Jurassic Jurassic The Jurassic (; from Jura Mountains) was a geologic period and system that spanned 56 million years from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period Mya. The Jurassic constitutes the middle period of the Mesozoic Era, also known as the Age of Reptiles. The start of the period was marked by the major Triassic–Jurassic extinction event. Two other extinction events occurred during the period: the Pliensbachian-Toarcian extinction in the Early Jurassic, and the Tithonian event at the end; however, neither event ranks among the "Big Five" mass extinctions. The
Geology of Arizona The subduction of the Farallon Plate beneath western North America remains a poorly understood event, although new seismic and computer tomography modeling is shedding light on the event. As the oceanic plate subsided under North America, it generated partial melting conditions that formed volcanoes on the surface in California and Arizona. Volcanic activity commenced in western Arizona around 205 million years ago, kicking off the Nevadan orogeny, which lasted until 145 million years ago, spanning the Triassic into the Jurassic. Granite intrusions 190 million years ago enriched many veins with minerals and generated copper porphyry gold deposits. Bisbee, Arizona, 92