How were the Sierra Nevada's mountains formed?

Answer 1

The Sierra Nevada Mountains formed by a thinning of the North American Plate which then cracked as a result of upward pressure. Huge blocks of rock containing huge chunks of granite welled up at this crack and tilted up along it. The upward tilting edges became the Sierra Nevada ridge line less than 4mya. Erosion exposed granite at places such as Yosemite's Half Dome.

A 2004 study of the Southern Sierras concluded that a massive amount of eclogite (basaltic) rock broke off the bottom of the continental crust and dropped into the molten mantle about 3.5mya. This may account for the thinning and weakening of the North American Plate responsible for the rise of the Sierra Nevadas; although many geologists lay blame on undefined "tectonic forces" that were responsible for the Basin and Range topology of Nevada.

Originally, basaltic rock formed under the North American Plate when the Farollan and Juan de Fuca Plates collided with it and were subducted beneath it. Portions of this rock melted and intruded into cracks in the country rock of the North American Plate at 200mya and 140mya. Other portions stuck to the bottom of the Plate. The intrusive melt was transformed by heat and pressure into granite, which eventually became the core of the Sierra Nevadas.