explain why gneiss is not converted back to shale as temperature and pressure decrease?

Answer 1

Two reasons:

1.) High grade metamorphism radically alters the mineral and grain structure of gneiss; AFAIK, it cannot become shale again.

2.) As someone else already mentioned, the minerals in gneiss are metastable; at the Earth's surface, it would take practically forever for them to revert back to low temperature and pressure equilibrium.

Answer 2

Gneiss can turn into shale through weathering since shale is essentially a sedimentary rock made out of clays. The feldspars and micas in the gneiss will physically and chemically weather down into clay eventually.

However, gneiss doesn't come from shale. Slate does. For one thing, heat and pressure alone cannot add all that was lost in the weathering process. Second, gneiss is (for the most part) a very coarse-grained metamorphic rock. What grains a shale has are very, very small. Now, it can melt completely, be extruded as a granite, then metamorphose into a gneiss. But I'm pretty sure that it can't go directly from shale to gneiss.

As an aside, a diamond wants to be graphite on the surface of the earth. Under intense heat and pressure, pure carbon takes the form of a diamond. If it rises slowly through the layers to the surface, it will eventually turn to graphite (which is also pure carbon...not charcoal). However, if it rises quickly to the surface, it will stay a diamond until sufficient energy is supplied to it to turn it into it's stable low-pressure form...graphite.

Answer 3

Gneiss can form from shale.
http://volcano.und.edu/vwdocs/vwlessons/lessons/Slideshow/Merocks/Merock5.html

If you revisit the rock cycle you will discover that gneiss does indeed change back to shale, except the process is not direct, it involves weathering of the minerals in the gneiss to form clays, which in turn form shales.

The gneiss is in fact in a meta-stable condition, meaning that the chemical compounds within it at the surface are no longer in the temperature and pressure regime at which they are stable. Weathering will return them to a stable condition until they are once again cycled into a different pressure-temperature regime and metamorphosed.

Answer 4

shale does not give rise to gneiss. Granite does.
Why doesnt bread turn into flour when it comes out of the oven?

Answer 5

Shale To Gneiss

Answer 6

Gneiss contains too much feldspar, and too much biotite mica. And its particles cannot decrease as prassure decreases.

Answer 7

many chemical processes are not reversible

consider how seldom you see a diamond turn into a charcoal like substance because it has been at low heat and pressure for awhile

Answer 8

Where do you go to school? Are you on the varsity basketball team? I hope so. I hope you are taking this class because you thought it would be an easy C.