DID CRATER LAKE, OREGON, ORIGINATE BY A 

 VOLCANIC SUBSIDENCE OR AN EX- 

 PLOSIVE ERUPTION? 



J. S. DELLER 



United States Geological Survey, Washington, D.C. 



The great volcanic eruption of 19 12 at Katmai, Alaska,' has 

 again called the attention of geologists to Crater Lake, Oregon, with 

 the suggestion of a similar explosive origin of the great depression 

 containing that lake, which depression had previously been ascribed 

 by the present writer^ to subsidence. 



Crater Lake is in a deep abrupt basin about 5 miles in diameter 

 and 4,000 feet deep in the plateau-shaped summit of the Cascade 

 Range. 



Crater Lake is completely encircled by a very bold rim which 

 rises to approximately 2,000 feet around the lake, which is about 

 2,000 feet deep. The rim has a very steep slope inside toward the 

 lake but a gentle slope outside to the summit-plateau on which 

 Mount Mazama was built up by many successively overlapping 

 lava flows and sheets of ejected volcanic fragments, all erupted from 

 practically the same volcanic orifice. 



Nearly all this rim, however, was built up by eruptions of 

 andesite before the eruptions of dacite began. 



Much dacite-pumice formed a partial top-rim of Crater Lake, 

 but the succeeding dacite flow from the summit of Mount Mazama 

 finally broke from the surface of the lava tunnel at Rugged Crest and 

 overflowed down the inside of the rim into the head of Cleetwood 

 Cove of Crater Lake as Mount Mazama sank away into engulfment, 

 leaving the outer slope of the rim with its fresh flow of dacite and 

 the glaciated rim with its glacial striae and moraines, as shown on 

 the map, Figure 13, Geological History of Crater Lake National 



' Robert Fiske Griggs, The Valley of Ten Thousand Smokes. The National 

 Geographical Society, 1922. 



= "Geology of Crater Lake National Park 1902," U .S .Geol. Survey Prof essional Paper 

 No. J. Also Geological History of Crater Lake National Park. Dept. of Interior, 1912. 



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