Some of volcanic hazards include gases, lava and pyroclastic flows, landslides and earthquakes and explosive eruptions.
The eruptions, while sometimes are relatively quiet they produce lava flows that creep across the land moving between 2 and 10 miles per hour. These resulting lava flows stream molten rock which destroys everything in its path. It usually moves slowly enough the people can move out of its way. These eruptions can trigger other natural hazards such as earthquakes, mudflows and flash floods, rock falls and landslides. Wild fires and tsunamis are additional hazards that might be triggered by a volcanic eruption.
The mudflows and the flow of debris from these are often one of the deadliest volcano hazards. This is the flow of mud, rock and water that rush down valley and stream channels reaching speeds of up to 40 mph and can travel more than 50 miles. These mudflows look like fast-moving rivers of wet concrete. These can occur during a volcano’s eruption or when the volcano is quiet. The water creating the flow occurs from melting snow and ice or intense rainfall. Glacier-clad volcanoes, such as Mount Rainier in Washington carry the greatest potential for such a mudflow. http://www.disastersrus.org.
The explosive eruptions can shoot columns of gases, ash and rock fragments miles into the atmosphere reaching hundreds of miles downwind.
Volcanic ash, which is the fine glassy rock fragments can affect people and equipment hundreds of miles away from the volcano. This ash contaminates water supplies, can cause electrical storms, disrupt operation of machinery and air traffic.
For this reason many federal agencies, including the U.S. Geological Survey (USGS), the Federal Aviation Administration, and the NOAA/National Weather Service, work together to issue warnings to airports and pilots. http://www.disastersrus.org.
Frederick K. Lutgens and Edward J. Tarbuck (2011). Foundations of Earth Science Sixth Edition. New Jersey: Pearson Education inc.