The structure and appearance of a volcano are a direct consequence of the molten rock material, or lava, that erupts from within the Earth. Different types of lava possess unique chemical and physical properties that determine how they flow, spread, and cool to construct the volcanic edifice. This relationship means the lava is the primary architect of the mountain, determining profiles ranging from steep sides to the wide, gently sloping profile of the shield volcano.
Identifying the Shield Volcano
The shield volcano is easily recognized by its distinctive morphology. It is named for its resemblance to a warrior’s shield lying on the ground, characterized by an enormous, broad base and a relatively low profile. The slopes of this type of volcano are notably gentle, typically ranging from a shallow two degrees near the base to about ten degrees closer to the summit area.
This low-angle profile is the accumulated result of countless eruptions, making shield volcanoes some of the largest volcanic structures on Earth in terms of volume. Unlike the sharp peak of a composite volcano, the shield shape is built up gradually by thin, widespread layers of lava. This construction style creates a mountain that is far wider than it is tall, with a convex shape that becomes even flatter near the central vent.
Basalt: The Low-Silica Lava
The type of lava that builds shield volcanoes is known as basalt, defined by its specific chemical makeup and high temperature. Basaltic lava is classified as a mafic rock, meaning it is rich in magnesium and iron. Its defining chemical characteristic is its low content of silica (SiO2), which typically ranges from 45 to 52 weight percent.
This low silica composition means the lava is less sticky than the high-silica andesitic or rhyolitic lavas that form explosive volcanoes. Basaltic lava also erupts at extremely high temperatures, generally between 1100°C and 1250°C. This combination of low silica and high temperature ensures the material remains highly fluid upon reaching the surface. The easy escape of gases from this fluid material contributes to the gentle, effusive nature of shield volcano eruptions.
How Low Viscosity Shapes the Volcano
The defining physical property of basaltic lava is its low viscosity, which is a measure of a fluid’s resistance to flow. Low viscosity means the lava is thin and extremely runny, often compared to the consistency of thin syrup or even ketchup. This fluidity is a direct result of the lava’s low silica content and its high temperature.
When erupted, this highly fluid lava flows quickly and can travel immense distances away from the vent, sometimes spreading more than 20 kilometers before solidifying. Because the lava spreads so widely and easily, it does not pile up steeply around the central vent. Instead, each successive flow forms a new, relatively thin layer that adds to the volcano’s ever-widening base, constructing the characteristic, low-angle shield shape.
The Two Primary Flow Types
Although all shield volcano lava is basaltic, it can solidify into two visually distinct surface textures depending on the conditions of the flow. These two primary flow types are known by the Hawaiian terms Pāhoehoe and ‘A’a. The difference between them is primarily due to the lava’s temperature, gas content, and the rate of shear strain as it moves.
Pāhoehoe lava forms when the flow is relatively slow and hot, resulting in a smooth, glassy, billowy, or ropey surface texture. This type of flow is often fed by well-insulated lava tubes, which keep the interior molten and prevent rapid cooling. In contrast, ‘A’a lava forms under conditions of faster movement, greater turbulence, or when the lava has cooled slightly and lost more gas. The increased shear strain causes the surface crust to tear and break into a rough, jagged, and clinkery heap of loose rock as the flow continues to move beneath.