Environmental Engineering Reference
In-Depth Information
influences on radiation and moisture balances. Annual global average yields of 1-3 km
3
of new magma and 20 million tonnes of related sulphur dioxide can be matched or
exceeded in single large eruptions considered capable of a small but significant role in
global climatic change.
Plinian-style stratovolcano eruptions, named after Pliny the Younger's graphic
account of the eruption of Vesuvius (Italy) in AD 79, are the most violent. Blast flattened
a large area of forest on Mount St Helens (Washington State, USA) in 1980 and slope
disturbance disrupted hydrological and vegetation systems, generating landslides and
debris avalanches aided by catastrophic ice melt. A cubic kilometre of erupted magma
spread ash over 600 km
2
. Pinatubo (Philippines) erupted ten times this volume of ash and
a further 3 km
3
of dacite magma in pyroclastic flows and ash columns during 1991.
Herculaneum and Pompeii were destroyed by pyroclastic flows and ash falls during the
AD 79 eruption of Vesuvius, with over 25,000 casualties, and ash flows travelling at up
to 500 km h
−1
from Mount Pelée in the Caribbean killed 26,000 in 1902. Lahars of
liquefied ash and other debris, named from Indonesia, where they are a major hazard,
killed over 21,000 people around Nevado del Ruiz (Colombia) in 1985. Area impacts and
other destructive effects from the blast of these and other recent volcanoes often cover
10
3-4
km
2
.
magmas, with the uppermost 4 km above sea level. Other island shield volcanoes are
located in the Galapagos (Pacific Ocean), Azores, Cape Verde, Madeira and Canary
island groups (Atlantic Ocean). Basalt effusion rates are more easily assessed on land,
where they appear as narrow, linear fissures or vast flood basalts. The 100 km-long Laki
fissure, where the Atlantic mid-ocean ridge goes onshore in Iceland, discharged over
5000 m
3
per second for several weeks in 1793. Its sulphurous exhalations spoiled
Benjamin Franklin's visit to Paris, 2000 km away. More impressive still, the Late
Cenozoic Roza eruption in Oregon probably contributed peak flows of 1500 km
3
per
week to the Columbia basalt plateau.
THE ROCK CYCLE (2) METAMORPHIC PROCESSES AND
LANDSYSTEMS
Rock material is subjected during its formation to immediate
syngenesis
or progressive
diagenesis
. These changes in form commonly involve low-magnitude compaction by
rock or water overburden pressures,
dewatering
(dehydration),
degassing
and even small
thermal effects. They create textural and chemical changes generally regarded as the final
stages of lithification of previously unconsolidated rock material. Higher
temperature/pressure levels such as those experienced in tectonic activity, however,
trigger more substantial changes. Magmatization is the ultimate response to
temperature/pressure changes in the lithosphere. Similarly, tremendous mechanical forces
generate wholesale crustal reorganization through subduction or uplift. Both interlinked
processes transform original rock character beyond recognition.
Metamorphism
alters
texture or mineralogy permanently, without a liquid phase - i.e. short of melting.
Metasomatism
is minerochemical change through infusion by high-temperature fluids
