Relief map of the Crater Lake area
Volcanic activity in this area is fed by subduction off the coast of Oregon as the Juan de Fuca Plate slips below the North American Plate (see plate tectonics). Heat and compression generated by this movement has created a mountain chain topped by a series of volcanoes, which together are called the Cascade Range. The large volcanoes in the range are called the High Cascades. However, there are many other volcanoes in the range as well, most of which are much smaller.
About 400,000 years ago, Mount Mazama began its existence in much the same way as the other mountains of the High Cascades, as overlapping shield volcanoes. Over time, alternating layers of lava flows and pyroclastic flows built Mazama's overlapping cones until it reached about 11,000 feet (3,400 m) in height.
As the young stratovolcano grew, many smaller volcanoes and volcanic vents were built in the area of the park and just outside what are now the park's borders. Chief among these were cinder cones. Although the early examples are gone—cinder cones erode easily—there are at least 13 much younger cinder cones in the park, and at least another 11 or so outside its borders, that still retain their distinctive cinder cone appearance. There continues to be debate as to whether these minor volcanoes and vents were parasitic to Mazama's magma chamber and system or if they were related to background Oregon Cascade volcanism.
After a period of dormancy, Mazama became active again. Then, around 5700 BC, Mazama collapsed into itself during a tremendous volcanic eruption, losing 2,500 to 3,500 feet (760 to 1,070 m) in height. The eruption formed a large caldera that, depending on the prevailing climate, was filled in about 740 years, forming a beautiful lake with a deep blue hue, known today as Crater Lake.
The eruptive period that decapitated Mazama also laid waste to much of the greater Crater Lake area and deposited ash as far east as the northwest corner of what is now Yellowstone National Park, as far south as central Nevada, and as far north as southern British Columbia. It produced more than 150 times as much ash as the May 18, 1980 eruption of Mount St. Helens.
This ash has since developed a soil type called andisol. Soils in Crater Lake National Park are brown, dark brown or dark grayish-brown sandy loams or loamy sands which have plentiful cobbles, gravel and stones. They are slightly to moderately acidic and their drainage is somewhat excessive.