If a stratovolcano is the dramatic, steep, dangerous cone, a shield volcano is its opposite — broad, gently sloped, and (despite being capable of producing large eruptions) far less violently explosive. Shield volcanoes are built by the steady flow of low-viscosity lava that spreads out over wide areas before solidifying. The result is a broad, dome-shaped or shield-shaped mountain — much like a Greek warrior's shield lying flat on the ground. The Hawaiian Islands are the most famous shield volcanoes. Mauna Loa on the Big Island of Hawaii is by some measures the largest volcano on Earth — it rises 4,170 metres above sea level and extends about 9 kilometres further down to the ocean floor, with a total volume of approximately 75,000 cubic kilometres of rock. Yet its slopes are gentle — typically 4-8 degrees, easy enough that hiking trails wind up its flanks. Iceland's shield volcanoes (Skjaldbreiður and others) are similar in form. Mount Etna on Sicily, despite often being called a stratovolcano, has shield-volcano characteristics overlaid on a more complex base. Understanding shield volcanoes — what they are, why they look so different from stratovolcanoes, and what their eruptions are like — is fundamental to understanding volcanism more broadly.
The Geometry
Shield volcanoes are broad and gently sloped — typically much wider than they are tall. Mauna Loa is approximately 120 kilometres long, 80 kilometres wide, and only 4 kilometres above sea level (10 kilometres above the seafloor). The ratio of width to height is approximately 30:1 — vastly different from a stratovolcano's 3-5:1 ratio. The slopes are mostly less than 10 degrees, with the steepest sections (near the summit) reaching perhaps 12-15 degrees. The overall shape is like a flattened dome, with the highest point at the summit and the slopes descending gradually in all directions. The summit area typically has one or more calderas — broad depressions where the volcano's upper magma chamber has partially collapsed. Multiple subsidiary cones, fissures, and craters can be present on the flanks.
Why They Are Different from Stratovolcanoes
The key difference between shield volcanoes and stratovolcanoes is the composition and viscosity of the magma they erupt. Shield volcanoes erupt low-viscosity basaltic lava — fluid, with viscosities low enough that the lava can flow for kilometres before cooling. Stratovolcanoes erupt much more viscous andesitic or dacitic magma. The viscosity difference is enormous — basaltic lava is roughly 100,000 times less viscous than typical stratovolcano magma. The low-viscosity basalt of shield volcanoes can flow easily, spreading out into thin sheets across the landscape and building up the gently-sloped shield form. The high-viscosity magma of stratovolcanoes cannot flow far before solidifying, so it builds up into the steep cone form characteristic of those volcanoes. The composition difference reflects the different tectonic settings — shield volcanoes are typically intraplate or at divergent plate boundaries (Iceland, Hawaii, the Mid-Atlantic Ridge), while stratovolcanoes are at convergent boundaries.
How They Erupt
Shield volcano eruptions are typically effusive rather than explosive. Lava flows steadily from vents or fissures, sometimes for months or years at a time. The eruptions are generally not dangerous in the immediate sense — the lava flows slowly enough (typically less than a few km/h) that people and most animals can move out of the way. Property destruction is more common than human deaths. The 2018 eruption of Kīlauea on the Big Island of Hawaii destroyed approximately 700 homes and altered the geometry of the volcano substantially, but caused only one direct fatality. The 2021 eruption of Cumbre Vieja on La Palma in the Canary Islands destroyed 1,600 homes and caused massive economic damage but no direct fatalities. The 1973 eruption of Eldfell on Heimaey in Iceland buried much of the town of Vestmannaeyjar but killed only one person. The slow pace of shield volcano eruptions allows for evacuation and orderly response, in contrast to the sudden explosive violence of stratovolcanoes.
Lava Flows and Their Types
Shield volcano lava typically takes two distinct surface forms. "Pahoehoe" lava is smooth, ropy, and undulating — formed by lava that retains heat and fluidity as it flows. "Aa" lava is rough, blocky, and sharp-edged — formed by lava that has cooled and become more viscous as it flowed. Both names are from Hawaiian, reflecting Hawaii's extensive volcanism and the local terminology developed for describing it. The two types form at different stages of a lava flow's lifetime and at different distances from the vent — pahoehoe typically near the vent (where the lava is hottest and most fluid) and aa typically further down (where the lava has cooled and become more viscous). A single eruption can produce both types in sequence. The morphology of the resulting flows differs substantially — pahoehoe forms thin, undulating sheets; aa forms thick, rubbly flow fronts.
