Page 441 - Global Tectonics

IMPLICATIONS OF PLATE TECTONICS 423

have an appropriate orientation with respect to the ardentes. Silica- and volatile-poor tholeiitic magmas,

present day stress field. The recurrence interval such as those of Iceland and Hawaii, give rise to quiet

between earthquakes in a particular intra-plate region extrusion of low viscosity lavas that flow readily at their
is typically very long – hundreds or thousands of years. source but slow as they cool and fill topographic depres-

The long intervals between earthquakes in a within- sions. Affected populations can usually avoid advancing
plate region and between very large magnitude earth- lava flows but not clouds of incandescent ash. The

quakes in the main earthquake regions means that pumice and ash that build up on the slopes of explosive
populations are often surprised and ill-prepared when volcanoes can also be remobilized by torrential rains
an earthquake does occur. that are triggered by the cloud of dust particles that
Major volcanic eruptions are even less frequent but accompanies the eruption. The resulting mudfl ows

unlike the situation for earthquakes the techniques for then sweep down the flanks with great speed, gathering
providing useful predictions of volcanic eruptions are more material and ultimately engulfing towns and vil-

showing greater promise and should increasingly save lages in their path.
lives. The nature of volcanic eruptions depends on the Devastating as the natural hazards associated with
chemistry of the magma involved. The relatively silica- earthquakes and volcanoes are they are an inevitable
and volatile-rich magmas of island arc volcanoes give consequence of the dynamic state of the Earth’s inte-
rise to explosive eruptions as a result of their relatively rior. Without such processes the Earth would not be
low temperature, high viscosity, and high gas content. such a distinctive planet, not only in terms of its surface
Such volcanoes give rise to large volumes of ash, slow features and the concentration of energy and mineral
moving, viscous lavas, and at worst turbulent clouds of resources at and near its surface, but also, in all probabil-
superheated gases and incandescent ash known as nuees ity, in terms of the origin and evolution of life.