A towering fireball from a ruptured pipeline in the most prolific shale gas region of the US has agitated energy markets and provided new fuel for critics of fracking. On Friday the Texas Eastern pipeline, a major artery in North America’s natural gas network, burst into flames outside Pittsburgh, Pennsylvania. The explosion seriously burned a man fleeing his house and carved a crater into the ground, authorities said. The blast occurred in the Marcellus and Utica shales, layers of rock which since 2010 have grown to provide one-quarter of US natural gas production. It came as new pipelines in the region faced protests from some residents, environmentalists and politicians. The Texas Eastern system runs from Texas to the New York metropolitan area. Spectra Energy, the pipeline’s owner, said the section that caught fire was built in 1981. An inspection in 2012 revealed no areas in need of repair, the company said.