Sea-floor spreading

Topic: Continental Drift, Sea-floor Spreading and Plate Tectonics · NCERT: Class 11, Ch 4 "Distribution of Oceans and Continents"

Meaning

Sea-floor spreading is the idea that new ocean crust (the rocky outer layer under the oceans) is made at mid-oceanic ridges and moves away on both sides, so the ocean floor grows wider, while old crust sinks and is used up at oceanic trenches. Harry Hess put forward this idea in 1961. It matters because it gave the missing answer to how continents move, which Wegener's continental drift (1912) could not explain. It also led to the theory of plate tectonics in 1967.

Explanation

How it works (cause → process → result)

• Cause: eruptions crack the crust at the ridge
• Volcanic eruptions keep happening along the crest (the top) of a mid-oceanic ridge. A mid-oceanic ridge is a long underwater mountain chain in the middle of an ocean.

• These eruptions crack (rupture) the ocean crust along a rift running down the crest.

• Process: new crust pushes old crust aside

• Lava (molten rock) pushes into the crack. It cools and becomes new crust.

• This new crust pushes the older crust away on both sides.

• Result: the ocean floor spreads

• The floor moves outwards from the ridge, like two conveyor belts moving apart.
• The sea floor spreads at a rate of several centimetres per year [2].

Where the old crust goes

• When one ocean spreads, another ocean does not shrink to make room. So the earth is not simply getting bigger.
• This means crust must be destroyed somewhere.
• Hess said the floor pushed away from the crest sinks at oceanic trenches and is used up (consumed) there (Figure 4.3, Class 11).

• An oceanic trench is a long, narrow and very deep hollow in the sea floor. It usually lies near the edge of a continent or a chain of islands.

• Old sea floor is pulled down (subducted) under other plates and replaced by new surface [3].

• Beyond NCERT: Hess's paper "History of Ocean Basins" came out in 1962. Robert Dietz first used the term "sea-floor spreading" in 1961.

Evidence: Class 11's five facts

• (i) Volcanic eruptions along ridges. Eruptions happen all along the mid-oceanic ridges and bring up huge amounts of lava.
• (ii) Matching rocks on both sides of the crest
• Rocks at equal distances on either side of the crest are the same in age, chemistry and magnetism.
• Rocks nearest the crest are the youngest. They have normal polarity, which means their magnetism points the same way as the earth's magnetic field does today.

• Rocks get older as you move away from the crest on either side.

• (iii) The ocean crust is young

• Ocean crust is nowhere older than 200 million years. Some continental rocks are 3,200 million years old.

• The oldest dated sections of ocean floor are about 180 million years old [2]. Very little oceanic crust is older than 150 million years [3].

• (iv) Thin, young sediments

• Sediments (mud, sand and the remains of sea life that settle on the sea floor) are surprisingly thin, and nowhere older than 200 million years.

• If the ocean floor were very old, much thicker layers would have piled up.

• (v) Earthquake depth

• Trenches have deep earthquakes, because crust is sinking deep into the mantle there.
• Ridges have shallow earthquakes, because new crust is being pulled apart near the surface.

Feature	Mid-oceanic ridge	Oceanic trench
What happens to crust	New crust is made	Old crust is destroyed (sinks)
Age of rocks	Youngest	Oldest ocean floor nearby
Earthquake depth	Shallow	Deep
Volcanic activity	Lava comes out all along the crest	Crust goes down into the mantle

The deciding proof: magnetic striping

• Palaeomagnetism is the study of the earth's past magnetic field as it is recorded in old rocks.

• When lava cools, its iron-rich grains line up with the earth's magnetic field, like tiny compass needles. Once the rock is solid, the grains are locked in place [2].

• Polarity reversal is a time when the earth's magnetic field flips, so the magnetic north and south poles swap places. This has happened many times.

• How the stripes form
• Each batch of new crust at the ridge locks in the polarity of its own time.
• Spreading then carries that crust away on both sides of the ridge.

• The result is parallel bands of normal and reversed polarity that run parallel to the ridge and are mirror images on both sides. This pattern is called magnetic striping.

