JOSHUA TREE NATIONAL PARK
FIELD TRIP STOP - Spheroidal weathering of Mesozoic Granite intruded into a PreCambrian Gneiss. Nearby exposure of the San-Andreas Fault.
LOCATION: Joshua Tree National Park is located in southern California, about 140 miles east of Los Angeles. There are three park entrances, Two are located in the northern part of the park, off of Route 62, at the towns of Joshua Tree and Twentynine Palms. The southern entrance is located about 5 miles west of Chiriaco Summit and just north of I-10.
GEOLOGIC FEATURES: Granite (Monzogranite), Gneiss, Spheroidal Weathering, Hydrolysis, Chemical Weathering, Jointing, San Andreas Fault, Dikes
DESCRIPTION:
Major Rock Types
Joshua Tree N.P. is perhaps best known for its stacked boulders of Granite (Monzogranite) viewed largely in the upper part of the park extending along Park Boulevard from the town of Joshua Tree to its intersection with Pinto Basin Road. These rocks are of Cretaceous age, having been formed between about 250 to 75 million years ago. Another prominent visible rock type is the Pinto Gneiss, a much older dark-colored foliated (layered) metamorphic rock formed about 1.4 to 1.7 billion years ago. This rock forms distinctive outcrops along Pinto Basin Road in the central part of the park. This gneiss formed by much older tectonic events that occurred during the Precambrian.
The granites ultimately resulted from the subduction of the Farallon Plate under rocks of the North American Continent. Molten rock generated at this convergent boundary rose towards the surface, eventually intruding into the preexisting Precambrian Pinto Gneiss before cooling and solidifying. Cooling of the granite in conjunction with regional stresses caused sets of parallel cracks (joints) in the rock that often intersect each other obliquely. In addition, uplift and erosion of rocks at the surface can release overlying pressure and thus form fractures somewhat parallel to the ground surface.
Formation of Distinctive Granite Boulders
Eventually, infiltration of ground water through the numerous intersecting fracture sets, chemically weather joint intersections into clay (through a process called hydrolysis), widening cracks and rounding edges, eventually transforming the solid rectangular blocks into spherical shapes. Subsequent uplift and erosion by floods washed away the weathered clay causing the eroded boulders to settle on top of one another into the piles of rounded granite boulders that we see today.
The San Andreas Fault and Transverse Ranges
Another important geologic feature of Joshua Tree National Park is its proximity to the San Andreas Fault, which is located just outside the park boundary and is strikingly visible from Keys Point. Movement along the San Andreas along with the motion of a few associated parallel faults within the Park boundaries, compressed and uplifted the east-west trending mountain ranges (the Eastern Transverse Ranges) that we see within the park.
STUDENT QUESTIONS:
(1) Define the geological meaning of a Joint including how it differs from a Fault. Describe the process that creates Jointing in rocks.
(2) Define the chemical process of Spheroidal Weathering and indicate its role in the formation of Joshua Trees’s many unique Granite formations.
(3) The San Andreas Fault is a Strike-Slip Fault. How does fault movement along this type of fault differ from a Normal or Reverse Fault? Construct a diagram that illustrates such a fault movement.
(4) How are thin, light-colored dikes formed within the granite?
(5) Construct a cross-sectional diagram that illustrates (a) subduction of the Farallon Plane beneath the North American continent, and (b) the areas of generation and subsequent emplacement of the Monzogranite within the preexisting Pinto Gneiss miles below the ground surface,
(6) CHALLENGE: Describe the chemical weathering changes involved in Hydrolysis of Granite surfaces.
(7) CHALLENGE: What is different about a Monzogranite that might differentiate it from other types of Granite?
(8) CHALLENGE: Using cross-sectional diagrams, illustrate the development of a present-day granite boulder-ridden outcrop starting with its initial solidification from a magma.
