Amphibolite blocks in folded Grenville Marble
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Approximately 1.5 miles east of Gouverneur on Route 58 is a road cut feature of amphibolite blocks in folded Grenville marble. The road has wide paved shoulders, and the base of the road cut is approximately 20 feet or more from the pavement.
A rock is naturally occurring and coherent aggregate of one or more minerals-although there are a few rocks made from other substance, such as the decayed vegetation of which coal is composed. There are three major classes of rocks-igneous, sedimentary and metamorphic- and each of these three classes is further subdivided into groups and types, principally based on difference in their mineral composition and texture.
All rocks can be changed by heat and pressure. Such changes are called metamorphism. The most significant and transformed occur deep within the Earth, but local changes can be produced nearer the surface. Strong pressure creates dynamic metamorphism, typified by the compression of mud-rocks into slate. On a smaller scale, such pressure may be localized—for example along fault planes, where rocks are distorted by the Earth’s movement. Intense heat produces thermal metamorphism, which ranges from localized heating of rocks around a small igneous body to more widespread heating that produces mile-wide zones of recrystallization, called thermal aureoles, around major, deep-seated plutonic intrusions. Together, heat and pressure on a large scale create regional metamorphism, which is graded from low to high, with increasing pressure and temperatures triggering the formation of assemblages of new mineral, and so converting the original rock texture into a metamorphic texture, often dominated by planar foliation.
Marble is the product of thermal or regional metamorphism of limestone. Heat and pressure cause the carbonate minerals to recrystallize into compact and often homogeneous marble of various colors. Superficially, it may be confused with quartzite (another metamorphic rock), but marble can be scratched by a steel point, and also reacts to weak hydrochloric or acetic acid. Recrystallization tends to remove any cracks or cleavage, but traces of folds—often highly contorted—maybe preserved, indicating that the rock flowed during metamorphism. Also, marble that is the product of low-grade metamorphism may preserve traces of fossil and sedimentary rocks. It is compose mainly of calcite but may contain other minerals rich in magnesium, calcium, and iron; such is serpentine, dolomite, phlogopite mica, and amphibole tremolite, which can produce distinctive colors and textures.
Basalt is the most common igneous rock on the Earth’s surface; it forms the rock floor of most of the oceans. It also occurs in continental settings as extensive basalt plateaus. In both settings, basalt is found as intrusive and extrusive bodies of rock. It weathers to pale green, brown, or gray or where oxidized, to red. It occurs mostly as extrusive lavas, but also forms intrusions such as dikes and sills. On cooling, it may form distinctive joined columns.
Amphibolite is one of the few metamorphic rock types that are named after a single mineral (amphibole). This mineral is extremely important in metamorphic rocks that are derived from extrusive rocks (lavas), such as basalt or andesite, and intrusive rocks, such as gabbro and diorite. This is because the mineral structure of amphibole can incorporate a large variety of elements. Despite changes in its composition, amphibole remains a stable mineral during metamorphism of mafic (basic) to intermediate igneous rocks. Amphibolites are formed at moderate temperatures (900 – 1,300F/500-700C) and across a wide range of pressure conditions, which typically occur 6-19 miles (10-30 km) below sea level.
At the above coordinates is a large white marble road cut, on the east side of the road, with a close examination and little imagination one can see a “Train Wreck”. This is part of the same Grenville marble unit seen in the Igneous Dike in Grenville Marble Road cut, but at this location it is strongly and complexly folded. The configuration of the folds can be determined only by following individual marble bands or layers for some distance. A large anticlinal form appears in the center part of the section, with conformable banding extending for some distance on either side. The folding is not as simple as it appears. Note, for example, that the pitchouts of parallel bands on the right delineate another tighter fold. See if you can distinguish other folds on either side of the cut. The character of all the folds is decidedly plastic, with rock flowage resulting in thinning and thickening of layers. The layer spacing is widest around the crest of the central fold, and narrowest on the limbs to the right.
Most important are the trains and clusters of dark rectangular blocks include in the marble. Close examination reveals that these are amphibolites, coarse-grained metamorphic equivalent of basalt. The original rock probable was similar to post Grenville dikes, but it intruded the marble either before or during the deformation and metamorphism. A possible sequence of events could be the formation of the marble, followed by a folding episode,then the intrusion of basaltic dikes, followed by another folding during which the dikes were metamorphose, broken separated and redistributed.
The train of blocks on the east side of the road cut is most helpful in visualizing this chronological sequence. Since most dikes (or sills) are tabular, these block show remarkable little displacement when compared to the high degree of distortation of the marble. Therefore, the folds probably existed in essentially its present form when the dikes intrude. Its lower part appears to have been a sill, while the upper part cuts across the marble foliation subsequent refolding, the shallow, broad syncline on the right is a refold cause by brecciation and displacement of the dike blocks. The second folding episode was quite certainly associated with intense metamorphism. Metamorphic reactions between dike material and marble is also evident in the form of course-texture and other minerals in the marble near the block.
To log this Earthcache as a find: submit a photo of you with the train wreck in the back ground. 2. What are the main colors of the marble, 3. How many amphibolites blocks can you count, more or less than 20, 3. What color(s) is/are the amphibolites blocks.
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