Igneous Dike in Grenville Marble
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Approximately 9/10 mile east of Gouverneur on route 58 is a road cut feature of an igneous dike in Grenville marble. The road has wide pave shoulders and the base of the road cut is approximately 20 feet from the pavement.
A rock is a naturally occurring and coherent aggregate of one or more minerals-although there are a few rocks made from other substances, 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.
Igneous rocks are defined as extrusive or intrusive depending on whether or not the molten magma from which they were formed emerged at Earth’s surface before crystallizing. Extrusive rocks form on the surface, intrusive rocks form below it. Intrusive rocks are categorized as plutonic if formed deep inside the crust and hypabyssal if formed at shallow depths. Hypabyssal intrusive rocks are characterized by the fine crystallization. They are found in dikes, sills, volcanic plugs and other relatively small formations. A dike is a sheet like body that cuts vertically or at a steep angle to the surrounding rocks. Dikes range from less than half an inch to many yards in thickness and can be hundreds of miles long. Sills are similar to dikes, except that they are form parallel to the enclosing rocks, intruding between two strata. Plugs are formed from magma solidified inside volcanic vents.
Marble is the product of thermal or regional metamorphism of limestone. Heat and pressure cause to carbonate minerals to recrystallize into compact and often homogeneous marble of various colors. Superficially, it may be confuse with quartzite (another metamorphic rock), but marble can be scratched by a steel point, and also reacts to weak hydrochloric or acetic acids. Recrystallization tends to remove any crack or cleavage, but traces of folds—often highly—contorted may be preserved, indication 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 composed 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 either is to pale green, brown, or gray or where oxidized, to red. It occurs mostly as extrusive lavas, but also forms intrusions as such as dikes and sills. On cooling, it may form distinctive joints.
Dolerite is equivalent to basalt and gabbro in composition, although it is intermediate between them in grain size, dolerite is from 1/3 to 2/3 calcium-rich plagioclase feldspar, the remained being principally pyroxene. It has a silica content of less than 55%, and its quartz contents is usually less than 10%. Magnetite and olivine-dolerite may be present if olivine is present the rock is called olivine-dolerite. Extremely hard and tough, it occurs in dikes and sills intruded into fissures of other rocks.
At the above coordinate you will find an igneous dike in Grenville marble. A thick dark green basaltic dike exposed near the middle of both sides of this road cut. It is fairly uniform in width and nearly vertical. If the dike could be removed, the two sides would fit together like pieces of a jigsaw puzzle. This feature shows that the basalt intruded along a fracture in the marble and either forced it open or simply filled a void that was already there. Look closely at the texture of the dike on freshly broken unweathered surfaces. In the dike core, you will see a spotted texture with dark green pyroxene or amphibole crystals “floating” in a finer matrix. At the edges, the spots are absent, and the rock appears much fined grained. The basalt intruded in the molten state and was chilled, or quenched, along its contact with the marble, while in the better insulated core, larger crystals had time to grow. In some place along the edges, the marble and basalt are interfeathered because to magma worked into small fractures.
To log this Earthcach as a find: Email a photo of you and your GPSr standing beside the dike on the log web page. Answer the following questions: 1. what is the average width, in inches, of the dike on the west side of the road? 2. What is its approximate height? 3. On which side of the road is the dike more uniform in width, East or West?
The shoulders of the road may be wide, but take extra caution, as the traffic is fast.
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