The Orogeny and Rock Types of the San Gabriel Mountains in the Angeles National Forest: A Geologic Overview

The San Gabriel Mountains are a mountain range located in Southern California, within the Angeles National Forest. The range extends approximately 56 miles from east to west and is composed primarily of igneous and metamorphic rocks. The San Gabriel Mountains are part of the Transverse Ranges, a series of mountain ranges that run perpendicular to the San Andreas Fault. The geology of the San Gabriel Mountains is complex, with a rich history of geologic processes, including orogeny, that have shaped the landscape over millions of years.

Orogeny of the San Gabriel Mountains

The San Gabriel Mountains were formed as a result of the Transverse Ranges Orogeny, a period of mountain building that occurred during the Late Cretaceous and Early Tertiary periods, approximately 70 to 30 million years ago. The orogeny was caused by the subduction of the Farallon Plate beneath the North American Plate. The subduction created a volcanic island arc that collided with the North American Plate, causing the uplift and deformation of the Transverse Ranges, including the San Gabriel Mountains.

During the orogeny, the rocks of the San Gabriel Mountains were subjected to intense pressure and heat, resulting in the formation of metamorphic rocks such as schist, gneiss, and quartzite. The pressure and heat also caused the rocks to fold and fault, creating the complex geologic structures visible in the mountains today.

Rock Types of the San Gabriel Mountains

The San Gabriel Mountains are primarily composed of igneous and metamorphic rocks. The metamorphic rocks in the San Gabriel Mountains were formed during orogeny and include schist, gneiss, and quartzite. Schist is a metamorphic rock characterized by its foliation, or layering, which is a result of the intense pressure and heat the rock was subjected to during the orogeny. Gneiss is another metamorphic rock characterized by its banded appearance, also a result of intense pressure and heat. Quartzite is a metamorphic rock formed from sandstone that has been subjected to pressure and heat, causing the quartz grains to recrystallize.

The igneous rocks of the San Gabriel Mountains were formed from magma that intruded into the metamorphic rocks during orogeny. The most common igneous rock in the San Gabriel Mountains is granite, a coarse-grained rock composed of quartz, feldspar and mica. The granite in the San Gabriel Mountains is part of a larger batholith known as the Sierra Madre Batholith, which extends about 160 miles along the southern California coast.

The significance of the San Gabriel Mountains

The San Gabriel Mountains are an important part of Southern California’s geology. The mountains provide a natural barrier between the Los Angeles Basin and the Mojave Desert, and are a source of water for surrounding communities. The San Gabriel Mountains are also a popular destination for outdoor recreation, including hiking, camping, and skiing.

In addition to their recreational and ecological importance, the San Gabriel Mountains are also important from a scientific perspective. The complex geologic structures and diverse rock types provide a window into the geologic history of Southern California and are an important area of study for geologists and earth scientists.

Conclusion

The San Gabriel Mountains in the Angeles National Forest are a complex and fascinating example of the geologic processes that have shaped the Earth over millions of years. The mountains were formed during the Transverse Ranges Orogeny and are composed primarily of igneous and metamorphic rocks. The geology of the San Gabriel Mountains provides important insights into the geologic history of Southern California and is a valuable area of study for earth scientists.

FAQs

What is the geologic history of the San Gabriel Mountains?

The San Gabriel Mountains were formed during the Transverse Ranges Orogeny, a period of mountain-building that occurred during the Late Cretaceous and Early Tertiary periods, approximately 70 to 30 million years ago. The orogeny was caused by the subduction of the Farallon Plate beneath the North American Plate, resulting in the formation of a volcanic island arc that collided with the North American Plate, causing the uplift and deformation of the Transverse Ranges, including the San Gabriel Mountains.

What types of rocks are found in the San Gabriel Mountains?

The San Gabriel Mountains are primarily composed of igneous and metamorphic rocks. The metamorphic rocks in the San Gabriel Mountains were formed during the orogeny and include schist, gneiss, and quartzite. The igneous rocks in the San Gabriel Mountains were formed from magma that intruded into the metamorphic rocks during the orogeny and include granite, diorite, and gabbro.

How were the metamorphic rocks in the San Gabriel Mountains formed?

The metamorphic rocks in the San Gabriel Mountains were formed as a result of the intense pressure and heat the rocks were subjected to during the orogeny. Schist is characterized by its foliation, or layering, which is a result of the intense pressure and heat. Gneiss is characterized by its banded appearance, which is also a result of the intense pressure and heat. Quartzite is formed from sandstone that has been subjected to pressure and heat, resulting in the recrystallization of the quartz grains.

What is the most common igneous rock found in the San Gabriel Mountains?

The most common igneous rock in the San Gabriel Mountains is granite, which is a coarse-grained rock that is composed of quartz, feldspar, and mica. The granite in the San Gabriel Mountains is part of a larger batholith, known as the Sierra Madre Batholith, which extends for about 160 miles along the southern California coast.

What is the significance of the San Gabriel Mountains?

The San Gabriel Mountains are an important part of the geology of Southern California. The mountains provide a natural barrier between the Los Angeles Basin and the Mojave Desert, and are a source of water for the surrounding communities. The San Gabriel Mountains are also a popular destination for outdoor recreation, including hiking, camping, and skiing. In addition to their recreational and ecological importance, the San Gabriel Mountains are also important from a scientific perspective, providing insights into the geologic history of Southern California.

What is the Sierra Madre Batholith and how is it related to the San Gabriel Mountains?

The Sierra Madre Batholith is a large body of igneous rock that extends for about 160 miles along the southern California coast. The granite in the San Gabriel Mountains is part of the Sierra Madre Batholith, and was formed from magma that intruded into the metamorphic rocks during the Transverse Ranges Orogeny. The Sierra Madre Batholith is an important feature of the geology of Southern California, and is related to the formation of the San Gabriel Mountains and other mountain ranges in the region.