Crater Lake was once a tall standing mountain similar to Mt. Rainier known as Mt. Mazama. After a large amount of violent eruptions, Mt. Mazama lost its top. When this happened, it created a huge depression that filled with water creating Crater Lake. Collapsed volcanoes like this are called calderas. Most calderas average 1-15 miles in diameter but some can be as much as 60 miles in diameter.

The island toward the rim of the crater is called Wizard Island .