Lava domes are composed of highly viscous lavas with high silica contents such as dacite or rhyolite. They are typically found in the craters of stratovolcanoes such as Mt.St. Helens or Soufriere Hills. The lava is exuded rather than erupted and is squeezed out like thick toothpaste. If they become unstable and collapse pyroclastic flows can be produced or tall eruption clouds.
Lassen peak in the cascade range is the worlds largest lava dome at over 10000 feet, and is made of dacite.