GEOLOGY OF THE SAN JACINTO MOUNTAINS

Together the San Jacinto and Santa Rosa Mountains form the Peninsular Ranges Province, extending about 900 miles from San Jacinto Peak down to the tip of Baja California. The geologic history of the Peninsular Ranges is a complex story. To understand how the mountains formed, we need to step way back in time. Imagine the Earth’s crust is like an eggshell—cracked. The pieces, called tectonic plates, float over the Earth’s fluid surface, joining and separating, sometimes combining and creating a super continent. The floor of the Pacific Ocean is also composed of a number of crustal plates. New floor is added as molten basalt flows out of the ocean floor cracks. As it cools and solidifies it becomes part of the plates, which are continuously moving. This movement causes gigantic continental collisions, creating mountain ranges such as the Himalayas or earthquake fault zones like the San Andreas. When two plates grind past one another, subduction occurs—the heavier plates dive beneath the edges of the lighter continental masses. Subduction zones occur where slabs of ocean crust slide under continental edges. The subduction zone in southern California occurs where the oceanic Pacific Plate slides under the edge of the continental North American Plate. During this process rocky debris was scrapped off the top of the slab and collected along the subduction zone. The Coastal Ranges of California consist in part of this debris. The formation of volcanoes is one of the most important results of subduction activity. The friction created between the subducting slab and the continent is hot enough to melt rock. Masses of molten rock rise to the Earth’s surface, escaping as lava, building volcanic cones. The melted rock remains under the surface as magma. As it rises, it cools very slowly forming solid granitic rocks called plutons. The core of the Peninsular Ranges is composed of a series of plutions; collectively they form what is known as a batholith.

The rocks we see around us in the park were created in such a process, beneath the surface of the Earth. They began as molten magma below the Earth’s crust, or are transformed through heat and pressure. Once formed, they are pushed through the surface during mountain-building periods occurring over millions of years.

As magma moved toward the surface it cooled, forming a batholith that increases in size downward and doesn’t have a definite floor. It has a surface exposure of more than 40 square miles. Slow cooling of the magma permitted large grains to form. This is the core of the San Jacinto Mountains we know today. What we now call the Southern California Batholith remained underground for millions of years. The range was first exposed about 95 million years ago; the overlying rock eroded during the following years. This was followed by earth-building activity that took place along several parallel faults (running northwest to southeast) including the San Andreas and the San Jacinto Faults. This earthquake activity lifted the eastern face of the batholith more dramatically than the western face, creating a sharp escarpment on the northern and eastern side of the range.

BATHOLITHS

A batholith is a unique geologic formation. It is formed deep under the Earth’s crust as viscous magma solidifies in a process taking millions of years. The formation is a large discordant pluton that increases in size as it extends downward with no determinate floor. The word comes from Greek, bathos meaning “deep” and lithos, meaning “stone” (or rock). Batholiths are usually composed of granite (light colored igneous rock) or granodiorite (a relatively homogeneous combination of granite and another igneous rock). Generally, batholiths are located in mountain ranges; their size is directly related to intensity of folding and crumpling of the rock that has occurred. They usually run parallel to axis of the mountain range, have irregular dome-shaped roofs, and contain a great deal of rock.

The Southern California Batholith is the heart of the rock mass of the San Jacinto Mountains, and is classified as Mesozoic granite. This light-colored igneous rock is easily observed throughout the mountains at the mid to higher elevations. Rock that originally covered the batholith while it was still a reservoir of magma has been stripped away by erosion over millions of years. Also, as the batholith rose, it pushed through overlying sedimentary layers. The second most abundant rock of the San Jacinto Mountains is metamorphic. Intense heat and pressure exerted on the layers caused changes to the sedimentary rock. Unlike igneous rock, metamorphic rocks have never been molten. Metamorphic rock can be seen along the eastern edges of the San Jacintos, adjacent to Palm Springs (up into Chino Canyon)

GEOLOGY GLOSSARY

Subduction
the heavier crustal plates dive beneath the edges of the lighter continental masses.
Batholiths
a rock mass made up of plutons. The rock bodies formed under the Earth’s crust; cooled very slowly. Slow cooling permitted large mineral grains to form; not surprising that batholiths are composed mainly of granitic rocks with large crystals. Smaller bodies of coarse-grained igneous rocks are called stocks.
Pluton
a body of igneous rock that is formed beneath the surface of the earth by consolidation of magma. Sometimes extended to include bodies formed beneath the surface of the earth by metasomatic replacement of older rock. All igneous rock masses that were formed when magma solidified within the Earth’s crust are called plutons.
Tabular plutons are those whose thickness is small in relation to its other dimensions.
Sill is a tabular concordant pluton; range in size from sheets less than an inch thick to masses hundreds of feet thick; an intrusion of younger rock into already existing rock; example, Palisades along Hudson River, NY (magmas intruded into flat-lying sediments); rock is fine-grained
Dike is a tabular discordant pluton; width ranges from a few inches to many feet. They are generally longer than wide, from a few feet to tens of feet thick. Dikes originate when magma forces its way through the fractures of adjacent rocks. Most dikes occupy cracks and have straight, parallel walls. Because they intrusive rock is generally resistant, dikes often form ridges when exposed by erosion; rock is coarse-grained
A discordant pluton increases in size downward, has no determinable floor, and shows an area of surface exposure exceeding 40 square miles; boundaries cut across layering.
A concordant pluton has definite layering with boundaries parallel to layering.
Inclusion
hot magma causes blocks of the overlying, enclosing rock to be broken off and fall through the melt. Most batholiths contain many inclusions.
Magma
any naturally occurring silicate melt, whether or not it contains suspended crystals or dissolved gases.
Pegmatite
small pluton of exceptionally coarse texture; crystals can be up to 40 ft in length; commonly formed at the margin of a batholith; 90% of pegmatites consist of quartz, orthoclase and small amounts of mica. Commonly occur as dikes; often contain large, beautiful minerals, such as beryl, tourmaline, topaz; some rare minerals
Stoping
a mechanism by which batholiths have moved into the crust by the breaking off and foundering of blocks of rock surrounding the magma chamber.