A Mohr circle construction for the opening of a pre-existing fracture

The opening of a fracture is dependent on the fluid pressure and the local stress regime around the fracture. For a fracture to open, the fluid pressure must exceed the normal stress acting on the fracture. The Mohr circle provides a useful tool allowing the visualization of these relationships. This construction determines the range of fracture orientations that are able to open, as well as determining the opening direction of these fractures. This is then used to produce ratios of the magma pressure and the principal stresses for some dykes from the Lizard ophiolite U.K.