A chain of five islands- Anacapa, Santa Cruz, Santa Rosa, San Miguel, and Santa Barbara Island- form the Channel Islands National Park.  The park consists of these five islands and their offshore waters for a distance of approximately 1 nautical mile. The four islands together lie in a chain which forms the southern boundary of the passage known as the Santa Barbara Channel. On the California mainland, north of San Miguel Island,  is Point Conception, which marks important cultural, biological, and geological boundaries.  To the east of the islands lie Santa Monica and San Pedro Bays.

GEOLOGIC HISTORY

    The channel islands and the Santa Barbara Basin are the western extensions of the Transverse Ranges of south-central California.  The Transverse Ranges are thought to be relatively modern and developed no earlier than the middle Miocene, about 12 million years ago. Transverse ranges can be explained as strike- slip movement on the San Andreas and east- west transverse range faults. The geologic process which creates the distinctive features of the transverse range is compression.  The Pacific and North American plates in this region are converging because of the Big Bend in the San Andreas fault.  As a result, the entire region is being squeezed together, with the compression oriented in a north south direction. As the crust is compressed, it is being squeezed and thickened. This process is shown at the surface, where some areas are being uplifted to form mountains, while others are being pushed down to form basins. (Harden, 1997))
    During Miocene time, volcanic activity occurred in the Transverse Range. Sills, dikes, and flows of andesite and basalt were extruded.  The Santa Monica Mountains were the site of numerous volcanoes, which extended north to form the Channel Islands.  Pillow basalt, andesite, breccia, and tuff are abundant on Anacapa, Santa Cruz, Santa Rosa, and San Miguel Islands. Volcanic rocks total 2,440 meters (8,000 feet) on Santa Cruz Island and 730 meters (2,400 feet) on San Miguel Island.  Lesser amounts occur on the other islands.
    Also during the Miocene, the Santa Monica and Channel Islands area was the site of a deep marine trough where 15,000 feet of sedimentary rocks were deposited. The basin is thought to have deepened during the Miocene because earlier rocks are coarse shallow marine conglomerates and breccias, and later rocks are fine grained deep water shales.  These Miocene beds, which were rich in marine diatom remains and organically rich muds, are the most likely source for much of the natural gas and petroleum products found in the Transverse Range between the Santa Monica Mountains and the Channel Islands.  (Norris and Webb, 1990)

ACTIVE FAULTING AND FOLDING IN THE SANTA BARBARA CHANNEL
 

Map from Norris and Webb, 1990.
 

    The Santa Barbara Basin and Channel Islands marks the southern part of an active fold and thrust belt (see the map) which developed during contraction in the Pliocene through Quaternary period. Present day measurements indicate that the regional contraction and compression are directed north- south which is subnormal to the San Andreas Fault.  Studies of Pliocene and Quaternary folding by Jackson and Yeats in 1998, suggest that per year several millimeters of northeast-southwest shortening is occurring across this fold-and-thrust belt in the eastern Santa Barbara Channel which lies above a subducting mantle lithospheric slab. Historical events, such as the 1978 Santa Barbara Earthquake with magnitude 5.1, hint that a component of shortening is happening seismically.  Deformed sea floor sediments along the fold trends further support this prediction.  Therefor a study was conducted in 1994 by Shaw and Suppe to assess the seismic hazards posed to southern California and nearby regions due to these underlying thrusts. They attempted to recognize and determine slip rates and active faults in this area.
    In their study, Shaw and Suppe identified several active blind- thrust faults in the Santa Barbara basin, which lack surface breaks but they have deformed Quaternary sediments. The Offshore Oak Ridge and Blue Bottle trends (see map above) in the east Santa Barbara Channel are two trends which indicate the presence of underlying active thrusts and pose a significant seismic hazard. The Offshore Oak Ridge trend was found to be generated by slip on an oblique, left lateral thrust that ramps upward from a depth of 16 km and uplifts the Santa Cruz and Anacapa Islands.  Correlation between individual fault surface area and earthquake magnitude suggest that rupture of this ramp may generate a 7.2 magnitude earthquake in the future.
    Active faults are also present at  depths of 2-5 km along the Blue Bottle trend.  The Blue Bottle faults ramp downward in the northern channel and run parallel to the Santa Barbara coastline, where they also pose a significant earthquake threat.
    It is estimated that these earthquakes, if periodic, should occur approximately every 1,500 years to accommodate long term fault slip. Therefor these faults should continue to pose an earthquake risk to the city of Santa Barbara and other populated regions for many years( Shaw and Suppe, 1994)
 

Press and Siecver, 1998.

