Wind carries ash; water turns loose volcanic debris into lahars. Hazards can continue after the eruption has ended.
Tephra means volcanic fragments, from large blocks to fine ash. Ash is made of tiny rock, mineral and glass fragments. Carried downwind, it can reduce visibility, contaminate water, damage machinery and disrupt flights. Thick deposits damage crops and vegetation. Wet ash is especially heavy and can overload roofs.
Volcanoes release water vapour, carbon dioxide and sulfur dioxide. Their danger depends on concentration and location. Carbon dioxide can collect in low or confined places and displace breathable air. Sulfur dioxide irritates breathing passages and contributes to acidic pollution. Gases may be released during unrest or steadily between major explosions.
A lahar is flowing water mixed with volcanic sediment and debris. The water may come from rain, melting snow or ice, crater lakes or saturated ground. Lahars can rush along valleys, bury farms and settlements, destroy bridges and change river channels. Their water-rich mixture differs from a hot pyroclastic flow.
Rain can remobilise loose deposits long after an eruption, exposing downstream communities repeatedly. At Mount Rainier, USGS identifies snow and ice, weakened rock and downstream valleys as important factors. The example explains the hazard; it does not predict an event on a particular date.
Step by step
Material is released
Eruptions deposit tephra; volcanoes can also release gas between eruptions.
Wind spreads ash
Downwind communities and flight routes may be exposed far from the vent.
Water moves debris
Rain or meltwater can turn loose deposits into lahars long after the eruption.
Worked example: The wind changes the footprint
Two settlements are equally far from a volcano. If ash-bearing winds blow towards one, it may receive much heavier ash fall than the other. Distance remains useful, but wind direction helps explain the unequal exposure.
Watch out for this
Lahars can happen only while fresh lava is erupting.
Rain, meltwater or other water sources can mobilise existing volcanic deposits, including after the main eruption.
Check your understanding
Heavy rain falls on loose volcanic deposits several months after an eruption. Which hazard can develop?
- Only a new earthquake, because old deposits cannot move.
- A lahar carrying water and volcanic debris down valleys.
- A magnetic reversal caused by wet ash.