The Sea Floor Spreading Theory explains how the ocean floor is formed and changes over time. According to this theory, new oceanic crust is created at mid-oceanic ridges and old crust is destroyed at trenches. This process works like a moving conveyor belt and helps us understand the movement of plates and changes on the Earth’s surface.
BACKGROUND
During the exploration of the ocean floor during World War II, a large amount of evidence was found regarding the presence of different relief features under the oceans. It was also observed that the temperature is high around a continuous mountain system, known as Mid-Oceanic Ridges.
BASIC INTRODUCTION
During World War II, sonar radiation was used to map the ocean floor, and magnetic anomalies were also studied. It was found that the ideal ocean floor has certain features, as shown in the diagram below:-
→ Thus, Harry Hess gave his Sea Floor Spreading Theory in 1960. According to him, the sea floor behaves like a conveyor belt, where new oceanic crust is created along the Mid-Oceanic Ridges and the old oceanic crust is destroyed along the trenches.
→ The hot convectional currents arising from the mantle cause the oceanic crust to break along the point where Mid-Oceanic Ridges are formed.
→ The magma rises up or escapes through ruptures or rifts, comes into contact with seawater, and solidifies to form parallel mountain systems, known as Mid-Oceanic Ridges.
→ Thus, a new oceanic crust is created along the Mid-Oceanic Ridges (MOR).
→ The rocks equidistant away from the MOR belong to the same age, because as sea-floor spreading continues, older MOR move away and form a part of the oceanic crust, while new MOR are created along the divergent plate boundary.
→ At the point of convergence of oceanic crust with continental crust, the oceanic crust, being heavier, is subducted into the mantle and melts. These regions form deep oceans or trenches.
EVIDENCES
1. Presence of Mid-Oceanic Ridges
Presence of Mid-Oceanic Ridges, which is the longest mountain chain on the Earth. This mid-oceanic ridge is present in all oceans except the Pacific Ocean (because the Pacific Ocean was once Panthalassa and now it is reducing in size).
2. Presence of continuous volcanic eruptions (silent or fissure type) and therefore high temperature.
3. Presence of Earthquakes
Presence of earthquakes, which explains that the sea floor spreads. It has now been proved that the Indian Ocean spreads at the rate of 2 cm per year, while the Atlantic Ocean spreads at 5–6 cm per year.
4. Age and Height Pattern of Rocks
Rocks equidistant away from the Mid-Oceanic Ridges are of the same age, and also the height of ridges goes on decreasing away from the Mid-Oceanic Ridges.
5. Magnetic Field Studies
Studies of magnetic fields (isochrones), meaning equal magnetic fields, have shown alternate normal and reverse magnetic anomalies.
Vine and Matthews conducted a survey of the floor of the Indian Ocean and found that rocks equidistant away from the Mid-Oceanic Ridges show normal and reverse polarity.
As the magnetic field reverses after every one lakh years, therefore the rocks formed have different magnetic polarity.
6. Age Difference Between Continental and Oceanic Crust
Numerous theories regarding the origin of the Earth have proved that the continental crust and oceanic crust were formed at the same time, but studies have shown that the continental crust is about 4.2 billion years old, similar to the age of the Earth, whereas the oceanic crust is only about 200 million years old.
This proves that oceanic crust is continuously created along the Mid-Oceanic Ridges (MOR) and destroyed along the trenches.
CONCLUSION
Tuzo Wilson used or combined the concepts of Continental Drift and Sea Floor Spreading to develop the Theory of Plate Tectonics.
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