Final answer:
The Hartford Basin in Connecticut is the geological formation that was formed by the extension of Pangean crust, whereas Mount St. Helens was formed by volcanic activity and the Caledonian Mountains were produced by continental collision.
Step-by-step explanation:
The student has asked which geological formations are the result of the extension of Pangean crust. Of the options provided: Mount St. Helens, Hartford Basin, Connecticut, and the Caledonian Mountains, the Hartford Basin in Connecticut is the feature formed by the extension of the crust that was once part of the supercontinent Pangea. Mount St. Helens is part of the Cascade Range which was formed by subduction zone volcanism and not by crustal extension of Pangea. The Caledonian Mountains were formed by the collision of the ancient continents of Laurentia and Baltica, but this was not an extension event. Therefore, the main answer is that the Hartford Basin is the geological formation formed by the extension of Pangean crust.Explanation in more than 100 words: The Hartford Basin formed during the breakup of the supercontinent Pangea in the early Mesozoic era. This basin is a result of a rift in the Earth's crust as Pangea began to pull apart, leading to faults and the thinning of the crust. As the lithosphere thinned, it sank and created space to be filled with sediments, forming the basin. This extensional tectonic event contrasts with the processes that created Mount St. Helens, which involved subduction-related volcanic activity along the Pacific Ring of Fire. The formation of the Caledonian Mountains was a product of continental collision, reflecting compressional tectonics rather than extensional forces.Conclusion: In summary, the geologic feature among the options given, which was formed by the extensional forces that acted upon Pangean crust, is undeniably the Hartford Basin in Connecticut.