I visited Maui last week & this was one stop @10,000ft. I only wish that the camera could capture what the eye actually sees, as when I walked up the ridge and looked over this view, I was brought to tears from the beauty of it. Within 10 minutes the entire area was encased in a low lying cloud with 0 visibility, so I as lucky to catch this image at all.
What you are seeing:
"The depression at the top of Haleakala is not actually a volcanic crater, but an erosional valley. During a period of inactivity erosion became the dominant force. Wind, ice and water carved the top of Haleakala, which may have been 3,000 feet taller than the summit is today. After the valley was created, Haleakala entered a "renewed volcanism" period. This renewed volcanic activity partially filled the valley with lava flows and small hills called cinder cones. True craters exist at the tops of some of the cinder cones."