Seafloor Spreading and Paleomagnetism

At a Glance

• Proposed by Harry Hess, early 1960s
• New crust forms at Mid-ocean ridges
• Old crust destroyed at Ocean trenches
• Key evidence Magnetic stripes and age of rocks
• Spreading rate About 2 to 15 cm a year
• Exam link UPSC Prelims, Geography: Geomorphic Movements and Plate Tectonics

Where Does It Fit?

This is the fourth topic of the category on geomorphic movements and plate tectonics. The previous topic described continental drift, which lacked a mechanism, and this one gives the mechanism. The next topic covers plate tectonics and the types of plate boundaries. Previous topic: Continental Drift Theory of Alfred Wegener. Next topic: Plate Tectonics and Types of Plate Boundaries.

What Is Seafloor Spreading?

In short: the ocean floor is created at ridges, moves outward like a conveyor belt and is destroyed at trenches.

Detailed mapping of the ocean floor after the Second World War, using echo sounders, revealed a continuous chain of underwater mountains, the mid-ocean ridge, with a central rift valley. In 1961 and 1962 the American geologist Harry Hess proposed that molten rock rises from the mantle along the ridge, cools and forms new oceanic crust. As more material rises, the older crust is pushed away on both sides, so the ocean floor spreads. Since the Earth does not grow in size, the same amount of crust must be consumed somewhere, and this happens at deep ocean trenches, where the crust descends into the mantle. Convection currents in the mantle were suggested as the driving force. The features are described in Major Ocean Relief Features.

What Evidence Supports Seafloor Spreading?

How Does Paleomagnetism Prove It?

When lava cools, tiny magnetic minerals such as magnetite align with the Earth’s magnetic field and are locked in the rock. This fossil magnetism is the basis of paleomagnetism. As the field has reversed many times, as explained in Geomagnetic Reversal and Geomagnetic Poles, the newly formed crust records the polarity of the time. Surveys by ships towing magnetometers found that the ocean floor shows magnetic anomalies in the form of long stripes of rock with alternating normal and reversed polarity, running parallel to the ridge and forming a mirror image on either side. In 1963 Fred Vine and Drummond Matthews, and independently Lawrence Morley, explained these stripes: each stripe formed at the ridge when the field had that polarity, and then moved away. The pattern matched the known timetable of reversals, which both supported the idea and made it possible to date the ocean floor.

How Fast Does the Seafloor Spread?

The rate differs from ridge to ridge. Slow-spreading ridges, such as the Mid-Atlantic Ridge, move at about 2 to 5 cm a year, so the Atlantic is widening at roughly the rate at which fingernails grow. Fast-spreading ridges, such as the East Pacific Rise, move at about 10 to 15 cm a year, producing smoother and broader ridges. The Indian Ocean ridges are of intermediate speed. These rates agree with the ages of the stripes and with direct measurement by satellite geodesy, which can detect movements of a few centimetres a year.

Why Is This Theory Important?

Seafloor spreading explained how continents could move: they are not ploughing through the ocean floor, but riding on it as it moves. It therefore solved the weakness of the continental drift theory and led directly to plate tectonics, discussed in Plate Tectonics and Types of Plate Boundaries. As an analytical point, it is a case where new technology, here sonar mapping, magnetometers and deep drilling, produced data that no earlier theory could have predicted, and the match of independent lines of evidence made it persuasive within a few years.

Why Is This Topic Important?

• Mechanism Explains plate movement
• Evidence Stripes and age asked often
• Link Connects to magnetism and ridges

What Came Next?

The next topic describes plate tectonics and the types of plate boundaries. Next topic: Plate Tectonics and Types of Plate Boundaries.

Key People and Terms

• Mid-ocean ridge Underwater mountain chain with new crust
• Trench Deep ocean groove where crust sinks
• Magnetometer Instrument measuring magnetic field
• Magnetite Magnetic mineral in basalt
• Polarity Direction of the magnetic field
• Convection current Slow circulation in the mantle

Exam Corner

Mains Angle

Questions ask for evidence and the link to plate tectonics. These points add depth.

• Evidence Age, sediment and stripes
• Mechanism Convection and spreading
• Link Foundation of plate tectonics