The fire is about 70 percent contained by a fire line, and full containment is expected by Sunday, said Scott Isaacson, a National Park Service employee.

The fire was discovered Thursday by a helicopter crew checking on the 100-acre Desert Ridge fire, which was also started from the July 3 lightning storm, Isaacson said.

However, the NPS is not fighting that fire, which is about one mile west of the Pumice Desert and just north of the caldera, he said.

"It was decided we would manage that fire as a natural fire for forest health," he said, referring to a program in which the NPS uses fire as part of the natural process to improve forest health by permitting low-intensity fires to burn brush and small trees without torching whole forests.

The Desert Ridge fire is about 40 percent contained. Containment is expected by mid-August, Isaacson said.