Talk:Stark's Knob

Stark's Knob is commonly and mistakenly said to have formed in the Connecticut River Valley during the Taconic orogeny. Rather, the Knob must be seen as a giant (80 m-long, c. 40 m-thick) lens of basalt that is mixed into a melange (a contorted layer of rock) under the older slates of the Taconic overthrust. As a Japan-like volcanic island arc converged on Laurentia (ancient North America), the Laurentian margin underthrust the volcanic arc and went down a subduction zone. By a modern interpretation based on geologic field relations and rock geochemistry, the relatively cold and dense Laurentian margin was actually pulled into the subduction zone and was broken by deep faults as it stretched. Magma (liquid rock) rose upward along the faults and was extruded as lava on the sea floor. This formed a small volcanic edifice that is Stark's Knob and the similar and age-equivalent Jonestown volcanics in eastern Pennsylvania. Stark's Knob is important in plate tectonics theory as it provided the first evidence of volcanism on the subducting plate, not only on the overriding plate--as Japan, Mount St. Helens, and the Aleutian arc. Stark's Knob developed when the shrinking Iapetus Ocean featured movement of a volcanic arc that also pushed deep water marine sediments (now the Taconic overthrust and slate belt) onto the margin of the North American continent. Fossils of snails found in limestone sandwiched between the pillows indicate that the lava formed during the Late Ordovician age about 460 million years ago. The snails seem to suggest that the lava was formed in shallow seas, as the species of snail was native to shallow water. However, the ancient gas bubbles in the Stark's Knob pillow basalt are tiny and indicate, with the geochemistry of the basalt, a depth as great as 2 kilometers. This means the snail was likely transported as an empty (dead) shell by marine currents onto Stark's Knob. The limestones on the pillows were formed from normal marine water (not from hot fluids derived from the basalt) by simple heating by the pillows that drove carbon dioxide out of the sea water. Timy lime (calcium carbonate) crystals settled onto the pillows almost like a "submarine snow." The pillows are the natural form that liquid basalt forms underwater as it is forced out of a lava vent. Eventually the rocks were pushed up and out of the water. Over time, the rock was upthrust and tilted to an easterly direction. The thrusting also pushed the rock of the Taconic allochthon (overthrust) from the Connecticut River Valley to its present location, just west of the Hudson River. LANDING, E., G. PE-PIPER, W.S.F. KIDD, K. AZMY. 2003. Tectonic setting of outer trench slope volcanism: pillow basalt and limestone in the Ordovician Taconian orogen of eastern New York. Canadian Journal of Earth Sciences, 40:1173–1187, Figs. 1–8, Tables1–3. LANDING, E. 2022. Tropical seas and volcanic fire in ancient New York. Natural History 130 (9): 36–41, 6 figs., https://community.geosociety.org/gsa2021/program/technical/geoheritage