Rates and drivers of plate movement; mantle plumes

Detail

1. How plate speed is measured: magnetic stripes

• Palaeomagnetic stripes are bands of ocean-floor rock that run parallel to a mid-ocean ridge. When lava cools at the ridge, its iron minerals line up with the earth's magnetic field at that time.
• The earth's magnetic field flips from time to time, between normal polarity (like today) and reverse polarity (the opposite).
• So the rocks form a series of normal and reverse strips. The pattern on one side of the ridge mirrors the pattern on the other side.

• Scientists know the age of each flip. They divide the width of a strip by the time it took to form, and that gives the spreading rate: how fast the two plates move apart.

• The result: spreading rates differ a lot from one ridge to another.

Ridge	Location	Spreading rate	Note
Arctic Ridge	Arctic Ocean	Under 2.5 cm a year	Slowest
Mid-Atlantic Ridge	Middle of the Atlantic Ocean, North and South	About 2.5 cm a year, which is 25 km in a million years [2]	Slow
East Pacific Rise	South Pacific, near Easter Island, about 3,400 km west of Chile	Over 15 cm a year [2]	Fastest

• Spreading seen on land: at the Krafla fissure zone in Iceland, where the Mid-Atlantic Ridge rises above the sea, rifting between 1975 and 1984 pulled the ground apart by about 7 m in total [2].
• How slow this is: plates move at about the speed your fingernails grow. (NCERT Class 7 compares India's movement to how fast hair grows.)

2. Measuring plate motion directly: GPS

• GPS (Global Positioning System) uses satellites that orbit at about 20,000 km. Fixed ground stations record their exact position again and again, so even a shift of a few millimetres a year can be measured [2].
• Space geodesy means measuring the shape and movement of the earth from space. It has shown that plate speeds and directions averaged over a few years match the speeds worked out from magnetic stripes averaged over millions of years [2]. This means plate movement is steady over very long periods.
• India today: the Indian plate moves north-northeast at about 4-5 cm a year. (NCERT Class 7: about 5 cm a year.)

3. India's journey: a speed record

• About 200 million years ago: Pangaea broke apart. India began to move north [3].
• About 80 million years ago: India lay about 6,400 km south of Asia. It was moving north at about 9 m every 100 years, which is about 9 cm a year [3].
• About 40-50 million years ago: India hit Asia. Its northward speed dropped by about half, to about 4.5 m every 100 years [3].
• The result: the Himalayas are still rising by more than 1 cm a year, which adds up to 10 km in a million years. Peaks such as Mt Everest have grown more than 9 km high in about 50 million years [3].

4. What drives the plates: the NCERT view (convection)

• The earth is dynamic, not still. Its inside is hot and moving, and the plates ride on top.
• Mantle convection is the slow, circular flow of hot, softened mantle rock below the rigid plates.
• Cause: heat from radioactive decay (unstable elements breaking down and giving off heat) and residual heat (heat left over from when the earth formed) [4].
• Process: hot rock rises, spreads out sideways under the plates, cools, becomes heavier and sinks again. Each loop is called a convection cell. It works like a pot of boiling soup, but far, far slower [4].

• Result: the flow drags the rigid plates along above it.

• Who proposed it: Arthur Holmes first suggested convection in the 1930s. Harry Hess used it in the 1960s to explain sea-floor spreading.

• The limit: scientists still do not know how many convection cells there are, or exactly how they are shaped. These deep forces cannot be tested directly [4].

5. The modern view: slab pull and ridge push

• Slab pull: at a subduction zone (where one plate sinks below another), the old, cold oceanic plate is heavier than the mantle around it.
• It sinks under its own weight.
• As it sinks, it drags the rest of the plate behind it, like a tablecloth sliding off a table.

• Most scientists now think this is the largest force [4].

• Ridge push: new crust at a mid-ocean ridge is hot and light, so the ridge stands high.

• The young plate slides down and away from this raised ridge under gravity.
• Magma pushing in at the ridge adds to this push [4].

• This force is smaller than slab pull.

• Mantle convection now acts mainly as the background setting. Plates and mantle work as one linked system.

• Proof from plate speeds: the fastest plates have long subducting edges.
• The Pacific plate sinks along trenches on almost all sides, so it is fast.
• Plates with few sinking edges, such as those around the slow Arctic and Atlantic ridges, move slowly.

Force	Where it acts	How it works	Relative size
Slab pull	Subduction zones (trenches)	A cold, heavy slab sinks and drags the plate	Largest
Ridge push	Mid-ocean ridges	A raised young plate slides away under gravity	Smaller
Mantle convection	Whole mantle	Heat-driven circular flow	Background setting

6. Mantle plumes and hotspots

• Mantle plume: a narrow column of unusually hot rock rising from deep in the mantle. As it nears the surface, the pressure drops and the rock melts.
• Hotspot: a small, long-lasting, very hot area under a plate where a plume supplies magma. It forms volcanoes on the surface [5].
• More than 100 hotspots have been active in the past 10 million years [5].

