page_title: Los Angeles metropolitan area
section_title: Geology
hierarchical_section_title: Los Angeles metropolitan area / Geology
caption_attribution_description: English: Amboy Crater — in the Mojave Desert of SE California.
context_page_description: The Los Angeles metropolitan area, also known as Metropolitan Los Angeles, Metro LA or the Southland, is the 18th largest metropolitan area in the world and the second-largest metropolitan area in the United States. It is the 3rd largest city by nominal GDP in the world with a $1 trillion+ economy. It is entirely within Southern California.
  The metropolitan area is defined by the U.S. Office of Management and Budget as the Los Angeles-Long Beach-Anaheim, CA Metropolitan Statistical Area, consisting of Los Angeles and Orange counties, a metropolitan statistical area used for statistical purposes by the United States Census Bureau and other agencies. Its land area is 4,850 sq. mi and its estimated 2016 population was 13,310,447.
  Los Angeles and Orange counties are the first- and third-most populous counties in California respectively, and Los Angeles, with 9,819,000 people in 2010, is the most populous county in the United States. The Los Angeles metropolitan area is the most populous metropolitan area in the western United States and the largest in area in the United States.
context_section_description: Much of the west coast of North America was part of a large convergent plate boundary between the Farallon Plate and North American Plate from 165 to 55 million years ago. Here, the Farallon Plate subducted under the North American Plate creating volcanoes about 100 miles east of this boundary which can still be seen as the Sierra Nevada which it has its southern border about 30 miles east of Grapevine, California in the Tejon Pass. The Farallon Plate was subjected to high temperatures and pressures as it subducted under the North American Plate. This led to the formation of molten plutons which rose because they were less dense than the surrounding magma. When they rose, they cooled and some formed enormous granite monoliths. Only less than 1% of these plutons ever made it to the surface out of a volcano or fissure vent. The 1% that did make it all the way to the surface erupted in andesitic lava which would pile on top of each previous flow. This would create steep volcanoes with extremely high elevations. Most of the ejecta that came out of a volcano is gas. About 60% is just carbon (C) and water vapor (H2O). About 30% is sulfur (S). The sulfur mixes with the water vapor to form sulfuric acid which is notorious for eating away at almost anything from plants to rocks.
  For the 99% percent of plutons that didn't make it to the surface they cooled to form granite monoliths under these volcanoes. When subduction activities ceased about 55 million years ago, these volcanoes were subject to erosion due to their steep slopes. Because granite is classified as a hard igneous rock, it is the only remnant of the volcanic chain from this subduction zone. These enormous granite monoliths can still be seen in Yosemite National Park as Half Dome and El Capitan about 300 miles from Los Angeles. Please refer to the Geologic History of Yosemite page to learn more specifically of the area's local geology.
  When the subduction zone ceased about 55 million years ago, the Farallon Plate was split into two distinctive plates which can still be observed today as the Juan de Fuca Plate and the Cocos Plate. Both were part of the same plate, but were discovered independently before this connection was made. At the time of this break off, the Pacific Plate had a general north west movement while the North American Plate had a general south east movement. This created a new fault zone when a weak point gave way between these two plates. This is the beginnings of the infamous San Andreas Fault. The San Andreas Fault is a right-lateral strike strip transverse fault. When this fault was just created, a volcano from the ancient subduction zone was situated about 3/4th on the Pacific Plate and 1/4th on the North American plate somewhere in what is today Central California. Nearly 55 million years later, this volcano was offset by about 250 miles. It is the largest known offset of the San Andreas Fault and it help geologist determine important information such as average slip movement and the age of the fault. The northern part of this offset is now called Pinnacles National Park near Soledad, California. The other half of Pinnacles is in Three Points, California in Los Angeles County.
  The Pacific Plate is the largest known plate on Earth. It is considered an oceanic plate because it is much more dense than a continental plate. That is the reason why oceanic plates always subduct under another plate. There are only a few places where the Pacific Plate is actually above the ocean. Most of the coastline of the state, below Eureka, California is part of the Pacific plate. The thickest part on land in California can be observed as far inland as the Salton Sea. To the south of the Salon Sea, there is a gap between plate boundaries. This gap acts like a divergent plate boundary where the land is being pulled apart. Mud volcanoes can be observed just at the southern edge of the sea as well as hot springs. Geothermal energy plants are abundant in the area, which power much of the local rural communities.
  When the San Andreas Fault originally formed, tension on t
caption: Amboy Crater in San Bernardino County, California along Route 66
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