Nonexplosive eruptions occur when little to no gas is contained within the magma. These events produce small fire fountains and lava flows, such as those erupting from Kilauea and Mauna Loa.

Nonexplosive eruptions tend to be less deadly than explosive eruptions, but can still cause great disruption and destruction. Eruptions at Hawaiian-style volcanoes can occur at the summit or along the flanks. New eruptions typically begin with the opening of a fissure or long crack that spews molten lava into the air and sometimes forms lava flows.

Lava tends to flow rather slowly. Typically it is easy to outrun a lava flow but impossible to stop or divert it. People can escape, but homes and property are vulnerable.

Both explosive and nonexplosive eruptions release volcanic gases, producing a hazardous blend called volcanic fog or VOG. VOG contains aerosols — fine particles created when sulfur dioxide reacts with moisture in the air. It can cause health problems, damage crops and pollute water supplies. The health department warned residents in November that air quality around Mauna Loa would deteriorate because of hazards like VOG.

These particles have global consequences when eruptions eject them into the stratosphere, where they block sunlight, cooling Earth’s climate. This effect can cause widespread crop failure and famine and is responsible for many historic, volcanic-related deaths. For example, the 1815 explosive eruption of Tambora in Indonesia caused 92,000 starvation-related deaths.

Snow-capped volcanoes, such as those in the Cascades and Alaska, can produce mudflows or lahars. These hazards form when ice and snow melt during an eruption, or ash is washed loose from the surface by heavy rain.

Mudflows have tremendous energy and can travel up to 60 miles per hour down river valleys. They can destroy bridges, structures and anything else in their path. A mudflow from the 1985 eruption of Nevado del Ruiz in Colombia killed 25,000 people.