Yosemite N.P. | GeologyVirtualTrips

YOSEMITE NATIONAL PARK

FIELD TRIP STOP – INSIDE A MESOZOIC GRANITIC PLUTON EXPOSED DUE TO TECTONIC ACTIVITY

LOCATION: Located in the central Sierra Nevada of California. It is found about 100 mi east of Gustine, California (located along I-5).

GEOLOGIC FEATURES: Batholith; Pluton; “Granitic”; Phaneritic and Porphyritic igneous textures; Hanging Valleys; Exfoliation Domes; Glacial Erratics and Striations; Fault-Block Mountains

DESCRIPTION: Nearly all the rocks exposed within Yosemite N.P. are igneous crystalline rocks of the Sierra Nevada Batholith, a Fault-Block Mountain Range.

Magma forming this batholith originated several miles below the surface. The magma began to rise and intrude the overlying rocks in three sets of Mesozoic intrusions occurring between 210 mya and 80 mya (Upper Triassic to Upper Cretaceous) that were related to a nearby subduction zone. Most rocks in the park are between 105-80 myo (Cretaceous). Multiple intrusive events were responsible for the variety of igneous rock types throughout. Some of the rocks are granite, others are quartz diorite, quartz monzonite, granodiorite, and tonalite among others. All of these are commonly grouped as “granites” in published articles. General slow cooling at depth caused these rocks to be coarse-grained (large crystals) and sometimes porphyritic.

By 65 m.y.a., after subduction had ended, this Ancestral Sierra Nevada mountain range began to erode the underlying granitic rock which were well-exposed by mid-Tertiary. Beginning less than 20-25 mya, an 11,000 ft. uplift of faults along the eastern side of the present mountain range tilted the rocks of this fault block towards the west-southwest accounting for the present-day topography.

Jointing occurred in places within the park commonly controlled by the rock type present. Some joints are vertical and occur in sets (e.g. at Cathedral Spires), others are sheets (e.g. exfoliation seen in part at Half Dome). In other lithologies, there is massive, unjointed rock (e.g. El Capitan).

Continued weathering of exposed rock, mass-wasting, and stream erosion continued until glaciation began about 3 mya which scoured the valleys. Many glacial features in the park reflect the most recent episode of glaciation (Tioga Stage) which spanned an interval between about 26,000 and 18,000 years ago. Glaciation resulted in U-shaped Valleys, hanging valleys (responsible for waterfalls), peaks and domes, polished granites (slickensides), and moraines and outwash.

STUDENT QUESTIONS:

(1) What is a Batholith? Does a Batholith differ from a Pluton? Explain.

(2) Define Exfoliation and describe how Exfoliation Sheets are formed.

(3) Why are U-shaped Valleys associated with Glaciation?

(4) How are Hanging Valleys formed?

(5) Why do large crystals indicate slow-cooling of a magma?

(6) What is meant by a Porphyritic Texture?

(7) What is a likely reason for the original generation of the Mesozoic magmas that rose to ultimately form the Sierra Nevada?

(8) CHALLENGE: Several types of igneous rock are listed in the Description above. What are the major differences among them? (You may use a triangular diagram with Quartz, Potassium Feldspar, and Plagioclase Feldspar as vertices of the triangle.)

(9) CHALLENGE: Give a possible way that a rock with a Porphyritic texture can form.

(10) CHALLENGE: What might account for the differences in magma type (and therefore rock types) occurring in the Sierra Nevada mountains?

SELECTED REFERENCES:

-Huber, N. King. 1987. The Geologic Story of Yosemite National Park; USGS Bull. 1595. Accessed on Jan. 28, 2020: https://pubs.usgs.gov/bul/1595/report.pdf

-Harris, A. and E. Tuttle. 1983 (3rd ed).Geology of the National Parks. Kendall/Hunt Publishing Co., Dubuque, IO. 554 pp.

-Harris, D.V. and E.P. Kiver. 1985 (4th ed.). The Geologic Story of the National Parks and Monuments, John Wiley and Sons, New York, 464 pp.

-National Park Service. Yosemite National Park – Geology. Accessed on Aug.1, 2023 from https:/nps.gov/yose/learn/nature/geology.htm

-USGS. Geology of Yosemite National Park. Accessed on Aug. 1, 2023 from https://www.usgs.gov/geology-and-ecology-of-national-parks/geology-yosemite-national-park

PHOTOS:

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Figure 1 - View of Yosemite Valley from Tunnel View. EC = El Capitan; CLR = Clouds Rest; HD = Half-Dome; CR = Cathedral Rocks; SR = Sentinel Rock; BV = Bridal Veil Fall.

Figure 2 - El Capitan. Also known as El Cap, this is a coarse-grained granite emplaced about 100 mya. It is massive and free of joints. El Cap rises about 3000 ft. from its base and is well known for the fact that on June 3, 2017, Alex Honnold "Free-soloed" its wall without a partner, a rope, or any protective gear.

Figure 3 - Horsetail Fall falls 617 ft from the eastern part of El Capitan. In mid to late February the Fall glows red-orange ("Firefall") when illuminated by the sunset.

Figure 4 - Bridalveil Fall

Figure 5 - Upper and Lower Yosemite Falls. The Upper Falls drops 1430 ft. The Lower Fall drops 320 ft. The Cascades between the two drop 675 ft for a total of 2425 ft.

Figure 6 - From Glacier Point we see Half Dome in the upper right.

Figure 7- Half Dome rises 4737 ft. above the valley floor. Vertical jointing on either side of the dome along with glaciation have isolated the rock structure and exfoliation sheets have rounded the outcrop top. Close examination of the domal surface show the "onionskin-like" sheeting caused by the release of overlying pressure.

Figure 8 - A telescopic view of Half Dome looking southwest taken from Olmsted Pont located on Tioga Road.

Figure 9 - Close-up view of typical "granitic" rock of Yosemite N.P. sampled at Olmsted Point. Quartz crystals are gray to white and glassy. Plagioclase feldspar is white (and rectangular). Hornblende (Amphibole) and Biotite Mica are black. Hornblende forms blades while biotite occurs in book-like sheets.

Figure 10 - Close-up view of typical Exfoliation Dome Sheeting exposed along Tioga Road

Figure 11 - Glacially transported "erratic" boulders at Olmsted Point that rest on Granodioritic rock which displays jointing and glacial striations (from bottom right to upper left of photo).