Page 402 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

366 part III The Earth–Atmosphere Interface
Age (millions of years)
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▲Figure 12.17 relative ages of oceanic crust. Compare the width of the red colour (young crust) near the East Pacific rise in the eastern Pacific Ocean with the width of the red colour along the Mid-Atlantic ridge. What does the difference tell you about the rates of plate motion in the two locations? [Image by Elliot Lim, CIrES.]
discoveries were an important step on the way to com- pleting the theory of plate tectonics.
Age of the Seafloor The youngest crust anywhere on Earth is at the spreading centres of the mid-ocean ridges, and with increasing distance from these cen- tres, the crust gets steadily older (Figure 12.17). Over- all, the seafloor is relatively young; nowhere is it more than 280 million years old, which is remarkable when you remember that Earth is 4.6 billion years old. In the Atlantic Ocean, the oldest large-scale area of seafloor is along the continental margins, farthest from the Mid- Atlantic Ridge. In the Pacific, the oldest seafloor is in the western region near Japan (dating to the Jurassic Pe- riod). Note on the map the distance between this part of the basin and its spreading centre in the South Pacific, west of South America. Parts of the Mediterranean Sea contain the oldest seafloor remnants, which may have been part of the Tethys Sea, dating to about 280 m.y.a (see Figure 12.14).
Today, scientists know that mid-ocean ridges occur where plates are moving apart (Figure 12.18). As the sea- floor spreads, magma rises and accumulates in magma chambers beneath the centreline of the ridge. Some of the magma rises and erupts through fractures and small volcanoes along the ridge, forming new oceanic crust. Scientists now think that this upward movement of material beneath an ocean ridge is a consequence of
seafloor spreading rather than the cause. As the plates continue to move apart, more magma rises from below to fill the gaps.
Subduction
When one portion of the lithosphere descends beneath another and dives downward into the mantle, the pro- cess is called subduction and the area is a subduction zone. As discussed earlier, the basaltic ocean crust has an average density of 3.0 g·cm−3, whereas continental crust averages a lighter 2.7 g·cm−3. As a result, when continental crust and oceanic crust slowly collide, the denser ocean floor will grind beneath the lighter conti- nental crust, thus forming a subduction zone.
The world’s deep-ocean trenches coincide with these subduction zones and are the lowest features on Earth’s surface. The Mariana Trench near Guam is the deep- est, descending below sea level to –11030 m. The Tonga Trench, also in the Pacific, is the next deepest, dropping to –10882 m. For comparison, in the Atlantic Ocean, the Puerto Rico Trench drops to –8605 m, and in the Indian Ocean, the Java Trench drops to –7125 m.
Subduction occurs where plates are colliding. The subducting slab of crust exerts a gravitational pull on the rest of the plate—a pull now known to be an important driving force in plate motion. The subducted portion trav- els down into the asthenosphere, where it remelts and