Unlike many united states cities, where a river is no longer the focal point of urban life, the river in san antonio winds through the middle of the business district, and the river walk, or paseo del rio, is the city's most popular attraction. (a) unlike many united states cities, where a river is no longer the focal point of urban life, the river in san antonio (b) unlike the river in many cities in the united states, which is no longer the focal point of urban life, in san antonio the river (c) today the river in many cities in the united states is no longer the focal point of urban life, unlike san antonio, where it (d) in few united states cities today, a river is the focal point of urban life, but the river in san antonio (e) no longer do many cities in the united states have a river as the focal point of urban life, but in san antonio the river

the earth’s crust is broken into separate pieces called tectonic plates (fig. 7.14). recall that the crust is the solid, rocky, outer shell of the planet. it is composed of two distinctly different types of material: the less-dense continental crust and the more-dense oceanic crust. both types of crust rest atop solid, upper mantle material. the upper mantle, in turn, floats on a denser layer of lower mantle that is much like thick molten tar.

each tectonic plate is free-floating and can move independently. earthquakes and volcanoes are the direct result of the movement of tectonic plates at fault lines. the term fault is used to describe the boundary between tectonic plates. most of the earthquakes and volcanoes around the pacific ocean basin—a pattern known as the “ring of fire”—are due to the movement of tectonic plates in this region. other observable results of short-term plate movement include the gradual widening of the great rift lakes in eastern africa and the rising of the himalayan mountain range. the motion of plates can be described in four general patterns:

collision: when two continental plates are shoved together

subduction: when one plate plunges beneath another (fig. 7.15)

spreading: when two plates are pushed apart (fig. 7.15)

transform faulting: when two plates slide past each other (fig. 7.15)

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