History Channel's How the Earth Was Made is a wonderful earth science documentary series. The episode titled "New York" is the story of the famed city and the geological forces that contributed to its founding and flourishing. Modern investigation reveals New York to be the product of colliding and splitting continents, volcanism, and glacial activity. The episode is set up as a type of detective story, with evidence shown to back up a hypothesis. The episode is available on YouTube
The video worksheet is a one-page, double-sided handout consisting of 41 multiple choice questions that track the progress of the video. This format enables the students to pay attention to the video while quickly recording their answers. In this way, the students are not bogged down in writing long responses, and they can better enjoy watching the video. A key is included, and the files are provided in both MS Word and PDF formats.
History Channel’s How the Earth Was Made: New York Overview
The modern city of New York is constructed upon ancient rocks dating back 450 million years. This bedrock, known as Manhattan schist, is seen as exposures in Central Park. Elongated crystals in the rocks provide evidence of an ancient mountain building episode. The region had once been subjected to the immense pressures associated with burial to a depth of 20-25 miles beneath the earth’s surface. The Manhattan schist represents the last remnants of a soaring mountain range that resulted from the collision of North America with Africa 450 million years ago. The schist is also important in that it forms a solid anchor for the iconic skyscrapers of New York. The two concentrations of tall buildings known as Uptown and Midtown reflect this subterranean geology, with the city’s smaller structures being constructed upon less stable glacial debris.
Later in time, the region was subjected to the exact opposite process—continental rifting. The nascent Atlantic Ocean began to form about 200 million years ago initiating the breakup of the supercontinent Pangaea. At the time, New York would have been located near the center of Pangaea.
Immense outpourings of lava associated with the breakup of Pangaea may also have contributed to a mass extinction at the end of the Triassic period. This is when postosuchus, a crocodile-like predator became extinct. Jurassic age footprints (230 ma*) of postosuchus were discovered in a quarry near New York City and are shown in the video.
The lava flows of the end-Triassic period are seen today as the Hudson River Pallisades, imposing cliffs of basaltic lava located near New York City. The video also documents the peculiar rock formations known as columnar basalt, resembling long, pencil-like structures that form in slowly cooling lava. The Pallisades have also been linked to similar-aged lava in Africa indicating that these now widely separated areas were once connected. With the breakup of Pangaea, New York became a coastal location.
Evidence for the breakup of Pangaea and the formation of the Atlantic Ocean has been provided by the discovery of paleomagnetism, or “fossil-magnetism”, in volcanic or igneous rocks. In the 1950s, it was recognized that minerals in cooling lava or magma become aligned with earth’s magnetic field. These magnetic minerals act as the geologic equivalent of compasses that indicate the past direction of earth’s magnetic field. Changes and reversals of earth’s magnetic field can be traced by the paleomagnetic study of igneous rocks, and these alterations are useful in dating the formation and subsequent changes of earth’s oceans and continents.
The video jumps forward to the near past and examines how the recent ice age affected the area around New York City. Evidence for past glaciers can be seen in Central Park. For example, boulders termed glacial erratics can be seen lying atop the Manhattan schist bedrock of the park. These erratics were carried by glacial ice and then deposited in areas far removed from their original locations. Another type of glacial evidence consists of the striations, or glacial scratches, seen in Central Park rocks. These striations are linked in the video to similar markings on nearby mountaintops. The height of these mountains suggests that an ice sheet the height of four Empire State Buildings, a continental glacier, once covered the entire area!
The motions of glacial ice also carved the Hudson River valley into a wide, U-shaped structure terminating in a deep harbor. The harbor was originally cut off from the ocean by a 220-foot tall wall of glacial debris left by melting ice. It appears that a titanic flood later removed this dam of glacial material. The flood was unleashed upriver along the Hudson by the draining of an immense glacial lake now known as Lake Iroquois. Traces of the flood are revealed in the denuded soil of Covey Hill in the Adirondack Mountains, and by the discovery of large boulders and mammoth tusks lying on the river bottom near the mouth of the Hudson. Today, this newly opened gap between the Hudson and the ocean is termed the Narrows (which is spanned by the Verrazano Bridge).
Earth related hazards could potentially create disasters for New York City. For example, proximity to the coast, and a bottleneck formed by adjacent Long Island, leads to the city’s vulnerability to hurricanes with their attendant storm surges. In 1842, Canal Street was flooded for three hours due to a category 1 hurricane, and the city briefly became two islands. A direct hurricane hit on the city in the modern world would create an immense problem for New York City and the United States with potential catastrophic costs in lives, money, and infrastructure.
Earthquakes can also affect New York, and small fault lines can be seen in maintenance tunnels. New York experienced small earthquakes in 1884 and 2001. Unfortunately, New York’s buildings have not been built to withstand a large earthquake.
Regardless of hurricanes or earthquakes, the city may eventually become buried in glaciers. Scientists predict that another continental ice sheet will advance and cover the region in 40,000 years. Any survivors of the next glacial advance will then be treated to another collision between North America and Africa in 250 million years. The collision will result in the formation of a soaring mountain range, and New York City will be relegated to the status of a geologic layer within this structure.
*Mega-annum, or million years