Lava domes are most commonly composed of sticky (high viscosity) lava, they are usually rhyolite or dacite in composition. This lava is to viscous to flow very far so it piles up over the opening to form mound shaped hills that very from the height of a human to those that are more than 2000 feet high.
Domes are steep sided and are usually emplaced following explosive discharge of more fluid, volatile-rich magma. Dome emplacement characterizes the late stage activity of mature volcanoes. Domes occur within the summit craters of composite volcanoes and within or along the margins of calderas.
Domes are glass-rich (lots of obsidian) and commonly exhibit flow banding. The upper surface of a dome is covered by fragments formed as the outer part of the dome cooled and then the hot interior expanded upward. Dome growth may be so rapid that the sides become over steepened and collapse. This sends avalanches of blocky debris down the flanks of the volcano. The Dome can also explode leading to the formation of small pyroclastic flows.