Sea-Floor Spreading 6:Plate Tectonics

Subduction at Deep-Ocean Trenches

Although the new ocean floor is constantly being added at the mid-ocean ridges, our planet is not growing larger. Earth's total surface area remains the same. How can that be? To accommodate newly created lithosphere, older portions of the ocean floor return to the mantle.

Magnetic Strips 2

As certain rocks form, they acquire the polarity that Earth's magnetic field has at the time. These rocks possess paleomagnetism. How does a rock become magnetized? Many igneous rocks contain magnetite, an iron-rich mineral. As the rock cools and hardens, the iron-rich mineral grains become magnetized in the same direction as the existing magnetic field. You can see this process in the figure. Once the rock has formed, its polarity remains frozen unless the rock is reheated above a certain temperature. But what if the rock is moved or if the magnetic pole changes its position? The rock's paleomagnetism does not change.

Deep-Ocean Trenches

As scientists mapped the ocean floor, they found long, curved valleys along the edges of some ocean basins called deep-ocean trenches.

Movement of the Ocean Floor

As the new ocean floor is added along mid-ocean ridges, the older ocean floor moves outward and away from the ridge on both sides. Rates of sea-floor spreading average about 5 centimeters per year. These rates are slow on a human time scale. But they are fast enough that all of Earth's ocean basins could have been formed within the last 200 million years.

Mid-Ocean Ridges

By the late 1950s, scientists had constructed a more complete map of Earth's ocean floor. The map showed that the mountain range in the middle of the Atlantic Ocean was not an isolated feature. Instead, it formed a mid-ocean ridge, a long chain of mountains extending the length of the ocean.

The Age of the Ocean Floor

Drilling into sediment on the ocean floor and the crust beneath it provided some of the best evidence for sea-floor spreading. Beginning in 1968, the drilling ship Glomar Challenger collected data on both sides of mid-ocean ridges. The data confirmed what the sea-floor spreading hypothesis predicted. The ocean floor is youngest along the central valley of the mid-ocean ridge. The ocean floor is oldest in subduction zones or near the edges of continents far from the ridge. The data also confirmed that none of the ocean floor is more than about 180 million years old. Older oceanic rock would have returned to the mantle through subduction.

Exploring the Ocean Floor 2

During the early 1900s, new technology made it easier to map the ocean floor. Sonar, which stands for sound navigation and ranging, is a system that uses sound waves to calculate the distance to an object. The sonar equipment on a ship sends out pulses of sound that bounce off the ocean floor. The equipment then measures how quickly the sound waves return to the ship. The deeper the water, the longer it takes the sound waves to return to the ship.

Exploring the Ocean Floor

During the mid-1800s, several nations sent ships on scientific expeditions to gather data about the oceans. Scientists wanted to know more about the topography of the ocean floor. They measured ocean depths in many areas. Data from the middle of the Atlantic Ocean, where scientists expected the water to be very deep, revealed large undersea mountains. This discovery helped to fuel interest in mapping the ocean floor.

Mid-Ocean Ridges 2

Earth's mid-ocean ridge system forms the longest feature on Earth's surface. The system winds more than 70,000 kilometers through all the major ocean basins like the seam on a baseball. The term ridge may be misleading. These features are not narrow like a typical ridge. They range from 1,000 to 4,000 kilometers wide. In a few places, such as Iceland, the mid-ocean ridge rises above the surface. Often, a deep, central valley runs down the center of a ridge. Called a rift valley, the central valley of a mid-ocean ridge resembles a long canyon. Some parts of the ridge system lack a rift valley.

Composition of the Ocean Floor

Earth's ocean floors are made of igneous rocks of basaltic composition. Basalt forms when lava reaches the surface and hardens to form solid rock. Most of the ocean floor is covered with a thick layer of sediment. Scientists found that the sediment layer became thinner closer to mid-ocean ridges and that along the ridge there was no sediment.

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Evidence for Sea-Floor Spreading

Hess's hypothesis got the attention of geologists. Sea-floor spreading explained the formation and destruction of the ocean floor and how ocean basins could grow wider or closer together. But what evidence was there to support Hess's hypothesis? Evidence for sea-floor spreading included magnetic stripes in ocean-floor rock, earthquake patterns, and measurements of the ages of ocean floor rocks.

Eruptions Along Mid-Ocean Ridges

How did the mid-ocean ridges form? Scientists have found evidence that the mid-ocean ridges formed as the result of volcanic activity. Fractures along the central valley of a mid-ocean ridge fill with magma that wells up from the hot mantle below. (Magma is molten rock that forms in the upper mantle and rises through the crust). Gradually, the magma cools to produce new slivers of the ocean floor. The spreading and upwelling of magma continuously adds a new ocean floor. The process can also begin on land when a rift valley forms and splits a continental landmass. Over millions of years, the rift valley widens to form a new ocean basin—like the Red Sea.

