Diabaseville EarthCache
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This is the 5th of a series of my “Earthcaches” on Staten Island. This cache is located in an old “traprock” rock quarry in the town known as Graniteville. Park your vehicle safely on the south side of Forest Avenue or onto Eunice Avenue off Forest Avenue or on Wilcox Street; park where allowable. Follow any of the trails into the woods; beware of poison ivy and catbriar patches! Not pet-friendly due to the abundant broken glass!
Townspeople who lived in this area and were not knowledgeable in geology referred to this rock as “granite” and hence called the town “Graniteville”, a name that continues today. However, this rock is NOT granite but a darker-colored variety of gabbro known as diabase. Granites are usually light-colored and contain low density minerals like quartz, biotite/muscovite mica, and several types of feldspar while gabbros are dark-colored and contain denser minerals such as pyroxene (augite) and plagioclase feldspar (labradorite/oligoclase). Minor amounts of olivine, biotite, hornblende, apatite, ilmenite, chromite, and magnetite also occur in the diabase. In reality the town should be re-named “Diabaseville” but I don’t think that name change will happen anytime soon. Geologically, this is the Palisades Diabase which is an igneous intrusive rock sheet injected into the Triassic-aged Newark Group series of sedimentary rocks. The 325-meter thick intrusion occurred during the early Jurassic Period or about 201 million years ago. Formerly described as a “sill”, further geologic work by Dr. C. Merguerian at Hofstra University has confirmed its morphology as a “lopolith” or spoon-shaped intrusive sheet (minus the handle). The Palisades Diabase can be traced southwards for 65 kilometers from Haverstraw, New York, along the west side of the Hudson River forming 160-meter high cliffs (Palisades), into Bayonne New Jersey, through Staten Island in Port Richmond near the Bayonne Bridge Toll Plaza, into and into the wetlands along Travis Avenue, beneath the Teleport Communication facility in Travis, appearing at the surface of the “Graniteville Quarry” adjacent to Forest Avenue, diving beneath the Arthur Kill River Carteret, New Jersey, and finally into Pennsylvania within the Delaware Subbasin. Within a small portion of the lopolith at this quarry is a geological feature known as a “xenolith” or “strange rock”. At this locality it represents a portion of the once surrounding sedimentary rock, most likely a black Triassic-age shale known as the Lockatong Argillite. During the intrusion of the lopolith, a piece of this shale was broken off and incompletely melted or vertically fused within the molten mass, analogous to a raisin in a cake. The heat of the surrounding molten rock was enough to cause chemical and physical changes such that the contact metamorphic conditions created a new and rare rock type called a “trondhjemite” and falls into the tonalite family of rocks. It was first identified on Staten Island in 1978 by a college lab technician. There are no collectible minerals to be found in the outcrop and collecting pieces of the xenolith is highly discouraged. Please note, nearby there may be some "urban campers"!
To log this EarthCache: email me the number of horizontal drill holes at the waypoint over the tan-colored xenolith.
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