UNIT 31
Plate Movement : Causes
and Effects
Figure 31.1 A highly simplified view of convection cells in the mantle. Hot rock rises at A, spreads toward B, and in the process,
pushes or drags a lithospheric plate in the same direction. At the spreading midoceanic ridge, new oceanic lithosphere is being
created from some of the upwelling rock. As the rock below the lithosphere slowly spreads toward B, it cools and eventually sinks
into the mantle where it is reheated. Litho-sphere destroyed in the subduction zone at B may become part of the cell of flowing rock
as well. the depth reached by the lower part of each cell is currently a matter of scientific debate.
Figure 31.2 The Himalayas form an awesome mountain wall when seen from the south. the gravitational pull of these massive
mountains caused errors in the British survey of India. this is central Nepal (a small country wedged between India’s Ganges Plain
and the Himalayan massif) in the vicinity of Annapurna, the world’s eleventh tallest peak.
Figure 31.3 Isostasy. The distribution and behavior of crust and mantle are analogous to blocks of ice floating in water (A).
Note that no matter how thick the block, the same percentage (10 per-cent/90 percent) floats above and below the surface.
each block is therefore in balance. the ratio for mountains and their “roots” of crustal rock is a little higher, but the principle is
the same (b).
Figure 31.3 Isostasy. The distribution and behavior of crust and mantle are analogous to blocks of ice floating in water (A). Note
that no matter how thick the block, the same percentage (10 per-cent/90 percent) floats above and below the surface. each block
is therefore in balance. the ratio for mountains and their “roots” of crustal rock is a little higher, but the principle is the same (b).
Figure 31.4 View from Pilot Mountain, North carolina, overlooking the Piedmont, which yields eastward to the coastal plain. A vista like this
suggests that the mountains are being eroded into lowland topography flanking them, but as the text points out, things are not that simple.
Figure 31.5 A plume of muddy water enters the ocean at the mouth of china’s Huang He (Yellow River). this river carries vast quantities
of material eroded from the wind-blown silt deposits of the Loess Plateau and landslide-prone mountain valleys in western china. Much of
the sediment is deposited in shallow water near the river mouth, and it accumulates so rapidly that the coastline has shifted tens of
kilometers out into the ocean in the last 200 years alone. As this sediment piles up, the lithosphere flexes downward so that the river
delta always remains near sea level.
Figure 31.6 “Traversing Glacier bay in southeast Alaska, we were given a seminar by a National Park service guide who enjoyed
asking challenging questions. Knowing some physical geography helped, but here she had me stumped. ‘Look at that outcrop,’ she
said. ‘What can you tell me about it?’ I said that the rocks looked darker than the regional gray-granite masses rising steeply from
the bay, but in the absence of any knowledge of volcanic activity here, I could not do any better. You’re looking at lava,’ she
explained; ‘this is a fragment of one of those suspect terranes, an old basaltic island arc welded onto the local regional geology. it’s
called Wrangellia, and pieces of it can be identified from mainland Alaska all the way down the coast to British Columbia and the
U.S. Northwest. it’s out of place and we don’t know how it got here, but it sure stands out in this landscape.’ that is the kind of field
experience you don’t forget.”
Figure 31.7 Specimen of gold from a quartz vein. Weathering and erosion of the rock that contained this vein would eventually have
released nuggets and flakes of gold to be carried by streams.