A weak or soft rock layer, particularly used in geology to describe the asthenosphere or weak portions of Earth's crust.
From Greek 'asthenos' (weak) + 'lithos' (stone), a geological term for mechanically weak rock material.
The asthenosphere beneath your feet is literally an 'asthenolith'—it's weak enough that the hard lithosphere can slide around on top of it, which is what drives plate tectonics and earthquakes.
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