The Geology Of The Garfield Trail
By Carl R. Swartzlow, Ranger-Naturalist
The Garfield Trail speaks in many languages. The song of the birds and the rustle of the breezes through the hemlocks and pines are the first sounds to great the ear, and they follow one all the way. This language is also expressed in less musical tones. As one approaches the Rim and looks toward the Lake, a rumbling sound is heard and attracts the attention to boulders, loosened by erosion, tumbling to the narrow beach at the water's edge. Jets of dust rise at points where the boulders strike on their downward journey. It is only after gazing at the inner Rim for several minutes that the grandeur of the scene unfolds and one realizes that the landscape of color dominates this trailside.
The Garfield Peak Trail may be called a study in brown. From the point where one first ascends the trail, thence to the top of the peak, one is constantly impressed by the ever changing shades of red, brown, and yellow. The causes for this particular series of colors are related to the most fundamental processes of geology an should be a part of the knowledge of every lover of the out of doors.
The reds and browns of the rocks along the trail are usually a result of the first stages of rock decay, and all subsequent compounds released are stained with these colors. The constituents of the pigments are oxygen, iron, and water. The iron oxides (iron united with oxygen) are a constituent of nearly every variety of igneous rock. Iron may not be a diagnostic element in some varieties of rock but its presence is all but universal. The abundance of iron oxide determines in a large measure the depth of color found in the weathered rock products.
Usually the iron in fresh rocks is not fully united with oxygen, i.e., oxidation is not complete and in this state the iron is soluble and is relatively colorless. Moisture that comes in contact with rocks has previously dissolved varying amounts of the atmospheric gases, of which oxygen is a common constituent. The oxygen in the water unites with the iron of the rocks and produces a compound in which the iron has taken on all the oxygen possibly and in this latter state is one of nature's most insoluble compounds. If water as such has united with the iron oxide the resulting compound (the mineral limonite) has a brown color, and if the compound is diluted the resulting color is yellow. If no water has entered into the reaction, the color of the resulting rock is red - the mineral hematite. If the soluble and insoluble iron compounds are mixed a greenish color is produced. It can be seen readily that combinations of these colors in various stages of dilution can produce an infinite variety of shades that enhance the beauty of the trailside.
At the beginning of the Garfield Trail one walks over a mass of buff-to-tan pumice dust. Undecomposed fragments of this volcanic glass reflect the sunlight as if the pumice contained myriads of diamond chips. In time their lustre will be dulled by the chemical action of the atmosphere or by organic acids released by decaying vegetation. The pumice soils contribute little essential plant food, and only the more hardy grasses and flowers are found growing upon them. Where abundant vegetation appears to be growing in pumice, the plants are usually rooted in more fertile soils below.
A few yards beyond the point where the trail first touches the Rim, and at several other points along the trail, the lava rocks (mainly andesite agglomerate) have been decomposed and young soils have been formed. These soils have a chalky appearance in contrast to the usual yellow or brown soil along the trail. These formations are seldom a result of normal weathering processes, but are probably due to the action of heated waters that escaped along the slopes of Mt. Mazama. The rocks have been almost completely decomposed. The residual material is the mineral kaolin or some variety of it. (Kaolin is the chief mineral constituent of clay). If one moistens his fingers and rubs them over some of these particles, a greasy or doughy ball of clay is readily formed.
About halfway to the top of the Peak there are additional examples of rock weathering that are more common but are none the less interesting. Large masses of volcanic agglomerate have been weathered for long periods of time by normal processes. In the construction of the trail, cuts have been made through the rock mass, and cross sections of many boulders are left to tell the steps that nature has used to bring about the changes from solid rock to soil. The sub-angular fragments of rock have clear outlines, but they can be crushed easily with the fingers.
In many cases a series of concentric bands, similar to the peelings of an onion, surround a central portion of rock that is relatively undecomposed. The cause of the banding is a common process observed in moist climates. Moisture penetrates the pore spaces of the rock. The depth of penetration depends upon the size of the pores, temperature, and character of the solutions. When the optimum depth has been reached, the moisture tends to decompose the rock. The new products formed are of greater volume than the original rock minerals. Consequently, swelling occurs and the shell of altered rock, as thick as the depth of moisture penetration, cracks away from the original rock. After the first shell has been released there is a ready passage for the succeeding influxes of moisture to penetrate the rock below and the same process repeats itself. This type of rock decay is called spheroidal weathering. Upon prolonged exposure to the elements the banding disappears and a homogeneous mass of soil is found where the boulder was situated originally. In a few instances the bands may appear to be of different colors. This is probably due to the amount of iron oxide absorbed by the soil during the breakdown of the boulders.
The foregoing examples can be readily contrasted with the fresh unaltered rocks along the talus slopes and the rock cuts along the trail. One is impressed by the ceaseless effort of natural forces to break down the rocks on the earth's surface. This is to provide soil and plant food so that the fauna and flora of the earth may carry on their life functions.
The names of several rocks have been mentioned above. The first, pumice, is present in varying amounts along most of the trail. Its most common mode of occurrence here is as a fine buff-colored material. In a few places fragments several cubic inches sin volume may be found.
These larger fragments exhibit all of the common characteristics of pumice; namely, light buff-to-tan color, glassy texture, and high porosity.
Pumice is invariably associated with explosive vulcanism and consequently the original magma is charged with gases, usually water vapor and carbon dioxide. The gases, under pressure, are forced into the viscous lava, thus producing the characteristic porous texture. The lava hardens before the pore spaces are eliminated.v
The next most abundant rock is andesite. It differs markedly from pumice, both in composition and the manner of its formation. Andesite, the common flow rock of Crater Lake, is rich in iron, magnesium, and calcium, and poor in silica. Pumice, on the other hand, has a high silica content with very minor amounts of the other elements. The pressure of the iron, magnesium, and calcium in the lavas increases their fluidity, thus permitting them to flow over wide areas. Common with most andesites, those of Crater Lake are porphyries. That is, there are large crystals embedded in a more dense background called the ground mass. The most common cause for the formation of porphyries is a change in the rate of cooling. When deep within the earth, the lava started to solidify or crystallize and numerous minerals (feldspars) grew to the size shown in the rocks. Then pressure from below forced the lava to the cool surface where rapid solidification stopped the mineral growth and caused the running lava to harden about the earlier formed minerals.
The only other rocks of importance is a volcanic agglomerate. This is a rock composed of fragments of the various igneous (lava) rocks of the region. During periods of explosive action all of the types of rock present were thrown into the air and then upon descent filled cracks and gullies in the sides of Mt. Mazama. Later, lava flows or percolating waters caused the fragments to be more or less consolidated. Two-thirds of the way up the trail an excellent view station is located where one can observe masses of boulders caught in the lavas. This shows that in some parts of Mt. Mazama, lava flows and explosive eruptions were simultaneous. Perhaps the boulders merely tumbled down the mountain side and were caught in the flow, or else were engulfed as the lava moved along.
Fragments of the mineral quartz may be seen among the talus debris along the trail. The mineral is a variety known as milky quartz and is common the world over. It has formed by solutions rich in silica rising through cracks in the sides of Mt. Mazama. The rarity of quartz as well as other secondary minerals is significant.
In several places along the Garfield Peak Trail are large white blotches on the rocks. This is especially true of the areas of volcanic agglomerate. These white spots represent areas where hot moist gases escaped to the surface of Mt. Mazama.
The rocks are largely kaolinized, but with the outline of the rock fragments retained. If the rocks are rubbed with the fingers the typical clayey feel can be readily recognized.
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