A plug dome volcano is a rounded volcano containing very thick lava that congests or “plugs” easily. Plug dome volcanoes come in many different forms. However, they all possess viscous lava that cannot flow readily. These structures form through repeated outpourings of bulbous lava masses around the volcanoes’ vents.
Plug dome volcanoes expand from within. As they grow, the outer surface cools and hardens, then shatters, spilling fragments down its sides. The size and shape of plug dome volcanoes vary from one volcano to the next. The most notable differences between plug dome volcanoes are found in the shape of the dome. The form of the dome depends on a number of factors including the strength and viscosity of the lava, as well as the slope of the surrounding area.
Plug dome volcanoes can form wherever volcanic activity is present. Typically these formations occur in the craters of larger volcanoes. However, they also can form on the flanks or sides of a volcano. The location of the dome depends on the magmatic characteristics of the surrounding areas.
The Novarupta Dome in Alaska is one of the most widely researched and visited lava domes in the world. Formed during the 1912 eruption of the Katmai Volcano, the Novarupta Dome measures 200 feet high and 800 feet across.