chapter

sections of the East Pacific Rise. Although these rates of
lithospheric production are slow on a human time scale,
they are nevertheless rapid enough so that all of Earth’s
ocean basins could have been generated within the last
200 million years. In fact, none of the ocean floor that has
been dated exceeds 180 million years in age.
During seafloor spreading, the magma that is injected
into newly developed fractures forms dikes that tend
to cool from their outer borders inward toward their centers.
Because the warm interiors of these newly formed
dikes are weak, continued spreading produces new fractures
that split these young rocks roughly in half. As a result,
new material is added about equally to the two
diverging plates. Consequently, new ocean floor grows
symmetrically on each side of a centrally located ridge
A.
B.
C.
D.
Continental crust
Rift
valley
Upwarping
Linear sea
Oceanic ridge
Chapter 7 Plate Tectonics 203
crest. Indeed, the ridge systems of the Atlantic and Indian
oceans are located near the middle of these water bodies
and as a consequence are called mid-ocean ridges.
However, the East Pacific Rise is situated far from the
center of the Pacific Ocean. Despite uniform spreading
along the East Pacific Rise, much of the Pacific Basin that
once lay east of this spreading center has been overridden
by the westward migration of the American plates.
Spreading Rates
and Ridge Topography
When various segments of the oceanic ridge system
were studied in detail, some topographic differences
came to light. Most of these differences appear to be
Rift
Oceanic crust
Continental
crust
Figure 7.10 A. Rising magma forces the crust upward, causing numerous cracks in the rigid lithosphere. B. As the crust is
pulled apart, large slabs of rock sink, generating a rift zone. C. Further spreading generates a narrow sea. D. Eventually, an
expansive ocean basin and ridge system are created.