Lines of weakness - joints and bedding planes?

Question

Hey - it would be great if I can find out more about the question above to assist me with my revision - I've been looking for hours on Google and I can't find anything. Can anyone provide a link or share their knowledge with me? Thnx, Matt :)

Answer 1

Joints are fractures in rock perpendicular to the bedding plane.  They are usually when the rock is moved (as in a fold or fault) although some may result from desiccation (such as in a mudstone).  These can usually be easily distinguished from one another because the desiccation cracks will not be perpendicular to one another (as in mud crack polygons).

The joints are indeed lines of weakness because that is where the rock has broken before.

Bedding planes can be lines of weakness in that beds may slide over one another in a fold situation.  This is greatly dependent on the types of beds involved.  For example, a limestone may have joints from flexure, but it could slide laterally if sitting on a shale.  The bedding plane would be one line of weakness and the joints another.

If you were mapping a fault seen by offset in the top of the limestone and followed that down through the limestone, it might seem to disappear when the fault hits the shale.  However, the fault may move along the bedding plane and continue on down at some distance away from the joint you were tracing down through the limestone.