted) out of a volcano.[22]

Eruption – When any solid, liquid, or gas materials are expelled out of a volcano and into the air, or onto the ground.[18]

Extinct volcano – Any volcano that is thought to have zero chance of erupting ever again.[20]

Fumarole – A volcanic vent out of which gases escape to the atmosphere.[22]

Geyser – A hot spring heated by volcanic forces, characterized by water periodically being ejected out of the ground along with steam. Similar to a hydrothermal explosion, but does not blow away earth. The boiling water periodically erupts out to relieve pressure.[19]

Hydrothermal explosion – When water beneath the ground is heated by magma it can boil and explode, blowing away a portion of earth, and often forming a crater.[18]

Lahar – A flow of mud or wet debris caused by volcanic activity, which may or may not be mixed with lava.[18]

Lava vs magma – Magma is molten or semi-molten rock that is still within the Earth. Lava is magma that has been erupted out and is in a liquid state, and it is still called lava after it cools and hardens.[18]

Maar – A broad crater with low relief caused by groundwater, heated by magma, boiling and then creating an explosion. Maars typically fill with lakes.[22]

Pyroclastic flow – A mixture of volcanic gases, rock fragments, and ash that travels away from the volcano rapidly (up to several hundred miles per hour). They occur with some explosive eruptions.[22]

Tephra – Any type or size of rock fragments that were erupted out of a volcano and traveled through the air before landing on the ground. This includes ash.[22]

Vent – Any opening in the Earth, out of which magma is erupted or volcanic gases are emitted. [22]

Devil Mountain Lakes in Alaska
Credit: CC BY-SA 4.0.

The Devil Mountain Lakes on Alaska's Seward Peninsula are the largest maar-based lakes on Earth.[23]