SELECTED REFERENCES:
-Cactus Atlas. 2024. Geology Tour Road | Exploring Earth's Wonders in Joshua Tree National Park (Video). Accessed on Dec. 19, 2024 from https://www.youtube.com/watch?v=QjqSDkfL_7w
-Jay Chapman. 2020. Geology of Joshua Tree National Park (Video). Accessed on Dec. 19, 2024 from
https://www.youtube.com/watch?v=T9eHpYAGlH8
-JoshuaTreeNPS. Geology of Joshua Tree Animation (Video). Accessed Dec. 19, 2024 from
https://m.youtube.com/watch?v=2Yj9pQprm8I
-National Parks Traveler. Joshua Tree’s Geology. Accessed on Dec. 19, 2024 from https://www.nationalparkstraveler.org/parks/joshua-trees-geology
-USA Safari. Geology of Joshua Tree National Park. Oct, 28, 2024. Accessed on Dec. 19, 2024 from https://www.usasafari.com/2024/10/geology-of-joshua-tree-national-park.html
-USGS. Joshua Tree National Park in Geology and Ecology of National Parks. U.S. Geological Survey. Accessed on Dec. 19, 2024 from https://www.usgs.gov/geology-and-ecology-of-national-parks/joshua-tree-national-park. (Includes Sections on the Geology and Ecology of the Park, as well as a Park Map.)
WHERE TO STAY:
While visiting Joshua Tree, why not stay at the beautiful Red Tail Ranch, a 5-acre desert retreat with serene mountain views, located in nearby historic Pioneertown. Visit https://www.theredtailranch.com for more details!!
FIGURE A - A general layout of Joshua Tree National Park. The northwest area of the park is the most visited one. .
PHOTOS:
Figure 1 - Numerous oblique joints cutting through the granite are ultimately responsible for spheroidal (spherical) weathering as well as the accumulation of weathered blocks at the base of the outcrop. A Joshua Tree stands in the foreground. [Photo at 4.3 miles from West Entrance]
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Figure 2 - Rounded blocks of granite are characteristic of the National Park. Photo at Quail Springs Picnic Area, about 5.2 miles from West Entrance.
Figure 3 - Rounded granite boulders produced by oblique jointing patterns. Note the narrow light-colored dikes cutting through some boulders. Photo at the Quail Springs Picnic Area.
Figure 4 - Rock climbing is a common hobby in the park. More than 5000 climbing routes are described within the park. Photo at Quail Springs Picnic Area.
Figure 5 - Note the prevalence of near horizontal jointing, Photo taken from Hemingway parking area.
Indio Hills
Figure 6 - Looking southwestward into Coachella Valley from Keys View. The San Andreas Fault splits into two closely spaced parallel faults (Mission Creek and Manning Faults). The Indio Hills form a ridge between the two segments, having been pushed upwards by fault action.
Figure 7- A sign displayed at Keys View illustrating the geography and position of the faults.
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Figure 8 - Close up of Indio Hills in the middle of Coachella Valley, bound on either side by segments of the San Andreas Fault (delineated by the parallel dotted lines). The San Jacinto Mts. are in the background. [Photo by Fran Marintsch.]
Figure 9 - Cap Rock. This picnic area is named for the outcrop having a relatively flat surface.
Figure 10 - Erosion of the overlying Pinto Gneiss (dark rock) has exposed the much younger (Mesozoic) granitic rock which has been intruded into the PreCambrian Gneiss. Photo taken near Ryan Mountain, between Cap Rock and Jumbo Rocks.
Figure 11 - Large granite boulders found at Jumbo Rocks Campground area.
Figure 12 - Pinto Gneiss exposure viewed from Pinto Basin Road, Wilson Canyon, north of Cholla Cactus Gardens. Note the foliation (layering) within this metamorphic rock.
Figure 13 - Cholla Cactus Garden viewed from Pinto Basin Road. Branches seem to glow in the sunlight. Don't be fooled by this cactus being called "teddy bear" cactus. Their spines easily penetrate your skin with the barbed ends being very difficult and painful to remove!
Figure 14 - Pinto Gneiss. Photo from Pinto Basin Road south of the Cottonwood Visitor Center at the southern end of the Park. Cottonwood Mountains.