Ed
Thanks for the research and explanation. It sounds as if the rocks
in Mohawk are rich in minerals. Charlie Cogbill considers amphibolite
to be one of the best rocks for a rich understory.
Bob
Sent from my iPhone
On Aug 13, 2009, at 2:16 PM, "Edward Frank" <[email protected]> wrote:
> 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
>
> >
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