4 THE MID-OCEANIC RIDGE
䊳 11.4 THE MID-OCEANIC RIDGE
ridge (Fig. 11–6). To return to our analogy, imagine that a can of white spray paint were mounted above
The extensive use of submarines during World War II two black conveyor belts moving apart. If someone
made it essential to have topographic maps of the sea sprayed paint at regular intervals as the conveyor
floor. Those maps, made with early versions of the echo belts moved, symmetric white and black stripes
sounder, were kept secret by the military. When they be- would appear on both belts.
came available to the public after peace was restored, scientists were surprised to learn that the ocean floor has
At about the same time that oceanographers discov- at least as much topographic diversity and relief as the ered the magnetic stripes on the sea floor, they also be-
continents (Fig. 11–7a). Broad plains, high peaks, and gan to sample the mud lying on the sea floor. They
deep valleys form a varied and fascinating submarine discovered that the mud is thinnest at the mid-Atlantic
landscape, but the mid-oceanic ridge is the most impres- ridge and becomes progressively thicker at greater dis-
sive feature of the deep sea floor. tances from the ridge. Mud falls to the sea floor at about
The mid-oceanic ridge system is a continuous sub- the same rate everywhere in the open ocean. It is thinnest
marine mountain chain that encircles the globe (Fig. at the ridge because the sea floor is youngest there; the
11–7b). Its total length exceeds 80,000 kilometers and it mud thickens with increasing distance from the ridge
is more than 1500 kilometers wide in places. The ridge because the sea floor becomes progressively older away
rises an average of 3 kilometers above the surrounding from the ridge.
Oceanographers soon recognized similar magnetic stripes and sediment thickness trends along other por- 1 Hess and Dietz proposed the sea-floor spreading hypothesis in 1960,
tions of the mid-oceanic ridge. As a result, they proposed prior to Vine and Matthews’s 1963 interpretation of sea-floor mag-
netic stripes, but the hypothesis received widespread attention only
the hypothesis of sea-floor spreading to explain the ori-
after 1963.
(a)
Mid- Atlantic Ridge
East Pacific Rise
Mid-Indian Ridge
(b) Figure 11–7 (a) Sea-floor topography is dominated by undersea mountain chains called mid-oceanic ridges and deep trenches called subduction zones. The green areas represent the relatively level portion of the ocean floor; the yellow-orange-red hues are mountains, and the blue-violet-magenta areas are trenches. (Scripps Institution of Oceanography, University of California, San Diego.) (b) A map of the ocean floor showing the mid-oceanic ridge in red. Double lines show the ridge axis; single lines are transform faults. Note that the deep sea mountain chain shown in (a) corresponds to the mid-oceanic ridge shown in (b).
The Mid-Oceanic Ridge 185
1 Mid-oceanic ridge
Figure 11–8 The sea floor sinks as it grows older. At the mid-
4 oceanic ridge, new lithosphere is buoyant because it is hot and of
5 low density. It ages, cools, thickens, Depth of sea floor (km)
and becomes denser as it moves 6
away from the ridge and conse- 7
quently sinks. The central portion of 0 20 40 60 80 10 0120140160180 20 0 the sea floor lies at a depth of
Age of oceanic crust (millions of years)
about 5 kilometers.
deep sea floor. It covers more than 20 percent of the separate (Fig. 11–9). Blocks of crust drop downward Earth’s surface—nearly as much as all continents com-
along the sea-floor cracks, forming the rift valley. bined—and is made up primarily of undeformed basalt.
Hundreds of fractures called transform faults cut On the mid-Atlantic ridge, which is a segment of the
across the rift valley and the ridge (Fig. 11–10). These mid-oceanic ridge, a rift valley 1 to 2 kilometers deep
fractures extend through the entire thickness of the litho- and several kilometers wide splits the ridge crest. In
sphere. They develop because the mid-oceanic ridge 1974, French and American scientists used small re-
actually consists of many short segments. Each seg- search submarines to dive into the rift valley. They saw
ment is slightly offset from adjacent segments by a trans- gaping vertical cracks up to 3 meters wide on the floor
form fault. Transform faults are original features of the of the rift. Nearby were basalt flows so young that they
mid-oceanic ridge; they develop when lithospheric were not covered by any mud. Recall that the mid-oceanic
spreading begins.
ridge is a spreading center. The cracks formed when brit- Some transform faults displace the ridge by less tle oceanic crust separated at the ridge axis. Basaltic
than a kilometer, but others offset the ridge by hundreds magma then rose through the cracks and flowed onto the floor of the rift valley. This basalt became new oceanic crust as two lithospheric plates spread outward from the ridge axis. Not all spreading centers have rift valleys as
Mid-oceanic ridge
deep and as wide as those of the mid-Atlantic ridge. The new crust (and the underlying lithosphere) at
Earthquakes
the ridge axis is hot and therefore of relatively low den-
along faults
Rift
sity. Its buoyancy causes it to float high above the sur- valley rounding sea floor, forming the submarine mountain
Oceanic
crust
chain called the mid-oceanic ridge system. The new
lithosphere cools as it spreads away from the ridge. As a ★ result of cooling, it becomes thicker and denser and sinks
to lower elevations, forming the deeper sea floor on both sides of the ridge (Fig. 11–8).
The heat flow (the rate at which heat flows outward from the Earth’s surface) at the ridge is several times greater than that in other parts of the ocean basins. A
Normal
great deal of Earth heat escapes from the ridge axis be-
faults
Basaltic
cause the spreading lithosphere is stretched thin under
magma
the ridge, and the hot asthenosphere bulges toward the
A cross-sectional view of the central rift valley surface. Rising magma carries additional heat upward.
Figure 11–9
in the mid-oceanic ridge. As the plates separate, blocks of rock Shallow earthquakes are common at the mid-oceanic
drop down along the fractures to form the rift valley. The ridge because oceanic crust fractures as the two plates
moving blocks cause earthquakes.
186 CHAPTER 11 OCEAN BASINS
Mid-oceanic ridge
Rift valley
Transform fault
Steep cliffs on transform fault
Figure 11–10 Transform faults offset segments of the mid-oceanic ridge. Adjacent seg- ments of the ridge may be separated by steep cliffs 3 kilometers high. Note the flat abyssal plain far from the ridge.
of kilometers. In some cases, a transform fault can grow ocean floor on each side of the fault moves in opposite so large that it forms a transform plate boundary. The
directions (Fig. 11–11). Earthquakes rarely occur on San Andreas fault in California is a transform plate
transform faults beyond the ridge crest, however, be- boundary.
cause there the sea floor on both sides of the fault moves Shallow earthquakes occur along a transform fault
in the same direction.
between offset segments of the ridge crest. Here the
No earthquakes
Rising magma
Figure 11–11 Shallow earthquakes (stars) occur along the ridge axis and on the trans- form faults between ridge segments, where plates move in opposite directions; no earth- quakes occur on the transform faults beyond the ridge segments, where plates move in the same direction.
The Mid-Oceanic Ridge 187