California Division of Mines and Geology Self-Guided Geologic Tour in Joshua Tree National Monument

The known geologic history of the Monument spans about 1.5 billion years and includes extensive periods of sedimentary deposition, igneous intrusions, metamorphism, and erosion. The oldest rocks in the Monument, the Pinto Gneiss (pronounced "nice"), have been dated at about 1.5 billion years old and contain deformed sediments. Since these early sediments were deposited, this region has undergone several periods of both tectonic uplift and submergence beneath the sea. Although the complete geologic history of this area may never be fully known, scientists have been able to decipher a partial chronicle from the rocks that occur in the Monument (Photo 5).

Photo 5. Outcrop of Pinto Gneiss, the oldest rock in the Monument. Note the 2-inch diameter lens cap in the upper middle of photo for scale. This photo was taken at Stop 14 on the Geology Tour Road.

Pinto Gneiss

The Pinto Gneiss varies from prominently foliated dark gray to a faintly foliated much lighter gray. The deformed sediments and possible volcanic material included in this rock provide the earliest geologic record in the Monument. Although the absolute age of these sediments is difficult to determine, geologists have radiometrically dated the Pinto Gneiss at between 1.65 billion years old and 1.4 billion years old (Trent, 1984). This unit originally covered a very extensive area of ancestral southern California. After deposition of the sediments, granitic intrusive rocks invaded this unit, metamorphosed it, and were subsequently incorporated within it to form the gneiss we see today (Rogers, 1961; Wanrow, 1975; Trent, 1984).

Igneous Rocks

At least four separate large igneous intrusions, called "plutons," later invaded the Pinto Gneiss. The oldest intrusion occurred during the Jurassic Period, and the three youngest intrusions occurred during the Cretaceous Period. The exact dates of these regionally significant intrusions, however, is not known. From abundant radiometric dates of other California plutonic intrusions during this period, it is estimated that these intrusions occurred between 186 million years ago (Mid Jurassic) and 125 million years ago (Early Cretaceous) (Trent, 1984).

Each of these four regionally significant plutonic intrusive episodes is distinguished by a specific rock type. From oldest to youngest these rock types include: (1) Twentynine Palms monzonite, (2) Queen Mountain monzonite, (3) White Tank monzonite, and (4) Oasis monzonite. Monzonite is a granitic light-colored igneous rock that is predominately composed of potassium feldspar and plagioclase feldspar minerals. Feldspar minerals are aluminum silicates containing one or two metals and are the most common rock forming minerals on Earth. Feldspar minerals are nearly five times more common than quartz. Plagioclase feldspar minerals contain varying percentages of sodium and calcium metals.

The oldest plutonic intrusion in the Monument is represented by the Jurassic Twentynine Palms monzonite and is characterized by large crystals of potassium feldspar with lengths of up to two inches (Trent, 1984). The second oldest plutonic intrusion in the Monument is the Cretaceous Queen Mountain monzonite.

Outcrops of the younger lighter-colored Cretaceous White Tank monzonite are prevalent in the western region of the Monument. Although this unit resembles the Queen Mountain monzonite from a distance, it differs from the Queen Mountain monzonite in several ways. The White Tank monzonite is finer-grained than the Queen Mountain monzonite; it contains small amounts of biotite and/or muscovite and, unlike the Queen Mountain monzonite, it contains no hornblende (Trent, 1984).

The most conspicuous rock outcrops that visitors see as they drive through the Monument are the bold White Tank monzonite boulders (Photos 1 and 2). These conspicuous outcrops provide an arresting landscape panorama. The rounded and eroded rock outcrops in the western region of the Monument were formed by erosive agents along joint planes (subparallel fractures) in the White Tank monzonite. This granitic unit is feldspar-rich and quartz-rich igneous rock that has characteristic joint sets that intersect at nearly right angles (Rogers, 1961).

The youngest plutonic intrusive rock in the Monument is the Cretaceous Oasis monzonite. Unlike the three older plutonic rock units, the Oasis monzonite contains a characteristic assemblage of muscovite and blood-red garnet minerals that, although small, can be seen without magnification. The muscovite grains impart a glittery appearance to the rock on sunny days (Trent, 1984).

In addition to the regionally significant monzonite plutons, numerous smaller igneous intrusive rocks occur throughout the Monument. One of the most extensive of these is the Gold Park diorite. Diorite is a dark colored granitic rock that is rich in plagioclase feldspar and contains a small amount of quartz. Basalt occurs at three places within the Monument: (1) near Pinto Basin where it formed as an extrusive flow, (2) at Malapai Hill where it cooled in place and did not penetrate the surface, and (3) in the Lost Horse Mountains where it formed a volcanic dome and cooled to form distinctive columnar joints (Figure 2; Trent, 1984).

The most recent of all igneous intrusive rocks in the Monument include numerous dikes of various widths and rock compositions. These dikes are common throughout the Monument and cross-cut all of the previously mentioned rock units.

Faults and Joints

Faults are fractures in rock where discernable movement has occurred. The major faults within the Monument, such as the Eagle Mountain fault that can be seen along the Geology Tour Road, trend east-west and are associated with fault escarpments or scarps. These major fault scarps form a linear trend at the base of mountain cliffs and were produced by fault movement. Fault movement occurs as an up-and-down motion, horizontal sliding motion, or a combination of up-and-down and horizontal sliding movement. Alluvial fans and pediments, geomorphic features formed by an accumulation of loose rock material deposited onto broad valleys or plains, subsequently developed adjacent to these fault scarps.

Unlike faults, no appreciable movement occurs along rock joints. Joints are fissures in rocks and commonly occur in recognizable patterns (Figure 3). Within the Monument, joints have often formed parallel sets, and joints with two or more parallel sets, called "joint systems," also occur.

Figure 3. Schematic diagram illustrating the formation of White Tank monzonite outcrops in the Monument. Adapted from Trent (1984). (A) Front view of a cross-section through the monzonite after the overlying Pinto Gneiss had eroded away about 20 million years ago. As this overlying unit eroded over geologic time, the release in pressure from the massive weight of gneiss caused the monzonite to form extensive sets of joints. (B) Following a long period of a much wetter climate than exists here today, the monzonite underwent an extensive period of decomposition that was caused by downward percolating groundwater along the joints. (C) With continued erosion, boulder-mantled outcrops formed during the past few million years. (D) The White Tank monzonite today.

Joint patterns are easily observed in the White Tank monzonite. Three dominant joint sets produced spectacular rock forms in this unit (Photos 1 and 2). One joint set is oriented horizontally and was caused by the release of pressure when the thick layer of original overlying rocks was removed by extensive erosion (Trent, 1984). Another set of joints in the White Tank monzonite occurs vertically and roughly parallels the contact of this unit with surrounding rock units. The third joint set in the White Tank monzonite is also vertical but is approximately perpendicular to the other vertical set. This system of joint sets results in rectangular-shaped blocks that can be observed along the Geology Tour Road.