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Dome - lava 

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Mt St Helens lava dome and eruption USGS

Domes or lava domes are extrusions of very viscous lava typically dacite or rhyolite.To form a dome the lava has to have the consistency of toothpaste. The dome of lava can grow rapidly and may even engulf the original crater. As the dome grows it may become unstable and collapses then pyroclastic flows can and eruption columns can occur.Mt St Helens has extruded a lava dome within the 1980 crater. More recently the Soufriere Hills volcano on Montserrat has had an extended period of dome building and collapse since 1995. The pyroclastic flows generated have devastated most of this tiny Caribbean Island.