If you found a line of volcanic peaks a few hundred miles long in which the volcanoes were progressively older toward one end of the line, what probably created them?

The geological process that could be responsible for this phenomenon would be the plate tectonics. The plate tectonic theories are believed to be the cause of the landforms forming on the earth's surface. What is described in the questions would be an oceanic ridge which is formed from converging oceanic crusts.