A view of the Santa Monica Mountains looking northeast from Point Dume in Malibu, CA
Geology of the Santa Monica Mountain
Range
A view of the Santa Monica Mountains looking northeast from Point
Dume in Malibu, CA
The Santa Monica Mountains are a well-defined
topographical and geological unit of the Transverse Mountain Ranges of Southern
California. As suggested by the name, these ranges lie perpendicular to the
Coast, Sierra Nevada, and Peninsula Ranges. The Santa Monica Mountains are
a part of the only east-west belt of mountains in California and one of only
two mountain ranges in North America so oriented.
The 46-mile long chain of peaks and ridges of the Santa Monica Mountains extend from Griffith Park to Point Mugu. The range is ten miles wide at its broadest part and reaches an elevation of 3,111 feet at Sandstone Peak at the eastern end of Boney Ridge; which is located near the western end of the range. The entire physiographic unit of the Santa Monica Mountains covers about 225,000 acres.
The geology of the Santa Monica Mountains is as complicated as the forces that created them. The Santa Monica Range is essentially a broad anticline that has been severely ruptured by faulting and intruded by sills and dikes of various materials. There are numerous faults; some of them have earthquake history and earthquake potential. The rocks found in the Santa Monica Mountains are from 12 to nearly 200 million years of age.
The mountains contain a variety of scenic features exemplified by the distinctive sites of Boney Ridge, Saddle Rock, Saddle Peak, Ladyface Mountain, and Malibu Canyon. These features are complemented by the small, deeply incised canyons, upland meadows, streams, glassy ponds, crashing rapids, numerous waterfalls, and rolling hills covered with an ever changing array of flowers, grasses and chaparral. The rugged Santa Monica range is completely bisected in only one place by the steady flow of water that flows through Malibu Canyon. Malibu Creek has cut gorges hundreds of feet deep on it's way to the Pacific. Malibu Creek, which originates in the Simi Hills, is thought to have flowed in its present course before the mountains existed, but was able to erode the growing mountains fast enough to maintain its position through the ages.
Long, wide beaches stretch along most of the coast, occasionally giving way to outcroppings of rocks into the ocean and high solid points such as Point Dume and Mugu Rock. In some places along the coastline high bluffs back narrow, excluded pocket beaches. The relentless action of the waves and tides have taken their toll and eroded these mountains, creating an abrupt shoreline and steep cliffs.
The scenic beauty of the Santa Monica Mountains is largely the result of the region's unique geologic history. The Santa Monica Mountains are young, but their structure is very complex. They are the result of tectonic interactions of the North American Plate and Pacific Plate, which induced periods of folding, faulting, and volcanism. They have been uplifted and then eroded many times over a period extending back to nearly 200 million years ago.
Most of the rock comprising this mountain range is Cretaceous to Miocene-age sedimentary and volcanic rock. Cretaceous deposits are of the third and last period of the Mesozoic era, which was between 66.4 million and 144 million years ago, and was characterized by the development of flowering plants and the disappearance of the dinosaurs. Miocene sedimentary deposits are of the fourth epoch of the Tertiary period, which was between 5 million and 24 million years ago, and was characterized by the appearance of primitive apes, whales and grazing animals.
Some of the oldest rocks can be observed where they have been exposed by erosion in Griffith Park. The dark Santa Monica slates can be seen in Topanga State Park from Temescal Canyon to Franklin Canyon, and in the roadcuts along the San Diego Freeway (I-405). These slates were originally formed when mud was deposited on the floor of a shallow ocean and through the process of lithication became shale. This took place during the Cretaceous period, about 140 million years ago. About 120 million years ago molten rock intruded inducing the metamorphism of the shale into slate and then after cooling, into granite. After much uplifting and erosion, oceans again covered the land and deposited the materials that today are the rocks that form the cliffs in lower Topanga Canyon.
About 60 million years ago during the Paleocene epoch of the Tertiary period, there was yet another cycle of erosion and a returning of the ocean. This cycle resulted in the deposition of material, which is called the Martinez Formation. The Martinez Formation can also be seen in Topanga Canyon. The Sespe Formation, which is easily recognized by its predominant red-purple color, followed the erosion of the Martinez Formation and can be seen in the sea cliffs along Pacific Coast Highway and in Malibu Canyon.
About 14 million years ago during the Miocene epoch of the Tertiary period, the ocean again covered the central part of the Santa Monica Mountains and deposited sediment which formed thick-bedded, coarse grained, buff-colored sandstones. During this same time there was extensive volcanic activity, which established a north-south trend; and has since been rotated 90 degrees and compressed into the present east-west trend of the Transverse Ranges.
Brown or reddish volcanic rock can be seen in many places in the western part of the mountain range. Modelo shale, which forms a mantle over the older rock layers on the north flank, is the youngest formation in the Santa Monica Mountains. It is light-colored, bedded and commonly contains fossils of marine animals and algae.
