The shield volcano is the gentle giant of volcanoes. The tallest volcano on Earth (Mauna Kea) and the tallest volcano in the solar system (Olympus Mons on Mars) are both shield volcanoes. While these volcanoes tower over the surrounding landscape, they also slope outward. Their eruptions sometimes feature fountains of lava and appear dramatic, but people and animals have a chance to evacuate before lava reaches them. Even so, shield volcanoes are hazardous because they release toxic gases and sharp glass particles into the air and the lava covers everything in its path.
What Is a Shield Volcano?
A shield volcano is a type of volcano characterized by its broad, shield-like profile. Unlike the steep, conical shape of stratovolcanoes, shield volcanoes have gentle slopes formed by the flow of low-viscosity basaltic lava. This lava travels long distances before cooling and solidifying, resulting in the extensive, gently sloping landforms associated with these volcanoes.
Ten Examples of Shield Volcanoes Around the World
Shield volcanoes form all over the world, wherever low-silica magma gets to the surface. They typically form at hot spots and divergent plate boundaries. In hot spots, such as those found in Hawaii, magma from deep within the Earth’s mantle rises through the crust. As the Pacific Plate moves over the hot spot, successive eruptions build up layers of basaltic lava, creating a shield volcano. At divergent boundaries, like the Mid-Atlantic Ridge, tectonic plates pull apart, allowing magma to rise and form new crust, sometimes via shield volcanoes.
Mauna Kea (Hawaii, USA) is a prime example of a shield volcano. It towers 4,207.3 m (13,803 ft) above sea level and 9,330 m (30,610 ft) from its base, yet features the characteristic gentle slope.
Here are ten more examples of shield volcanoes world-wide:
- Mauna Loa (Hawaii, USA): One of the largest shield volcanoes on Earth, Mauna Loa rises over 4,170 meters above sea level and covers an area of approximately 5,271 square kilometers.
- Kīlauea (Hawaii, USA): Kilauea is known for its persistent activity.
- Skjaldbreiður (Iceland): This Icelandic volcano has the classic shield volcano shape.
- Fernandina (Galápagos Islands, Ecuador): An active shield volcano, Fernandina is the youngest and most volcanically active of the Galápagos Islands.
- Pico Island (Azores, Portugal): The highest mountain in Portugal, Pico is a shield volcano with a prominent summit crater.
- Nyamuragira (Democratic Republic of Congo): One of Africa’s most active volcanoes, Nyamuragira produces extensive lava flows.
- Sierra Negra (Galápagos Islands, Ecuador): Another active Galápagos volcano, Sierra Negra has one of the largest calderas in the world.
- Hualālai (Hawaii, USA): An active shield volcano on the Big Island of Hawaii, Hualālai overlooks the city of Kailua-Kona. It last erupted in 1801.
- Ascension Island (South Atlantic Ocean, UK): This island’s shield volcano forms the central part of the island and has a relatively low profile.
- Erta Ale (Ethiopia): Erta Ale is one of the few volcanoes in the world with a continuously active lava lake.
Shield Volcanoes Elsewhere in the Solar System
Shield volcanoes are not exclusive to Earth. Mars hosts Olympus Mons, which stands about 22 kilometers high and is roughly 600 kilometers in diameter, making it the tallest volcano in the Solar System. Venus features numerous shield volcanoes as well, like Sif Mons and Maat Mons, suggesting extensive volcanic activity and a similar formation process involving low-viscosity lava flows.
Distinguishing Features of Shield Volcanoes
The shape of a shield volcano and the nature of its lava go hand-in-hand. The lava has a low silica content, so it readily flows. The way the lava flows leads to the sloped shape of the volcano. The eruption is also distinctive, featuring a lot of lava.
- Gentle Slopes: Shield volcanoes have low-angle slopes, typically between 2 and 10 degrees, due to the fluid nature of basaltic lava.
- Broad Base: They cover a large area, with diameters often reaching several kilometers.
- Basaltic Lava: The lava is low in silica, making it less viscous so it flows easily over long distances.
- Frequent Eruptions: Eruptions are typically non-explosive and frequent, producing extensive lava flows rather than violent pyroclastic activity.
Formation and Eruption of Shield Volcanoes
A key characteristic of a shield volcano eruption is the flow of lava. Shield volcanoes form primarily through the effusion of basaltic lava, which is low in silica and highly fluid. This lava originates from the mantle, rising through the crust at hotspots or divergent plate boundaries. Each eruption lasts from days to years. Over time, successive eruptions build up broad, gently sloping layers of solidified lava, creating the characteristic shield-like shape.
Eruption Process
Volcanologists detect several warning signs before a shield volcano erupts. The surface deforms, the volcano releases gases, and earthquakes become more common:
1. Magma Ascent:
- Magma Chamber Filling: Magma accumulates in a reservoir beneath the volcano, increasing pressure.
- Ground Deformation: The surface bulges and tilts as the magma chamber expands.
2. Warning Signs:
- Seismic Activity: There is an increased frequency and intensity of earthquakes as magma moves upward.