The Hawaiian System
The Hawaiian Islands are the most studied shield volcano system on Earth. They form a chain because they sit over a long-lived mantle plume — a column of unusually hot mantle material that has been producing magma for at least 80 million years. As the Pacific plate moves northwest over the stationary plume, new volcanoes are produced at the leading edge of the plate and older volcanoes become inactive as they move off the plume. The result is the Hawaiian-Emperor seamount chain — extending from the active volcanoes of the Big Island northwestward through the older Hawaiian Islands, through the Northwestern Hawaiian Islands (now small atolls because the volcanoes have eroded), and finally through the Emperor Seamounts (now entirely submerged) ending at the Aleutian Trench. The chain is approximately 6,000 kilometres long. The most recent volcano in the chain is Loihi, a submarine volcano southeast of the Big Island that will eventually emerge above sea level to become the newest Hawaiian island in tens of thousands of years.
The Calderas at the Summit
Shield volcanoes often have calderas at their summits — broad depressions where the upper part of the volcano has subsided into the partially-emptied magma chamber below. These summit calderas can be several kilometres in diameter. Kilauea Caldera on the Big Island is approximately 4 km long and 3 km wide. Mauna Loa's Moku'āweoweo Caldera is similar in size. The Halema'uma'u crater at the summit of Kilauea has contained lava lakes for much of recent history; the lava lake from 2008-2018 was one of the world's largest and most accessible (visitors could view it from observation areas a few hundred metres away). The calderas are dynamic features that subside and refill as the underlying magma chamber expands and contracts. The 2018 eruption of Kilauea was preceded by the collapse of the Halema'uma'u crater floor by hundreds of metres as magma drained from the summit chamber.
Iceland's Shield Volcanoes
Iceland sits at the intersection of the Mid-Atlantic Ridge (a divergent plate boundary) and the Iceland mantle plume. The combination produces extensive volcanism, much of it in the form of shield volcanoes. Skjaldbreiður (literally "broad shield") is the iconic Icelandic shield volcano. The 1783 eruption of Laki — actually a fissure system rather than a single shield volcano — produced one of the most consequential eruptions of the past several centuries, killing approximately 25 percent of Iceland's population from poisonous gases and crop failure, and producing a haze that affected European climate for years. Modern Iceland continues to experience regular shield-volcano-type eruptions, including the 2021 Fagradalsfjall eruption that produced about 150 days of moderate effusive activity.
Mount Etna
Mount Etna in Sicily is sometimes classified as a stratovolcano but has many shield-volcano characteristics. It is the largest volcano in Europe (about 3,330 metres tall) and one of the most active in the world. Its slopes are dominated by frequent basaltic eruptions that build up the volcano's flanks in a way more characteristic of a shield volcano than a typical stratovolcano. Etna's eruptions are typically effusive rather than explosive, though it has produced significant pyroclastic activity at times. The mountain has been continuously active for at least 500,000 years and shows no sign of stopping. The fertile volcanic soils of its lower slopes support extensive agriculture (vineyards, citrus groves), and the volcano itself is a major tourist destination.
Shield Volcanoes Elsewhere
Shield volcanoes occur in many other settings beyond Hawaii and Iceland. The volcanoes of the Galapagos Islands are shield volcanoes formed over the Galapagos hotspot. Réunion in the Indian Ocean has the active Piton de la Fournaise (a shield volcano). The Canary Islands include several shield volcanoes including the Cumbre Vieja volcano that erupted in 2021. The Cape Verde Islands have shield volcanoes. African volcanism includes the shield volcano Erta Ale in Ethiopia. The Tristan da Cunha islands in the South Atlantic are shield volcanoes. Even Mars has shield volcanoes — Olympus Mons, the largest volcano in the solar system at about 24 kilometres tall and 600 kilometres across, is a shield volcano. The Martian shield volcanoes have similar basic morphology to terrestrial ones but are much larger because Mars lacks plate tectonics (so a single volcano can grow over much longer time periods).
Living with Shield Volcanoes
Shield volcanoes, despite their lower direct hazard compared to stratovolcanoes, are not benign. Lava flows can destroy property and infrastructure even if they rarely kill directly. Sulfur dioxide emissions during sustained eruptions can produce serious air quality problems (the "vog" — volcanic smog — that affects Hawaii during major eruptions). The combination of fertile volcanic soils, scenic beauty, and tourism means that shield volcanoes are surrounded by substantial human populations and infrastructure. Hawaii has the worst of both worlds — densely populated areas on the flanks of active volcanoes that can erupt unexpectedly. The 2018 Kilauea eruption forced the evacuation of thousands of residents and reshaped the lower flanks of the volcano with new lava flows. Living on a shield volcano is a different kind of challenge than living on a stratovolcano, but it is a challenge nonetheless. The continued activity of shield volcanoes will continue to shape Hawaiian and Icelandic and Réunion and various other landscapes for as long as the underlying volcanism continues.