Palaeomagnetism

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

Meaning

Palaeomagnetism (also spelt paleomagnetism) is the study of the earth's past magnetic field, as it is recorded in old rocks. When lava cools, its magnetic minerals line up with the earth's field and stay locked that way.

It matters because each such rock works like a fossil compass. It tells us where a continent was in the past, such as India. Its mirror-image magnetic stripes on the sea floor gave the deciding proof for sea-floor spreading, which led to plate tectonics in 1967.

Explanation

How a rock records the magnetic field (cause → process → result)

• Cause: hot lava has no fixed magnetism. It contains magnetic minerals, such as iron-rich grains.
• Process:
• As the lava cools, these grains line up with the earth's magnetic field, like tiny compass needles.

• Once the rock is solid, the grains are locked in place.

• Result: the rock keeps the direction of the field as it was at that time [2]. Every such rock becomes a fossil compass.

Rebuilding the old positions of continents

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

Polarity reversals: the "time marks" in rocks

• Polarity reversal means 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 it does today.

• Reversed polarity means it points the opposite way.

• The field has flipped many times in the past.

• Dating the flips:
• USGS scientists Allan Cox, Richard Doell and Brent Dalrymple dated these flips in volcanic rocks.
• They used the potassium-argon method. In this method, potassium-40 slowly turns into argon-40. It has a half-life (the time it takes for half of it to change) of 1,310 million years.
• They built a history of reversals going back about 4 million years [2].

Magnetic striping on the sea floor (cause → process → result)

• Cause: lava keeps coming out at the mid-oceanic ridge (a long underwater mountain chain with a crack along its top, the crest). At the same time, the field keeps flipping from time to time.
• Process:
• Each batch of new crust locks in the polarity of its own time.

• Sea-floor spreading then carries the crust away on both sides of the ridge.

• Result: magnetic striping, a zebra-like pattern of normal and reversed bands on the ocean floor.

• The stripes run parallel to the ridge.
• They are mirror images on both sides.

• Rocks nearest the crest are the youngest and have normal polarity. Rocks get older as you move away from the crest.

• How the discovery happened (beyond NCERT):

• 1950s: ocean exploration 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 (British), and separately Lawrence Morley (Geological Survey of Canada), said the stripes came from repeated reversals of the field [2].

• 1966: the dated reversals matched 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.

Where it occurs

• India: rocks of the Nagpur area, Maharashtra, are used to trace India's past positions (Figure 4.4, Class 11).
• Mid-Atlantic Ridge (Atlantic Ocean): an example of the youngest sea floor at a divergent boundary (a place where two plates move apart) [3]. Here the stripes form mirror-image bands on both sides of the crest.
• East Pacific Rise (Pacific Ocean): the dated reversals were matched to its stripe pattern in 1966 [2].
• The world's ocean floors:
• Magnetic polarity is now used to find the age of the sea floor. Age maps are drawn with lines (contours) at 20-million-year steps [3].
• The oldest dated sections of ocean floor are about 180 million years old [2]. NCERT says ocean crust is nowhere older than 200 million years.
• Spreading is not even everywhere. This creates linear features at right angles to the divergent boundaries [3].

Don't confuse with

• Polarity reversal: this is the event in which the earth's poles swap. Palaeomagnetism is the study of the record of such events, and of the old field in general, as kept in rocks.
• Magnetic striping: this is one result that palaeomagnetism reads on the sea floor. It shows mirror-image bands parallel to a ridge and proves sea-floor spreading. Palaeomagnetism in continental rocks, such as the Nagpur rocks, shows a continent's past position.
• Sea-floor spreading (Hess, 1961): this is the theory that new crust forms at ridges and moves away. Palaeomagnetism is the evidence that proved it.
• Potassium-argon dating: this method gives a rock's age [2]. Palaeomagnetism gives the direction of the old magnetic field. Putting the two together dated the reversals.

Prelims Hooks

• Rocks nearest the mid-oceanic ridge crest are the youngest and have normal polarity. They get older on both sides as you move away from the crest.
• Magnetic stripes run parallel to the ridge and are mirror images (symmetrical) on both sides. A common trap says "at right angles", so watch for it.
• India's past positions are traced from palaeomagnetic study of rocks of the Nagpur area, Maharashtra (Figure 4.4, Class 11).
• The stripes were explained by repeated field reversals by Vine, Matthews and Morley (1963) [2].
• Order of ideas: continental drift (1912) → sea-floor spreading (Hess, 1961–62) → Vine–Matthews–Morley (1963) → plate tectonics (1967).
• Cox, Doell and Dalrymple dated the reversals with the potassium-argon method, going back about 4 million years [2].

Mains Points

• Why palaeomagnetism gave the deciding proof:
• Wegener's continental drift (1912) could not explain how continents move.

• Mirror-image stripes parallel to the ridges showed that crust is made at the ridge and carried away on both sides. This made sea-floor spreading measurable and led to plate tectonics in 1967.

• Palaeomagnetism as a tool:

• It rebuilds the past positions of continents. One example is India's northward journey traced from the Nagpur rocks, which links to the formation of the Himalayas.

• It dates the ocean floor [3]. It also measures spreading rates of several centimetres per year [2].

• Crust is made and destroyed (GS-I):

• Polarity-dated sea floor is nowhere older than 200 million years, while some continental rocks are 3,200 million years old.
• This shows that ocean crust is recycled at trenches. That is why the oldest geological record lies on land, for example in the Peninsular shield of India.

Related concepts

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

Read more

• Sea-floor spreading and palaeomagnetism