El Capitan EarthCache

Welcome to Yosemite National Park

El Capitan towers above Yosemite Valley as a massive granite wall rising nearly 3,000 feet from the valley floor. From El Capitan Meadow, visitors can clearly observe the scale, structure, and surface features of this iconic cliff. This location provides an excellent vantage point to examine how deep-seated igneous rock, natural fractures, and Ice Age glaciers combined to shape one of the most dramatic rock faces in the world. This EarthCache focuses on the granite composition of El Capitan and the geologic processes responsible for its steep, vertical form.

This EarthCache was created in correspondence with and with permission from the Yosemite National Park Service. Respect and follow all park regulations during your visit. Please help protect Yosemite by picking up your trash, staying on designated trails, and never feeding wildlife.

The Story of El Capitan

El Capitan is composed almost entirely of granitic rock that formed deep within the Earth’s crust during the Mesozoic Era, approximately 90 to 100 million years ago. At that time, molten magma slowly cooled far below the surface as part of the Sierra Nevada batholith. The slow cooling process allowed large mineral crystals to develop, producing a strong, coarse-grained rock. Over tens of millions of years, erosion stripped away the overlying layers of rock, eventually exposing this granite at the surface. Because granite is highly resistant to erosion, it commonly forms steep cliffs and large monolithic features like El Capitan.

Although El Capitan appears smooth and uniform from a distance, the granite contains numerous joints and fractures. These cracks formed as the magma cooled, as tectonic stresses affected the region, and as pressure was released when overlying rock was removed. Jointing divides the granite into large slabs and blocks, controlling where weathering and erosion occur. The spacing and orientation of these joints strongly influence the shape of the cliff face and contribute to its sheer, planar surfaces.

During the Pleistocene Epoch, Yosemite Valley was repeatedly filled by thick alpine glaciers flowing down from the high Sierra Nevada. These glaciers scoured the valley floor and walls, deepening and widening the valley into a broad U-shaped trough. As the ice moved past El Capitan, it exploited fractures in the granite, plucking loose blocks and steepening the cliff face. Unlike nearby rock faces that were more heavily rounded by glacial abrasion, El Capitan remained steep and angular due to its massive, well-jointed granite. From El Capitan Meadow, the near-vertical face reflects both the resistance of the rock and the powerful erosional role of glacial ice.

Rock Types of El Capitan

El Capitan is composed of multiple intrusive igneous rock units that record a complex magmatic history within the Sierra Nevada batholith. These rock types differ in mineral composition, texture, color, and intrusive relationships, all of which can be observed directly on the cliff face.

• El Capitan Granite — A coarse-grained, light gray to pale tan biotite granite composed mainly of quartz, potassium feldspar, plagioclase, and minor biotite; it is massive, relatively uniform, and forms much of the clean, unbroken cliff face.
• Taft Granite — A coarse-grained biotite granite to granodiorite that is slightly darker and more compositionally variable than El Capitan Granite, often showing a salt-and-pepper appearance due to increased mafic mineral content.
• Leaning Tower Granite — A medium- to coarse-grained granitic rock, typically medium gray, with a higher proportion of mafic minerals and more closely spaced fractures, giving it a rougher surface expression.
• Tonalite of the Gray Bands — A medium-grained tonalite rich in plagioclase and hornblende with lesser quartz, distinctly darker gray in color, occurring as tabular intrusions that form prominent vertical bands across El Capitan.
• Diorite — A fine- to medium-grained, dark gray to black intrusive rock dominated by plagioclase and hornblende ± biotite, occurring as irregular bodies and narrow intrusions that cut the surrounding granitic rocks.
• Aplite — A very fine-grained, light-colored igneous rock composed primarily of quartz and feldspar, forming thin, sharp-edged dikes and veins that represent late-stage felsic melts.
• Pegmatite — An extremely coarse-grained felsic intrusive rock mineralogically similar to granite, characterized by very large crystals and occurring as dikes or pods formed during the final stages of magma crystallization.

Tasks for This EarthCache

To log this EarthCache, visit the site and answer the following questions. Make your observations from El Capitan Meadow. Send your answers via Geocaching or email.

1. Include “El Capitan - GCBJ9J9” on the first line of your message.

2. Observe the overall appearance of El Capitan and describe its (a) height, (b) colors, and (c) textures. What features suggest this rock formed slowly beneath the Earth’s surface?

3. Look closely at the cliff face. Do you see large cracks, joints, or vertical divisions in the rock? In your own words, explain why these features appear limited or widely spaced on El Capitan.

4. Choose two different colors you can see on the cliff face and describe the (a) location and (b) abundance of each. Based on the diagram and rock descriptions, which rock types do you think correspond to these colors?

5. In your log, attach a photo of yourself or a personal belonging at the posted coordinates with El Capitan in the background. (Note: photos predating the publication of this EarthCache are not accepted.)

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Works Cited

https://en.wikipedia.org/wiki/El_Capitan

https://en.wikipedia.org/wiki/El_Capitan_Granite

https://www.nps.gov/yose/learn/nature/granite.htm