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examBoard: Cambridge
examType: IGCSE
lessonTitle: Volcano Types and Classification
Volcanoes are one of Earth's most spectacular and dangerous natural features. They come in different shapes and sizes and understanding how they're classified helps us predict their behaviour and manage the risks they pose to people living nearby.
Key Definitions:
Volcanoes typically form at plate boundaries where one tectonic plate moves under another (subduction zones) or where plates move apart (divergent boundaries). They can also form at hotspots, where plumes of magma rise from deep within the Earth's mantle, such as the Hawaiian Islands.
The type of volcano that forms depends on several factors, including the magma's viscosity (thickness), gas content and eruption style. These factors are influenced by the tectonic setting and the chemical composition of the magma.
Volcanoes are primarily classified by their shape, which is determined by the type of material they erupt and how it builds up over time.
Shape: Broad, gently sloping sides
Magma: Low viscosity (runny) basaltic lava
Example: Mauna Loa, Hawaii
Shield volcanoes form when fluid lava flows easily and spreads widely before cooling. They're named for their shape, which resembles a warrior's shield laid on the ground.
Shape: Steep, symmetrical sides
Magma: Medium to high viscosity
Example: Mount Fuji, Japan
Also called stratovolcanoes, these are built up of alternating layers of lava flows, ash and other volcanic debris. They're typically found at subduction zones.
Shape: Steep-sided, rounded
Magma: Very high viscosity
Example: Lassen Peak, USA
Dome volcanoes form when thick, sticky lava piles up around the vent instead of flowing away. They can be extremely explosive.
These are the simplest type of volcano, formed from eruptions of small fragments of lava that cool before hitting the ground. They typically have:
Example: Paricutin in Mexico, which famously grew in a farmer's field in 1943.
These are large, basin-shaped depressions formed when a volcano's magma chamber empties and the overlying land collapses. Features include:
Example: Crater Lake in Oregon, USA, which formed after Mount Mazama's explosive eruption about 7,700 years ago.
Volcanoes are also classified by how they erupt, which is largely determined by the viscosity of the magma and its gas content.
These eruptions involve the relatively gentle outpouring of fluid lava. Characteristics include:
Examples: Hawaiian volcanoes like Kilauea
These violent eruptions occur when thick, gas-rich magma reaches the surface. Features include:
Examples: Mount St. Helens (1980), Mount Vesuvius (79 CE)
Scientists use the Volcanic Explosivity Index to measure and compare the size and power of volcanic eruptions. It's similar to the Richter scale for earthquakes.
| VEI | Description | Plume Height | Volume of Ejected Material | Example |
|---|---|---|---|---|
| 0-1 | Non-explosive to gentle | <1 km | <10,000 m³ | Kilauea, Hawaii |
| 2 | Explosive | 1-5 km | 1 million m³ | Galeras, Colombia (1993) |
| 3 | Severe | 3-15 km | 10 million m³ | Nevado del Ruiz, Colombia (1985) |
| 4 | Cataclysmic | 10-25 km | 100 million m³ | Eyjafjallajökull, Iceland (2010) |
| 5 | Paroxysmal | >25 km | 1 billion m³ | Mount St. Helens, USA (1980) |
| 6 | Colossal | >25 km | 10 billion m³ | Krakatoa, Indonesia (1883) |
| 7 | Super-colossal | >25 km | 100 billion m³ | Tambora, Indonesia (1815) |
| 8 | Mega-colossal | >25 km | >1 trillion m³ | Yellowstone (640,000 years ago) |
Mount Pinatubo in the Philippines erupted in June 1991 after being dormant for 500 years. It was one of the largest eruptions of the 20th century.
Understanding volcano types helps scientists assess the risks they pose:
The type of volcano influences:
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