Volcano Types: Shield vs Composite Volcanoes

A volcano is an opening in Earth’s crust through which magma (molten rock below the surface) erupts as lava, ash and gases; shield volcanoes are broad with gentle slopes built by runny basaltic lava and mostly gentle eruptions, while composite (stratovolcanoes) are steep-sided cones built by alternating layers of lava and ash and are prone to violent explosive eruptions. In brief: the type of volcano depends on the viscosity and gas content of the magma, which depends on its silica content and the tectonic setting. Shield volcanoes (such as Mauna Loa) form at hot spots and divergent boundaries; composite volcanoes (such as Mount Fuji, Mount St. Helens and Vesuvius) form at subduction zones. Other types are cinder cones, calderas and lava domes.

Volcanoes create new land and fertile soil, but also some of the most destructive hazards. This guide explains magma and lava, the two main types of volcano, the kinds of eruption, compares the types in a table, works through calculations of slope, volume and flow timing, outlines the history of volcanology and corrects common mistakes.

What are volcanoes, magma and lava?

Magma is molten rock with dissolved gases beneath the surface; lava is magma that has reached the surface. Magma forms where the mantle melts: at divergent boundaries (decompression melting), at subduction zones (water from the sinking plate lowers the melting point) and at hot spots (plumes of hot rock). Basaltic magma is hot (1,100–1,200 °C), low in silica and runny; rhyolitic magma is cooler, rich in silica and very viscous (sticky); andesitic magma is intermediate. The more viscous the magma and the more gas it contains, the more explosive the eruption. Volcanic products include lava flows, ash, pumice, volcanic bombs, gases (water vapor, carbon dioxide, sulfur dioxide) and pyroclastic flows (fast avalanches of hot gas, ash and rock). A crater is the bowl-shaped opening at the top, and a caldera is a much larger depression made by the collapse of the ground after a big eruption empties the magma chamber. Volcanoes are classified as active, dormant (sleeping) or extinct, though the boundaries are not sharp.

What are shield volcanoes?

Shield volcanoes are built by many thin flows of runny basaltic lava that spread far, forming a wide, gently sloping dome shaped like a warrior’s shield; slopes are typically 2–10°. Eruptions are mostly effusive (lava fountains and flows), rarely explosive, and are rarely very dangerous to life because the lava moves slowly, although it can destroy property. They form over hot spots (Hawaii) or at divergent boundaries (Iceland). Mauna Loa in Hawaii is the largest active volcano on Earth by volume, and with the sea floor on which it rests it is one of the largest mountains. Olympus Mons on Mars is the largest known shield volcano in the solar system. Related features are lava tubes, lava lakes and fissure eruptions.

What are composite volcanoes?

Composite volcanoes (stratovolcanoes) are steep, symmetrical cones, with slopes of about 30–35° near the summit, built of alternating layers of viscous lava and ash. The magma is andesitic to rhyolitic, rich in silica and gas, and it is formed above subduction zones. Because the magma is sticky, gas builds up under pressure, and eruptions are often explosive, producing ash columns tens of kilometers high, pyroclastic flows, lahars (mudflows of volcanic debris and water) and ash falls. Examples are Mount Fuji, Mount St. Helens (1980), Mount Pinatubo (1991), Vesuvius (79 CE, which buried Pompeii and Herculaneum), Krakatoa (1883) and Mount Etna (frequent activity). The explosivity of an eruption is described by the Volcanic Explosivity Index (VEI), 0 to 8, and each step represents roughly a tenfold increase in the volume of material erupted.

What kinds of eruption are there?

Eruption styles are named after volcanoes: Hawaiian (runny lava fountains and flows), Strombolian (regular moderate explosions with bombs), Vulcanian (short, violent explosions), Plinian (huge, sustained ash columns, as at Vesuvius in 79 CE) and Peléan (pyroclastic flows from collapsing domes). Cinder cones are small, steep cones of loose cinders thrown out of a vent, such as Paricutín in Mexico, which grew from a cornfield in 1943. Supervolcanoes are volcanoes capable of eruptions of VEI 8 (more than 1,000 km³ of material), such as Yellowstone and Toba, which are very rare.

Volcano types compared

Worked example: slope, volume and flows

1. Shield slope (hypothetical volcano): a shield rises 3,000 m over a horizontal distance of 60 km (60,000 m). The slope angle is arctan(3,000 ÷ 60,000) = arctan(0.05) = 2.9°.
2. Composite slope: a cone rises 2,000 m over a horizontal distance of 4 km (4,000 m): arctan(2,000 ÷ 4,000) = arctan(0.5) = 26.6°, much steeper.
3. Volume of the cones: a cone has a volume of one third of the base area times the height. For the composite cone with a base radius of 4 km and a height of 2 km: ⅓ × π × 4² × 2 = 33.5 km³. For the shield with a radius of 60 km and a height of 3 km: ⅓ × π × 60² × 3 = 11,310 km³, which is 11,310 ÷ 33.5 = about 340 times as much.
4. Lava flow timing: a basalt flow advances at 1 km/h. A village 5 km away is reached in 5 ÷ 1 = 5 hours, which gives time for evacuation.
5. Pyroclastic flow timing: a pyroclastic flow moving at 100 km/h reaches a town 10 km away in 10 ÷ 100 = 0.1 h = 6 minutes. Speeds of these flows range from about 100 to several hundred km/h, which is why they cannot be outrun and evacuation must be done before the eruption.
6. VEI: a VEI 5 eruption produces roughly 10² = 100 times the volume of ejecta of a VEI 3 eruption (about 1 km³ against 0.01 km³).

How has volcanology developed?

Pliny the Younger left a detailed account of the eruption of Vesuvius in 79 CE, in which his uncle Pliny the Elder died, and this gave the name to Plinian eruptions. Scientific volcano observatories were started in the 19th century (Vesuvius, 1841) and in Hawaii in 1912. The 1980 eruption of Mount St. Helens, with a sideways blast, was closely studied and improved the understanding of eruptions. The 1991 eruption of Pinatubo in the Philippines was successfully forecast, and the evacuation of tens of thousands of people saved many lives. The 1815 eruption of Tambora in Indonesia, with a VEI of 7, was followed by the “year without a summer” in 1816.

Which mistakes are common?

Mistake 1: Thinking that all volcanoes are explosive cones

Shield volcanoes are broad and erupt gently. Explosive eruptions are typical of composite volcanoes.

Mistake 2: Believing that lava is the main killer

Pyroclastic flows, ash fall, lahars and gases have caused most deaths. Lava flows are usually slow.

Mistake 3: Confusing magma and lava

Magma is underground. Lava is magma that has reached the surface.

Mistake 4: Assuming that a dormant volcano will never erupt again

Dormant volcanoes can reawaken after hundreds or thousands of years, as Mount Pinatubo did in 1991 after about 500 years of rest.