How Crater Lake Was Formed?

How Crater Lake Was Formed? Mount Mazama, a 12,000-foot-tall volcano, erupted and collapsed approximately 7,700 years ago, forming Crater Lake. What is a Crater Lake and how is it formed? Lakes in maars fill medium-sized craters where an eruption deposited debris around a vent. Crater lakes form as the created depression, within the crater rim,