Lava flows from Mount St. Helens typically affect areas within 6 mi (10 km) of the vent. However, two basalt flows erupted about 1,700 years ago extended about 10 mi (16 km) from the summit; one of them contains the Ape Cave lava tube.
Lava flows typically follow stream drainages and spread out in areas of low relief. Few people live in areas subject to lava flows, but lava flows originating from Mount St. Helens could start forest fires and destroy roads and bridges. Lava flows that encounter snow and ice could cause melting and generate lahars.
Lava domes
Viscous lava may pile up over ventsto form steep-sided lava domes or thick, stubby lava flows. On steep slopes, such as the volcano's upper flanks, the parts of these lava masses may collapse, spawning avalanches of hot volcanic debris (block and ash flows that generate pyroclastic flows and surges.) Dome building can go on episodically for decades to centuries, and the dome-growth episodes in 1980-86 and 2004-08 will likely be followed by others. As the lava dome grows surrounded by Crater Glacier, there is an increased probability that lava–dome collapse may occur, swiftly melting snow and ice and generating lahars. If past trends are repeated, renewed dome growth may be preceded by a few days to weeks of heightened seismicity, ground deformation, and gas emission. Small to moderate explosions often herald the start of new eruptive activity.