Volcanoes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 205

Non-explosive volcanoes produce two main types of eruptions.

(1) Flood lavas which are typically basaltic in composition. They are the

least hazardous type of volcanic eruption.

(2) Hawaiian-type eruptions which are similar to flood lavas except that

they can be faster flowing. Hawaiian eruptions also have the potential
to produce tephra. These volcanoes can also produce large cones due to
their aperiodic eruptions that build up successive deposits over time.

Impacts

There are six main hazards associated with volcanic eruptions. These
are lava flows, basaltic and tephra clouds, pyroclastic flows and base surges,
gases and acid rain, lahars (mud flows) and Jokülhlaups (glacier burst) (Bryant,
2005). Volcanic eruptions also have the potential to produce tsunamis. Volcanic
eruptions can be associated with earthquakes and may be triggered by such
events.

Lava f lows

The velocity at which lavas travel depends upon their viscosity. The low-
viscosity Hawaiian and Icelandic flows tend to move the fastest. They are difficult
to stop, depending upon the type of lava as some, such as Pahoehoe, cool more
quickly than aa lava. Attempts to cool lava down, build barriers or divert its
movement have had mixed levels of success.

Ballistics and tephra clouds

Volcanic ballistics include up to boulder size accumulations of magma
and hot rock material which can be projected relatively high into the air above
the volcano. Tephra or ash can be projected tens of kilometres into the atmo-
sphere and travel around the globe. This material can cover homes and farm
land for many kilometres around the erupting volcano.

Pyroclastic f lows and base surges

Pyroclastic flows occur as the ash column collapses under gravity during
Peleean-type eruptions. Basal surges are produced during the lateral explosion
of volcanoes and can pick up large amounts of dust in addition to ash from the
volcano itself to potentially reach speeds of more than 150 km h − 1 .Pyroclastic
flows are avalanches of an exceedingly dense mass of hot, highly charged gas
and fine particles. Pyroclastic avalanches can be very hot, having temperatures
exceeding 1000 ◦ Cinextremeeruptions. As pyroclastic flows can also travel
rapidly at speeds of upto200ms − 1 adestructive flow is created that can

overtop high topographic features.