Types of Volcanoes

Volcanoes come in a remarkable variety of shapes and sizes, from the broad, gently sloping shields of Hawaii to the steep, symmetrical cones of the Ring of Fire to the vast, basin-like calderas left behind by supereruptions. These differences are not cosmetic — they reflect fundamental differences in magma chemistry, eruption dynamics, and tectonic setting that determine how a volcano behaves and how dangerous it is. The Smithsonian Institution's Global Volcanism Program catalogues 1,222 Holocene volcanoes worldwide, classifying them into several morphological types.

Stratovolcanoes (composite volcanoes) dominate the global count with 661 entries (54% of all catalogued volcanoes), followed by volcanic fields (139, or 11%), shield volcanoes (111, or 9%), calderas (85, or 7%), complex volcanoes (55, or 4.5%), and various other forms including cinder cones, lava domes, fissure vents, and submarine volcanoes. Understanding these types is essential for assessing volcanic hazards, interpreting eruption histories, and predicting future behavior. A shield volcano and a stratovolcano sitting in the same region may pose completely different risks to nearby populations — the shield producing relatively gentle lava flows that allow evacuation, the stratovolcano capable of sudden, devastating pyroclastic flows.

**Stratovolcanoes** (also called composite volcanoes) are the iconic, steep-sided, conical mountains that most people picture when they think of a volcano. They are built by alternating layers of lava flows, tephra (volcanic debris), and ash, accumulated over thousands to hundreds of thousands of years. The name 'strato' comes from the Latin word for 'layer.'

Stratovolcanoes are the most common and most dangerous type of volcano on Earth. The Smithsonian database lists 661 stratovolcanoes (including multi-centered stratovolcano complexes), accounting for approximately 54% of all catalogued Holocene volcanoes. They form primarily at convergent plate boundaries — subduction zones where oceanic crust dives beneath continental or other oceanic crust — which is why the Ring of Fire is densely populated with them.

The magma feeding stratovolcanoes is typically intermediate to high in silica content (andesitic to dacitic), making it viscous and gas-rich. This combination produces explosive eruptions: the thick magma traps dissolved gases until pressure builds to a critical point, then releases them violently. Eruption styles range from moderate Strombolian and Vulcanian explosions to catastrophic Plinian events that send eruption columns 30+ km into the stratosphere.

Stratovolcanoes have been responsible for the majority of volcanic fatalities in recorded history. Notable examples include Mount Vesuvius (Italy), whose 79 AD eruption (VEI 5) buried Pompeii; Mount St. Helens (United States), whose 1980 lateral blast killed 57 people; Mount Pinatubo (Philippines), whose 1991 VEI 6 eruption was the second-largest of the 20th century; Mount Fuji (Japan), the world's most iconic stratovolcano; Cotopaxi (Ecuador), one of the highest active volcanoes in the world at 5,911 m (19,393 ft); and Mount Merapi (Indonesia), with 106 confirmed eruptions.

Typical characteristics: elevation 1,000–6,000 m, slope angles of 25–35 degrees, summit craters, and a history of alternating explosive and effusive eruptions. Stratovolcanoes can remain dormant for centuries before reawakening violently, as Pinatubo demonstrated in 1991 after roughly 500 years of quiet.

**Shield volcanoes** are broad, gently sloping volcanic edifices built almost entirely by fluid basaltic lava flows. Their low-angle profiles (typically 2–10 degree slopes) resemble a warrior's shield laid on the ground, giving them their name. They are the largest volcanoes on Earth by volume, though they lack the dramatic height-to-width ratio of stratovolcanoes.

The Smithsonian database lists 111 shield volcanoes, accounting for approximately 9% of Holocene volcanoes. They form primarily at hotspots (like Hawaii) and along rift zones (like Iceland and the East African Rift), where low-silica basaltic magma rises from the mantle with relatively little interaction with continental crust.

Because basaltic magma has low viscosity and low gas content, shield volcano eruptions are typically effusive rather than explosive. Lava emerges in rivers and fountains rather than violent blasts, and flows can travel tens of kilometers before solidifying. This makes shield eruptions spectacular but generally less immediately deadly than stratovolcano eruptions — though lava flows can destroy communities and infrastructure in their path, as demonstrated by Kilauea's 2018 eruption, which destroyed over 700 structures in Hawaii.

**Calderas** are large, basin-shaped depressions formed when a volcanic edifice collapses into a partially emptied magma chamber, typically during or after a massive eruption. The term comes from the Spanish word for 'cauldron.' Calderas range from a few kilometers to over 100 km in diameter and are often so large that they are not immediately recognizable as volcanic features from the ground.

