Crater Lake National Park's
Frequently Asked Questions
How was Crater Lake formed?
Answer: The phrase "GREW, BLEW, FELL, and FILL"
describes the process that created Crater Lake.
Grew - Mount Mazama was a large composite volcano that was built during the past 400,000
years by hundreds of smaller eruptions of lava flows. Mount Mazama rose to an
approximate height of 3,700 m (12,000 ft) above sea level.
BLEW - About 7,700 years ago, Mount Mazama erupted
catastrophically, blowing out about 50 km3 (12 mi3) of
magma in a few days. The volcanic ash covered parts of the northwestern states,
spreading as far as central Canada. Rare particles of Mazama ash have even been
found in ancient ice from Greenland. The airfall pumice and ash covered a total
surface area of more than 2,600,000 km2 (1,000,000 mi2) at
least 1 mm thick. A volume of 42-54 km3 (10-13
mi3) of the mountaintop had disappeared.
Where had all this mass gone? Did Mount Mazama
blow its top off?
FELL - Mount Mazama did not blow its top off; it
collapsed in on itself. As this enormous volume of magma was rapidly removed from the chamber to feed the climactic eruption, it created a huge void
underneath the mountain. Leaving no support for this massive dome, the roof of
the magma chamber collapsed, forming the bowl-shape depression known as a
caldera.FILL - About 5,000 to 6,000 years ago, the
accumulation of rain and snow filled the caldera. It took perhaps 250 years for
the caldera to fill to its present-day lake level, which is maintained by a
balance between precipitation and evaporation plus seepage.
