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PCT Natural History
Geology
It is very likely that the California section of the Pacific Crest Trail is unequaled in its diversity of geology. Many mountain trails cross glacial and subglacial landscapes, but which ones also cross arid and semi-arid landscapes? Some parts of your trail will have perennial snow; others are usually dry. Precipitation may be more than 80 inches per year in places, less than 5 inches in others. In each of the three major rock classes—igneous, sedimentary, and metamorphic—you’ll encounter dozens of rock types. Because the PCT provides such a good introduction to a wide spectrum of geology, we have added a liberal dose of geologic description to the basic text. By the end of your journey you’ll have developed a keen eye for rocks and understand the relations between the different rock types. Since we assume that many hikers will have only a minimal background in geology and its terminology, we’ll try to cover this broad subject for them in the next few pages. Those wishing to pursue the subject further should consult the list of references at the end of this book.
Rocks
First, you should get acquainted with the three major rock classes: igneous, sedimentary, and metamorphic.
Igneous rocks
Igneous rocks came into being when the liquid (molten) rock material (magma) solidified. If the material solidified beneath the earth’s surface, the rock is called intrusive, or plutonic, and a body of it is a pluton. If the material reached the surface and erupted as lava or ash, the rock is called extrusive, or volcanic.
Intrusive rocks: The classification of an igneous rock is based on its texture, what minerals are in it, and the relative amounts of each mineral present. Since intrusive rocks cool more slowly than extrusive rocks, their crystals have a longer time to grow. If, in a rock, you can see an abundance of individual crystals, odds are that it is an intrusive rock. These rocks may be classified by crystal size: fine, medium, or coarse-grained, to correspond to average diameters of less than 1 millimeter, 1–5, and greater than 5.
Some igneous rocks are composed of large crystals (phenocrysts) in a matrix of small crystals (groundmass). Such a rock is said to have a porphyritic texture. The Cathedral Peak pluton, which is well exposed on Lembert Dome at the east end of Tuolumne Meadows in Yosemite National Park, has some feldspar phenocrysts over four inches long. High up on the dome these phenocrysts protrude from the less resistant groundmass and provide rock climbers with the holds necessary to ascend the dome.
The common minerals in igneous rocks are quartz, feldspar, biotite, hornblende, pyroxene, and olivine. The first two are light-colored minerals; the rest are dark. Not all are likely to be present in a piece of rock; indeed, quartz and olivine are never found together. Intrusive rocks are grouped according to the percentages of minerals in them. The three common igneous groups are granite, diorite, and gabbro. Granite is rich in quartz and potassium feldspar and usually has only small amounts of biotite. Diorite is poor in quartz and rich in sodium feldspar, and may have three dark minerals. Gabbro, a mafic rock (rich in magnesium and iron), lacks quartz, but is rich in calcium feldspar and pyroxene, and may have pyroxene and olivine. You can subdivide the granite–diorite continuum into granite, quartz monzonite, granodiorite, quartz diorite and diorite. These rocks, which are usually called “granitic rocks” or just plain “granite,” are common in the Sierra Nevada and in most of the other ranges to the south.
Since it is unlikely that you’ll be carrying a polarizing microscope in your backpack, let alone a great deal of mineralogical expertise in your head, your best chance of identifying granitic rocks lies in making educated guesses based upon the following table.
At first you’ll probably estimate too high a percentage of dark minerals, partly because they are more eye-catching and partly because they show through the glassy light minerals. If the intrusive rock is composed entirely of dark minerals (no quartz or feldspar), then it is an ultramafic rock. This rock type, which can be subdivided further, is common along the trail from Interstate 5 at Castle Crags State Park northwest to the Oregon border.
Extrusive rocks: Extrusive, or volcanic, rocks are composed of about the same minerals as intrusive rocks. Rhyolite, andesite, and basalt have approximately the same chemical compositions as granite, diorite, and gabbro, respectively. As with the intrusive rocks, the three volcanics can be subdivided into many groups, so it is possible to find ordinary rocks with intimidating names like “quartz latite porphyry”—which is just a volcanic rock with quartz phenocrysts and a composition in between rhyolite and andesite.
Castle Crags, Section P
Texture is the key feature distinguishing volcanic from plutonic rocks. Whereas you can see the individual crystals in a plutonic rock, you’ll have a hard time finding them in a volcanic one. They may be entirely lacking, or so small, weathered and scarce that they’ll just frustrate your attempts to identify them. If you can’t recognize the crystals, then how can you identify the type of volcanic rock? Color is a poor indicator at best, for although rhyolites tend to be light gray, andesites dark gray, and basalts black, there is so much variation that each can be found in any shade of red, brown or gray.
One aid to identifying volcanic rock types is the landforms composed of them. For example the high silica (SiO2) content of rhyolite makes it very viscous, and hence the hot gases in rhyolite magma cause violent explosions when the magma nears the surface, forming explosion pits and associated rings of erupted material (ejecta). For the same reason, a rhyolite lava flow (degassed magma) is thick, short, and steep-sided and may not even flow down a moderately steep slope. The Mono and Inyo craters, north of Devils Postpile National Monument, are perhaps the best examples of this volcanic rock in California. You will find very little of it along the trail.
The landform characteristically associated with andesite is the composite cone, or stratovolcano. Mount Shasta and some of the peaks in the Lassen area, including Brokeoff Mountain, are examples. These mountains are built up by alternating flows and ejecta. In time parasitic vents may develop, such as the cone called Shastina on Mount Shasta; and the composition of the volcano may shift to more silica-rich dacite rock, an intermediate between rhyolite and andesite, which, like rhyolite, gives rise to tremendous eruptions, but also can produce lava domes such as Lassen Peak.
The least siliceous and also the least explosive of volcanic rocks is basalt. A basaltic eruption typically produces a very fluid, thin flow and a cinder cone, usually less than 1000 feet high. When in Lassen Volcanic National Park, take the alternate route up to the rim of the Cinder Cone. From this vantage point you can see what an extensive, relatively flat area its thin flows covered. Contrast this with Lassen Peak, to the west, California’s largest dacite dome.
Sedimentary rocks
We often think of rocks as being eternal—indeed, they do last a long time. But even the most resistant polished granite eventually succumbs to the effects of weathering, although on broad, unglaciated ridges and gentle slopes the rate of removal (denudation) is about a foot or less per million years. Granite rocks solidified under high pressures and rather high temperatures within the earth. At the surface, pressure and temperature are lower and the rock’s chemical environment is different, and in this environment it is unstable. The rock weathers, and the pieces are gradually transported to a place of deposition. This place may be a lake in the High Sierra, a closed basin with no outlet such as the Mono Lake basin, an open structure such as the great Central Valley, or even the continental shelf of the Pacific Ocean. The rocks formed of the sediment that collects in these basins are called sedimentary rocks.
Most sedimentary rocks are classified by the size of their particles: clay that has been compacted and cemented forms shale; silt forms siltstone, and sand forms sandstone. Sandstone derived from granitic rock superficially resembles its parent rock, but if you look closely you’ll notice that the grains are somewhat rounded and that the spaces between the grains are usually filled with