The Smithsonian database lists 85 calderas, representing about 7% of Holocene volcanoes. Calderas are particularly important because they include Earth's most powerful volcanic systems — the supervolcanoes. Several calderas in the database have produced VEI 7 or VEI 8 eruptions: Taupo (New Zealand) produced a VEI 8 event roughly 26,500 years ago; Aira (Japan) produced a VEI 7–8 event roughly 29,000 years ago; Crater Lake (United States) formed in a VEI 7 eruption around 5680 BCE.

Calderas form through several mechanisms. The most dramatic is explosive caldera collapse, where a massive eruption evacuates enough magma that the overlying rock can no longer support itself and drops into the void, sometimes accompanied by ring-fault eruptions. Other calderas form more gradually through summit collapse during sustained lava drainage, as at Kilauea's Halema'uma'u crater.

Many calderas remain actively dangerous. Campi Flegrei (Italy) is experiencing ongoing unrest with measurable ground uplift and increasing seismicity, with 1.5 million people living within the caldera. Taal (Philippines) last erupted in 2020 and sits just 50 km from Manila. Krakatau (Indonesia) generated one of the deadliest eruptions in recorded history in 1883 (over 36,000 dead) and its child volcano, Anak Krakatau, caused a deadly tsunami in 2018.

**Cinder cones** (also called scoria cones or pyroclastic cones) are the smallest and most common type of volcanic landform, though many are too small or short-lived to appear in the Smithsonian's Holocene database of major volcanoes. The database lists 39 pyroclastic cone entries. They are steep-sided (30–40 degree slopes), typically 30 to 300 m tall, and built by the accumulation of loose volcanic fragments (cinders, scoria, and bombs) ejected from a single vent during Strombolian-style eruptions.

Cinder cones form rapidly — often within days to months — and are frequently found on the flanks or in the vicinity of larger volcanoes. Paricutin in Mexico is the most famous cinder cone in history: it emerged in a cornfield on February 20, 1943 and grew to 424 m over nine years of continuous eruption, burying two villages in lava. Sunset Crater in Arizona (USA) erupted around 1085 AD and is one of the youngest volcanic features in the contiguous United States.

Because they are built of loose, unconsolidated material, cinder cones erode quickly and have relatively short geological lifespans compared to shield volcanoes or stratovolcanoes. They rarely produce highly dangerous eruptions, though lava flows from their bases can damage property, and their explosive phases can generate hazardous tephra fall within several kilometers.

**Lava domes** are mound-shaped volcanic features formed by the slow extrusion of highly viscous, silica-rich magma (typically dacitic or rhyolitic) that piles up around and over the vent rather than flowing away. The Smithsonian database lists 32 lava dome volcanoes. They are typically 100 to 1,000 m in diameter and grow over months to decades.

Lava domes are among the most hazardous volcanic features because they are prone to sudden, unpredictable collapse. When a growing dome becomes gravitationally unstable or is disrupted by internal gas pressure, parts of it can collapse catastrophically, generating pyroclastic density currents — fast-moving avalanches of superheated rock fragments and gas — that can travel at speeds exceeding 100 km/h and temperatures above 300 degrees C.

The deadliest lava dome collapse in modern history occurred at Mount Pelee in Martinique (France) on May 8, 1902, when a pyroclastic surge from a collapsing dome destroyed the city of Saint-Pierre and killed approximately 29,000 people in minutes. Unzen in Japan experienced a devastating dome collapse on June 3, 1991, killing 43 people including the noted volcanologists Katia and Maurice Krafft. The ongoing dome growth at Mount St.

Helens (2004–2008) and at Sheveluch in Russia illustrate the persistent hazard these features pose.

**Fissure vents** (also called fissure eruptions) are linear volcanic vents through which lava erupts along a crack in the Earth's surface rather than from a central cone. The Smithsonian database lists 53 fissure vents. They are most common along divergent plate boundaries and rift zones, particularly in Iceland and the East African Rift.

Fissure eruptions can produce enormous volumes of lava. The 1783–1784 Laki eruption in Iceland — technically a fissure eruption along a 27 km crack system — produced approximately 14.7 km3 of basaltic lava over eight months, one of the largest lava eruptions in recorded history. The sulfuric aerosol haze it generated caused crop failures and estimated death tolls of 9,000 in Iceland (roughly 25% of the population) and contributed to an estimated 23,000 excess deaths in England from air pollution.

