the east, forming the undulating Nevadaplano. As the Farallon Plate subducted, the Sevier Orogeny generated large mountain ranges in the east. Cenozoic (66 million years ago-present) In the Mesozoic, the subducting Farallon Plate had produced magma and played a role in building the Sierra Nevada mountains, but by 60 million years ago in the Cenozoic, its downward angle decreased and it moved further eastward without producing magma. The plate produced shear stress at the base of the North American Plate, driving the Laramide orogeny, which created the Rocky Mountains. Due to conditions in the underlying crust, inferred to be a thinner section of the Farallon, intense volcanic activity began in the Eocene in northern Nevada around 43 million years ago, reaching the center of the state by the Oligocene and the south by the Miocene. The volcanism was some of the most intense in Earth history, ejecting 17,000 cubic miles of material in 250 major eruptions and layering the landscape in tuff ash falls thousands of feet thick. Extinct calderas up to 35 miles wide are preserved in the mountains of south-central Nevada, particularly near the Tonopah range, as well as in the Hot Creek, Monitor, Reveille and Kawich ranges. Welded ignimbrite formed as hot ash formed natural glass on rock surfaces. Violent volcanism continued in the southwest until seven million years ago, forming the ancestral Cascade arc, named for its similarity to the Cascade Range in the Pacific Northwest. Additionally, the Walker Lane—an area of northwest trending right-lateral strike-slip faults formed 12 million years ago—associated with some of the most intense eruptions, such as the 16 to 6.5 million year old Southwest Nevada volcanic field. During the last 10 million years, volcanic activity shifted to bimodal volcanism with basalt lava flows alternating with rhyolite domes. Dark gray to black basalt and pink or brown rhyolite are particularly common in Elko, Washoe and Humboldt counties. Some small cinder cones formed as recently as the Pleistocene and the Nye County Lunar Crater volcanic field was active only 15,000 years ago. In Nevada's recent geologic past, tectonic changes have created normal faults and creating the basin and range horst and graben terrain. Thinning of the upper crust caused deeper, highly metamorphosed rock masses to rise to the surface, where it is overlain by younger faulted and domed rocks. There are more than 24 metamorphic core complexes in the Basin and Range Province