• Hotspots can lie far from plate boundaries. The Hawaiian Islands are more than 3,200 km from the nearest plate boundary [5].

• Types by setting [5]:

• In the middle of an ocean plate: Hawaii, on the Pacific plate.
• Under continental crust: Yellowstone, on the North American plate (USA).
• On or near mid-ocean ridges: Iceland, the Azores (both on the Mid-Atlantic Ridge) and the Galapagos Islands (East Pacific).

7. Hotspot tracks: how a chain of volcanoes forms

• J. Tuzo Wilson (1963) explained the straight line of the Hawaiian Islands and Emperor Seamounts. He said the Pacific plate moves over a hotspot that stays nearly fixed under Hawaii [5].
• Cause: the plume stays in place, but the plate above it keeps moving.
• Process: a volcano forms over the plume. The plate carries it away, the volcano dies, and a new one forms over the plume.

• Result: a chain of volcanoes that get older the farther they are from the plume.

• Hawaii, the evidence [5]:

• Kauai (north-west end): oldest rocks about 5.5 million years old.
• Big Island of Hawaii (south-east end): oldest exposed rocks less than 0.7 million years old, and it is still forming.

• The age gets younger towards the south-east, so the Pacific plate is moving north-west.

• A caution: since the 1990s, scientists have debated how deep plumes start and whether hotspots are truly fixed [5]. That is why they are called "nearly fixed".

• Flood basalts: when a large plume head first reaches the surface, huge amounts of runny lava pour out over wide areas and build thick, step-like layers called traps.

8. Plumes that shaped India

Plume	Present location	What it made in and near India
Reunion plume	Under Reunion Island, western Indian Ocean (east of Madagascar)	Deccan Traps: the flood basalts of the Deccan Plateau (Maharashtra and nearby states)
Kerguelen plume	Under the Kerguelen Islands, southern Indian Ocean	Ninety East Ridge: a straight undersea ridge running north-south along about 90°E in the Bay of Bengal and the Indian Ocean. It probably also made the Rajmahal Traps (Jharkhand). See Section 10.

• How it fits together: India moved north quickly over these two plumes.
• It passed over the Kerguelen plume first, which left the long Ninety East Ridge track.
• It passed over the Reunion plume later, which poured out the Deccan lavas.

Prelims Hooks

• Fastest spreading ridge: East Pacific Rise near Easter Island, over 15 cm a year. Slowest: Arctic Ridge, under 2.5 cm a year. The Mid-Atlantic Ridge spreads at about 2.5 cm a year [2].
• Order of spreading rate, fastest to slowest: East Pacific Rise > Mid-Atlantic Ridge > Arctic Ridge.
• Indian plate: moves north-northeast at about 4-5 cm a year. Before it hit Asia, about 80 million years ago, it moved at about 9 cm a year [3].
• Main driving force of plates (modern view): slab pull, not ridge push. Trap: "mantle convection directly pushes plates" is the older textbook view (Holmes, then Hess).
• Sources of heat for convection: radioactive decay and residual heat from the earth's formation [4].
• Hotspot idea: J. Tuzo Wilson (1963), based on the Hawaiian-Emperor chain [5].
• In a hotspot chain, the volcanoes get older away from the active hotspot. In Hawaii, Kauai is older than the Big Island [5].
• Plume and feature pairs: Reunion → Deccan Traps. Kerguelen → Ninety East Ridge (and probably the Rajmahal Traps).
• Hotspot and setting pairs: Yellowstone (continental crust). Iceland and the Azores (Mid-Atlantic Ridge). Hawaii (middle of the Pacific plate) [5].
• Trap: hotspot volcanoes can form far from plate boundaries. Hawaii is more than 3,200 km from the nearest boundary [5].

Mains Points

• Plate speeds are uneven, and the forces explain why: plates with long subducting edges, such as the Pacific plate, move fastest because slab pull is the largest force. Ridge push and mantle convection play smaller, supporting roles. This links the location of ocean trenches to plate speed, which helps answer GS-I questions on the distribution of earthquakes and volcanoes.
• India's tectonic story explains its landforms:
• India moved fast (about 9 cm a year) after Pangaea broke up [3].
• It passed over the Kerguelen and Reunion plumes, which gave it the Ninety East Ridge, the Rajmahal Traps and the Deccan Traps.

• It then hit Asia and slowed. The Himalayas still rise by more than 1 cm a year [3].

• Hazard angle (GS-III): India still moves about 4-5 cm a year towards Eurasia. This stress builds up along the Himalayan belt and causes earthquakes there. GPS monitoring of plate motion is therefore a tool for disaster preparedness.

• How science works: the theory moved from a single driver (convection, Holmes and Hess) to several linked forces (slab pull, ridge push, plumes). GPS has now confirmed that plate rates seen over a few years match those seen over millions of years [2]. This makes a good example of how measurement confirms a theory.