Pressure shadow
Appearance
A pressure shadow (also called strain shadow) is a term used in metamorphic geology to describe a microstructure in deformed rocks that occurs adjacent to a relatively large, undeformed particle, such as a porphyroclast. Pressure shadows often appear in thin sections as pairs of roughly triangular regions that are elongated parallel to the foliation around a clast of a different mineral.[1] Pressure shadows that contain fibrous mineral textures are also termed pressure fringes or strain fringes.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/4/4e/Aar02_pressureshadows.tif/lossless-page1-220px-Aar02_pressureshadows.tif.png)
Formation
During deformation, minerals can migrate by plastic flow or may grow by diffusive mass transport into the lower-stress regions created by a rigid porphyroclast or porphyroblast.[2]
References
- ^ Passchier and Trouw (2005). Microtectonics (2nd ed.). Springer. ISBN 978-3-540-29359-0.
- ^ Blenkinsop, T.G. (2007). Deformation Microstructures and Mechanisms in Minerals and Rocks. Kluwer Academic Publishers. p. 150. ISBN 0-306-47543-X.