Shield volcanoes are known for their gentle, non-explosive eruptions, a characteristic linked to the properties of their underlying magma. They are identifiable by their broad, low-profile shape, resembling a warrior’s shield. This form develops from the steady accumulation of highly fluid lava flows. Unlike steep, conical volcanoes, shield volcanoes expand horizontally, often becoming Earth’s largest by volume. Their gentle slopes result from effusive eruptions, where lava flows easily.
Understanding Eruption Styles
Volcanic eruptions range from explosive to effusive, with shield volcanoes exhibiting the latter. Effusive eruptions involve the steady outpouring of lava onto the Earth’s surface, creating flowing streams of molten rock. This contrasts sharply with explosive eruptions, which violently fragment magma and propel ash, gas, and rock fragments high into the atmosphere. Effusive eruptions are characterized by lava flows that spread widely, minimal ash production, and a lack of forceful explosions. The Volcanic Explosivity Index (VEI), which measures eruption magnitude, rates shield volcano eruptions at a low 0-1.
Magma Viscosity: The Primary Factor
Magma viscosity, its resistance to flow, is the primary determinant of a volcano’s eruption style. Shield volcanoes erupt highly fluid, low-viscosity basaltic magma. This low viscosity is attributed to two main factors: low silica content and high temperature. Basaltic magma contains a low silica content, typically 45% to 55% silicon dioxide (SiO2). Silica forms complex molecular chains, and a lower concentration of these chains allows the magma to flow more freely, like water compared to thick syrup.
Higher temperatures also significantly reduce magma viscosity. Basaltic magma erupts at high temperatures, 1000°C to 1200°C. This elevated temperature provides more energy for magma particles to move, further decreasing resistance to flow. The combination of low silica and high temperature results in magma that flows easily down the volcano’s gentle slopes, preventing pressure buildup and leading to effusive eruptions.
Gas Release: Preventing Explosive Pressure
The behavior of dissolved gases within magma also plays a significant role in determining the eruption’s intensity. Magma contains dissolved gases, such as water vapor and carbon dioxide, which are kept in solution by the immense pressure deep within the Earth. As magma rises towards the surface, the confining pressure decreases, allowing these dissolved gases to expand and form bubbles, similar to opening a carbonated drink.
In shield volcanoes’ low-viscosity magma, these gas bubbles can escape easily as the magma ascends. This efficient degassing prevents the accumulation of internal pressure that would otherwise lead to explosive eruptions. If the bubbles can rise and escape, the eruption remains effusive, producing lava flows. This contrasts with high-viscosity magmas, where trapped gases cannot escape easily, leading to a buildup of pressure that results in violent, explosive eruptions.