In our notes on Metamorphism there is a section on foliated vs. non-foliated. A metamorphic rock is produced when the physical and chemical properties change, so is physical metamorphism always recognized through foliation? Because some rocks become foliated and others do not, therefore are the ones that do not foliate always going to be chemically metamorphisized?

-- Kelly Wirth (kwirth@oswego.edu), March 16, 2001

Answers

I don't think foliation is related necessarily to the process of metamorphism as much as it is to the original form of the rock and to the grade of metamorphism. For foliation to exist, there must have been clear sedimentary layering in the original rock. The Manhattan Schist reflects in its foliation the original silt, mud and sand deposits that were lithified then metamorphosed in the Taconic orogeny. In contrast, the Inwood marble was metamorphosed at the same time but its original limestone probably had no clear sedimentary layering. Consequently the marble shows no foliation. Chemical metamorphosis can produce alternating layers in rock. The Manhattan schist shows bands of quarts that are scattered layer-like throughout the rock, but not necessarily in places where there were layers of sand in the original sediments. Quartz is a product of chemical metamorphosis and collects as a function of density differences between rock components at a given temperature and pressure. Layers of mica certainly metamorphose chemically from clay but again, they are scattered through the rock and dont' constitute foliation.

-- peter matt (peter@mattbrothers.com), October 12, 2004.