Stratification? | Foliation?

Discussion in 'Earth Science' started by kingwinner, Nov 28, 2005.

  1. kingwinner Registered Senior Member

    Messages:
    796
    1) I don't get the difference between Stratification and Foliation...and if a rock a foliated or non-foliated, what does it mean?

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    2) How are sedimentary and metamorphic rocks different in appearance? (In other words, how can you tell whether a rock is sedimentary or metamorphic?)

    3) How are cross-bedding in some sedimentary rocks formed?
    These are the quotes from my text book, but I don't understand...can someone elaborate?
    "cross-bedding is formed as inclined layers of sediment move forward across a horizontal surface"
    "cross-bedding is formed as sediment is carried forward across a layer of sediment, and cascades down the front face of the layer"


    Can someone explain? I would appreciate!

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  3. Laika Space Bitch Registered Senior Member

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    Foliation is the planar fabric that a rock can acquire during metamorphosis. It can be formed by the alignment of crystal grains (by physical rotation or dynamic crystallisation) under extreme pressure. It can also occur due to the preferential crystallisation of some minerals at certain sites along microscopic folds. The foliation plane produced is generally perpendicular to the direction of maximum compression.

    By stratification I presume you mean sedimentary stratification, as in bedding and laminations. Such stratification also produces a planar rock fabric due to similarly-orientated grains or by compositional changes between layers.

    The mineral composition of metamorphic rocks varies according to the protolith (the original, unmetamorphosed rock) and the temperature/pressure conditions during metamorphosis. Using a microscope, thin sections of metamorphic rocks can be distinguished fairly easily from sedimentary rocks by the mineral composition and fabric.

    If you don't have a microscope or thin sections, hand samples of metamorphic rock might be identified by the visible presence of minerals such as mica, garnet, kyanite, etc.

    Also, in sedimentary rocks compositional boundaries will generally coincide with bedding planes. In metamorphic rocks, however, compositional boundaries are rarely parallel to the cleavage plane.

    My knowledge in these areas is rusty so it'd be good if somebody could step in if I'm in error... Ophiolite, Facial?
     
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  5. kingwinner Registered Senior Member

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    So basically, stratification and foliation both mean layering of rocks, but stratificaiton is a term used for sedimentary rocks and foliation is a term used for metamorphic rocks!

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    Then, how can you distinguish between a sedimentary rock and a foliated metamorphic rock when both have observable layering?
     
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  7. Laika Space Bitch Registered Senior Member

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    Being pedantic, foliation doesn't really mean layering of rocks, but it can provide a planar fabric that is superficially similar.

    In sedimentary rocks layers are generally pretty uniform in composition over small distances. This reflects the fact that sedimentary environments can be very large. There are exceptions, of course.

    If a sedimentary rock has been metamorhosed, and bedding planes are still apparent, it is very unlikely that their strike and dip will coincide with that of the metamorphic foliation. So while you might see compositional/textural changes between bedding planes, the foliation will cut across these boundaries, and will be distinguishable from it.

    Probably the best indicator is the mineral composition. The minerals present in metamorphic rocks are usually different from those in their unmetamorphosed counterparts. The exact assemblage tells an experienced metamorphic geologist (not me) the tectonic history of the rock.
     
  8. valich Registered Senior Member

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    3,501
    Stratification, or layering, can be caused by folding or foliation, but can also just be caused by deposition, or the transport of a sedimentation layer through climatic weathering or other anomalies.

    Foliation can lead to stratification, but not always. It is a textual feature involving metamorphic rocks and cleavage. The cleavage can lead to parallel planes or nearly parallel surfaces that may or may not be stratified or layered.

    How does that sound? Feedback here.
     

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