Divergent boundaries: ridges and rifts
Sea-floor spreading with magnetic stripes, and a continent splitting into a new ocean.
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What you see
Mid-ocean ridge: mantle rises, melts and builds new crust that records reversals of Earth’s magnetic field as symmetric stripes; the anomaly a ship would measure is drawn above the sea. Change the spreading rate and the stripes widen. Continental rift: the crust domes, breaks into a rift valley, thins, erupts and finally splits into a new sea like the Red Sea.
Key ideas
- At divergent boundaries new lithosphere is created by decompression melting of rising mantle.
- Magnetic stripes mirrored on both sides of ridges confirmed sea-floor spreading (Vine & Matthews 1963).
- Ocean depth increases with the square root of the age of the floor as the plate cools.
- Continental rifting goes from dome to rift valley (East Africa) to narrow sea (Red Sea) to ocean (Atlantic).
Test yourself
1. Why are magnetic stripes symmetric about the ridge?
Each stripe forms at the axis and is split and carried away on both plates.
2. Which is an example of a young ocean formed by continental rifting?
The Red Sea floor has new oceanic crust between the Arabian and African margins.
3. A faster spreading rate makes the stripes…
More crust is produced during each polarity interval, so each stripe is wider.
References
- Vine, F.J. & Matthews, D.H. (1963). Magnetic anomalies over oceanic ridges. Nature, 199, 947–949.
- Parsons, B. & Sclater, J.G. (1977). An analysis of the variation of ocean floor bathymetry and heat flow with age. JGR, 82, 803–827.
- Kearey, P., Klepeis, K.A. & Vine, F.J. (2009). Global Tectonics, 3rd ed. Wiley-Blackwell.
- Bosworth, W., Huchon, P. & McClay, K. (2005). The Red Sea and Gulf of Aden basins. Journal of African Earth Sciences, 43, 334–378.