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 as a whole. In some cases, faulted blocks have shifted more than 50 miles from the apex of the dome. Along detachment surfaces, mylonite forms due to shear. The Ruby Mountains-East Humboldt, close to Elko, and the Snake Range, close to Utah, are the two most researched core complexes.