Most of Earth's mountain ranges have resulted either from the collision of plates (the Alps, the Himalayas, the Appalachians) or the development of volcanoes along subduction zones (the Andes, the Cascades). One exception is the Sierra Nevada Range which first developed as an underground tube of magma, known as a batholith.
The first pulse of this molten rock developed within the Smartsville Block, a large exotic terrain that loomed off the west coast during the Jurassic Period, 190 million years ago. This Block fused with the western edge of North America 25 million years later, forming what is now western Nevada and most of California. The second intrusion of magma occured 140 million years ago as an island arc, known as the Coastal Range Ophiolite, collided with the west coast, completing the assembly of central and northern California. The third and final pulse occurred during the Cretaceous reign of Tyranosaurus rex, some 80 million years ago.
The core of the Sierra Nevada is composed of granodiorite, indicating that the intruded magma was composed of melted rock from both oceanic and continental plates. Having cooled below the surface, the massive batholith began to rise through the Tertiary Peroid; this ascent accelerated about 4 million years ago and continues today. Once above the surface, wind, water and ice erosion began to sculpt the batholith, cutting through veins of silver and gold that formed as the magma cooled. The Pleistocene, which began 2 million years ago, brought a cold, moist climate to the region and glaciers developed along the summits of the range; as these ice sheets expanded and contracted, they carved the scenic "U shaped" valleys of the Sierra Nevada, exemplified by the splendor of Yosemite.
