Shield Volcano

Shield volcanoes are the “gentle giants” of the volcanic world. Named for their resemblance to a warrior’s shield lying flat on the ground, these massive structures are built almost entirely of fluid lava flows. While they lack the dramatic, conical profile of stratovolcanoes, they can grow to immense sizes, forming the largest mountains on Earth and in the solar system.

Geological Context and Formation

Shield volcanoes are primarily formed by basaltic magma, which is low in silica and has a low viscosity (runny). This allows the lava to flow easily and travel great distances before cooling, rather than piling up steeply around the vent.

They are typically found in two tectonic settings:

1. Hotspots: Mantle plumes that punch through the crust, such as the Hawaiian-Emperor seamount chain or the Galápagos Islands.
2. Divergent Plate Boundaries: Where tectonic plates spread apart, such as in Iceland and the East African Rift.

Structure and Morphology

The defining characteristic of a shield volcano is its profile. The slopes are extremely gentle, often tilting only 2° to 10° near the base.

• Dimensions: They are much wider than they are tall. For example, Mauna Loa is about 120 km (75 miles) in diameter at its base on the ocean floor.
• Summit Calderas: unlike the simple craters of cinder cones, shield volcanoes often have large, steep-walled calderas at their summits, formed by the collapse of the ground above a drained magma chamber.
• Rift Zones: Eruptions often occur along linear fracture zones (rift zones) that radiate from the summit, distributing lava across the flanks.

Eruptive Style

Eruptions are predominantly effusive rather than explosive. The low gas content and fluid nature of the magma result in fountains of fire and steady rivers of lava.

• Lava Tubes: As the surface of a lava flow cools and hardens, the interior remains hot and fluid. This forms natural pipelines called lava tubes, which insulate the lava and allow it to flow tens of kilometers without cooling significantly. This mechanism is key to building the broad base of the shield.
• Texture: Flows typically solidify as pahoehoe (smooth, ropy) or ‘a’a (rough, blocky) lava.

The Hawaiian Life Cycle

Geologists have identified a distinct life cycle for hotspot shield volcanoes:

1. Submarine Stage: The volcano grows underwater (e.g., Lōʻihi Seamount).
2. Emergent Stage: It breaches the ocean surface, often interacting explosively with water.
3. Shield-Building Stage: 95% of the volcano’s mass is built during this phase of frequent, high-volume effusive eruptions (e.g., Mauna Loa, Kīlauea).
4. Post-Shield Stage: Activity slows, and lava becomes slightly more viscous and explosive (e.g., Mauna Kea).
5. Erosional Stage: The volcano goes dormant and is worn down by weather and subsidence.

Famous Examples

• Mauna Loa (Hawaii): The largest active volcano on Earth. From its base on the seafloor to its summit, it rises over 9,000 meters (30,000 ft), making it taller than Mount Everest.
• Skjaldbreiður (Iceland): The “broad shield” that gave this type of volcano its name. It formed in a single massive eruptive period about 9,000 years ago.
• Olympus Mons (Mars): The largest volcano in the solar system, proving that shield volcanism is a fundamental planetary process.