One Hundred Hikes in Yosemite

Evolution of the Yosemite Landscape — The Present as a Key to the Past

We can examine what geological processes are occurring today and apply them to the past. Along most of California, the San Andreas fault is the approximate boundary between the Pacific plate, on the west, and the North American plate, on the east. This primary fault is one of several nearly parallel major faults of the San Andreas fault system. Movement on these major faults is right-lateral-the block of crust on the west side of each fault moves north with respect to the block of crust on the east side. (In right-lateral faulting, the land on one side of the fault moves during an earthquake to the right with respect to an observer on the other side. It makes no difference which side of the fault the observer is on.) Currently the San Andreas fault exhibits the most movement, but in the past other faults were more active, and in the future still other faults probably will become more active.

About 5 million years ago, the direction of the Pacific plate's motion began to change, so that instead of its east edge sliding smoothly north past the west edge of the North American plate, the east edge was increasingly compressed against it. The continental crust riding on the two plates, being of relatively low density, could not sink out of the way, so the compressive forces caused uplift, and the Coast Ranges were born. Because the major faults aren't perfectly parallel, irregularities have arisen. Where parallel faults tend to converge, as through west-central California, a block of submerged continental crust was raised thousands of feet above sea level to create the Santa Lucia Range, among others. But to the north, where parallel faults tend to diverge, a lowland developed, the San Francisco Bay. Notice that in the same fault system, opposite events can occur at the same time-compressional uplift here, extensional subsidence there. When right-lateral or left-lateral faulting is associated with compression and extension, the resulting motion is called, respectively, transpression and transtension. These motions occurred in the early Sierra Nevada, just as they do in the Coast Ranges today.

If the change in the direction of the Pacific plate's motion continues, the Coast Ranges will grow to rival the Sierra Nevada in only a few million years. Eventually the angle between the two plates may increase to the point that the Coast Ranges will be thrust across the Central Valley, obliterating it. If so, they will be compressed against the western Sierra Nevada, and this compression will cause an orogeny, or mountain-building episode. During it, the Coast Ranges will become attached, or accreted, to the range, which then will become a bit wider. This accreted belt of continental crust will be similar to previously accreted belts, each called a terrane-a usually complex, fault-bounded geologic unit (as opposed to a terrain-a geographical area). The compression that leads to accretion also causes a transformation in a terrane's rocks through increased heat and pressure, and through the movement of super-hot fluids. This compression is on a grand scale, resulting in regional metamorphism. Sedimen tary rocks are metamorphosed to metasediments, volcanic rocks to metavolcanics.

While the continental crust can't sink, the denser oceanic crust as well as the dense, upper-mantle rock underlying both crusts can, and so an oceanic plate will dive under a continental plate, a process called subduction. With increasing pressure and depth, the diving plate will begin to undergo partial melting, and the melt, or magma, being of relatively low density, will ascend through the continental crust. As it ascends through preexisting rock of the upper crust, it alters the existing crustal rocks. This alteration is called contact metamorphism, since in these rocks it is greatest near the contact with the magma. In the Sierra, older rocks show signs of being metamorphosed two or more times. Magma that solidifies beneath the surface forms plutonic rock, and a body of this rock is called a pluton. In Sierra Nevada lands, most of this rock is light-gray granitic rock.

If magma reaches the surface as eruptions of ash or lava, it becomes volcanic rock. Volumi nous eruptions can create a mountain range, such as the Cascade Range of Washing ton, Oregon, and northern California. That range is largely the result of an oceanic plate diving eastward beneath the Pacific North west. Together, volcanic and plutonic rocks are classified as igneous rocks, which are any that have solidified from a molten mass. The common types of igneous rocks are listed in the accompanying table.


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