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Subduction at Deep-Ocean Trenches 2

In the process of subduction, ocean floor returns to the mantle as it sinks beneath a deep ocean trench. The areas where subduction occurs are called subduction zones.

Earthquake Patterns

More evidence for sea-floor spreading came from studies of the depth at which certain earthquakes occur. Scientists knew that there were many earthquakes in subduction zones. Two scientists, Kiyoo Wadati and Hugo Benioff found a pattern when they plotted the depth of earthquakes in relation to their distance from deep-ocean trenches.

Deep-Ocean Trenches 3

Most trenches occur around the edges of the Pacific Ocean, although others occur in the Indian and Atlantic oceans.

Magnetic Strips 3

Scientists collected data on the paleomagnetism of the basalt that makes up the ocean floor. Ships towed instruments called magnetometers across the ocean floor. The data revealed a pattern of alternating strips of magnetized rock. Strips of rock with normal polarity alternated with strips of rock having reverse polarity. Scientists inferred that as new basalt forms along the mid-ocean ridges, it becomes magnetized according to the polarity of Earth's magnetic field at the time. The matching pattern of strips on both sides of a ridge is evidence that sea-floor spreading occurs.

Earthquake Patterns 3

Scientists considered the pattern of earthquakes in Wadati-Benioff zones in relation to sea-floor spreading. The pattern was what scientists expected would result from the subduction of the ocean floor. These data convinced scientists that slabs of ocean floor return to the mantle in subduction zones.

Earthquake Patterns 2

Shallow-focus earthquakes occur in and around a trench. Wadati and Benioff observed that intermediate-focus and deep-focus earthquakes occur in a belt about 50 kilometers thick. This belt extends through the lithosphere and deep into the asthenosphere. The deeper the earthquake, the farther away its focus is from the deep-ocean trench. No earthquakes have been recorded below about 700 kilometers. At this depth, the subducting slab of ocean floor has been heated enough to soften.

How do strips of magnetized rock on the ocean floor provide evidence of sea-floor spreading?

Strips of ocean-floor basalt record the polarity of Earth's magnetic field at the time the rock formed. These strips form a pattern that is the same on both sides of the mid-ocean ridge. The pattern shows that ocean floor forms along mid-ocean ridges and then moves away from the ridge.

As the hypothesis of sea-floor spreading predicts, the age of the ocean floor differs with location. What pattern does the ocean floor's age form?

The floor is youngest near the mid-ocean ridge, then becomes older further away.

Magnetic Strips

The magnetic properties of the rock that makes up the ocean floor provided evidence for sea-floor spreading. To understand this evidence, you need to understand how some rocks can become magnetized. Earth's magnetic field is much like that of a bar magnet. Geophysicists learned that Earth's magnetic field occasionally reverses polarity. That is, the north magnetic pole becomes the south magnetic pole, and vice versa. Scientists graphed these reversals of polarity going back millions of years. When Earth's magnetic field lines up in the same direction as the present magnetic field, it is said to have normal polarity. When the magnetic field lines up in the opposite direction, it is said to have reverse polarity.

The Process of Sea-Floor Spreading

The new map of the ocean floor aroused the curiosity of many scientists. One geologist, Harry Hess, thought that the mid-ocean ridges and deep-ocean trenches might help to explain how the ocean floor was formed. In 1963, Hess published his hypothesis of sea-floor spreading. In the process of sea-floor spreading, new ocean floor forms along Earth's mid-ocean ridges slowly moves outward across ocean basins, and finally sinks back into the mantle beneath deep-ocean trenches. During sea-floor spreading, the new oceanic lithosphere is formed, and the ocean floor gets wider. Today, the Atlantic Ocean is thousands of kilometers wide. Millions of years ago, the Atlantic would have been a narrow sea, like the Red Sea.

Deep-Ocean Trenches 2

Trenches form the deepest parts of Earth's oceans. For example, the Marianas Trench in the Pacific Ocean is more than 11 kilometers deep.

Where is one example of a deep-ocean trench?

around the western edge of the Pacific Ocean

What are deep-ocean trenches?

long, curved valleys along the edges of the ocean basin

What did scientists discover when they studied the paleomagnetism of the ocean floor?

matching strips of alternating polarity on both sides of the mid-ocean ridge

What is the length of the mid-ocean ridge system?

more than 70,000 kilometers

In the process of sea-floor spreading, how does the ocean floor become wider?

new floor forms and spreads from mid-ocean ridges

In which direction does the ocean floor move in the process of sea-floor spreading?

outward from mid-ocean ridges