EARTH238 Lecture Notes - Lecture 15: Crenulation, Schist, Phyllite
30 Aug 2016
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Lecture 15
- Differentiate b/w slate, phylite, gneiss
- Know geologic time scale (divisions and recent time periods)
- Cleavage
o Low grade rocks
- Disjunctive cleavage
o Not continuous
o Microlithon
▪ Initial cleavage is preserved
- Pencil Cleavage
o As easy to break along the bedding surface as it is to break along the cleavage
o Rock – one side defined by the bedding surface and other two sides by the cleavage
o Possibility to break along 2 directions ~same
- Slatey Cleavage
- Phyllicitc cleavage and schistosity
o Phyllite more shiny than slate
- Crenulation cleavage
o Small scale folding (crenulation)
o Foliation and differentiation
o Layers are deformed and folded creating the crenulated feature
- Gneissic layering and migmatization
o Orthogneiss – metamorphosis of intrusions
Folds and Foliation
- Can have folds without foliation but rarely ever see foliation without folding
- Axial planar cleavage
o Parallel or symmetrical to the axial plane
- Intersection cleavage – formed by the intersection of two surfaces (ex. Bedding and cleavage)
Cleavage refraction
- Due to change in medium
- Low competency layer (ex. Siltstone)– smaller angle cleavage
- More competent(ex. Sandstone) – larger angle cleavage
Formation of cleavage
- Preferred orientation
o
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