Pacific Crest Trail: Northern California. Jeffrey P. Schaffer

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      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 a cement, usually calcite. Pebbles, cobbles and boulders may be cemented in a sand or gravel matrix to form a conglomerate. If these particles are deposited on an alluvial fan and then gradually cemented together to form a hard rock, collectively they become fanglomerate. Alluvial fans are usually formed where a stream debouches from the mouth of a canyon and drops its sedimentary load, or alluvium, over a fan-shaped area. Alluvial fans are seen along the south edge of the Mojave Desert, where it abuts the north base of the San Bernardino and the San Gabriel mountains. If the larger particles in a conglomerate or fanglomerate are angular rather than rounded, the sedimentary rock is called a breccia.

      Granitic rock walls above Smedberg Lake, Yosemite National Park, Section I

      Limestone, another type of sedimentary rock, is formed in some marine environments as a chemical precipitate of dissolved calcium carbonate or as cemented fragments of shells, corals and foraminifers. The individual grains are usually microscopic. If the calcium in limestone is partly replaced by magnesium, the result is dolomite.

      Since the PCT attempts to follow a crest, you’ll usually find yourself in an area being eroded, rather than in a basin of deposition, so you’ll find very ephemeral sediments or very old ones. The young ones may be in the form of alluvium, talus slopes, glacial moraines, or lake sediments. The old ones are usually resistant sediments that the intruding granitic plutons bent (folded), broke (faulted) and changed (metamorphosed).

      Metamorphic rocks

      A volcanic or a sedimentary rock can undergo enough alteration (metamorphism) due to heat, pressure, and superhot, corrosive fluids that it loses its original characteristics and becomes a metavolcanic or a metasedimentary rock. Metamorphism may be slight or it may be complete. A shale undergoing progressive metamorphism becomes first a slate, second a phyllite, then a schist, and finally a gneiss. The slate resembles the shale but is noticeably harder. The schist bears little resemblance and is well-foliated, with flaky minerals such as biotite or other micas clearly visible. The gneiss resembles granite, but has alternating layers of light and dark minerals.

      Hornfels is a hard, massive rock, common in parts of the High Sierra, formed by contact of an ascending pluton with the overlying sediments. It can take on a variety of forms. You might find one that looks and feels like a slate, but differs in that it breaks across the sediment layers rather than between them.

      Quartzite is a metamorphosed sandstone and resembles the parent rock. The spaces between the grains have become filled with silica, so that now if the rock is broken, the fracture passes through the quartz grains rather than between them as in sandstone. Metamorphism of limestone or dolomite yields marble, which is just a crystalline form of the parent rock. Check out Marble Mountain, in northern California, when you reach it.

      Geologic Time

      You cannot develop a feeling for geology unless you appreciate the great span of time that geologic processes have had to operate over. A few million years’ duration is little more than an instant on the vast geologic time scale (see following Geologic Time Scale). Within this duration a volcano may be born, die and erode away. Dozens of major “ice ages” may come and go.

      A mountain range takes longer to form. Granitic plutons of the Sierra Nevada first came into being about 240 million years ago, and intrusion of them continued until about 80 million years ago, a span of 160 million years. Usually there is a considerable gap in the geologic record between the granitic rocks and the older sediments and volcanics that they intrude and metamorphose—often more than 100 million years.

      Geologic History

      With the aid of a geologic section, like the one above, we can reconstruct in part the geologic history of an area. Our geologic section represents an idealized slice across the Sierra Nevada to reveal the rocks and their relations.

      Through dating methods that use radioactive materials, geologists can obtain the absolute ages of the two granitic plutons, the andesite flow, and the basalt flow, which respectively would likely be Cretaceous, Pliocene, and Holocene. The overlying, folded sediments intruded by the plutons would have to be СКАЧАТЬ