Sea-floor spreading and palaeomagnetism

Detail

Why the ocean floor became the key

• Wegener's continental drift (1912) said the continents move, but it could not explain how they move.
• After the 1950s, scientists mapped the ocean floor and measured the magnetism of its rocks. These two kinds of study gave the missing evidence.
• Mid-oceanic ridge: a long underwater mountain chain in the middle of an ocean. A crack (rift) runs along its top, the crest. Examples: the Mid-Atlantic Ridge in the Atlantic Ocean and the East Pacific Rise in the Pacific Ocean.
• Oceanic trench: a long, narrow and very deep hollow in the sea floor, usually near the edge of a continent or a chain of islands.

Evidence for sea-floor spreading: Class 11's five facts

(i) Volcanic eruptions along ridges

• Eruptions happen all along the mid-oceanic ridges.
• They bring up huge amounts of lava (molten rock).

(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 (their magnetism points the same way as the earth's magnetic field today).
• Rocks get older as you move away from the crest on both sides.
• So the ridge is where new crust is born.

(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]. (NCERT: nowhere older than 200 million years.)
• The youngest sea floor lies along divergent boundaries (places where two plates move apart), such as the Mid-Atlantic Ridge [3].

(iv) Thin, young sediments

• Sediments (mud, sand and the remains of sea life that settle on the sea floor) are surprisingly thin.
• They are also nowhere older than 200 million years.
• If the ocean floor were very old, much thicker layers would have piled up.

(v) Earthquakes are deep at trenches and shallow at ridges

• 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 there.

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

Sea-floor spreading: Hess's idea (1961)

Sea-floor spreading: new ocean crust forms at a mid-oceanic ridge and moves away on both sides, so the ocean floor grows wider.

How it works (cause → process → result)

• Cause: eruptions keep happening at the ridge crest, and they crack (rupture) the ocean crust.
• Process: new lava pushes into the crack and cools into new crust. This pushes the older crust aside on both sides.
• Result: the ocean floor spreads outwards from the ridge, like two conveyor belts moving apart.

Where does the extra crust go?

• One ocean spreading does not make another ocean shrink. 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).
• 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" was published in 1962. Robert Dietz first used the term "sea-floor spreading" in 1961.
• The sea floor spreads at a rate of several centimetres per year [2].

Palaeomagnetism: rocks as old compasses

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

How a rock records the field

• Hot lava has no fixed magnetism. As it cools, its magnetic minerals (such as 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. The rock keeps the direction of the field as it was at that time [2].
• So every rock is like a fossil compass.

Using it to rebuild old positions of continents

• The direction locked in a rock shows where that rock was when it formed, including how far it was from the poles.
• Scientists compare rocks of different ages to work out where a continent used to be.
• India: its past positions are traced from rocks of the Nagpur area (Maharashtra) (Figure 4.4, Class 11).

Magnetic reversals and magnetic striping

Polarity reversal: a time when the earth's magnetic field flips, so the magnetic north and south poles swap places.

• Normal polarity means the field points the same way as today. Reversed polarity means it points the opposite way.
• The earth's field has flipped many times in the past.
• USGS scientists Allan Cox, Richard Doell and Brent Dalrymple dated these flips in volcanic rocks using the potassium-argon method. (In this method, potassium-40 slowly turns into argon-40 and has a half-life of 1,310 million years: the time for half of it to change.) They built a history of reversals going back about 4 million years [2].

How the stripes form (cause → process → result)

• Cause: lava keeps coming out at the ridge, and the field keeps flipping from time to time.
• Process: each batch of new crust locks in the polarity of its own time. Then spreading carries it away on both sides of the ridge.
• Result: the sea floor shows parallel bands (stripes) of normal and reversed polarity. The stripes run parallel to the ridge and are mirror images on both sides.

Magnetic striping: this zebra-like pattern of alternating normal and reversed magnetic bands on the ocean floor.

History of the discovery (beyond NCERT)

• 1950s: exploration of the oceans found zebra-like magnetic patterns in ocean-floor rocks [2].
• 1962: the U.S. Naval Oceanographic Office reported on these magnetic stripes [2].
• 1963: Frederick Vine and Drummond Matthews (British) and, working separately, Lawrence Morley (Geological Survey of Canada) said the stripes came from repeated reversals of the earth's magnetic field [2].
• 1966: the dated reversals were matched to the stripe pattern, for example on the East Pacific Rise. This close match strongly supported sea-floor spreading [2].
• The stripes act like a "tape recorder" of spreading. This was the deciding proof, and it led to plate tectonics in 1967.
• Today the magnetic polarity of the sea floor is used to find its age. Age maps are drawn with lines (contours) at 20-million-year steps [3].
• Spreading is not even everywhere. This creates linear features at right angles to the divergent boundaries [3].

Term	What it means	What it proves
Palaeomagnetism	Old magnetic direction locked in rocks	Where a continent was in the past (e.g. India from Nagpur rocks)
Polarity reversal	Earth's magnetic poles swap places	Gives the "time marks" in the rock record
Magnetic striping	Mirror-image normal and reversed bands parallel to a ridge	New crust forms at the ridge and moves away (sea-floor spreading)

Prelims Hooks

• Rocks nearest the mid-oceanic ridge crest are the youngest and have normal polarity. Age increases on both sides as you move away from the crest.
• 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].
• Earthquakes: deep at trenches, shallow at ridges. This is a common trap, so do not reverse it.
• Crust is created at mid-oceanic ridges and destroyed at oceanic trenches.
• Sea-floor spreading idea: Harry Hess (1961; paper "History of Ocean Basins", 1962). The term "sea-floor spreading" was coined by Robert Dietz (1961).
• The magnetic stripes were explained by Vine, Matthews and Morley (1963) [2].
• India's past positions are traced from palaeomagnetic study of rocks of the Nagpur area (Figure 4.4, Class 11).
• Magnetic stripes run parallel to the ridge and are symmetrical (mirror images) on both sides.
• An example of the youngest sea floor at a divergent boundary is the Mid-Atlantic Ridge [3].
• 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 continents. Hess's idea gave a working process: crust is made at ridges and destroyed at trenches. Magnetic stripes, the young age of ocean crust and the thin sediments gave measurable proof.
• Palaeomagnetism as a tool: it rebuilds the past positions of continents, such as India's northward journey traced from Nagpur rocks. It also dates the ocean floor [3] and measures spreading rates of several centimetres per year [2]. This links ocean studies to the formation of the Himalayas.
• Pattern of earthquakes and volcanoes: shallow earthquakes and lava at ridges, and deep earthquakes at trenches, show that seismic and volcanic hazards follow plate edges. This helps in mapping hazard zones (GS-I physical geography, GS-III disasters).
• Balance of the earth's crust: the ocean floor is recycled within about 200 million years, while continents hold rocks 3,200 million years old. This explains why the oldest geological record lies on land, for example in the Peninsular shield of India.