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 You are here: Home > Online Library > Nature Notes > Volume 7, No. 1, July 1934 - The Geology of Wizard Island
   

Nature Notes From Crater Lake

Volume 7, No. 1, July 1934

 

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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.

The trail up the slope of the miniature volcano leads across this flow which is around the entire southwestern edge of the island. As the "explorer" continues upward along the trail, the character of the rocks changes to loose ash, pumice and scoria. These vary in color through shades of black to bright red. Such an accumulation indicates that the cone of Wizard Island was the site of explosive activity where the material was blown into the air and settled around the vent to build up the cone. Walking here becomes somewhat difficult. The angle of slope of the cone is as much as thirty five degrees. It is therefore typical of the usual cinder cone.

The steep slope of Wizard Island does not stop at the water's edge. It must be remembered that the island has been built up from the floor of the lake which is 2000 feet at its greatest depth. This means that Wizard Island is itself quite a substantial volcano over 2700 feet high. It is a monument to the last volcanic activity of this immediate vicinity. Here is a case of a volcano within a volcano. There are two other smaller cones now submerged in the lake which are probably contemporaneous with the Wizard Island cone. They are east of the larger vent and were discovered when the Lake was sounded in 1886.

As one nears the summit of Wizard Island there may be seen on the Rim to the west the darkness of Llao Rock capped by a light yellowish material. The base of the rock cannot be seen but is of interest because it is a different type of lava from the others so far noticed. It is younger and is the type called dacite; that is, it contains a little more silica than the others. The base of Llao Rock is curved or U-shaped where it comes in contact with the lighter colored and older lavas. The logical thought is that the dacite is occupying an old valley which at one time extended up Mazama's high slopes. This seems to be true, and what is more, the U shape indicates that it was a glacial valley. Mt. Mazama built a cone by successive outpourings of lava and explosions of ash and fragmental materials. Later, the cone accumulated snow and ice on its slopes and glacial valleys were formed. And, finally, one of these was filled by the lava which now forms Llao Rock. A glance to the east shows that glacial valleys were formed in other places too. The U shape of Sun Notch and Kerr Notch are particularly significant in this respect; and they are younger than the valley of Llao Rock. No lava flows over occupied their floors. On top of Llao Rock and other points around the Rim may be noticed a bright yellowish material which resembles a field of ripe wheat. This material is in reality a light rock of frothy appearance known as pumice. It was formed when molten lava containing a high percent of gas solidified while these gases were escaping, leaving it full of holes or air spaces.

On reaching the summit of Wizard Island a perfect crater is found. The crater is about five hundred feet in diameter and about one hundred feet in depth. The rim of the crater at some points is brilliant red, showing a high degree of oxidation. Most of the cinders, ash, and lava have a fused appearance, indicating the high temperatures which must have existed when eruption was in process.

By the use of a little imagination, one can here picture volcanic activity at its best. It is not hard to picture the crater emitting steam and hot gases and boiling sullenly with miniature explosions, while red hot molten lava restlessly moved about within it. Another striking thought is a realization that it could not have been so many years ago that such a scene would have been a reality. Certainly it could not have been many thousand years, and if the story told by the age of the trees on the island means anything, it was not many hundreds of years ago. The most recent estimate concerning the last activity of the Wizard Island cone, by tree ring count, is about 800 years. More than that could not be safely said, but certainly the lava and ash is as fresh as if put there yesterday and erosion shows practically no effects as yet. The general shape is that of a young volcano showing no dissection. It is in miniature an example of old Mt. Mazama which must have been of the same semi-explosive type. The absence of any signs of glaciation would tend to date Wizard Island as later than the last glacial period. At any rate, it is a very interesting thought to consider as the return down the slope is made and so back across the nor serene lake, once the scene of so many active and violent forces of nature.

 

 

 

 

 

 

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