- Gas Emissions: The volcano releases higher levels of volcanic gases, such as sulfur dioxide.
- Thermal Anomalies: Satellite or infrared imaging reveals increased heat flow at the surface.
3. Initial Eruption:
- Fissure Opening: Cracks or fissures open in the crust, allowing magma to escape as lava.
- Lava Fountains: Lava sometimes initially erupts as fountains from the fissure, creating spatter cones.
4. Lava Flow:
- Effusive Flow: Lava flows out of the fissures or vents, spreading over large areas due to its low viscosity.
- Formation of Lava Tubes: As the surface of the lava flow cools and solidifies, the molten lava beneath continues flowing, creating underground tubes.
5. Building the Shield:
- Layer Accumulation: Each eruption adds a new layer of basaltic lava, gradually building up the broad, gentle slopes.
- Caldera Formation: If the magma chamber empties significantly, the ground above may collapse, forming a caldera at the summit.
6. Eruption Sequence:
- Initial Eruption Phase: The initial eruption features vigorous lava fountaining and the formation of lava flows.
- Steady Effusion Phase: Next comes a sustained, steady flow of lava, creating extensive lava fields.
- Decline Phase: Finally, there is reduced lava output and cooling of the lava flows, ending the eruption cycle.
Key Geological Features of Shield Volcanoes
Aside from the overall shape of a shield volcano, it also has features such as lava lakes and lava tubes:
- Summit Caldera: A large, basin-like depression at the summit, formed when the magma chamber empties and the ground above collapses.
- Vents: Openings through which magma erupts. Vents occur at the summit or on the flanks of the volcano.
- Lava Lakes: Pools of molten lava sometimes form in the summit caldera. Notable examples include the lava lake in Erta Ale.
- Rift or Fissure Zones: These are areas where the crust cracks, allowing lava to erupt from elongated fissures rather than a central vent.
- Lava Flows: Lava flows are streams of molten rock that travel down the slopes of the volcano. These flows cover vast areas and create new landforms.
- Lava Tubes: Lava tube are natural conduits formed by flowing lava beneath the hardened surface. Through them, lava travels long distances underground.
- Vog: Vog is volcanic smog consisting of sulfur dioxide and other gases emitted by the volcano. Vog poses health risks and environmental problems.
- Laze Plumes: Laze plumes are steam plumes that form when lava enters the ocean, creating hydrochloric acid and volcanic glass particles that are hazardous to health.
How Shield Volcanoes Differ from Other Volcano Types
Basically, shield volcanoes display broad shield shapes, while stratovolcanoes and cinder cones have steep sides and lava domes are dome-shaped. Because these other types of volcanoes erupt lava with a higher viscosity, they are more likely to erupt ash clouds and pyroclastic fragments rather than just lava.
- Stratovolcanoes: These volcanoes have steep, conical shapes and are characterized by explosive eruptions due to high-viscosity lava. Examples include Mount St. Helens and Mount Fuji.
- Cinder Cone Volcanoes: Smaller volcanoes with steep sides, formed from pyroclastic fragments. They typically have short-lived, explosive eruptions. Examples include Parícutin and Sunset Crater.
- Lava Domes: Formed from high-viscosity lava that piles up near the vent, creating a dome shape. Examples include Mount St. Helens’ lava dome.
Hazards Associated with Shield Volcanoes
While shield volcanoes are generally less explosive than other types, they still pose significant hazards:
- Lava Flows: While typically slow-moving, lava flows destroy property and infrastructure in their path.
- Vog: Volcanic smog can cause respiratory issues and other health problems for people living downwind of the volcano.
- Laze Plumes: These plumes are hazardous to breathe and cause skin and eye irritation.
- Earthquakes: The movement of magma causes earthquakes, potentially damaging structures and triggering landslides.
- Gas Emissions: Shield volcanoes emit gases such as sulfur dioxide and carbon dioxide, which can be harmful to humans and animals.
Are Shield Volcanoes Dangerous?
Compared to stratovolcanoes, shield volcanoes are generally less dangerous due to their effusive eruption style. However, they still pose significant risks, particularly from lava flows and gas emissions. The relatively slow advance of lava flows often allows for evacuation and mitigation efforts, reducing the immediate threat to human life. Nonetheless, the earthquakes, long-term exposure to volcanic gases, and the potential for property damage make them hazardous.
References
- Keddie, Susan T.; Head, James W. (1994). “Sapas Mons, Venus: evolution of a large shield volcano”. Earth, Moon, and Planets. 65 (2): 129–190. doi:10.1007/BF00644896
- Plescia, J. B. (2004). “Morphometric properties of Martian volcanoes”. Journal of Geophysical Research. 109 (E3): E03003. doi:10.1029/2002JE002031
- Schmincke, Hans-Ulrich (2003). Volcanism. Berlin: Springer. ISBN 9783540436508.
- Wood, Charles A.; Kienle, Jürgen (1990). Volcanoes of North America: United States and Canada. Cambridge, England: Cambridge University Press. ISBN 0-521-43811-X.