Los Angeles Basin

The Los Angeles Basin is the coastal sediment-filled plain located between the peninsular and transverse ranges in southern California in the United States containing the central part of the city of Los Angeles as well as its southern and southeastern suburbs (both in Los Angeles and Orange counties). It is approximately convert|35|mi|km|0|sp=us long and convert|15|mi|km|0|sp=us wide, bounded on the north by the Santa Monica Mountains and Puente Hills, and on the east and south by the Santa Ana Mountains and San Joaquin Hills. The Palos Verdes Peninsula, formerly an island, marks the outer edge of the basin along the coast.

Geology

The sediment in the basin is up to convert|6|mi|km|0|sp=us deep. The basin began to form during the Neogene approximately 15 million years ago (mya), when the terrain was underwater, during a crustal upheaval caused by a clockwise shift in the surrounding mountains. The underlying crustal weakening resulted in the formation of the large bowl of the basin. Sediment from the sea and rivers accumulated in the undersea bowl, building up in thick layers.

Petroleum

The accumulation of micro-organisms during this time is believed to be the source of the large deposits of oil, including the large Wilmington Oil Field, that were once under the basin but have been largely extracted. Approximately 5 million years ago, the crustal stretching subsided and the ocean floor of the basin was forced to the surface. Additional sediment accumulated during the upswell resulting in the floor of the basin as it exists today.

Other large active oil fields include the Huntington Beach Oil Field, which underlies much of the city of Huntington Beach; and the Torrance Oil Field, adjacent to the Wilmington field on the northwest. Most of the numerous fields in the basin have either been abandoned or greatly scaled back in production since the early part of the 20th century; in the 1890s the oil field directly north of downtown Los Angeles, the Los Angeles City Oil Field, led the state of California in oil production. [ [ftp://ftp.consrv.ca.gov/pub/oil/history/History_of_Calif.pdf History of Oil Production in California: California Department of Conservation, Department of Oil and Gas] ]

Earthquakes

The sedimentary character of the basin is the principal reason why it is considered especially susceptible to excessive damage during earthquakes. The basin is often compared by geologists to a "a bowl of jelly" that can shake violently when driven by seismic activity.

ubsidence

Its loose rock structure has also led to numerous instances of subsidence as a result of petroleum extraction, the most spectacular examples being the Baldwin Hills dam collapse of 1963 and the sinking of the bed of Long Beach Harbor by several meters.

Air Quality

The basin consistently ranks as the most polluted in the nation, topping the American Lung Association lists for both ozone and particulate pollution. In 2007 Los Angeles ranked 4th and 3rd for long-term particle, and ozone pollution respectively.

Gallery

ee also

* Oxnard Plain
* San Joaquin Valley

References

External links

* [http://tectonics.harvard.edu/sgat/SGATlabasinI.html Harvard Univ.: 3D Model of the Los Angeles Basin]
* [http://www.losangelesalmanac.com/topics/Geography/ge08e.htm Los Angeles Almanac: Los Angeles Basin]