What causes a shield volcano to be shaped like a broad dome?

Question: What causes a shield volcano to be shaped like a broad dome?

Volcanoes are fascinating natural phenomena that can produce spectacular eruptions of lava, ash and gas. But not all volcanoes are alike. Some are tall and steep, while others are low and wide. What makes them different? One of the main factors is the type of lava they erupt.

Lava is molten rock that flows out of a volcano's vent. The viscosity, or resistance to flow, of lava depends on its temperature, composition and gas content. High-viscosity lava is thick and sticky, while low-viscosity lava is thin and runny.

Shield volcanoes are named for their resemblance to a warrior's shield lying on the ground. They are formed by the eruption of low-viscosity lava, which travels farther and forms thinner flows than the more viscous lava erupted from a stratovolcano. Repeated eruptions result in the steady accumulation of broad sheets of lava, building up the shield volcano's distinctive form.

Shield volcanoes have gentle slopes that are less than 10 degrees and can cover large areas. They are usually composed of basaltic magma, which is high in temperature, very low in silica and gas content. This makes the lava runny and non-acidic, allowing it to flow long distances before it solidifies.

Shield volcanoes are found wherever fluid, low-silica lava reaches the surface of a rocky planet. However, they are most characteristic of ocean island volcanism associated with hot spots or with continental rift volcanism. They include the largest volcanoes on Earth, such as Mauna Loa in Hawaii, which is about 120 km across and 9 km high.

Shield volcanoes are not very explosive, as their low gas content prevents the build-up of pressure inside the magma chamber. However, they can produce spectacular lava fountains and rivers, as well as occasional pyroclastic flows and surges when the lava interacts with water or ice.

Shield volcanoes are among the most long-lived and active volcanoes on Earth. They can erupt for millions of years, creating vast volcanic edifices that shape the landscape and influence the climate.

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