Stratovolcano or Composite Volcano – Definition and Examples


Stratovolcano or Composite Volcano

A stratovolcano or composite volcano is a dramatic volcano with a steep profile, layered composition, and explosive eruption. This type of volcano dominates the landscape and affects nearby populations and even the global climate.

What Is a Stratovolcano or Composite Volcano?

A stratovolcano or composite volcano is a large, steep-sided volcano that consists of multiple layers of hardened lava, tephra, pumice, and volcanic ash. These volcanoes have a conical shape and produce periodic, often explosive eruptions. The prefix “strato-” and term “composite” refer to the layered structure resulting from alternating eruptions of viscous lava and tephra (fragmented volcanic material).

Structure of a Stratovolcano

The structure of a stratovolcano includes several key components:

  • Magma Chamber: A reservoir of magma or molten rock beneath the surface.
  • Conduit: A passage through which magma travels to reach the surface.
  • Vent: The opening at the Earth’s surface through which volcanic material is expelled. The location of the main vent often changes over time. Parasitic vents and cones often form.
  • Crater: A bowl-shaped depression around the vent. Some craters contain fumaroles (vents that leak gases), lakes, or even ice.
  • Lava Flows: Streams of molten rock that solidify as they cool.
  • Pyroclastic Flows: Fast-moving currents of hot gas and volcanic matter.
  • Lahars: Volcanic mudflows or debris flows. The composition may be thick and slow-moving, or liquid and fast-flowing.

Major Characteristics of Stratovolcanoes

For the most part, identifying a stratovolcano is easy simply by looking at its size and shape. The eruptions are quite different from those of other volcanoes.

  1. Steep Profile: Stratovolcanoes have steep, conical slopes formed by the accumulation of viscous lava and tephra.
  2. Explosive Eruptions: These volcanoes often produce powerful, explosive eruptions due to the high viscosity and gas content of their magma.
  3. Composite Layers: Alternating layers of lava flows, ash, and tephra create a complex internal structure.
  4. Long Dormant Periods: Stratovolcanoes often remain dormant for centuries between eruptions.
  5. High Elevation: They can reach significant heights, often towering over surrounding landscapes. That being said, the tallest volcanoes on Earth (Mauna Kea in Hawaii) and in the Solar System (Olympus Mons on Mars) are actually shield volcanoes.

Differences From Other Types of Volcanoes

The two other common types of volcanoes are cinder cones and shield volcanoes:

  1. Cinder Cone Volcanoes:
    • Shape: Cinder cones are smaller with steep, conical profiles.
    • Eruption Type: They produce relatively small, short-lived eruptions.
    • Lava Composition: Typically basaltic and less viscous.
    • Examples: Parícutin (Mexico), Sunset Crater (USA).
  2. Shield Volcanoes:
    • Shape: Broad, gently sloping profiles.
    • Eruption Type: Effusive eruptions with low-viscosity lava.
    • Lava Composition: Primarily basaltic.
    • Examples: Mauna Loa (Hawaii), Galápagos Islands (Ecuador).
NameShapeEruption TypeLava CompositionHazardsExamples
StratovolcanoConicalExplosiveAndesitic to RhyoliticPyroclastic flows, lahars, ashfall, lava flows, volcanic bombsMount Fuji, Mount St. Helens
Cinder ConeConicalSmall, Short-livedBasalticLava flows, minor tephra fallParícutin, Sunset Crater
Shield VolcanoBroad, GentleEffusiveBasalticLava flowsMauna Loa, Galápagos Islands

Where Do Stratovolcanoes Form?

Stratovolcanoes primarily form at convergent tectonic plate boundaries where an oceanic plate subducts beneath a continental plate. This subduction produces melting of the mantle and the formation of magma, which rises to the surface and forms a volcano. The cyclical pattern of explosive eruptions and lava flows builds the stratified layers characteristic of composite volcanoes.

Ring of Fire

Global Distribution of Stratovolcanoes

Stratovolcanoes occur in volcanic arcs and mountain ranges associated with subduction zones. Notable regions include the Pacific Ring of Fire, the Andes, the Cascade Range, and the Mediterranean.

Examples of Stratovolcanoes

Stratovolcanoes typically tower over the landscape and have the classic conical shape. Here are some examples:

  1. Mount Fuji (Japan)
  2. Mount St. Helens (USA)
  3. Mount Vesuvius (Italy)
  4. Mount Etna (Italy)
  5. Mount Pinatubo (Philippines)
  6. Mount Kilimanjaro (Tanzania)
  7. Mount Shasta (USA)
  8. Mount Merapi (Indonesia)
  9. Cotopaxi (Ecuador)
  10. Mount Mayon (Philippines)

Hazards Associated with Stratovolcanoes

Most people associate lava with a volcanic eruption, but a stratovolcano or composite volcano does much more than just release molten rock:

  1. Pyroclastic Flows: Pyroclastic flows are extremely hot and fast-moving currents of gas and volcanic matter.
  2. Lahars: These are volcanic mudflows that sometimes bury communities.
  3. Ashfall: Dispersal of ash over large areas impacts air quality and aviation.
  4. Lava Flows: Stratovolcanoes sometimes produce destructive molten rock flows.
  5. Volcanic Bombs: Because of the pressure within the volcano, there are often large projectiles ejected during eruptions. These igneous rocks range in size from a baseball to that of a car.

