The coordinates are on the trail to Mount Whitney. It is at high elevation and requires a wilderness permit to enter, even for day hikes.
The Sierra Nevada Batholith began forming about 210 million years ago as a subduction zone formed west of the continent. It is thought that the region looked similar to the modern day Andes. An oceanic plate was subducting beneath the North American plate. Partial melting of the oceanic plate formed large bodies of granitic magma that moved up in the crust to cool slowly, creating a huge mass of granitoid rocks. The slow rate of cooling allowed large crystals to form in these plutonic rocks.
While the Sierra Nevada are commonly associated with granites, the Sierra Nevada Batholith is made up of more than 100 distinct granitoid units. Granitoids are plutonic rocks with between 20 and 60 percent quartz. The relative percent of feldspar minerals (potassium feldspar and plagioclase) determines the type of granitoid. Granitoids with predominantly plagioclase are called granodiorites and are not considered true granites.
One of the youngest and largest units in the Sierra Nevada Batholith is the Mount Whitney Intrusive Suite. The Mount Whitney Intrusive Suite is about 80.4 to 87 million years old and is made up of the Paradise Granodiorite, Sugarloaf Granodiorite, Lone Pine Granodiorite, and the Whitney Granodiorite. While these different granodiorites look similar, changes in the minor chemical composition and color of individual crystals within the rock are what are used to tell them apart.
The Whitney Granodiorite is characterized by large almost white potassium feldspar crystals. Some of the crystals get to be 4 to 8 cm long. The rest of the crystals are significantly smaller. Due to the noticeable difference in crystal sizes, the rock is considered porphyritic. The other units have smaller crystal dimensions and are generally darker in color.
As the Whitney Granodiorite cooled cracks formed in it and were filled the remaining magma. Since most of the other minerals had already crystallized out of the magma, this remaining magma was largely quartz. It continued more quickly, so the crystals are visibly smaller than the surrounding granodiorite. These smaller grained veins are called aplites.
Logging questions:
- The text "GC3PVQG Whitney Granodiorite" on the first line.
- The number of people in your group (put in the log as well).
- What is the length of the large crystals (potassium feldspar) in the granodiorite?
- What is the color, width, and orientation of the aplite vein that cuts across the granodiorite?
- What are the size of the grains in the aplite vein.
The following sources were used to generate this cache.
- Moore, James G. 1987, Mount Whitney Quadrangle,Inyo and Tulare Counties, California Analytic Data. U.S. GEOLOGICAL SURVEY BULLETIN 1760.
- Moore, James G. 2000. Exploring the Highest Sierra. Stanford University Press.
- Moore, J.G., 1981, Geologic map of the Mount Whitney quadrangle, Inyo and Tulare Counties, California: U.S. Geological Survey, Geologic Quadrangle Map GQ-1545, scale 1:62500.