CHEMISTRY · Stage 9 · Chapter 13: Planet Earth and Continental Drift
Chemistry 13.3 Evidence from seafloor spreading — Mid-Ocean Ridges and Palaeomagnetism
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Lesson summary
13.3 Evidence from seafloor spreading — Mid-Ocean Ridges and Palaeomagnetism — Alhussan International Academy — Chemistry (Stages 7–9) notes: Define oceanic ridge and seafloor spreading and explain how symmetrical magnetic stripes provide evidence for plate movement.Why Wegener w…
- Why Wegener was rejected
- Seafloor mapping
- Rift valley
- Oldest crust
- Creation and destruction
- Magnetised rock
Part 1 of 5Oceanic ridges
- Why Wegener was rejected — in spite of the evidence from fossils and continent shapes, most scientists did not know how continents could move apart.
- Seafloor mapping — in the 1950s scientists used instruments towed by ships that send sound waves to the seafloor, detect the reflections and process them into images.
- Oceanic ridge = a mountain chain on the ocean floor.
- A huge system of ridges runs through the Atlantic, Pacific and Indian Oceans, including the Mid-Atlantic Ridge.
- Rift valley — at the centre of a ridge, hot molten rock (magma) rises from the mantle, cools and solidifies to form new oceanic crust.
Part 2 of 5Seafloor spreading
- Seafloor spreading = the movement of the seafloor away from the two sides of an oceanic ridge.
- Process — as more magma rises, the older crust is pushed away on either side, driven by convection currents in the mantle.
- New rock forms at the ridge, and the rocks become increasingly older further from it.
- Example — at the Mid-Atlantic Ridge the South American plate and African plate move apart, so the Atlantic widens by a few centimetres a year.
Part 3 of 5Age of the ocean floor
- Rock agesrock samples drilled from the sea floor show the youngest rock is at the ridge and the rock becomes steadily older with distance from it.
- Oldest crustthe oldest oceanic crust is only about 180 million years old, much younger than continental rocks, which can be billions of years old.
- Creation and destructionocean crust is continually created at ridges and destroyed at deep ocean trenches, where it sinks back into the mantle.
- Sedimentit is thinnest at the ridge and thicker further away, because older crust has had longer to collect it.
Part 4 of 5Magnetic stripes (palaeomagnetism)
- Magnetised rock — much magma is rich in iron compounds, and when it cools and freezes at a ridge the new rock is magnetised in the direction of the Earth's magnetic field.
- Reversals — every now and again the Earth's magnetic poles flip, so the magnetic North pole becomes the magnetic South pole and vice versa.
- Rock formed at these times is magnetised in the opposite direction.
- Stripes — magnetometers towed by ships show stripes of rock with alternating normal and reversed magnetisation, parallel to the ridge.
- Symmetry — the stripe pattern on one side of the ridge is a mirror image of the other side.
- New crust forms at the ridge, records the magnetic field at that time, and moves away in both directions.
- Not the sea — the magnetism is the Earth's field recorded in the rock, not something caused by the sea.
Part 5 of 5Why the evidence mattered
- Mechanism — seafloor spreading gave the mechanism that Wegener lacked for how continents drift apart.
- Acceptance — by 1966 many scientists knew the magnetic stripe evidence, understood how continents could drift apart and accepted plate tectonics.
| Evidence | What it shows |
|---|---|
| Rift valley at ridge | Magma rises and new crust forms |
| Rock age increasing from ridge | Crust moves away over time |
| Symmetrical magnetic stripes | Field reversals recorded as crust spreads |
| Trenches | Crust is destroyed at plate edges |
- Speed of spreading — speed = distance ÷ time, and spreading is about 2 cm per year.
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