About 3 million to 10 thousand years ago, during the late Tertiary period, the Santa Monica Mountains were arched and thrust upward as a result of tectonic forces acting upon the area. This process built up the present day Santa Monica Mountain Range, which now extends offshore with their oldest visible peaks forming the Northern Channel Islands. This uplifting process of the Santa Monica Mountains is still ongoing.
The process of mountain uplifting has been accompanied with formation of fault planes, the movement along these faults, and the results of these movements, earthquakes. The main fault of the Santa Monica Mountains is the Malibu Coast Fault. The Malibu Coast Fault is a reverse fault 34 kilometers long, with a northern segment endpoint at (-118.93º W; 34.05º N), a southern segment endpoint at (-118.53º W; 34.03º N). It dips 75º to a depth of 13 kilometers, and strikes east. The last surface rupture of the Malibu Coast Fault occurred sometime during the last 10,000 years, the Holocene epoch. It is calculated to be capable of producing an earthquake with a moment magnitude of 6.0 to 7.1.
As you can now imagine, the geology of the Santa Monica Mountains and the processes that have and continue to build them are very complex. The Santa Monica Mountain Range is a result of the interactions between two plates, the Pacific Plate and the North American Plate. About 250 million years ago during the Permian period, these two plates which had been on a collision course for some time converged upon each other. The Pacific Plate's crust is oceanic and composed of basalt, which is denser than continental crust. Because of this fact, the Pacific Plate subducted under the North American Plate. As the subducting edge of the Pacific Plate moved deeper towards the earth's mantle, it began to heat and melt the plate into magma. Between 150 and 140 million years ago during the late Jurassic and early Cretaceous periods, this magma started to move up to the earth's surface, forming the Klamath and Peninsular ranges.
About 30 million years ago during the Oligocene epoch of the Tertiary period, the relative movements of the North American Plate and the Pacific Plate changed directions. The two plates began sliding against each other, the Pacific Plate moving to the north, and the North American Plate moving to the south; a strike slip plate boundary. This area where the two plates slip past each other is called the San Andreas Fault. Along the San Andreas Fault, the folding of the sea floor along the margin of the North American Plate resulted in the creation of the Coast and Transverse mountain ranges.
This is a simplified and general description based on the theory of plate tectonics. This theory describes how tectonic processes have and continue to form the Santa Monica Mountains, and most other geological features of the earth. To fully understand the geology of the Santa Monica Mountains, one must have some knowledge from many of the fields in geological sciences. To learn more about these fields, follow the links bellow.
Pictures of the Santa Monica Mountains:
Images of the Backbone Trail, Santa Monica
Mountains
Images of the Santa Monica Mountains during and
after a fire.
Images of flooding at the base of the Santa Monica
Mountains.
Images of landslides in the Santa Monica
Mountains.
Aerial Images of the Santa Monica
Mountains.
Plate Tectonics:
This Dynamic
Earth: The Story of Plate Tectonics: Find out more about
how tectonic plates shape our planet.
Volcanology:
Volcano
World: Learn all about volcanoes here.
Cascades Volcano
Observatory: Everything you have ever wanted to know about
the Cascade Volcanoes can be found here, including facts, figures, photos,
research, volcanic history, definitions and descriptions
Seismology:
SeismoCam:
A joint program between the USGS and CalTech that shows real time data of
earthquakes. See earthquakes as they happen or replay past events.
A great place to learn more about earthquakes.
Southern California Earthquake
Center: A research program dedicated to minimizing the earthquake
hazard in Southern California.
Other Links:
United States Geological
Survey: Everything you have ever wanted to know about geology
can be found somewhere within this web site; and if you can't find what your
looking for, click here to
Ask-A-Geologist at the
USGS.
Division of Mines
and Geology: Another good site with lots of information on
geologic topics.
Geological Survey of
Canada: Find out what the Canadians are doing in the field
of geology. Many interesting research projects and information on the
geology of Canada.
American Geological
Institute: Links to 31 geoscience societies. There is
a lot of information which can be found here.
Online
Resources for Earth Scientists: General Geology: Read geological
journals and periodicals and get other geological information here.
Santa Monica Mountains National
Recreation Area Home Page: Some general information
on the Santa Monica Mountains National Recreation Area. Find out where
you can camp, hike, research opportunities, etc. here.
Santa Monica Mountains
Recreation Area Weather Report: Before you head out
to the Santa Monica Mountains, check the weather report and forecast
here.
Santa Monica Mountains
Conservancy: Find out what the SMMC does, the programs
they offer, and some information about the parklands they operate.
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Information on the geology of the Santa Monica Mountains was compiled
from various reports and books found in the archives of the
Mountains Conservancy
Foundation
5775 Ramirez Canyon Road
Malibu, California 90265
(310) 589-2400
Page designed and written by
Joshua E.
Pace
upacej01@mcl.ucsb.edu
June 2,1997