THE PYGMY MAMMOTH

    It is believed that during the Miocene the sea had invaded the land and covered most of the Transverse Range. Then in the late Pliocene, during the last ice age, the sea level became much lower and large areas of the sea bed dried up.  During this time the Channel Islands were part of one vast island called the Santarosae which was five miles from the mainland and approximately 200,000 acres long.  It is believed that during this time large mainland mammoths, attracted by the smell of the Santarosae island grasses, swam across the Santa Barbara Channel to the large island.  Then as the sea levels began to rise again, about 12,000 years ago, the population was isolated from the mainland by rising sea levels. The dwarf version of the mammoths then began to evolve because of limited food supplies.  These dwarf species of the Channel Islands were only about 2 to 3 meters high at the shoulder, in comparison with the mainland species, Parelephas columbi, which stood about 4 meters at the shoulder. (Information found at http://www.nps.gov/chis/pygmy.htm
 
 

A nearly complete pygmy mammoth skeleton, missing only the right tusk, one foot, and a minor portion of the skull, was found on Santa Rosa Island in 1994. Radiocarbon dating proved that it lived 13,000 years ago. This 57 years old male specimen stood five and one half feet tall and weighed about 1 ton at the time of death. He had been walking along the northern coast of the island when he is thought to have laid down on his left side, and died, nearly 13,000 years ago.  The pygmy mammoth was then quickly covered by a sand dune while his skin was still intact, which accounts for the great preservation of his bones.     Pygmy mammoth populations have been found on islands in the Mediterranean and southeast Asia, but this was the first discovery of a complete skeleton. It will help scientists gain further understanding of this isolated species and the speciation which occurs in an island environment.

 
 
 

GLOSSARY

Andesite-A Volcanic rock type intermediate in composition between rhyolite and basalt. An extrusive intermediate rock. Basalt- A fine grained, dark, mafic igneous rock composed largely of plagioclase feldspar and pyroxine. Breccia- A pyroclastic rock in which all fragments are more than 2 mm in diameter. Big Bend(San Anreas Fault)- A bend in transform boundary where the Pacific Plate Slides past the North American Plate. Blind-Thrust Faults- Faults that are not visible on the surface and are not otherwise known until and earthquake occurs. Dikes- A tabular igneous intrusion that cuts across structures of surrounding rock.

Diatom- A small, multicellular, almost microscopic marine animal. Fold and Thrust Belt- An area where folding and faulting is occurring. In this case it is occurring below the surface of the earth and deforming quaternary sediments. Lithosphere-  The outermost, rigid part of the earth trhat constitutes plate techtonics. Thickness varies from 0 km at mid-ocean ridges to 70 km in aceanic lithosphere and 140 km in continents.  Oblique-Slip Fault- A fault that occurs from a combination of shear and compression or tension. Shales-  A very fine grained clastic rock composed of silt and clay that tends to part along bedding planes. Sills- A horizontal, tabular igneous intrusion running between parallel layers of bedded country rock. Transverse Ranges- An east-west mountain belt in California which is oriented diagonal to the prevailing structural trends in California. It includes California's highest peaks south of the central Sierra Nevada, and the four channel islands.

References:

Harden, D.R., 1997. California Geology, Prentice Hall, Inc., Upper Saddle River, New Jersey, USA

Norris, R.M, Webb, R.M., 1990. Geology of California, John Wiley and Sons, Inc., New York, New York, USA

Press, F., and Siever, R., 1998.  Understanding Earth, W.H. Freeman and Company, New York, New York, USA

Shaw, J.H., and Suppe, J., 1994. Active faulting and growth folding in the eastern Santa Barbara Channel, California: Geological Society of America Bulletin, vol.106, p. 607-626