LAVAS OF MOUNT MAZAMA.a
ANDESITE OF WIZARD ISLAND.
Wizard Island is a perfectly
preserved volcano consisting of a cinder cone (Pl. VIII, B), with cratera
above and lava field about its base. Amid such impressive surroundings it is one
of the most attractive and inviting spots of the region. The lava is andesite
and belongs not only to the last andesite flow, but to the final eruption
connected with this great volcanic center. The bulk of the older lavas is
andesite; then come the basalts from the cones around the base of the rim, and
the dacites on the mountain slope succeeded. Finally, after the great
catastrophe which engulfed the upper part of Mount Mazama, an andesitic eruption
on the floor of the caldera followed, which built up Wizard Island and
apparently covered almost the whole bottom of Crater Lake.
aThe saucer-shaped depression
in the summit of the cinder cone of Wizard Island is properly called a
crater. It marks an orifice from which lava reached the surface
to build up the cinder cone and lava field. From the fact that the lake
is called Crater Lake the term crater has been applied to the great
depression which the lake occupies. The large depression does not in its
present size mark the orifice from which the lavas of Mount Mazama
issued upon the surface to build up the mountain, but rather, as will be
shown in the sequel, the hole through which the summit of the mountain
sank into the earth. Great depressions like that containing Crater Lake,
originating by subsidence in connection with volcanic activity, are
often called pit craters, but better still calderas, which is
distinctive. Prof. W. M. Davis very appropriately remarks (Physical
Geography, p. 215) that the depression containing Crater Lake is "one of
the most superb calderas in the world."
Wizard Island has an area of
nearly nine-tenths of a square mile. The eastern half is a prominent cinder
cone, and the western is an extremely rough lava field. The cinder cone rising
from the lava field has very steep slopes, made up chiefly of fragmental
immaterial blown out of the crater, and rises to a height of 845 feet. The
crater in its summit is about 250 feet in diameter and 80 feet deep. Its bottom
is solid lava. Inside its rim upon the southwest slope is a great snow bank,
where the snow accumulates during the winter and lasts almost throughout the
The lapilli are rather coarse and usually dark or black, but many are red.
There are numerous fragments of lava and comparatively little sand. The solid
lava is brilliant red only upon the surface; within it is dark. When the
material is porous, the color may permeate the whole mass. One hundred feet
below the summit on the northwest slope a little stream of lava (30) 6 to 15
feet in thickness broke out and coursed down the slope for 150 feet. Its surface
is very rough and somewhat ropy, and along the under surface it picked and
inclosed numerous fragments from the slope over which it passed. Near the summit
is a mass of brilliantly red lava (56). The cone is therefore not wholly
composed of cinders, but contains, besides lapilli and chunks of lava, a number
of small flows, not of sufficient size, however, to interrupt the regularity of
the cone. Near the base of the cone, where it merges into the lava field,
several "volcanic bullets" were observed. They are round, and range in diameter
from 1 to 2 feet, with irregular fractures. They appear to have solidified
before ejection, like those so well developed about the base of the cinder cone
10 miles northeast of Lassen Peak, California.a
aBull. U. S. Geol. Survey No.
79, 1891, Pl. IV.
The lava flow which extends
westward is extremely rough and made up of large angular blocks of the broken
flow. It is a dark, somewhat basaltic-looking andesite (19), which is
occasionally streaked with lighter colored material (20) among the dark bands.
The lava escaped chiefly from the west base of the cinder cone and spread
westward beyond the present limit of the island, for beneath the clear water it
may be seen to extend far west toward the shore of the lake. Some escaped
eastward, and it is probable that lava from the Wizard Island center spread over
much of the floor of the lake.
The eastward flows are less
broken upon the surface, and are probably older than those to the west, for they
are well covered with trees. The flow is well exposed in section along one of
the small streams on the east shore of the island, and exhibits to a marked
degree an arched platy structure parallel to the surface of the flow. Such
structure is rare on the island, but has been observed curving around the narrow
and thick flows of the rim, especially in the neighborhood of Llao Rock.