Chapter 1, Section 4

Chapter 1, Section 4

Several types of evidence supported Hess’s theory of sea-floor spreading: eruptions of molten material, magnetic stripes in the rock of the ocean floor, and the ages of the rocks themselves. This evidence led scientists to look again at Wegener’s hypothesis of continental drift.

## Evidence From Molten Material

In the 1960s, scientists found evidence that new material is indeed erupting along mid-ocean ridges. The scientists dived to the ocean floor in Alvin, a small submarine built to withstand the crushing pressures four kilometers down in the ocean. In a ridge’s central valley, Alvin’s crew found strange rocks shaped like pillows or like toothpaste squeezed from a tube. Such rocks form only when molten material hardens quickly after erupting under water. These rocks showed that molten material has erupted again and again along the mid-ocean ridge.

## Evidence From Magnetic Stripes

When scientists studied patterns in the rocks of the ocean floor, they found more support for sea-floor spreading. You read earlier that Earth behaves like a giant magnet, with a north pole and a south pole. Surprisingly, Earth’s magnetic poles have reversed themselves many times during Earth’s history. The last reversal happened 780,000 years ago. If the magnetic poles suddenly reversed themselves today, you would find that your compass needle points south.

Scientists discovered that the rock that makes up the ocean floor lies in a pattern of magnetized “stripes.” These stripes hold a record of reversals in Earth’s magnetic field. The rock of the ocean floor contains iron. The rock began as molten material that cooled and hardened. As the rock cooled, the iron bits inside lined up in the direction of Earth’s magnetic poles. This locked the iron bits in place, giving the rocks a permanent “magnetic memory.”

Try This Activity

Reversing Poles

1. Cut six short pieces, each about 2.5 cm long, from a length of audiotape.

2. Tape one end of each piece of audiotape to a flat surface. The pieces should be spaced 1 cm apart and lined up lengthwise in a single row.

3. Touch a bar magnet’s north pole to the first piece of audiotape. Then reverse the magnet and touch its south pole to the next piece.

4. Repeat Step 3 until you have applied the magnet to each piece of audiotape.

5. Sweep one end of the magnet about 1 cm above the line of audiotape pieces. Observe what happens.

Making Models What characteristic of the ocean floor did you observe as you swept the magnet along the line of audiotape pieces?

Using sensitive instruments, scientists recorded the magnetic memory of rocks on both sides of a mid-ocean ridge. They found that stripes of rock that formed when Earth’s magnetic field pointed north alternate with stripes of rock that formed when the magnetic field pointed south. As shown in Figure 17, the pattern is the same on both sides of the ridge.

Figure 17Magnetic Stripes Magnetic stripes in the rock of the ocean floor show the direction of Earth’s magnetic field at the time the rock hardened. Interpreting Diagrams How are these matching stripes evidence of sea-floor spreading?

## Evidence From Drilling Samples

The final proof of sea-floor spreading came from rock samples obtained by drilling into the ocean floor. The Glomar Challenger, a drilling ship built in 1968, gathered the samples. The Glomar Challenger sent drilling pipes through water six kilometers deep to drill holes in the ocean floor. This feat has been compared to using a sharp-ended wire to dig a hole into a sidewalk from the top of the Empire State Building.

Figure 18Sea-Floor Drilling The Glomar Challenger was the first research ship designed to drill samples of rock from the deep-ocean floor.

Samples from the sea floor were brought up through the pipes. Then the scientists determined the age of the rocks in the samples. They found that the farther away from a ridge the samples were taken, the older the rocks were. The youngest rocks were always in the center of the ridges. This showed that sea-floor spreading really has taken place.