Granite domes are domical hills composed of granite with bare rock exposed over most of the surface. Generally, domical features such as these are known as bornhardts. Bornhardts can form in any type of plutonic rock but are typically composed of granite and granitic gneiss. As granitic plutons cool kilometers below the Earth's surface, minerals in the rock crystallize under uniform confining pressure. Erosion brings the rock closer to Earth's surface and the pressure from above the rock decreases; as a result the rock fractures. These fractures are known as exfoliation joints, or sheet fractures, and form in onionlike patterns that are parallel to the land surface. These sheets of rock peel off the exposed surface and in certain conditions develop domical structures. Additional theories on the origin of granite domes involve scarp-retreat and tectonic uplift.
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Granite dome in the context of Batholith
A batholith (from Ancient Greek βαθύς (bathús), meaning "deep", and λίθος (líthos), meaning "stone") is a large mass of intrusive igneous rock (also called plutonic rock), larger than 100 km (40 sq mi) in area, that forms from cooled magma deep in the Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock types, such as granite, quartz monzonite, or diorite (see also granite dome).
Granite dome in the context of Texas Hill Country
The Texas Hill Country is a geographic region of Central and South Texas, forming the southeast part of the Edwards Plateau. Given its location, climate, terrain, and vegetation, the Hill Country can be considered one of the borders between the American South and Southwest. The region represents the very remote rural countryside of Central Texas, but also is home to growing suburban neighborhoods and affluent retirement developments.
The region is notable for its karst topography and tall rugged hills of limestone or granite. Many of the hills rise to a height of 400–500 ft (120–150 m) above the surrounding plains and valleys, with Packsaddle Mountain rising to a height of 800 ft (240 m) above the Llano River in Kingsland. The Hill Country also includes the Llano Uplift and the second-largest granite dome in the United States, Enchanted Rock. The terrain throughout the region is characterized by a thin layer of topsoil and many exposed rocks and boulders, making the region very dry and prone to flash flooding. Native vegetation in the region includes various yucca, prickly pear cactus, desert spoon, and wildflowers in the Llano Uplift. The predominant trees in the region are Ashe juniper and Texas live oak.
Granite dome in the context of Sierra Nevada Batholith
The Sierra Nevada Batholith is a large batholith that is approximately 400 miles long and 60-80 miles wide which forms the core of the Sierra Nevada mountain range in California, exposed at the surface as granite.
The batholith is composed of many individual masses of rock called plutons, which formed deep underground during separate episodes of magma intrusion, millions of years before the Sierra itself first began to rise. The extremely hot, relatively buoyant plutons, also called plutonic diapirs, intruded through denser, native country rock and sediments, never reaching the surface. At the same time, some magma managed to reach the surface as volcanic lava flows, but most of it cooled and hardened below the surface and remained buried for millions of years.