Why do some volcanoes have steep, explosive cones while others spread out as broad, gentle slopes?
Quick Answer
The shape mostly depends on magma thickness and gas content. Runny basaltic lava makes broad shield volcanoes, while sticky, silica-rich magma traps gas and builds steep composite volcanoes.
The Full Story
Volcano shape is strongly controlled by how easily magma flows. Low-silica basalt magma is hot and fluid, so it travels far before cooling and forms wide, low-angle shield volcanoes like those in Hawaii. High-silica magma is more viscous, traps more gas, and tends to erupt explosively, piling up layers of ash and lava to form steep stratovolcanoes such as Japan’s Mount Fuji or Indonesia’s Mount Merapi. Repeated eruptions over thousands of years build these contrasting landforms. The chemistry of the magma, especially silica content, is the key reason volcanoes can look so different.
Key Facts
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