Somewhere Out There - JT Geology and Geomorphology

Geology and Geomorphology at JT

The great climbing at Joshua Tree is the result of processes that began 100 million years ago, and continue today.

Geology

The story begins at the Mesozoic Era. While dinosaurs were in their heyday, huge masses of molten rock (called plutons, after Pluto, Roman god of the underworld) rose like gigantic hot-air balloons far below much of what is now California and Baja California. These plutons intruded into older "country" rock. An excellent example can be seen at Saddle Rocks (home of Walk on the Wild Side), where Saddle Rock (a frozen pluton) is surrounded by far older dark gneiss.

The plutons cooled while still miles underground, insulated by the mass of surrounding rock. Differences in melt composition and cooling rates yield variations in the resultant rock types. While the general name for these rocks is "granitic", in the Joshua Tree area the rocks are specifically termed "Quartz Monzonite" or "Monzogranite". All this means is that if you look closely enough, you'll find approximately equal amounts of orthoclase and plagioclase feldspars in a JT rock.

Cooling rates varied enough to make dramatic differences in the rock texture. Some of the nastiest rock in the park is found at Jumbo Rocks, where relatively slow cooling allowed big, flesh-ripping crystals to form. Formations like Echo Rock cooled more quickly, yielding smaller crystals and smoother rock.

As the plutons cooled, they were cloven by big fractures called joints. The rock near the edges of the masses cooled, and shrank, more quickly than rock in the middles of the plutons. Huge, vertically-oriented joints resulted. Often these joints were injected with molten, quartz-rich rock. As this rock cooled it created the andesite, pegmatite, and aplite dikes that are the key to many routes (Tiptoe, 39 Steps, and Pops Goes Hawaian come to mind).

Over the eons, much of California was uplifted, accelerating the erosion of the country rock. Removal of this overburden created additional sets of joints in the plutons, parallel to the earth's surface. The igneous masses were now thoroughly jointed, cut into blocky shapes of various sizes.

Geomorphology

Concurrent with the surface erosion of country rock, groundwater percolated through the monzonite formations, travelling easily through the joint systems. Exposure to this groundwater caused feldspars to decompose into clay, while flushing away some of the more resistant minerals. In time, what were originally squarish blocks of monzonite were transformed into rounded and even spherical boulders.

The landscape continues to evolve. Weathering and erosion are the predominant factors today. The process of exfoliation, aggravated by daily and seasonal temperature variations, causes huge scabs of rock to peel off the faces of domes. Rainfall collects in pockets, intensifying small-scale weathering and leading to the formation of potholes and huecos. Flash floods and wind transport grains away from the formations. Even climbers contribute on a small scale, accidentally breaking off handholds, and more commonly, dislodging loose grains.

The hundred million year history of the Joshua Tree rocks may be of little concern when you're run out at the limits of your ability, but may help to put things in perspective. Long after we're gone, the monzonite domes, towers, and walls of JT will continue to stand, slowly succumbing to erosion.

This document was last updated on Monday, September 11, 2000