Convectional Current Theory and Mapping of the Ocean Floor

At a Glance

• Convection theory Arthur Holmes, late 1920s
• Sea-floor spreading Harry Hess, early 1960s
• Ocean floor mappers Heezen and Tharp, 1950s
• Evidence Magnetic stripes; age of ocean crust
• Oldest ocean floor About 180 to 200 million years
• Exam link UPSC Prelims, Geography: Plate Tectonics and Distribution of Continents and Oceans

Where Does It Fit?

This topic follows the failure of Wegener’s forces and shows how a better mechanism, mantle convection, was found and tested by exploring the ocean floor. It leads straight to the statement of plate tectonics. Previous topic: Forces of Drifting: Pole-Fleeing Force, Tidal Force. Next topic: Plate Tectonics: Major and Minor Lithospheric Plates.

What Is the Convectional Current Theory?

In short: the mantle is slowly stirred by heat, and its currents carry the plates above.

In the late 1920s and 1930s the British geologist Arthur Holmes suggested that heat from radioactive decay inside the Earth sets up slow convection currents in the mantle. Hot, light rock rises, spreads sideways under the crust, cools, and sinks again. Where two currents move apart under a continent, the crust is stretched and may split; where they move together, the crust is crumpled and pulled down. The currents carry the continents on top like a conveyor belt. Holmes thus offered the mechanism that Wegener lacked. The idea was developed by Harry Hess in the early 1960s as sea-floor spreading, and it is still part of the modern explanation in Rates and Forces of Plate Movement.

• Heat source: radioactive decay of uranium, thorium and potassium, and leftover heat from formation.
• Speed: a few centimetres a year, about as fast as fingernails grow.
• Solid, not liquid: the mantle is solid rock but flows over millions of years.

How Was the Ocean Floor Mapped?

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 Relief of the Ocean Floor: Continental Shelf, Continental Slope, Deep Sea Plain, Oceanic Deep and Trenches.

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, 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 Did This Change the Theory?

The ocean-floor discoveries showed several things at once. The sea floor is young compared with the continents, so it is made and destroyed continually. It spreads from the ridges, so the continents are carried along on moving plates and do not plough through anything. The magnetic stripes give a clock for the spreading. These findings revived Wegener’s idea in a changed form and led to the theory of plate tectonics in Plate Tectonics: Major and Minor Lithospheric Plates. As an analytical point, the theory was accepted not because it was argued better but because new data, mainly from oceans, left no other explanation.

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 sets out plate tectonics and names the major and minor lithospheric plates. Next topic: Plate Tectonics: Major and Minor Lithospheric Plates.

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