Volcanoes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 206

Gases and acid rains
Gas can be emitted both during volcanic eruptions and from passive vol-
canoes and can be transported as acid aerosols, compounds absorbed on tephra
particles or as salt particles. Large amounts of fluorine and chlorine are released
into the atmosphere by passive volcanoes. In the presence of water during an
eruption, hydrochloric, sulphuric, carbonic and hydrofluoric acids may develop.
These gas particles, released as light acid rains, can stick to vegetation and kill
crops or native vegetation. The acid rain can also cause skin irritations.
Lahars
Both primary and secondary lahars can occur during and after eruptions
respectively. Lahars are mudflows that can be generated by pyroclastic flows or
crater lake eruptions or saturation of fine deposited materials. These mudflows
have the potential to destroy farmland through burying and pose a great threat
to human lives depending on their size.
Glacier bursts
Glacier bursts are floods of heated water mixed with mud debris follow-
ing the melting of snow and ice caps during a volcanic eruption. These muddy
flows have the potential to threaten human lives. They may also flood the land-
scape and destroy crops and structures. Lahars can develop if these floods entrain
sufficient mud.
Magnitude
The eruption of Tambora on the island of Sumbawa, Indonesia is
regarded as the largest volcanic explosion of recent times (see Tables 8.1 and
8.2). In April 1815, it threw ash 20 km into the atmosphere and pieces of pumice
up to 13 cm across landed over 40 km away. Tephra deposits were greater than
1.5mthickat the base of the volcano and over 50 cm thick 150 km away. Tamb-
ora ejected over 100 km 3 of material and killed over 100 000 people. Fatalities
occurred directly from volcanic activity and also because of starvation due to
ash smothering agricultural land.
The eruption of Mount Vesuvius, in the Bay of Naples on the west coast of
Italy in AD 79, was one of the largest volcanic eruptions ever witnessed (Cioni
et al ., 2000). It was of the Plinian type. Plinian eruptions are labelled after the
Vesuvius eruption as the eyewitness Pliny the Younger was the first to describe
this type of eruption. After a day of large ash expulsion, a series of six surges
and pyroclastic flows occurred in the area below the volcano. Almost 50% of