Describe how workers are able to walk upon lava fields as they try to cool down the lava on Heimaey in 1973.

As they attempt to cool down the lava flows on Heimaey in 1973, workers can walk upon lava fields because the surface is cooled, forming a two-inch crust. They have to keep moving, though. If they stand in one place for too long, the heat of the lava flowing below the crust can burn their boots.