Volume 7 No. 1 – July 1, 1934
All material courtesy of the National Park Service.These publications can also be found at http://npshistory.com/
Nature Notes is produced by the National Park Service. © 1934
A Prefatory Note
By The Editors
An effort has been made to give unity to this issue of Nature Notes by a consideration of the different elements involved in the appreciation, information, and enjoyment to be derived from a small section of Crater Lake National Park. Since the Naturalist Division maintains, among other guided trips, a scheduled Ranger-Naturalist conducted hike and boat trip to Wizard Island, the area was selected for consideration in this issue. Because of its unique situation, its interesting biotic features, and its meaningful esthetic values, Wizard Island was chosen. The reader will find that the articles herein contained represent the meaning or significance of Wizard Island as taken from the notes of the geologist, the ornithologist, the botanist, and the artist.
The National Parks were established for the use and enjoyment of the people. It is hoped that this material will contribute to that ideal by motivating the use of this Park and contributing to the understanding of it. To summarize briefly, this issue represents a research problem in appreciation.
The Geology of Wizard Island
By Hugh H. Waesche, Ranger-Naturalist
The visitor to Crater Lake who wishes to make the trip to Wizard Island must first descend a trail which goes down from the rim of the lake by a gradually grade and numerous switchbacks. This trail is 1.6 miles long. It is an easy descent and the casual observer cannot refrain from looking at the rocks along the way. The close view of these though, would bean little to the untrained person. There is exhibited at various elevations and in no regular succession a series of rocks which in some places are much jointed lava flows and in other places masses of heterogeneous, fragmental materials. They are colored red, brown, yellow and gray, and the lavas are known as andesites. The fragmental material is known as agglomerate. The rim of Crater Lake at the trail is 900 feet above the water.
On entering the boat, the first inclination is to look into the water to see if it is as blue as when seen from the Rim. Toward the center as views horizontally, the Lake is as blue as ever. Under the boat, it is somewhat startling to find that the water is quite clear, but that objects beneath its surface are surrounded by a hale of rainbow colors. It is likewise startling to see that the shore slopes off precipitously into the Lake. Indeed, the slope seems to be as steep blow the water as on the walls of the Lake. Within a few feet of the shore the water becomes too deep to observe the rocks below, but whenever seen they appear to be boulders of various sizes and shapes, apparently derived as talus from the slopes of the Rim above. At best there are only a few beaches around the Lake. At the boat landing there is a pseudo-beach which is really a detrital fan of coarse talus about one-hundred to one hundred and fifty feet in width. The trail descends through a steep weathered valley to the water’s edge. The beach talus accumulation is derived from materials which have rolled down this valley.
As one looks back up the steep wall down which he has come and then follows it around the complete circle of the Lake, he begins to realize with some feeling of awe that he is doing something different, something that is not the usual thing. He is down inside of a volcano where its actual structure may be seen. Any direction one looks, layers of lava and agglomerate greet the eye, piled layer on layer, to the very top of the Rim. To the left of the boat as it moves towards Wizard Island and about halfway up the Rim may be seen one outstanding layer running all the way from the trail to The Watchman. Usually the layers are warped downward as though they had filled in pre-existing valley floors and in nearly all cases they have a tabular appearance, often curved, caused by cooling during flowage or by jointing from later pressures. As the boat continues, new views appear which had been hidden by promontories.
Not only is the observer seeing the cross-section of a once quite active volcano, but he is likewise in the heart of what was formerly a majestic mountain peak, Mt. Mazama, which towered above the surrounding country. It must have been 14,000 or more feet in height and may have rivaled Mt. Shasta, Mt. Rainier and other Cascade peaks in grandeur.
The angle of slope of the lava beds away from the lake rim, the glacial valleys, and the lava filled valleys, all indicate the existence of such a peak. What a sight it must have been until finally it was destroyed preparing the way for the present scenery. But how was it destroyed? There are several ways this might have happened. The two must accepted theories are explosion and subsidence. Numerous volcanos are known to have exploded, blowing large portions of their cones into the air leaving craters where a mountain had been. It is possible that Mt. Mazama may have done this, but many geologists refute this because they cannot find enough fragmental material in the surrounding country, which would result from such an explosion, to account for it. Many geologists think that such was the case; others prefer to think that the mountain caved in upon a receding mass of lava in its vent. Others think that possibly the process of destruction was similar to the enlarging of craters now going on in the Hawaiian Islands today by a process of undermining. In the latter case the molten lava in the conduit rises and falls and in so doing corrodes the walls of the crater as well as fracturing them, thus gradually enlarging the crater by encroachment. However, Crater Lake may have been formed, the problem is complex. Evidences are confused and meagre so that a true understanding is difficult. In any case, a former mountain did exist, and it was subsequently destroyed, and the yawning crater some five miles across has been filled by the rains and snows of countless years to reach the conditions now seen.
