Talk:Cleavage (geology)

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Cleavage and Foliation[edit]

From my understanding, cleavage in rocks is just a colloquial name for some types of metamorphic foliation, and that the term "cleavage" is only appropriate when talking about crystals. Anyone else have anything to say on this? Awickert (talk) 20:40, 26 October 2008 (UTC)

In my experience, cleavage is distinct from foliation, just about. It is used to describe planar (S) fabrics in low to very low-grade metamorphic rocks. Searching on google for various combinations yields the following: "slaty cleavage" 14600 hits v. "slaty foliation" 144 hits, "spaced cleavage" 2980 hits v. "spaced foliation" 412 hits, "crenulation cleavage" 8700 hits v. "crenulation foliation" 369 hits, "fracture cleavage" 6310 hits v. "fracture foliation" 85 hits and finally "axial planar cleavage" 2330 hits v. "axial planar foliation" 1120 hits. I also tried "cleavage geology -mineral" 70900 hits v. "foliation geology -plant" 92000 hits. IMO there is nothing 'colloquial' about the usage of this term in structural geology. Perhaps the current article should be changed to a cleavage (structural geology) article, leaving out the mineral part, although I think that this would be a little cumbersome. It's certainly true that the current page lacks content, partly my fault, I started the thing. Mikenorton (talk) 10:03, 27 October 2008 (UTC)
Replaced the mineralogy part with a dablink to cleavage (crystal). Now need to expand the structural part & add refs. Vsmith (talk) 12:11, 27 October 2008 (UTC)
Ah - so I was just meaning the metamorphic foliation-usage of cleavage, especially slaty cleavage. I generally wouldn't call fold axial planes, crenulation, spacing, or jointing a type of foliation: yes, planar feature, no foliation. And for the slaty cleavage - looks like you're right about general usage, although my professors were always adamant that the term cleavage should only apply to minerals, and that it was really just used to describe a particularly well-developed foliation in low-grade metamorphic rocks. When I'm in the field, I know that I mark slaty cleavage in my notes simply as a foliation, because it's related to the same processes that cause foliation in schists and phyllites. Maybe to reconcile this, we could say that cleavage is the name for particularly well-developed foliation in low-grade metamorphic rocks. Awickert (talk) 21:35, 27 October 2008 (UTC)
Somewhat expanded now. I'm still a bit unsure how to handle the cleavage/foliation terminology issue as there seem to be two main schools of thought, either all planar fabrics are foliations or there is some sort of progression from cleavage→foliation→banding (roughly equivalent to slate→schist→gneiss). Mikenorton (talk) 17:32, 14 September 2009 (UTC)
  • Though not a structural geologist, and 1/4 Century out of date, there is a precedent for cleavage-schistosity-banding, which would leave 'foliation' free. Cloos defined a lineation as a 'descriptive and non-genetic term for any kind of linear structure within or on a rock'. One could substitute 'foliate' for 'linear' and have a rational nomenclature. How this would offend current usage I don't know. Geologist (talk) 18:16, 1 October 2009 (UTC)
Found refs for these two end members and modified text accordingly. Mikenorton (talk) 17:54, 14 September 2009 (UTC)
You know, you can ignore my protests. At least in my experience, "cleavage" is reserved for "slaty cleavage" which is a subset of metamorphic foliation and is discouraged (e.g., used only for crystals), but I've said all that above. I'll always use "foliation" instead (e.g., for everything in the top right figure), but as long as some people use it, we might as well have the info. If I find a ref, I might add that "some geologists suggest that the term be restricted to crystallographic orientations". Awickert (talk) 19:25, 14 September 2009 (UTC)
Turns out that the usage I'm familiar with started with Darwin, who was the first to define the term 'foliation'. Note that the 'IUGS sub-commission on the systematics of metamorphic rocks', all europeans, have (perhaps reluctantly) agreed to adopt the north american usage of cleavage as a special case of foliation. I was taught by both Price & Cosgrove, so I cling to the alternative view that foliation should show a clear segregation, but I'm going to rewrite that paragraph to match the IUGS view. Mikenorton (talk) 06:30, 15 September 2009 (UTC)
Isn't that great about geology? In our terminology, the mothballs come out of the closet dressed in adventurous-looking tweed and bowties proclaiming the mismash history of science. Awickert (talk) 07:01, 15 September 2009 (UTC)