Seafloor Spreading

Have you ever seen the rings inside a tree that has been cut down? Do you think the rings closest to the center are the youngest or the oldest? Why?


Mapping the Ocean Floor

If you were to lower a rope from a boat until it reached the seafloor, you would know how deep the ocean was in that spot. This is the first way people mapped the ocean floor.


How did technology improve seafloor mapping?

The rope method was used until German scientists

discovered how to use sound waves to locate submarines.

Later, sound waves on ships were used to map the seafloor.

Sound waves travel through the water. When they hit the

seafloor, they bounce back. The longer it takes for the sound waves to bounce back to the ship, the deeper the water is.

The new sound wave technology made it easier to make

better maps of the seafloor.


What did sound waves help discover?

By using sound waves, scientists found that the seafloor

had an underwater system of ridges. These ridges are like

mountain ranges and valleys on land. In some of these

ridges are long rift valleys. These rift valleys are like rips in the ocean floor. Volcanic eruptions and earthquakes occur in the rift valleys from time to time. Underwater volcanic eruptions create underwater mountains. When these mountains push out of the water, they create islands.


What are mid-ocean ridges?

The mid-ocean ridges are a chain of ridges and valleys

stretching along Earth’s ocean floor. Many of these ridges

are connected. They circle Earth much like the stitching on a baseball.


Is the seafloor spreading?

In the 1960s, Harry Hess, an American scientist, proposed

that the ocean floor moves. He called his theory seafloor

spreading. The theory of seafloor spreading proposes that magma, or melted rock, under Earth’s crust is forced up toward the surface at the mid-ocean ridges, forming new seafloor. When the less dense magma hits Earth’s crust, it flows sideways. The magma carries the seafloor away from the central ridge in both directions. New seafloor is continuously being created. Older sea floor is pushed away from the central ridge as shown in the figure above.

Evidence for Spreading

In 1968, scientists began studying rocks on the seafloor.

They took rock samples from the mid-ocean ridges. They also took rock samples farther away from the ridge. They found that rocks near the mid-ocean ridge were the youngest rocks. Rocks farther away from the ridge were older.

According to Hess’s theory of seafloor spreading, the

seafloor near the ridge has formed more recently from

magma. The older seafloor is pushed away from the ridge.

Like tree rings, the further away the rocks, the older they

are. The age of the rocks and their distance from the

mid-ocean ridge supports the theory of seafloor spreading.

New life-forms have been discovered near the mid-ocean

ridges. These giant clams, mussels, and tube worms, get heat and chemicals from magma pouring out of rifts in

mid-ocean ridges.

Does Earth’s magnetic field change?

Earth’s magnetic field has a north pole and a south pole.

Invisible lines of magnetic force leave Earth near the south pole and enter Earth near the north pole. At this time, Earth’s magnetic field travels from south to north. This is not always true. At times, the lines of magnetic force have traveled in the opposite direction, north to south. These direction changes are called magnetic reversals. During a magnetic reversal, the lines of magnetic force run the opposite way. All of these magnetic reversals are recorded in rocks forming along mid-ocean ridges.


How does the seafloor record history?

Minerals containing iron, such as magnetite, are found

in rocks on the seafloor. Iron in the rock records the magnetic reversal. A device called a magnetometer (mag nuh TAH muh tur) tells scientists what direction a magnetic field has.

How do scientists know when a magnetic reversal happened and when it changed back? A strong magnetic reading is recorded when the polarity of a rock is the same as the polarity of Earth’s magnetic field. Look at the figure below. Normal polarities in rocks show up as large peaks. After the magnetic reversal, the magnetometer records a weak reading. Over time, the reversals are shown in strips parallel to mid-ocean ridges.

Changes in Earth’s magnetic field can be seen on both sides of mid-ocean ridges. This discovery adds to the evidence that the seafloor is spreading. The magnetic reversals showed  that new rock was being formed at mid-ocean ridges. This helped explain how Earth’s crust could move. It gives evidence that the continental drift hypothesis did not provide.

Normal magnetic polarity

Reverse magnetic polarity

Changes in Earth’s magnetic field are recorded in rock.