• Discovery timeline (beyond NCERT)

• 1950s: ocean surveys found zebra-like magnetic patterns in ocean-floor rocks [2].
• 1962: the U.S. Naval Oceanographic Office reported on these stripes [2].
• 1963: Frederick Vine and Drummond Matthews, and separately Lawrence Morley, said the stripes came from repeated reversals of the earth's magnetic field [2].

• 1966: dated reversals were matched to the stripe pattern, for example on the East Pacific Rise. This close match strongly supported sea-floor spreading [2].

• Dating the reversals. Allan Cox, Richard Doell and Brent Dalrymple dated the flips using the potassium-argon method. This method uses the slow change of potassium-40 into argon-40. They built a history of reversals going back about 4 million years [2].

• The stripes work like a "tape recorder" of spreading. Today, sea-floor polarity is used to find the age of the ocean floor, and age maps use contour lines at 20-million-year steps [3].

Where it occurs

• Mid-Atlantic Ridge (Atlantic Ocean): a mid-oceanic ridge where new crust forms. It is an example of the youngest sea floor at a divergent boundary (a place where two plates move apart) [3].
• East Pacific Rise (Pacific Ocean): a mid-oceanic ridge. The 1966 match between magnetic stripes and dated reversals was shown here [2].
• Oceanic trenches: usually found near the edges of continents or chains of islands. Old ocean floor sinks and is consumed at these trenches.
• Divergent boundaries in general: spreading is not even everywhere. This creates linear features at right angles to these boundaries [3].
• India (palaeomagnetism link): India's past positions are traced from the magnetism of rocks in the Nagpur area (Maharashtra) (Figure 4.4, Class 11). This is part of the same body of evidence that supports moving plates.

Don't confuse with

• Continental drift (Wegener, 1912): this idea said the continents move but gave no working force. Sea-floor spreading supplied the process: crust is made at ridges and destroyed at trenches.
• Plate tectonics (1967): this is the full theory that the whole outer shell is made of moving plates. Sea-floor spreading (1961–62) came earlier and was one of its building blocks.
• Palaeomagnetism: this is a tool, the study of old magnetism in rocks. Sea-floor spreading is the process. The magnetic stripes found through palaeomagnetism are the proof of that process.
• Ridge vs trench: crust is created at ridges, which have shallow earthquakes and the youngest rocks. Crust is destroyed at trenches, which have deep earthquakes. Do not swap the two.

Prelims Hooks

• The sea-floor spreading idea came from Harry Hess (1961; paper "History of Ocean Basins", 1962). The term was coined by Robert Dietz (1961).
• Rocks nearest the ridge crest are the youngest and have normal polarity. Age increases on both sides as you move away from the crest.
• Earthquakes are deep at trenches and shallow at ridges. A common trap is to reverse this.
• Ocean crust is nowhere older than 200 million years (NCERT). Some continental rocks are 3,200 million years old. The oldest dated ocean floor is about 180 million years old [2].
• Magnetic stripes run parallel to the ridge and are mirror images on both sides. They were explained by Vine, Matthews and Morley (1963) [2].
• Order of ideas: continental drift (1912) → sea-floor spreading (1961–62) → Vine–Matthews–Morley (1963) → plate tectonics (1967).

Mains Points

• Why sea-floor spreading succeeded where continental drift failed
• Wegener had no force to move the continents.
• Hess gave a working process: crust is made at ridges and destroyed at trenches, so the earth does not grow bigger.

• There was measurable proof: magnetic stripes, the young age of ocean crust and thin sediments. Spreading rates of several centimetres per year [2] turned drift into a testable idea that led to plate tectonics.

• Hazard pattern along plate edges (GS-I, GS-III disasters)

• Ridges have shallow earthquakes and lava eruptions. Trenches have deep earthquakes.

• This shows that earthquake and volcano hazards follow plate edges, which helps in mapping hazard zones.

• Recycling of the earth's crust

• The ocean floor is recycled within about 200 million years, while continents keep rocks 3,200 million years old.
• This explains why the oldest geological record lies on land, for example in the Peninsular shield of India. Palaeomagnetism of Nagpur rocks also traces India's northward journey, which links sea-floor spreading to the formation of the Himalayas.

Related concepts

• Evidence for sea-floor spreading
• Palaeomagnetism
• Magnetic striping

Read more

• Sea-floor spreading and palaeomagnetism