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How is deformation related to metamorphic rock structures?
deformation is related to metamorphic rock structures because deformation can create metamorphic rocks through the stress it creates, and also certain structures in the foliations of metamorphic rocks can indicate stress and stress direction from deformation. Some structures caused by stress are folds and faults, and the resulting indicators in metamorphic rocks include things such as sigma structures, Z structures, mica fish, boudinage structures and many others.
What happens when extension pulls rocks apart?
Depending on the scale and also on the pressure temperature conditions where the extension is occuring there are a number of diffrent results of tension in rocks. At higher temperatures and pressures a type of ductile deformation occurs which can form structures by boudinage which causes the stretching and breakup of preexisting structures wihin the rock (see the related link for more information) to form a sausage or barrel geometry. At shallower depths where materials behave in a brittle way, initially rocks will break "or crack" forming joints. If the tensile force continues to be applied and if it is large enough this may then generate normal faults and "horst and graben" structures. These are blocks of crust that are bounded by normal faults where the "graben" are downthrown blocks relative to the "horst", where this motion helps to accomadate the strecthing of the crust. See the realted links for more information and diagrams on this phenomenon. If the crust continues to experience tensile stress at a high enough level for a long enough time, then ultimately a rift valley marking the creation of a new ocean and a new plate boundary will form.
