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Iceland
Iceland

20

Cape Verde Islands
0

20

40 Mid-Atlantic
Ridge and
Rift Valley
120 100 80 60 40 20 0 20 40 60
0–5 million years
5–23 million years
FIGURE 18.10 The Mid-Atlantic Ridge divides the Atlantic
Ocean into two nearly equal parts. Where the ridge reaches above 23–38 million years
sea level, it makes oceanic islands, such as Iceland. 38–53 million years
53–65 million years
65–135 million years

Studies of the Mid-Atlantic Ridge found at least three related FIGURE 18.11 The pattern of seafloor ages on both sides of
the Mid-Atlantic Ridge reflects seafloor spreading activity. Younger
groups of data and observations: (1) Submarine earthquakes
rocks are found closer to the ridge.
were discovered and measured, but the earthquakes were all
observed to occur mostly in a narrow band under the crest of
the Mid-Atlantic Ridge. (2) A long, continuous rift, or valley,
was observed to run along the crest of the Mid- Atlantic Ridge to indicate that the Atlantic Ocean did not exist until 150 mil-
for its length. (3) A large amount of heat was found to be escap- lion years ago. At that time, a fissure formed between Africa
ing from the rift. One explanation of the related groups of find- and South America, and new materials have been continuously
ings is that the rift might be a crack in Earth’s crust, a fracture flowing, adding new crust to the edges of the fissure.
through which basaltic lava flowed to build up the ridge. The More convincing evidence for the support of seafloor
evidence of excessive heat flow, earthquakes along the crest of spreading came from the paleomagnetic discovery of patterns
the ridge, and the very presence of the ridge all led to a seafl oor of magnetic strips in the basaltic rocks of the ocean floor. Earth’s
spreading hypothesis (see the reading on Harry Hess on p. 469). magnetic field has been reversed many times in the last 150 mil-
This hypothesis explained that hot, molten rock moved up from lion years. The periods of time between each reversal were not
the interior of Earth to emerge along the rift, flowing out in both equal, ranging from thousands to millions of years. Since iron
directions to create new rocks along the ridge. The creation of minerals in molten basalt formed, became magnetized, then
new rock like this would tend to spread the seafloor in both froze in the orientation they had when the rock cooled, they
directions, thus the name. The test of this hypothesis would made a record of reversals in Earth’s ancient magnetic field
come from further studies, this time on the ages and magnetic (Figure 18.12). Analysis of the magnetic pattern in the rocks
properties of the seafloor along the ridge (Figure 18.11). along the Mid-Atlantic Ridge found identical patterns of mag-
Evidence of the age of sections of the seafloor was obtained netic bands on both sides of the ridge. This is just what you
by drilling into the ocean floor from a research ship. From would expect if molten rock flowed out of the rift, cooled to
these drillings, scientists were able to obtain samples of fossils solid basalt, then moved away from the rift on both sides. The
and sediments at progressive distances outward from the Mid- pattern of magnetic bands also matched patterns of reversals
Atlantic Ridge. They found thin layers of sediments near the measured elsewhere, providing a means of determining the age
ridge that became progressively thicker toward the continents. of the basalt. This showed that the oceanic crust is like a giant
This is a pattern you would expect if the seafloor were spread- conveyer belt that is moving away from the Mid- Atlantic Ridge
ing, because older layers would have more time to accumulate in both directions. It is moving at an average 5 cm (about 2 in) a
greater depths of sediments. The fossils and sediments in the year, which is about how fast your fingernails grow. This means
bottom of the layer were also progressively older at increas- that in 50 years, the seafloor will have moved 5 cm/yr × 50 yr,
ing distances from the ridge. The oldest, which were about or 2.5 m (about 8 ft). This slow rate is why most people do not
150 million years old, were near the continents. This would seem recognize that the seafloor—and the continents—move.

18-9 CHAPTER 18 Plate Tectonics 463