Returning to present observations, near Discovery Point there appears a new type of rock. This is a narrow dark rock which cuts at an angle across the lava layers. This is an andesite dike and is made up of material similar to that of the lavas, but it is much younger and is of different origin. The lavas flowed down the slopes of the old volcano, Mt. Mazama, but the dike in a molten condition was forced into a zone of weakness in the Crater wall in a nearly vertical position. As the molten rock welled up in the vent of the volcano, the pressure became very great below, and this pressure exhibited itself in dike formation. It may be noticed that the lava flows have their jointing in a vertical direction; those of the dikes are horizontal, giving the appearance of piled cord wood. A very prominent dike know as the Devil’s Backbone may be seen standing out from the Rim beyond Wizard Island and toward Llao Rock.
At almost every point around the Lake may be seen steep slopes of loose material which seems to have slid down from the Rim. These are known as talus slopes and are the result of weathering of the lavas, causing slide material to accumulate at the greatest angle of repose. The action of the rain and air on the materials composing the talus has caused their iron content to reach several stages of oxidation. In other words, the rocks have rusted and the tints of yellow, red, brown, and gray tell a story of varying oxygen content. A very striking example of this is seen in the wide talus slope beneath the pinnacles of The Watchman and Hillman Peak, southwest of Wizard Island. These peaks are themselves the result of the erosional activities of weathering. In connection with weathering activities it will be observed that the plant life along the Rim both aids and prevents erosion. Erosion is speeded up by growing plants which send their roots into the joints of the lava beds and break them off by the pressure exerted. The life processes cause plants to generate certain acids which help break down the rocks by chemical action. On the other hand, the matted nature of some of the plant life, as well as the binding action of the root systems, helps prevent disintegration of the rim slopes.
By this time the boat will have reached Wizard Island itself. Here one is greeted by a new bit of scenery. At the boat landing the visitor is confronted by a black, broken, and irregular mass of rock. This is an andesite lava which has flowed out of the volcanic cone which is Wizard Island. It apparently did not come from the summit of the cone, but came out of its side at or near the level of the lake. It is possible that the Lake was present at the time and the lava may have flowed into the water, but the absence of vertical pillow structure would seem to indicate that the Lake had not yet formed. At any rate it cooled rapidly. It is evident that the cooling was much more rapid on the surface, since it seems that the jumbled mass over which the island explorer must climb is the broken surface of a flow which continued to move after its surface had frozen enough to be fractured by sub-surface movement. The flow was quite irregular as to directional movement because the shore of the island is irregular, forming numerous bays and inlets. The lava is often of a vesicular or porous nature. It is also quite hilly and rugged.
Close examination of the broken lavas show them to have a dense texture and a nearly black color. Scattered through the black mass may be seen light colored, lath shaped crystals. These crystals are feldspars, one of a group of major igneous rock-forming minerals. Such a texture, in a rock where larger minerals (phenocrysts) are found in a dense or glassy groundmass, is called porphyritic. This type of texture is further evidence that the lavas cooled rather rapidly. A rock which has cooled very slowly is made up completely of crystallized minerals; one which has cooled very quickly is glassy, containing no definitely recognizable minerals. Rocks are colored by their chemical constituents which are directly related to their mineral compositions. Minerals like feldspars, containing sodium, potassium, aluminum, silicon and oxygen, are light colored and where predominant, produce light colored rocks. On the other hand; the ferromagnesium minerals such as biotite, hornblende, and pyroxene, high in iron, calcium and magnesium, are dark colored and tend to make rocks black. Where these two extremes are about equal, the rock is intermediate in color and composition. The rocks on Wizard Island and in the walls of Crater Lake belong to such an intermediate group.