Types of Volcano
Not all volcanoes look the same. The shape of a volcano depends on the kind of magma it produces and how it erupts. Geologists usually recognise four main types: shield volcanoes (wide and gentle), stratovolcanoes (tall and explosive), cinder cones (small and steep) and supervolcanoes (enormous but rare). Each type has its own typical eruption style, its own danger level and its own famous examples around the world.
- Main types4Shield, stratovolcano, cinder cone, supervolcano
- WidestShield volcanoesLike Mauna Loa, can be 100+ km wide
- TallestStratovolcanoesLike Mount Fuji
- SmallestCinder conesOften under 300 m tall
- Most dangerousStratovolcanoes and supervolcanoesExplosive eruptions
- RarestSupervolcanoesErupt every tens to hundreds of thousands of years
Shield volcanoes
Shield volcanoes are wide, gently sloping mountains built up from many flows of runny, low-viscosity lava. They are usually fed by basalt magma which flows easily and can travel long distances before cooling. Eruptions are typically gentle: spectacular but not very explosive. Lava flows can be watched from a safe distance, which is why they have become tourist attractions.
Examples: Mauna Loa and Mauna Kea in Hawaii. Mauna Loa is one of the largest volcanoes on Earth, covering over half of the island of Hawaii. Other shield volcanoes are found in Iceland and in the Galapagos Islands.
Stratovolcanoes (composite volcanoes)
Stratovolcanoes are tall, steep, classic cone-shaped mountains. They are built up from alternating layers of lava and volcanic ash. The magma is thicker and stickier than in shield volcanoes, often built up over centuries until pressure forces a violent explosive eruption. Stratovolcanoes can throw ash kilometres into the sky and produce dangerous pyroclastic flows: superheated avalanches of ash and rock.
Examples: Mount Vesuvius (Italy, destroyed Pompeii in 79 AD), Mount Fuji (Japan), Mount St Helens (USA), Mount Pinatubo (Philippines), Krakatoa (Indonesia). Most of the famous volcanic disasters in human history were caused by stratovolcanoes.
Cinder cones
Cinder cones are small, steep cone-shaped volcanoes built from blobs of lava (called cinders or tephra) that fall to the ground around a single eruption vent. Most cinder cones are less than 300 m tall and form quickly, sometimes during a single eruption that lasts months or years.
One of the most famous cinder cones is Paricutin in Mexico, which formed dramatically between 1943 and 1952. It started in a farmer's cornfield and grew to over 400 metres tall before activity stopped. Watching a brand new volcano form provided some of the most important volcanic observations of the 20th century.
Supervolcanoes
Supervolcanoes are enormous, rarely active volcanoes capable of producing eruptions thousands of times bigger than ordinary ones. Instead of a classic cone, a supervolcano usually forms a vast caldera: a depression in the ground caused by the collapse of an underground magma chamber after an enormous eruption.
The two most famous are Yellowstone (USA) and Toba (Indonesia). Yellowstone's last super-eruption was 640,000 years ago. Toba's last super-eruption was approximately 74,000 years ago and is thought to have caused a global volcanic winter that may have come close to wiping out early humans. Supervolcanoes erupt extremely rarely (perhaps once every 50,000 to 500,000 years), but when they do, the consequences are continental in scale.
Other special types
- Lava domes: small mounds of thick lava that pile up above a vent. Common inside the craters of larger volcanoes.
- Fissure vents: long cracks rather than central vents. Common in Iceland.
- Submarine volcanoes: underwater volcanoes that build up the ocean floor. Most volcanic activity on Earth is actually underwater.
- Mud volcanoes: not true volcanoes; they release mud and gas rather than magma. Common in Azerbaijan and Italy.
Deeper dive: how the same hot spot built every Hawaiian island in turn
The Hawaiian island chain is one of the most famous examples of volcanic geography in the world. All the Hawaiian islands are shield volcanoes, but they were not made all at once: they were built one by one by the same underground hot spot.
The hot spot itself is a stationary plume of unusually hot mantle rock that has been rising under the Pacific Plate for tens of millions of years. As the Pacific Plate slowly drifts northwest at approximately 7 to 10 cm per year, the hot spot stays fixed, but the surface above it moves. Each part of the plate that passes over the hot spot gets a new volcano. Once the plate moves on, the old volcano cools and slowly erodes.
The result is a long chain of progressively older volcanoes stretching northwest from the youngest island (the Big Island of Hawaii, where Mauna Loa and Mauna Kea are still active) all the way to Kauai (approximately 5 million years old) and beyond that into the Emperor Seamount Chain, where ancient extinct volcanoes are now submerged and worn down to underwater stumps. The very oldest extends 6,000 km away towards Russia.
Right now, the next Hawaiian volcano is Kamaehuakanaloa (formerly Loihi), a young underwater shield volcano about 35 km off the south coast of the Big Island. It is still 1,000 metres below the sea surface, but it is growing. In another 10,000 to 100,000 years, it will probably rise above the waves and become the next Hawaiian island. The hot spot keeps right on producing.
For more, see the Ring of Fire and volcanic eruptions.