Joints
- Created by: annayapp
- Created on: 18-01-17 17:10
Joints
- Joints are formed by tensional forces
- Can be as a result of folding, cooling and unloading.
- Rocks have a blocky apperance when they outcrop at the earths surface.
- Only form in competent rocks , brittle and break under tension, include limestone, sandstone and granite.
- Folds in sedimentary rocks form perpendicular to the beds.
Joints produced by folding (Tectonic Joints)
- Produced by tension created in rocks when they are folded by the earths movements.
- Compressive forces fold the rocks but cause some of the rocks around a fold to be under tension.
- Rocks fracture because the outer surface of the bed is stretched more than the inner surface.
- Rocks are competent so do not change thickness as the are bent, so brittle fractures form
- There are two types of tectonic joints: Tension joints (parallel to the axial plane trace of the fold) and Cross joints (at an angle to the axial plane trace of the fold).
Joints produced by cooling (cooling joints)
- Caused by tensional forces set up in an igneous rock as it cools steadily and contracts.
- Joints are perpendicular to the cooling surfaces of the thick basaltic lava flows, dykes and sills, so will often be vertical.
- The igneous rock is insulated and then cooled with evenly spaced cooling centres.
- As the rock cools and crystalizes it shrinks slightly and this results in a series of column like structures which are polygonal in plan.
Joints formed by unloading rocks (unloading joints
- Often form in granite, an igneous rock which cooled deep below the surface where the pressure is high.
- The weight from the overlying rocks 'compresses' the rock- called load pressure.
- When granite is exposed at the surface, as result of uplift and erosion, the lack of load pressure from the overlying rocks allows them to expand.
- Joints form roughly parallel to the earths surface.
- Most joints are approximately horizontal but can be vertical.
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