Introduction
No two volcanoes look exactly alike — and this variety is not random. The shape of a volcanic edifice is a direct physical consequence of eruption style, magma composition, and the balance between construction (lava and tephra added) and destruction (erosion, collapse). A gentle, broad shield volcano with slopes of 2–5° broadcasts its nature immediately: low-viscosity basaltic magma, effusive eruptions, a long history of lava flows accumulating at low angles. A steep-sided stratovolcano rising to 3,000 metres at a 30° angle says something completely different: high-viscosity andesitic or dacitic magma, alternating explosive and effusive eruptions, alternating layers of lava and pyroclastic material that build a composite cone over thousands of years.
The diversity of volcanic landforms also reflects the diversity of volcanic settings. Mid-ocean ridges produce pillow basalts — the most abundant volcanic rock on Earth — by submarine effusive eruption. Hotspot systems beneath oceanic plates build enormous shield volcanoes that can grow to heights of 10,000 metres from the seafloor (Mauna Loa is taller than Mt. Everest if measured from its base on the ocean floor). Subduction zone arcs produce the classic stratovolcanoes (composite cones) that most people picture when they think of volcanoes: Fuji, Rainier, Shasta, Merapi, Pinatubo, Popocatépetl. Continental hotspot systems produce the most dangerous volcanic landforms of all: calderas — collapsed craters formed by the catastrophic emptying of a shallow magma reservoir during a supervolcanic eruption.
Understanding volcanic landform types is one of the first skills a volcanologist develops — not just because it allows classification, but because each edifice type predicts the hazards it poses, the eruption style it is capable of, and the monitoring approach that is most relevant. A shield volcano monitored for lava flow paths requires different attention than a caldera system monitored for the first signs of magma chamber reactivation.
Key Terms
A broad, gently sloping volcanic edifice (slopes 2–8°) built by the accumulation of highly fluid basaltic lava flows. Named for resemblance to a warrior's shield lying flat. Built by effusive eruptions from a central vent and radial fissures. Can be enormous: Mauna Loa (Hawaii) is ~100 km (62 mi) wide and rises ~10,000 m (32810 ft) from the seafloor — the largest volcano on Earth by volume.
Also called composite volcano. A steep-sided (slopes 20–35°) cone built by alternating layers of lava flows and pyroclastic deposits (tephra, ash, pyroclastic flow deposits). Typical of subduction zone arcs. Moderately to highly silicic magma (andesitic to dacitic). Can reach 3,000–5,000 m (16405 ft) height. Examples: Mt. Fuji, Mt. Rainier, Mt. Pinatubo, Mt. Merapi. The most common type of large subaerial volcano.
A small (usually <300 m (984 ft) tall), steep-sided (30–40°) cone built from scoria (cinder) — pyroclastic basaltic material ejected during Strombolian eruptions. Very common, typically monogenetic (one eruption episode). May have a lava flow from the base. Usually short-lived (days to years). Parícutin (Mexico), which grew from a cornfield in 1943, is the most-observed birth of a cinder cone.
A large volcanic depression (1–100 km (62 mi) diameter) formed by the collapse of the ground into a partially emptied magma reservoir during or after a major eruption. Not a crater (which is a simple depression at a vent). Some calderas form incrementally over multiple eruptions; others form catastrophically in minutes during supervolcanic events. Examples: Crater Lake (Oregon), Yellowstone, Toba (Sumatra).
A mound of highly viscous lava that accumulates around a vent when magma is too viscous to flow far from its source. Grows by internal injection (endogenous dome) or by surface extrusion (exogenous lobe). Associated with dacitic to rhyolitic magma. Can be highly unstable and collapse, producing pyroclastic density currents (block-and-ash flows). The growing dome at Mt. St. Helens after the 1980 eruption is a classic example.