What is Stress and Strain?
There are four types of stresses that can act on rocks
- Confining stress – weight of all the rocks above
- Compression – forces that act towards each other make up compressional stress… such as at convergent / collisional boundaries of tectonic plates (Juan de Fuca Plate moving under North America, Indian Plate and Eurasian Plate forming the Himalayas…)
- Tension – forces that are acting to pull the rock apart make up tensional stress, causing rocks to lengthen and / or break… such as at divergent plate boundaries (East African Rift Valley, Red Sea Rift…)
- Shear – forces that are parallel but in opposite directions make up shear stress… such as at transform plate boundaries (San Andreas Fault in California, Alpine Fault on South Island of New Zealand…)
From the four types of forces above, one can imagine that rocks at different depths in different regions of the world are experiencing different stresses. Much stress data from reliable sources is compiled into the World Stress Map – http://www.world-stress-map.org/
When the stresses act upon a material / rock, they cause that material to strain / deform. This deformation can be broken down into two categories
- Ductile deformation
- Brittle deformation
Look for a follow-up post describing these deformations.