Lava dome

circular protrusion from slowly extruded viscous volcanic lava; 6% of eruptions form lava domes; content varies from basalt to rhyolite; the viscosity is due to either high silica levels or fluid magma degassing; most preserved domes are the former

In volcanology, a lava dome or plug dome is a round-shaped mound created by the eruption of viscous lava by a volcano.

Lava domes in the crater of Mount St. Helens.

Lava domes are formed when erupting lava is too thick to flow and makes a steep-sided mound as the lava piles up near the volcanic vent. Lava domes have formed in Mount St. Helens and experts say a lava dome is growing inside the Shinmoedake volcano in Japan.