Oil became a major California industry in the 20th century with the discovery of new fields around Los Angeles and the San Joaquin Valley, and the dramatic explosion in demand for gasoline to fuel the rapidly growing number of automobiles and trucks now being produced. Most of the oil production in California began in the late 19th century. At the turn of the century, oil production in California continued to rise at a booming rate. In 1900, the state of California produced 4 million barrels. In 1903, California became the leading oil-producing state in the US, and traded the number one position back and forth with Oklahoma through 1930. Production at the various oil fields increased to about 34 million barrels per year by 1904. By 1910 production had reached 78 million barrels. California drilling operations and oil production are concentrated primarily in Kern County, the San Joaquin Valley, and the Los Angeles Basin.