Could someone explain the formation of the foliation of Gneiss?

Answer 1

Gneiss is a high-grade metamorphic rock that can have many different parent rocks, with the most common being granite, diorite and schist. The buildup of intense heat and pressure over a very long period of time enables the migration of atoms within the rock leading to the formation of mineral bands that are oriented perpendicular to the directions of pressure. This banding develops a crude sort of foliation, typically as alternating layers of light minerals (quartz and feldspar) and dark minerals (hornblende and biotite mica). As gneiss approaches its melting point, the dark and light-colored mineral bands can bend and become quite contorted. Of course, if temperature rises too high, the rock will melt and return to the igneous phase of the rock cycle.

Answer 2

Gneissic banding probably has no one single cause, but is different in different cases.

Metamorphic segregation, where the minerals 'unmix' in the solid state is commonly cited (Kim's explanation), but the actual mechanisms behind this ain't too well understood.

Banding can also be derived from the parent rock (or rocks) that were metamorphosed to form the gneiss. Many gneisses in granitic gneiss terranes are formed from granites, doirites, tonalites etc. which often were intruded as as a series of veins, and commonly are themselves cut by younger intrusions. During intense deformation these original inhomogeneities get streaked out and flattened to form banding, but the original structure often survives in undeformed patches. Most of the gneisses I have seen/worked on have banding that originates this way.

Another cause is dehydration/migmatisation. High grade metamorphism almost always involves dehydration reactions. The escaping water can dissolve material and reprecipitate new minerals as veins, or promote the growth of new minerals. Likewise migmatisation (partial melting) can give rise to pale granitic layers (the melt) and usually darker residue (the stuff that didn't melt). Either, can form parallel to an existing fabric giving rise to layering, or can be subsequently flattened into layers during deformation.

Answer 3

Gneiss question (it is pronounced 'nice' or sometimes 'niece'). The foliate gneiss contains alternate layers of weak and strong rock like a stack of pancakes (weak syrup?) although the layers may wrap around. The weak layer of gneiss contains feldspar which has flake-like weak crystals. The link shows how the 'flakes' may be stacked like toothpicks during metamorphosis.

Answer 4

Kim's explanation is bang-on. Gneisses are my favorite rocks because of the wonderful color variations and the wild plastic contortions that are sometimes present. You can also see augen (eye) structures and roll structures, pressure shadows, and a whole slew of interesting features.

I think I like gneisses because they show some much evidence for plastic behavior, yet rocks seem to be such durable, hard substances to us.

Answer 5

I am not sure that the absolute answer to ths is known. It was originally supposed to be a rough conformation with original bedding planes but that is not held to be the case of late since the striae/folia follow mineral species patterns more closely, and can be explained with mineral crystalizaion sequencing--in short the idosyncracies of Nature (God ?) play an important role but no one was there to witness it when it was happening so which criteria is truest can be argued.

Answer 6

You should explain that you want the real answer not the cheap out "God made it" that religious folks will give you. No wonder religion is popular, makes it so easy to be lazy.