Rock Types

Metamorphic Rocks

The oldest rocks of the Joshua Tree region are those of the Precambrian Pinto Gneiss which range in composition from quartz monzonite to quartz diorite. Much of the rock is dark-gray and prominently foliated, but some is much lighter, sometimes nearly white and only faintly foliated. The gneiss is composed primarily of quartz, feldspar, and biotite. Where there is a significant amount of biotite the Pinto Gneiss is very dark and this distinguishes it from the other rocks in the area (Photo 1).

Photo 1. Pinto Gneiss on the trail to the Lost Horse Mine.

The Pinto Gneiss, which is exposed in much of the high desert of Joshua Tree National Monument, formed from the metamorphism of pre-existing sedimentary and igneous rocks. Radiometric dates give two ages of metamorphism, 1650 million years and 1400 million years (Powell, 1982, p. 120-121).

The Pinto Gneiss served as the host rock into which the younger plutonic rocks intruded. Examples of the intrusive contact can be seen along the trail to Fortynine Palms Oasis and northeast of Sheep Pass (Photo 2).

Photo 2. The contact between the White Tank monzogranite and the Pinto Gneiss on the east side of Lost Horse Valley.

Igneous Rocks

At least four different major plutons have intruded the Pinto Gneiss (Figure 2). The oldest are Jurassic, and the youngest are Cretaceous. The exact dates of these Mesozoic intrusions, however, are poorly known. Isotopic ages of Jurassic plutons in California fall generally between 186 and 155 million years, and the ages of the Cretaceous plutons are mainly between 155 and 125 million years (Bateman, 1981). The Mesozoic plutons in Joshua Tree National Monument, in common with the granitic plutons of the Sierra Nevada, the Peninsular Ranges, the Klamath Mountains, and the White-Inyo Mountains, are generally believed to have originated in an Andean-type tectonic environment (Ernst, 1981).

Figure 2. Geology of Joshua Tree National Monument.

The oldest pluton is the Twentynine Palms porphyritic quartz monzonite (Brand and Anderson, 1982). It consists of a matrix of small mineral grains which enclose large spectacular phenocrysts of potassium feldspar crystals that attain lengths of up to two inches. This pluton belongs to the important Jurassic plutonic belt of the western United States (Powell, 1982), which is significant because it may signal the onset of Andean-type tectonics on the continental margin (Miller, 1977). The rock crops out at the beginning of the trail to Fortynine Palms Oasis and is exposed along the arroyo on the east side of the Indian Cove campground.

The earliest of the Cretaceous plutons is the Queen Mountain monzogranite, which is exposed over a large area around Queen Mountain (Figure 2). It is coarse-grained, and consists of plagioclase, potassium feldspar, quartz, and either biotite or biotite and hornblende (Brand and Anderson, 1982).

The light-colored, Cretaceous White Tank monzogranite predominates in the more accessible parts of the Monument. The rock was originally recognized as a monzonite by Miller (1938), later as a quartz monzonite by Rogers (1954, 1961) and Dibblee (1968), but is now named a monzogranite (Brand and Anderson, 1982; Powell, 1982) in accordance with the modified version of Streckeisen's classification (1973) of igneous rocks. It resembles the Queen Mountain monzogranite but differs by being finer-grained, and by containing very small amounts of biotite and/or muscovite but no hornblende (Brand, personal communication). Areas underlain by the White Tank monzogranite include Indian Cove, the Wonderland of Rocks, Jumbo Rocks, White Tank, and Lost Horse Valley (Photo 3).

Photo 3. Rock sheeting in the White Tank monzogranite forming a dome-like landform at Barker Dam in the Wonderland of Rocks. Photo by Walter R. Stephens.

The youngest of the Cretaceous plutons, the Oasis monzogranite, is a distinctive garnet-muscovite-bearing pluton that is exposed in the area of Fortynine Palms Oasis (Brand and Anderson, 1982). The garnets are blood red and small, but large enough, nevertheless, to be visible without magnification. The muscovite grains impart a glittery appearance to the rock on sunny days, even in shadows.

In addition to large monzogranite and quartz monzonite plutons, there are masses of a similar granitic rock called granodiorite, and small dark plutons called the Gold Park diorite (Rogers, 1954, 1961; Brand and Anderson, 1982). Cutting across all of these rock masses, and thus being younger in age, are dikes of felsite, aplite, pegmatite, andesite, and diorite (Photo 4).

Photo 4. Aplite dike cutting White Tank monzogranite on the Barker Dam trail. The horizontal shelf-like development of the cliff is from thick slabs of monzogranite blocked out by rock sheeting.

Even younger than these dikes are veins of milky quartz which over the years have been prospected for gold. The quartz is sometimes stained reddish brown from the weathering of pyrite, or fools gold. Pyrite is a common mineral in quartz veins and is sometimes associated with gold or other valuable minerals. Chemical alteration of the pyrite produces reddish iron oxides that stain the rock and serve the prospector as a clue that gold, silver, copper, and other metals may be present.

The pegmatite dikes are mainly quartz and potassium feldspar and have a composition close to that of granite. What makes them distinctive is the very large size attained by the mineral grains, often three or four inches long. Pegmatites may have large books of biotite and muscovite mica (isinglass).

Basalt occurs at three places in the Monument: (1) near Pinto Basin, where the basalt probably originated as extrusive flows, (2) at Malapai Hill on the Geology Tour Road (Photos 5 and 6), and (3) in the Lost Horse Mountains (Photos 7 and 8). In addition to basalt, another rock named lherzolite occurs as inclusions within the basalt at Malapai Hill and in the Lost Horse Mountains. Lherzolite is about 75 percent olivine with the remainder being mostly bronzite and diopside. It is considered to be derived from the mantle; thus the basalt has risen some 35 to 50 miles to carry the lherzolite inclusions to the surface.

Photo 5. Malapai Hill, an eroded endogenous volcanic dome, rises above the pediment of southern Queen Valley. The Hexie Mountains, behind Malapai Hill, reveal the contact between the Pinto Gneiss (dark rocks) and the White Tank monzogranite.

Photo 6. Columnar jointing in the basalt at Malapai Hill.

Photo 7. Columnar jointing in the endogenous volcanic dome in the Lost Horse Mountains. Columns are 10 to 12 inches in diameter. The white encrusting material has formed by 'wicking' of water from the joints and evaporation leaving a thin coating of calcium carbonate. Photo by Walter R. Stephens.

Photo 8. Detail of the sides of the basalt columns in the Lost Horse Mountains. The step-like pattern marks the incremental progress of the crack during the cooling of the basalt.

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