6.1 Stratovolcanoes and Shield Volcanoes

Stratovolcanoes and shield volcanoes are two distinct types of volcanoes with unique shapes and behaviors. Stratovolcanoes are steep and cone-shaped, prone to explosive eruptions. Shield volcanoes are flatter and wider, known for their gentle lava flows.

The differences stem from their magma composition and eruptive styles. Stratovolcanoes have silica-rich, viscous magma that builds steep slopes. Shield volcanoes have fluid, basaltic lava that spreads out, creating broad, gentle slopes.

Stratovolcanoes vs Shield Volcanoes

Morphology and Composition

• Stratovolcanoes, also known as composite volcanoes, are characterized by their steep-sided, conical shape resulting from the buildup of viscous lava flows, ash, and tephra during eruptions
  • Example: Mount Fuji in Japan, known for its nearly perfect conical shape
• Shield volcanoes have a broad, gently sloping morphology resembling a warrior's shield, formed by the accumulation of fluid, low-viscosity basaltic lava flows
  • Example: Mauna Loa in Hawaii, which has a diameter of about 120 km and a height of 4,169 m above sea level
• The magma in stratovolcanoes is often andesitic to rhyolitic in composition, containing higher silica content and dissolved gases
  • Higher silica content makes the magma more viscous and prone to explosive eruptions
• The magma in shield volcanoes is typically basaltic, with lower silica content and lower gas content compared to stratovolcanoes
  • Lower silica content results in more fluid lava that can flow easily and create the gentle slopes of shield volcanoes

Eruptive Characteristics

• Stratovolcanoes typically produce explosive eruptions due to the high viscosity and gas content of their magma, which can result in pyroclastic flows, ash clouds, and lahars
  • Example: The 1980 eruption of Mount St. Helens in the United States, which produced a massive ash cloud and devastating pyroclastic flows
• Shield volcanoes are associated with effusive eruptions, producing relatively gentle outpourings of fluid, low-viscosity basaltic lava that can travel long distances before solidifying
  • Example: The ongoing eruptions at Kilauea in Hawaii, which have been continuously producing lava flows since 1983
• The eruptive behavior of stratovolcanoes is often episodic, with periods of dormancy punctuated by violent, explosive eruptions, while shield volcanoes tend to have more continuous, steady eruptive activity
  • Stratovolcanoes may have centuries or even millennia between major eruptions, while shield volcanoes can have nearly continuous lava flow activity

Volcano Development Factors

Magma Properties and Tectonic Setting

• Magma composition plays a crucial role in determining the type of volcano formed, with stratovolcanoes associated with more silica-rich, viscous magmas and shield volcanoes with less silica-rich, fluid magmas
  • Silica content affects the viscosity and gas content of the magma, influencing the eruptive style and morphology of the volcano
• Tectonic setting influences volcano formation, with stratovolcanoes commonly found in subduction zones where oceanic crust is subducted beneath continental crust, while shield volcanoes are often associated with hot spots or mid-ocean ridges
  • Example: The Cascade Range in the western United States, which includes several stratovolcanoes like Mount Rainier and Mount Shasta, is located along a subduction zone
  • Example: The Hawaiian Islands, which are home to numerous shield volcanoes, are situated over a hot spot in the middle of the Pacific Plate

Magma Supply and Volcanic System Evolution

• Magma supply rate affects volcano morphology, with shield volcanoes typically having a higher and more consistent magma supply compared to stratovolcanoes
  • Higher magma supply rates contribute to the formation of the broad, gently sloping shield shape
• Magma chamber depth and size can influence the eruptive style and frequency of volcanic activity, with shallower and smaller magma chambers often associated with more explosive eruptions in stratovolcanoes
  • Larger, deeper magma chambers in shield volcanoes allow for the storage and steady supply of magma to the surface
• The presence of groundwater or surface water can contribute to the explosive nature of stratovolcano eruptions through phreatomagmatic interactions
  • Water coming into contact with hot magma can cause steam explosions and contribute to the formation of ash and pyroclastic material
• The age and evolution of the volcanic system can impact the morphology and eruptive characteristics, with older, more mature volcanoes often exhibiting more complex structures and eruptive histories
  • Example: Mount Etna in Italy, which has been active for over 500,000 years, displays characteristics of both stratovolcanoes and shield volcanoes due to its complex evolution

