Natural Hazards: Causes and Effects - Disaster Management Center ...

A map of the world’s 500 or so volcanoes that have erupted in historic times shows clearly
defined volcanic belts (see Fig. 4.1), with the circum-Pacific belt containing the majority. The
other obvious belt is in a line down the middle of the Atlantic Ocean, and these volcanic zones
correlate with two other worldwide patterns—the distribution of earthquakes and the plate
boundaries of the earth’s crust. Like earthquakes, volcanoes are essentially plate-boundary
phenomena that indicate the enormous geological forces involved where the plates of the
earth’s crust move against each other. The Pacific “Ring of Fire” and the mid-Atlantic line are
well-known plate boundaries, and the Mediterranean and East and West Indies boundaries also
have their share of volcanoes. But there are other volcano areas, and the concentration in East
Africa is particularly obvious. The volcanoes there are associated with the Rift Valley system,
which is probably a very young plate boundary where Africa would be splitting apart if it were
not for the far more powerful mid-Atlantic plate boundary compressing Africa. But even if we
call the African Rift Valley a plate boundary, other volcanoes are exceptions to the rule. The
temporarily inactive volcanoes of the Saharan Tibesti mountains, as well as the better known
and more active ones on Hawaii, are situated in the middle of plates. Totally unrelated to plate
boundaries, they owe their origins to hot spots in the earth’s internal structure, that have
penetrated through the overlying plates. 7
Natural Preconditions for Disaster
To understand the forces and potential dangers of volcanoes it is necessary to understand what
they are and what causes them to occur. (See Fig. 4.2.)
The basic ingredients of a volcanic eruption are molten rock and an accumulation of gases.
Driven by buoyancy and gas pressure, the molten rock, which is lighter than the surrounding
solid rock, forces its way upward and may ultimately break through zones of weakness in the
earth’s crust. If so, an eruption begins, and the molten rock may pour from the vent as
nonexplosive lava flows, or it may shoot violently into the air as dense clouds of lava fragments.
Larger fragments fall back around the vent, and accumulations of fall-back fragments may move
downslope as ash flows under the force of gravity. Some of the finer ejected materials may be
carried by the wind only to fall to the ground many kilometers away. The finest ash particles
may be injected kilometers into the atmosphere and carried many times around the world by
stratospheric winds before settling out.
Molten rock that rises in volcanic vents from below the earth’s surface is known as magma.
After it erupts from a volcano it is called lava. Originating many tens of kilometers beneath the
ground, the ascending magma commonly contains some crystals, fragments of surrounding
(unmelted) rocks, and dissolved gases, but it is primarily a liquid composed principally of
oxygen, silicon, aluminum, iron, magnesium, calcium, sodium, potassium, titanium, and
manganese. Magmas also contain many other chemical elements in trace quantities. Upon
cooling, the liquid magma may precipitate crystals of various minerals until solidification is
complete, forming igneous rock.
Lava is red hot when it pours or blasts out of a vent but it soon changes to dark red, gray, black,
or some other color as it cools and solidifies. Very hot, gas-rich lava containing abundant iron
and magnesium is fluid and flows like hot tar, whereas cooler, gas-poor lava high in silicon,
sodium, and potassium flows sluggishly, like thick honey in some cases or in others like pasty,
blocky masses.
All magmas contain dissolved gases, and as they rise to the surface to erupt, the confining
pressures are reduced and the dissolved gases are liberated either quietly or explosively. If the