Areas around volcanoes are at risk from a number of different hazards. Lava flows produced during an eruption can flow overland or down stream valleys and destroy everything in their path. The risk areas for lava flows are determied by the terrain - where the land surface is flat, lava flows will spread out over broad areas, but where the terrain is hilly the lava flows will move down stream valleys. A study of where lava flows have advanced during past eruptions can provide good information of their potential future path and travel distance. These areas can be plotted on maps and such maps can serve as important educational and land-use planning tools.
Lahars are mudflows produced during or after a volcanic eruption. They are the greatest threat to people and property around many volcanoes. If a volcanic eruption flash melts the snowcap on a volcanic peak the meltwater can quickly mix with ashfall debris and soil to produce a swift- moving mudflow. Lahars generally follow stream valleys, and can move at speeds over 50 miles per hour. They are deadly and very destructive and can travel over 50 miles downstream from the volcano that produced them. Heavy rains during or following an eruption can also produce lahars. Again, a study of historic eruptions can reveal where past lahars have travelled and the distance that they have reached beyond the base of the mountain.
Ashfalls, pyroclastic flows and landslides are additional hazards that can be shown on a volcanic hazard map. An overview of these types of hazards can be found at the US Geological Survey's Volcanic Hazards website. The map above is one of many produced by USGS for the Mount Rainier area. For more details on the map above visit their Volcanic Hazards from Mount Rainier webiste.