Chapter 6: Problem 16

This photograph shows a nearly horizontal layer of basalt that now crops out as a cliff along the Hudson River, across from New York City. It is parallel to layers of sandstone above and below, and is younger than all of those layers. The basalt intruded at \(190 \mathrm{Ma}\), as Pangaea was breaking apart. What is this body of basalt, and in what geologic setting did it form? (E)

Short Answer

Expert verified

The body of basalt in the photograph is a sill, an igneous intrusion formed horizontally between layers of pre-existing rocks. It was formed during the Early Jurassic Period in the Mesozoic Era, when the supercontinent Pangaea was breaking apart. The geologic setting of this sill is characterized by rifting and continental breakup, as it is found along the Hudson River, across from New York City.

Step by step solution

The photograph shows a nearly horizontal layer of basalt, which is surrounded by layers of sandstone above and below it. The basalt layer is parallel to the other layers and younger. In geological terms, this indicates that the body of basalt is an igneous intrusion - specifically a sill. Sills form when magma intrudes into pre-existing rocks and solidifies horizontally between the layers. #Step 2: Determine when the basalt formed#

With a known age of \(190 \mathrm{Ma}\) for the basalt intrusion, we can pin down the geological period when it formed. The given age corresponds to an era within the Early Jurassic Period during the Mesozoic Era. #Step 3: Identify the geologic setting of the basalt formation#

The age of the sill and the fact that Pangaea was breaking apart during that time, indicate that this body of basalt likely formed during a period of rifting and continental breakup. The splitting apart of supercontinent Pangaea created tensional forces that lead to crustal thinning, allowing for the formation of rift valleys. Magma moving through the crust intruded into the rift valleys and formed sills and other igneous intrusions. #Step 4: Summarize the solution#

The body of basalt is a geological feature called a sill, formed during the Early Jurassic Period in the Mesozoic Era. It was created during the process of Pangaea breaking apart, forming in a geologic setting characterized by rifting and continental breakup. This particular sill is found along the Hudson River, across from New York City.