One Hundred Hikes in Yosemite

Evolution of the Yosemite Landscape — Extension, Plutonism, and Volcanism

Each of the assortment of plates that comprises the outer layer of our planet is composed of crust atop underlying upper mantle. The crust can be either thin oceanic crust, thick continental, or both. The continental crust is thick enough so that about 6+ miles down the heat and pressure are sufficient to cause it to deform. The lower crust, which in the Yosemite area extends to a depth of about 25 miles, can slowly flow, up to several inches per year (about as fast as your fingernails grow). The crustal flow can be due either to flow in the underlying upper mantle or to flow deeper within the earth. The upper crust (the top 6+ miles), being brittle, cannot flow, and its base can only accommodate so much strain in response to underlying stresses before it breaks. A break, or fault, then grows upward from the base of the upper crust, generating an earthquake in the process. Where extension has been extreme, such as in the Great Basin, so much stress can develop that the base of the upper crust detaches from the top of the lower crust. This fairly horizontal breaking apart between the upper and lower crust is called a detachment, and the movement along it is detachment faulting. As we will soon see, this faulting played a very important part in the formation of today's Sierra Nevada.

Major changes were needed to create today's largely granitic range, and these were accomplished in part when the major compression that brought about the Nevadan orogeny gave way to major extension. Faults rifted the upper crust apart, providing space for ascending magma. Extension and plutonism went hand in hand, and from about 115 to 85 million years ago, plutonism occurred on an unprecedented scale. At first plutonism occurred in a north-south belt from north of the Park's northwest corner southward along its western boundary and beyond it to the edge of today's San Joaquin Valley. Over time the locus of magma generation migrated episodically eastward across the ancestral Sierra Nevada, perhaps in response to a shallowing angle of an east-diving oceanic plate. Consequently, granitic rocks east of the boundary, say in the upper part of the Merced Gorge and the west half of Yosemite Valley, are about 105 to 100 million years old. Those in its east half are about 100 to 90 million years old, while those farther east, say around Tuolumne Meadows, are about 90 to 85 million years old.

The actual patterns of the spatial distribution and composition of plutons are complex. Some magma intruded older plutons, resulting in younger plutons partly displacing them. And some magma, working upward over millions of years, intruded and displaced outward newly solidified magma. This resulted in a nested, composite pluton called an intrusive suite, whose composition of magma has evolved over time. The largest one in our area, about 450 square miles, is the Tuolumne intrusive suite, which today is exposed over most of the eastern third of the Park, but also includes the eastern part of Yosemite Valley, Tenaya Canyon, Little Yosemite Valley, and most of the Illilouette Creek drainage. Its plutonism began about 91 million years ago and continued in surges until about 86 million years ago, and the magma's composition evolved toward increasing amounts of lighter-colored minerals (quartz and alkali feldspar).

During the 30-million-year period of abundant plutonism, volcanism locally dominated when extension and plutonism were minimal. Towering stratovolcanoes such as today's Mts. Shasta, Hood, and Rainier likely stood at various times above the range's core. During the last surge of magma in the Tuolumne intrusive suite, one large volcano rose high above what is now Johnson Peak, just south of Tuolumne Meadows. However, a more-impressive volcano straddled the Park's southeast border about 100 million years ago, centered on the Minarets caldera. A caldera is a large, more-or-less circular basin at the center of a large volcano. The Minarets caldera was giant, about 18 miles across from near Chiquito Creek northeast to Thousand Island Lake, and 14 miles across from near the San Joaquin River northwest to Washburn Lake. Over its existence this eruptive center produced an estimated 360 cubic miles of ash and lava. This volume is enough to bury the San Francisco Bay metropolitan area under about 1100 feet of deposits, the Los Angeles-Orange counties metropolitan areas (excluding the San Fer nando Valley) under about 1700 feet, or the San Diego metropolitan area under about 6600 feet!

Magmatism waned on in the Sierra Nevada until about 80 million years ago, when the oceanic plate greatly increased its velocity and shallowed its angle of diving. The result of these changes was that the locus of magma generation shifted far east of the range, and it ultimately caused a mountain-building episode in the Rocky Mountains, the Laramide orogeny. The Sierra Nevada now had a core composed of dozens of generally light-gray plutons. These, however, typically lay several miles beneath a largely volcanic landscape-definitely not the Range of Light. More change was in order.


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