Laboratory Manual for Introductory Geology 4e

EXERCISE 2.8
Continental Rifting: Splitting East Africa to Form the Next Ocean (continued)
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A new mid-ocean ridge and oceanic lithosphere have formed in the southern two-thirds of the Red Sea, where a narrow
belt of slightly deeper water defines the trace of the ridge. At the northern end of the Red Sea, the ridge/rift axis is cut by a
transform fault that runs along the eastern side of the Sinai Peninsula and through the Dead Sea.
(a) Draw a red line on the map to show the trace of the Red Sea ridge/rift axis. Draw a purple line to show the trace of
the Dead Sea transform fault.
(b) The narrow arm of the Indian Ocean bordering the southeastern edge of the Arabian Peninsula is the Gulf of Aden.
Draw red and purple lines to trace the ridge segments and transform fault in this narrow sea.
(c) Consider the geometry of the ridges and transform faults in the Red Sea and the Gulf of Aden, and draw an arrow
showing the motion of the Arabian Peninsula (the Arabian Plate) relative to Africa.
2.5.3 Subduction Zones: Deducing the Steepness
of Subduction
When two oceanic plates collide head-on at a convergent boundary, one is subducted
beneath the other and returns to the mantle, completing the tectonic
cycle begun when the oceanic crust initially erupted at a mid-ocean ridge. Melting
occurs in the mantle above the subducted plate when the plate reaches depths of
100 to 150 km. As a result, the volcanic arc associated with a subduction zone forms
at some distance from the deep-sea trench. The area between the volcanic arc
and the trench, called the arc-trench gap, typically contains an accretionary prism,
which is composed of highly deformed sediment scraped off the subducting plate,
and a forearc basin, in which debris eroded from the arc accumulates (FIG. 2.12).
No two subduction zones are identical. Their differences may include the width
of the accretionary prism, the rate of subduction, and the steepness of the subducted
plate as it moves into the mantle. Earthquakes occur in the subducted plate
as it moves, concentrated in what geologists call a Wadati-Benioff zone. We can track
the plate as it is subducted by the depths and locations of earthquake foci (points
where energy is released).
FIGURE 2.12 Anatomy of an island arc–trench system.
Arc trench gap
Volcanic island arc
Fore-arc
basin
Accretionary
prism
Trench
Oceananic lithosphere
Asthenosphere
2.5 PROCESSES AT PLATE BOUNDARIES REVEALED BY EARTH FEATURES
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