Notable Stratovolcano Eruptions

Mayon stratovolcano

Stratovolcano eruptions are among the most destructive natural disasters:

  1. Mount Vesuvius, AD 79 (Italy)
    • Effect: The eruption buried the cities of Pompeii and Herculaneum under ash and pumice, killing thousands of people and preserving the cities under a thick layer of volcanic material.
  2. Mount St. Helens, 1980 (USA)
    • Effect: The eruption caused a massive landslide, followed by a lateral blast that devastated 230 square miles of forest, killed 57 people, and caused significant economic damage.
  3. Mount Pinatubo, 1991 (Philippines)
    • Effect: The massive ash cloud circled the globe, reducing global temperatures by about 0.5°C, and causing widespread destruction and displacement of communities.
  4. Krakatoa, 1883 (Indonesia)
    • Effect: The eruption and subsequent tsunamis killed over 36,000 people, generated loud explosions heard thousands of kilometers away, affected global climate, and produced spectacular sunsets.
  5. Mount Pelée, 1902 (Martinique)
    • Effect: A pyroclastic flow destroyed the city of Saint-Pierre, killing nearly 30,000 people in one of the deadliest volcanic disasters in history.
  6. Nevado del Ruiz, 1985 (Colombia)
    • Effect: The eruption melted the mountain’s ice cap, triggering lahars that buried the town of Armero and killed approximately 23,000 people.
  7. Mount Tambora, 1815 (Indonesia)
    • Effect: The eruption caused the “Year Without a Summer,” with global climate anomalies leading to food shortages and severe weather patterns. Tens of thousands of people died due to direct and indirect effects.
  8. Mount Etna, ongoing (Italy)
    • Effect: Frequent eruptions of Mount Etna continuously shape the landscape, destroy infrastructure, and affect air travel, while also providing fertile soil for agriculture in the region.
  9. Sakurajima, ongoing (Japan)
    • Effect: Regular eruptions produce ashfall affecting local communities, agriculture, and air quality.
  10. Mount Merapi, 2010 (Indonesia)
    • Effect: Pyroclastic flows and ashfall resulted in the evacuation of thousands of people, significant loss of life, and disruption to the local economy and environment.

Life Cycle of a Stratovolcano

The life cycle of a stratovolcano involves several stages:

  1. Magma Accumulation: Heat and pressure melt rock into magma. Magma accumulates in a chamber beneath the Earth’s crust. The magma in stratovolcanoes is typically andesitic to rhyolitic in composition, meaning it has higher silica content than basaltic magma. This higher silica content makes the magma more viscous and prone to trapping gases, which is a key factor in the explosive nature of stratovolcano eruptions.
  2. Eruption: Pressure builds until there is an explosive eruption. As pressure increases, it drives water out of hydrated minerals. This “dewatering” lowers the melting point of the rock. As it melts, its lower density makes it rise through the crust. But, the chemical composition of the magma makes it thick and viscous, so it does not readily escape to the surface and flow. Various triggers permit the explosive eruption, such as earthquakes, interactions with groundwater, or collapse of chambers.
  3. Lava Flow: Viscous lava flows solidify, forming layers. The characteristic stratified (layered) structure of a stratovolcano results from a repeated alternation between explosive and effusive eruptions.
  4. Dormancy: Stratovolcanoes enter long periods of dormancy, during which there is little to no eruptive activity. These periods last for decades, centuries, or even millennia. However, the magma chamber beneath the volcano remains active, with magma slowly accumulating and gradually deforming the volcano’s surface.
  5. Renewed Activity: There are often warning signs indicating that the volcano is reawakening. These signs include increased seismic activity (earthquakes), ground deformation (bulging or uplift of the volcano’s surface), changes in gas emissions (such as increased sulfur dioxide), and the formation of new fumaroles (vents releasing volcanic gases). Subsequent eruptions add layers to the volcano.
  6. Erosion: Erosion gradually wears down the volcanic structure over time. Despite the ongoing erosion, stratovolcanoes can reawaken and enter new phases of growth and eruption.
  7. Extinction: Eventually, the magma supply dwindles and the volcano becomes extinct.

FAQs about Stratovolcanoes

  • What causes stratovolcanoes to erupt?
    Pressure buildup from magma and volcanic gases leads to explosive eruptions.
  • How dangerous are stratovolcano eruptions?
    They are potentially extremely dangerous due to pyroclastic flows, lahars, and ashfall.
  • How long can stratovolcanoes remain dormant?
    They often remain dormant for centuries between eruptions.
  • Can stratovolcanoes form under the ocean?
    Yes, they can form at subduction zones beneath the ocean, eventually emerging above sea level.
  • Are there any warning signs before a stratovolcano erupts?
    Common signs include increased seismic activity, gas emissions, and ground deformation.
  • What is the largest stratovolcano in the world?
    Ojos del Salado in the Andes is the stratovolcano with the highest summit elevation (22,615 feet or 6893 meters). However, in terms of sheer size, Mount Kilimanjaro in Africa (16,076) feet is bigger. But, Mount Kilimanjaro actually consists of three volcanoes that share one peak.
  • Can stratovolcanoes affect climate?
    Yes, large eruptions release ash and gases into the atmosphere, affecting global climate.
  • How do scientists monitor stratovolcanoes?
    They use seismographs, gas sensors, satellite imagery, and ground-based observations.
  • What is a lahar?
    A lahar is a volcanic mudflow composed of water, ash, and debris.
  • Why are stratovolcanoes typically found at subduction zones?
    Subduction zones provide the necessary conditions for magma generation and explosive eruptions.

References

  • Cañón-Tapia, Edgardo (2014). “Volcanic eruption triggers: A hierarchical classification”. Earth-Science Reviews. 129: 100–119. doi:10.1016/j.earscirev.2013.11.011
  • Schmincke, Hans-Ulrich (2003). Volcanism. Berlin: Springer. ISBN 9783540436508.
  • Sigurðsson, Haraldur, ed. (2015). The Encyclopedia of Volcanoes (2nd ed.). Academic Press. ISBN 978-0-12-385938-9.