Volcano Hazards

Stratovolcano Hazards

• Stratovolcanoes pose significant hazards due to their explosive eruptions, which can generate pyroclastic flows, ash falls, and volcanic bombs that can cause destruction, burial, and incineration of surrounding areas
  • Example: The 79 AD eruption of Mount Vesuvius in Italy buried the cities of Pompeii and Herculaneum under ash and pyroclastic flows
• Pyroclastic flows are fast-moving, ground-hugging avalanches of hot ash, pumice, and volcanic gases that can travel at speeds up to 700 km/h and reach temperatures of 1,000°C
  • These flows can cause complete destruction and loss of life in their path
• Lahars, or volcanic mudflows, are another major hazard associated with stratovolcanoes, formed by the mixing of volcanic ash, debris, and water from melting snow or heavy rainfall
  • Example: The 1985 eruption of Nevado del Ruiz in Colombia triggered lahars that killed over 23,000 people in the town of Armero

Shield Volcano and Other Hazards

• Shield volcanoes, while generally less hazardous than stratovolcanoes, can still pose risks such as lava flows that can destroy infrastructure and vegetation, as well as the emission of volcanic gases that can affect air quality
  • Example: The 2018 eruption of Kilauea in Hawaii destroyed over 700 homes and covered large areas with lava flows
• Both types of volcanoes can cause volcanic earthquakes and ground deformation, which can damage buildings and infrastructure in nearby areas
  • Volcanic earthquakes are caused by the movement of magma and volcanic gases within the volcano
• The collapse of volcanic edifices, particularly in the case of stratovolcanoes with unstable slopes, can trigger massive landslides and debris avalanches
  • Example: The collapse of the northern flank of Mount St. Helens in 1980 triggered a massive debris avalanche that traveled up to 25 km from the volcano
• Secondary hazards, such as the formation of acid rain from volcanic gases and the contamination of water sources by volcanic ash, can have long-term environmental and health impacts in the affected regions
  • Volcanic gases such as sulfur dioxide can react with water vapor in the atmosphere to form acid rain, which can harm vegetation and aquatic life

Notable Volcano Examples

Stratovolcanoes

• Mount Fuji in Japan: An iconic stratovolcano known for its nearly perfect conical shape and cultural significance
• Mount Vesuvius in Italy: Famous for its eruption in 79 AD that buried the cities of Pompeii and Herculaneum
• Mount St. Helens in the United States: Experienced a catastrophic eruption in 1980 that removed the upper 400 m of the volcano and triggered a massive debris avalanche
• Popocatépetl in Mexico: An active stratovolcano located near Mexico City, with frequent small-scale eruptions and ash emissions
• Krakatoa in Indonesia: Its 1883 eruption was one of the most violent in recorded history, generating massive tsunamis and affecting global climate

Shield Volcanoes

• Mauna Loa and Kilauea in Hawaii, United States: Two of the world's most active volcanoes, known for their frequent lava flow eruptions and ongoing volcanic activity
• Olympus Mons on Mars: The largest known shield volcano in the solar system, with a height of nearly 22 km and a diameter of over 600 km
• Fernandina in the Galapagos Islands, Ecuador: A young, active shield volcano that has experienced frequent eruptions since its discovery in the 16th century
• Skjaldbreiður in Iceland: A classic example of a shield volcano, with a broad, flat summit area and gently sloping flanks formed by numerous basaltic lava flows Some volcanoes, such as Mount Etna in Italy, exhibit characteristics of both stratovolcanoes and shield volcanoes, showcasing the complexity and diversity of volcanic systems. Mount Etna has a complex morphology resulting from its long eruptive history and the interplay between explosive and effusive eruptions.