Meet the Manhattan SchistHave you ever noticed that the high-rise buildings in New York City cluster

Meet the Manhattan SchistHave you ever noticed that the high-rise buildings in New York City cluster into two areas; near Midtown and the Financial District? Turns out the reason for this clustering isn’t just “money”, it’s also bedrock.Skyscrapers in New York are cheaper and easier to build when there is solid bedrock to support them. There is some topography to the bedrock unit and it pretty well matches where the skyscrapers are. That rock unit underlies much of the island, including a number of outcrops in Central Park. It shows up behind this worker of the NY Metropolitan Transportation Authority, and is known as the Manhattan Schist.The Manhattan Schist is a product of hundreds of millions of years of tectonics. Its story started innocently enough 600 million years ago as part of the floor of the Iapetus Ocean; the predecessor to the modern Atlantic Ocean. At the time, the rocks sat offshore away from the heart of the Laurentian continent (the continental fragment that would evolve into North America), and sediments from Laurentia piled on top of igneous rocks on the seafloor.Then something happened…a piece of land came slamming in. In the first case, it was an island arc; the plate that made up the Iapetus Ocean was subducted, forming an island arc offshore, and eventually that arc ran into Laurentia. When that arc slammed in, pieces of the seafloor were broken off, stuck in the middle, and accreted to the continent.If this sounds like a violent process for the rocks, it is. This event was known as the Taconic Orogeny, and it was one of the major events that helped build the Appalachian Mountains. During this orogeny, the rocks that would become the Manhattan Schist were heated, twisted, and folded. They stopped being sediments and became metamorphic rocks. New minerals such as garnet and kyanite grew when the rocks were heated at pressure, changing their color and texture. That’s why its now called a schist; new minerals grew to the size where they could be seen and the rock developed a foliation (layering formed due to metamorphic mineral growth).From there, the rocks that are today were buried to shallow depths and eventually exhumed following several other mountain-building events farther out along the coast.Today the Manhattan Schist forms the backbone of Manhattan itself. In this image you can see a portion of the schist being torn apart by man. The NY MTA is building a new subway tunnel on the East side of the island, and that requires digging into the schist, identifying structures within it, and mapping out the fine scale details. In this case, of course, using a Brunton compass.More than half a billion years of geologic history has played its part in building this great city, from the subways to the skyscrapers.(Meet the schist, meet the schist, step right up and greet the schist…)-JBBMTA Photo, Metropolitan Transportation Authority / Patrick Cashin.http://www.flickr.com/photos/mtaphotos/8639449015/SUNY Stony Brook field guide:http://www.geo.sunysb.edu/reports/ny-city/BBC News Article:http://www.bbc.co.uk/news/science-environment-22798563 -- source link

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