The ongoing eruptions on the Reykjanes Peninsula in Iceland since 2021 are fissure-style events.

On a much grander scale, fissure eruptions produced the enormous continental flood basalt provinces found in the geological record — the Deccan Traps of India, the Siberian Traps of Russia, and the Columbia River Basalt Group of the northwestern United States. These events, each involving thousands of cubic kilometers of lava over millions of years, are associated with mass extinction events in Earth's history.

**Volcanic fields** are areas containing clusters of small volcanic vents — typically cinder cones, maars, and small shield volcanoes — distributed across a broad region rather than concentrated at a single central edifice. The Smithsonian database lists 139 volcanic fields, making them the second most common volcano type at approximately 11% of the total.

Volcanic fields form in a variety of tectonic settings, including continental rift zones, back-arc basins, and intraplate settings where magma exploits pre-existing weaknesses in the crust. Individual vents within a field are typically monogenetic — each erupts once and then becomes extinct — but the field as a whole may remain active for hundreds of thousands of years as new vents open.

Notable volcanic fields include the Auckland Volcanic Field in New Zealand, which contains approximately 53 volcanic centers beneath a city of 1.7 million people; the Michoacan-Guanajuato Volcanic Field in Mexico, which includes Paricutin and contains over 1,000 individual vents; and the Eifel Volcanic Field in Germany, which includes the Laacher See caldera that last erupted approximately 12,900 years ago.

**Complex volcanoes** are volcanic centers that do not fit neatly into a single morphological category. They may combine characteristics of stratovolcanoes, calderas, shield volcanoes, and lava domes in a single edifice, often reflecting a long and varied eruptive history. The Smithsonian database lists 55 complex volcanoes and 12 compound volcanoes.

A complex volcano may have begun as a shield volcano, built a stratovolcanic cone, then collapsed into a caldera, and subsequently grown lava domes within the caldera — all within a single volcanic center. Galeras in Colombia is classified as a complex volcano, as is Unzen in Japan.

**Compound volcanoes** are closely spaced volcanic vents that overlap to form a single, multi-summit edifice. They are distinguished from volcanic fields by the close proximity and structural interconnection of their vents. Colima in Mexico, with its paired Nevado de Colima and Volcan de Fuego summits, is a classic compound volcano.

**Submarine volcanoes** erupt beneath the ocean surface and are among the least studied volcanic features because of their inaccessibility. They include underwater seamounts, mid-ocean ridge volcanic centers, and shallow submarine vents that occasionally breach the surface to form new islands.

The 2022 eruption of Hunga Tonga-Hunga Ha'apai (Tonga) — a submarine caldera volcano — demonstrated that underwater volcanoes can produce globally significant eruptions. Its VEI 5 explosion generated an atmospheric shockwave that circled the globe multiple times and injected roughly 146 teragrams of water vapor into the stratosphere. Kavachi in the Solomon Islands is one of the most active submarine volcanoes in the Pacific, repeatedly building and destroying temporary islands.

An estimated 75% of all volcanic eruptions on Earth occur along mid-ocean ridges beneath the sea, but the vast majority go undetected. The Smithsonian database lists only a fraction of known submarine volcanic centers because most lack sufficient evidence of Holocene activity.

The type of volcano that forms at any given location is primarily determined by magma chemistry, which in turn is controlled by tectonic setting. This connection between chemistry, eruption style, and landform is the central organizing principle of volcanology.

**Basaltic magma** (low silica, approximately 45–52% SiO2) is hot (roughly 1,100–1,250 degrees C), fluid, and gas-poor. It flows easily, producing effusive eruptions, lava rivers, and lava fountains. Over time, these flows build shield volcanoes and flood basalt plains.

Basaltic magma dominates at hotspots (Hawaii), divergent boundaries (Iceland, mid-ocean ridges), and some rift zones.

**Andesitic magma** (intermediate silica, approximately 52–63% SiO2) is cooler, more viscous, and more gas-rich. It produces moderately explosive eruptions (Strombolian to Vulcanian) and builds the classic layered cone of a stratovolcano. Andesitic magma is the hallmark of subduction zone volcanism.

**Dacitic and rhyolitic magma** (high silica, above 63% SiO2) is the most viscous and gas-charged. It produces catastrophically explosive Plinian eruptions, lava domes, ignimbrite flows, and caldera collapses. The most devastating eruptions in history — Tambora, Krakatau, Pinatubo — involved dacitic or rhyolitic magma. Supervolcanic systems like Yellowstone and Toba are fueled primarily by rhyolitic magma.