93 The Caldera Floor

The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams

The Caldera Floor


THE bottom of Crater Lake is hidden from view over an area of approximately 20 square miles. To obtain a clear picture of its form it would be necessary to make many more soundings than are now available. Nevertheless, the records collected by Dutton’s party in 1886 and by the naturalist staff of the Park Service during the seasons 1938-1940 suffice to indicate the principal features (see map, figure 30). What is now most needed is a series of closely spaced soundings along radial lines within a mile from shore, in order to determine the presence or absence of marginal fault scarps.

The following are among the principal features revealed by soundings:

I. The sides of the caldera are steep. In many places the hidden slopes are steeper than the cliffs above the water’s edge. Except near Wizard Island and along the south side of the caldera, the walls drop at least 1000 feet within 1/2 mile of the shore.

2. Not only are the sides of the basin steep, but most of the floor lies more than 1500 feet below the level of the lake. Before Wizard Island was formed, more than three-quarters of the floor may have been that deep. In other words, by far the greater part of the original bottom may have lain more than 3500 feet below the highest point on the rim.

3. The deepest part of the caldera lies northeast of the center. Here, in an area measuring 2 by 1 1/2 miles in maximum dimensions, the soundings exceed 19 feet. Over most of this deep area the floor is surprisingly flat. Perhaps it was in this region that activity was first renewed after the formation of the caldera, and it may be that a lava lake existed there for a time.

4. Much of the shallow floor near Wizard Island is covered by post-caldera lavas erupted from fissures at the base of that cone. Possibly some of these flows extend 2 miles east of the island, almost to the center of the lake. Within the 500-foot contour, where the soundings are most numerous (figure 30), the surface of the submerged lavas seems to be as rugged as that of the lavas exposed above water. Among lavas of this type, the snouts of successive flows are generally both high and steep. Yet there are plates on the submerged flows and along their margins where the surface falls with unusual rapidity. For instance, in one locality the surface falls 311 feet within a distance of 500 feet; in another, the drop is 235 feet in less than 200 feet. Such high, steep slopes can scarcely be the fronts of blocky lavas; more likely they mark fault scarps. If so, Wizard Island may have been built on a platform above the general level of the caldera floor.