Lava flows destroy everything in their path, but most move slowly enough that people can move out of the way. Volcanic Gases are released by explosive eruptions and lava flows. Despite the toxic nature of these gases they have only rarely been the direct cause of a major disaster and death. Ground Deformation, Lahars, Landslides, Tsunamis Ground deformation occurs as volcanoes swell due to the magma welling up within and as slopes bend as this expelled out onto them. Such deformation is not a hazard in itself but can cause various mass movement hazards.
Lahars are volcanic mudflows.
They occur in the flanks of volcanoes, especially in the wet tropics. Apart from pyroclastic flows they are the greatest threat to human life. They can occur in any volcanic event when large quantities of water are present on the steep sides of a volcano. Landslides and debris avalanches are a common feature of volcanic-related ground failure. As mentioned an eruption can cause ground deformation which can cause landslides.