KRAFLA, Iceland — Soon after work began here on a power plant to harness some of the vast reserves of energy stored at the earth’s crust, the ground moved and, along a six-mile-long fissure, began belching red-hot lava. The eruptions continued for nine years, prompting the construction of a stone and soil barrier to make sure that molten rock did not incinerate Iceland’s first geothermal power station.