ENTS, Determining what the original rock was in a metamorphic rock sequence is often very difficult to downright impossible. If it is only slightly metamorphosed then some of the original structure may still e detected. If it has been heavily metamorphosed any original structure is long gone. In metamorphic rocks the larger the grain the higher the degree of metamorphism. When sandstone is metamorphosed you will always get quartzite - the quartz in the sandstone will simply reform to other sizes of quartz grains under metamorphosis. Similarly limestone will form marble when metamorphosed as it consists of just the mineral calcite. Since limestone layers may be relatively thin under metamorphosis they may intermix with layers above and below and not form a pure marble. When you have rocks with mixed minerals in them - like shale and siltstone, or many of the igneous rock you get a variety of different metamorphic rocks. Shale is the most common sedimentary rock making up 70% of the total mass of sedimentary rocks followed by siltstone.
During metamorphosis the rocks are subject to heat and pressure. The pore space space is lost. The minerals in the rocks or sediments melt under the pressure and recrystalize in a different orientation, or combine with other minerals originally to form new minerals. The more heat and pressure the more dramatic the changes, and the larger the crystal grains that form. The specific minerals present are a function of the original mineral composition of the rock and the the heat and pressure to which it was subjected. In lightly metamorphosed rock the grains are impossible to see - these are slates and greenstones. In the next greater stage you can almost see the individual grains - you can convince yourself you can see them - they are often shinny metallic looking muscovite mica - these are called phylites. The next greater degree of metamorphoses forms schist. These have grains easily seen by the naked eye. The minerals in slates, phyllites and schists form layers that dependant of any original bedding called cleavage. The layers in slate for example, are not the same orientation as the layers in shale that likely was the original source. The grain orientation is dependant on the directions of stress and strain in the folding process. Under the greatest degree of metamorphosis Gneiss is formed, It is coarse grained rock and the layers are folded and contorted within the rock pieces. In general the minerals in a schist or gneiss may be exactly the same as those found in a granite. The difference is that in a schist and gneiss the minerals are arranged in layers, while in igneous rocks the minerals are randomly distributed and oriented. Greenstones are a metamorphic rock that are derived by low grade metamorphosis of basalt (lava) and associated massive clay deposits. Remember that water is often incorporated into the basalt material when exposed at or near the surface ad prior to metamorphosis. The only good way to determine the original rock material of most metamorphic rocks is to correlate or trace the deposits to areas outside the region of metamorphism where they have not be altered (or altered as much) by heat and pressure and see what they were. The Hawley Formation at Mohawk Trails State Forest are described as: Hawley Formation - Interbedded amphibolite, greenstone, feldspathic schist and granofels. Coarse plagioclase in some amphibolite near top; local coarse hornblende blades or sprays. Sparse coticule (Emerson, 1917, p. 43). As used here the Hawley includes amphibolite, sulfidic rusty schists, abundant coticules, silvery schists, quartzites and quartz conglomerates, and quartz, feldspar, biotite granulites. The quartzites and quartz conglomerates occur at two positions in rocks here assigned to the Hawley. Amphibolite is technically really an igneous rock comprised primarily of the mineral amphibole, but as used here I believe to refer to a metamorphic rock with amphibole as the primary mineral The schist, greenstone, gneiss and granofels are all metamorphic rocks. (In granofels the layeres are indistinct because of the near uniformity of the mineral composition). Ed "The most beautiful thing we can experience is the mysterious. It is the source of all true art and all science." - Albert Einstein --~--~---------~--~----~------------~-------~--~----~ Eastern Native Tree Society http://www.nativetreesociety.org Send email to [email protected] Visit this group at http://groups.google.com/group/entstrees?hl=en To unsubscribe send email to [email protected] -~----------~----~----~----~------~----~------~--~---
