Rock Park: Central Section

Featuring “plutonic” (deep-Earth-formed) igneous rocks

The realm of Pluto, The Greek god of Hell, is one hot place, deep GCC Central Rock Park Section What rocks would form there? Certainly they would be molten and, being deep in the Earth, would cool slowly allowing minerals to form rather large sizes. Many common rocks form in this manner, for example, granite. The heat and pressure colliding tectonic plates during the Paleozoic Era melted a lot of pre-existing rocks which then cooled slowly into a variety of granite types. Rocks that did not melt became changed into metamorphic types. This section represents rocks melted or metamorphosed due to the Paleozoic assembling of Pangea.

#1 Stone bench, composed of granite curbing, quarry location unknown. Besides the gray granite, note the intrusions of coarse-grained material, called “pegmatite”. These represent minerals from the final stages of cooling of the magma chamber and have large sizes.

#2 Two polished granite pieces, discards from central Vermont’s monument industry. One sample is a pink, due to pink feldspar.


#3 Granite Porphyry samples (2), Kinsman Granite, Devonian Period, glacial erratics from Winchester, NH where this rock type occurs (Ashuelot Pluton). The large crystals of feldspar formed early in the crystalization of this sample, and were able to grow to large sizes. This is the defining feature of a “porphyry”. The large crystals are called “phenocrysts “.

#4 Granite Porphyry, Coys Hill Pluton, Worcester County, Devonian. This granite porphyry sample has been slightly metamorphosed — the fabric of the rock is changed from the usual granite pattern of randomness. Crystals of feldspar form the “phenocrysts”.


#5 Granite (pink) intrusion into gneiss. Original location unknown, from construction co. in Orange.

#6 Granite (tonalite or quartz diorite subtype), Hatfield Pluton, Hatfield, MA, Devonian age. Note intrusions of quartz with minor fluorite (purple).


#7 “Big Peg”, central sample. Goshen formation, Devonian Period, Goshen, MA. This large sample is an excellent example of an intrusion. The Goshen Stone (a mica schist with small garnets — also see sample #11 in the North Park area) has been intruded by pegmatite, a coarse-grained granite. When the sample was delivered, it broke. The pegmatite pieces 9a and 9b were formerly attached to “Big Peg”.

#8 Pegmatite intruding gneiss. This large sample came from a construction company site in Orange. It is probably the Monson Gneiss, a late Ordovician Period metamorphosed granite rock.

#9 Pegmatite samples, Devonian age, from Gilsum, NH and Goshen, MA Pegmatites are dominated by the common rock-forming minerals quartz, feldspar, and muscovite mica, but interesting accessory minerals can often be found. This is because at the final stages of cooling of a magma chamber, the atoms that didn’t fit into previous minerals, will be concentrated. In this group of samples, look for long black crystals of tourmaline and (in sample 9a) green apatite.


Unnumbered: smaller samples against brick wall. These samples represent several things: (1) Graphic Granite, Devonian Period, Gilsum, NH. The quartz and feldspar intergrow to form this unusual rock. (2) Vein Quartz, West Northfield and other locations. Quartz is a common intrusion (vein) because it is one of the first minerals to melt during metamorphic heating, and also the last to crystallize during magma cooling. So, many times, there is a lot of quartz that’s able to move through the rocks and crystallize in veins. Sometimes you can see the crystal points, but usually the quartz completely fills the vein opening. (3) Calcite veins, West Northfield. Groundwater fluids rich in calcium carbonate will deposit calcite in veins.

Calcite has cleavages and is softer than glass, so it can easily be differentiated from quartz, if you know what to look for. Do you?

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