Chapter 8 Plate Tectonics

196 CHAPTER 8: PLATE TECTONICS
sition similar to granite.That is, the continental rocks are generally
felsic, light-colored rocks with a relatively low density.
The low-density material of which continents are composed
tends to rise above the ocean basins. Most oceanic rocks have a
composition similar to basalt. They are mafic. You may recall
from Chapter 5 that mafic rocks are darker in color and relatively
dense.This is why the ocean basins tend to ride lower on
the rocks of Earth’s interior and are covered by the oceans.
At the time Wegener worked on his hypothesis, the oceans
were vast expanses of unknown depth. After his death, the
strongest evidence for his theory of moving continents came
from the oceans. During World War II, there were important
advances in technology and oceanic exploration. A moving
ship could make a continuous record of the depth of the ocean
by bouncing sound waves off the ocean bottom. After the
war, scientists used this technology to map the world’s ocean
bottoms. Much of the ocean bottom is flat and featureless.
But scientists discovered an underwater system of mountain
ranges that circles Earth like the seams on a baseball. These
are the mid-ocean ridges. Most of these 64,000-km-long
features are under water, but they rise above sea level in Iceland
and on several smaller islands. By looking at the landscape
of Iceland, scientists can investigate processes that
occur at the bottom of the ocean hidden from view.
In 1960, Princeton University geologist Harry Hess reconsidered
Wegener’s ideas. Hess had commanded a ship during
World War II that measured ocean depths. He had seen
the newly discovered shape of ocean bottoms including the
mid-ocean ridges. Hess suggested that molten magma from
the mantle rises to the surface at the mid-ocean ridges and
erupts onto the ocean bottom. In many places, the ocean
ridges are like two mountain chains separated by a valley. It
is in the valley that the most active eruptions are taking
place to form a strip of new lithosphere. Some of the magma
solidifies as it comes into contact with the cold ocean water
making pillow-shaped rocks. Wherever scientists find pillow
lavas, they can be sure that magma moved into water and solidified
quickly. Pillow lavas are common along the mid-ocean
ridges but are uncommon elsewhere in the oceans. Hess suggested
that the mid-ocean ridges are the places where new
lithosphere is made and adds on to older material that moves