Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 29

FIGURE 2.7 Schematic view of plate movement through sea-floor spreading and subduction. New
crustal material is being created at the mid-ocean ridge (diverging and accreting plate margin),
whereas oceanic crust and lithosphere are being consumed back into the Earth at the deep-sea
trenches (converging plate boundary). Accordingly, the central ocean in this sketch is increasing
in size while the one on the left is closing. (Adapted from various sources.)
Although the discoveries about sea-floor spreading fitted into the context of contin-
ental drift as it was conceived in the 1960s, they also raised new questions. If new ma-
terial was rising and moving away from the mid-ocean ridges that formed the divergent
margin of a plate, what was happening at the other end—the convergent margin of a
plate? It was soon recognized that where plates came together a deep-sea trench de-
veloped and one plate sank, or subducted, beneath the other, to be consumed deep in
the mantle. Subduction involes dense oceanic lithosphere sinking back into the mantle,
largely under its own weight (Fig. 2.7). The evidence for this process comes mainly from
seismology (Tatsumi 2005).
Most volcanic and earthquake activity occurs along plate edges as direct by-products
of rifting, plate movement, and subduction. Four basic types of areas have earthquakes
(Turcotte and Schuster 2002). Along the mid-ocean ridges, high heat flow and volcanic
activity is caused by the stretching of the Earth's surface (Fig. 2.8). Mid-ocean earth-
quakes there are generally of shallow focus, originating at depths of less than 70 km
(∼44 mi). Second, shallow earthquakes occur along transform faults, where one section
of the Earth is sliding past another; examples include the San Andreas Fault in Califor-
nia and the Dead Sea-Jordan Fault in the Middle East, along which so many earthquakes
occurred in antiquity. Third, a belt of shallow-and intermediate-focus earthquakes ex-
tends from the Alps to the Himalayas; it is apparently associated with compressive
forces responsible for the creation of these mountains. In general, shallow-focus earth-
quakes pose the greatest danger because they are the most numerous and closest to
where people live. Other earthquake areas are the deep-sea trenches and volcanic is-
land arcs (Fig. 2.9). The foci of earthquakes occurring in these regions may be shallow,
intermediate, or as deep as 700 km (∼440 mi), depending upon their exact location in
the subduction zone (Strahler 1998; Lliboutry 1999). Independent tracing of the differ-