Geology of Catalina Island
Santa Catalina Island, often called Catalina Island, or just Catalina, is a rocky island off the coast of the U.S. state of California. The island is 22 miles (35 km) long and 8 miles (13 km) across at its greatest width. The island is located about 22 miles (35 km) south-southwest of Los Angeles. The highest point on the island is 2,097 feet (639 m) Mt. Orizaba. One of the Channel Islands of California archipelago, Catalina lies within Los Angeles County. Most of the island is administered by the Catalina Island Conservancy. The total population as of the 2000 census was 3,696 persons, almost 85 percent of whom live in the island's only incorporated city, Avalon. The second center of population is the unincorporated village of Two Harbors, in the north, with a population of 298. The island has an overall population density of 49.29/mi² (19.03/km²).
The island is very rich in quartz, to the point that some beaches on the seaward side have silvery-grey sand. Catalina is primarily composed of two distinct rock units, Catalina Schist and volcanic and intrusive igneous rocks. In the simplest terms, Catalina consists of a base of metamorphic rock (schist and amphibolite) originally laid down in the Mesozoic era (250 – 65 mya). This is overlain with igneous rock of the volcanic origin from the Miocene era (23 – 5 mya) and this is mostly covered with sedementary rock from more recent times (Tertiary period). Geologic maps of Catalina show that the Northwestern end of the Island has predominant deposits of metamorphic Blueschist (approximately 100 million years old) while the South eastern portion contains predominantly quartz deposits (approx. 19 million years old).
Plate Tectonics
Catalina Island was formed by the subduction of the ancient Farallon tectonic plate below the continental North American Plate. Very little of this ancient plate exists today. Catalina Island is a rare example of Farralon rock which when subducted under the North American plate, came under intense pressure and volcanic heat. Volcanic activity expelled the metamorphic shist to form the basal layer of rock on the island. This subduction resulted in the uplifting of the North American and Pacific plates and the rising of Catalina Island from the ocean some 2 million years ago. Today the continental North American Plate meets the oceanic Pacific Plate at the infamous San Andreas Fault in central California.
The Pacific Plate is an oceanic tectonic plate (yellow above) that lies beneath the Pacific Ocean. At 103 million square kilometres, it is the largest tectonic plate. Of great importance to Californians is the collision of the Pacific Plate with the North American Plate (brown above) at the San Andreas Fault. The evolution of the San Andreas dates back to the mid Cenozoic, to about 30 Ma. At this time, a spreading center between the Pacific Plate and the Farallon Plate was beginning to interact with the subduction zone off the western coast of North America. The relative motion between the Pacific and North American Plates was different from the relative motion between the Farallon and North American Plates, so when the spreading ridge was 'subducted', a new relative motion caused a new style of deformation. This style is chiefly the San Andreas Fault.
All land west of the fault on the Pacific Plate is moving slowly to the northwest while all land east of the fault is moving southwest (relatively southeast as measured at the fault) under the influence of plate tectonics. The rate of slippage averages approximately 33 to 37 millimetres (1.3 to 1.5 in) annually across California. This raises the intriguing possibility of Catalina Island and western Los Angeles passing San Fransisco in approximately 20 million years time.
Studies of the relative motions of the Pacific and North American plates have shown that only about 75 percent of the motion can be accounted for in the movements of the San Andreas and its various branch faults. The rest of the motion has been found in an area east of the Sierra Nevada mountains called the Walker Lane or Eastern California Shear Zone. The reason for this is not as yet clear, although several hypotheses have been offered and research is ongoing. One hypothesis which gained some currency following the Landers Earthquake in 1992 is that the plate boundary may be shifting eastward, away from the San Andreas to the Walker Lane. Interestingly the Pacific Plate contains an interior hot spot forming the Hawaiian Islands.
Ferries to Avalon from Los Angeles leave from Long Beach and Dana Point and run everyday of the week – see the companies websites for up to date schedules and pricing.
Logging Requirements.
Email me the answers to the following questions, please do not post them in your log. Photo's posted with your log, are optional.
1. Using your compass, look at the geological feature at a bearing of 330 degrees magnetic from GZ. What is the rock at this site primarily composed of ?
2. Assuming that the architectural feature immediately to your north is 100 ft tall, estimate the height of the geological feature.
3. Which tectonic plate are you currently standing on
4. How does the rock around Avalon in the south differ from Two Harbors in the North of the Island.
References
1. Stephen M. Rowland “Geology of Santa Catalina Island” California Geology Nov 1984.