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12.2 What is the evidence that plates are rigid?
- Is the whole lithosphere really strong?
- Rock strength decreases with increasing temperature
- Rocks should thus be very weak below 15-40 km (Section 11.26)
- But plate tectonics invokes a strong lithosphere throughout, down to ~100 km
- Seismic waves indicate an abrupt decrease in rock strength at the top of the asthenosphere (Section 8.4) at the melting point of peridotite. So the lithosphere is stronger and more rigid than the asthenosphere beneath.