Crater Lake's serenity belies a violent origin. Its blue waters fill a caldera nine kilometers in diameter, whose floor is 600 meters below the lake level, while the encompassing walls rise steeply 600 meters above it. Howel Williams, a distinguished American volcanologist of Welsh extraction, concluded in a classic study that the present caldera occupies the site of an older volcanic cone, about 3600 meters high, called Mt. Mazama. 6,800 years ago, a huge plinian eruption destroyed the original volcano, formed the present caldera and showered tephra as far as Alberta in Canada. After the plinian phase, a series of ignimbrites was erupted, filling the valleys radiating from the old cone. Wizard Island, a small volcanic cone constructed after the eruption, now occupies one corner of the lake. (Fig. 10.11; 14.4).

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