The Llano Uplift is a roughly circular geologic dome In structural geology, a dome is a deformational feature consisting of symmetrically-dipping anticlines; their general outline on a geologic map is circular or oval. The strata in a dome are upwarped in the center; if the top of a dome is eroded off, the result will be a series of concentric strata that grow progressively older from the outside-in, of Precambrian The Precambrian is an informal name for the supereon comprising the eons of the geologic timescale that came before the current Phanerozoic eon. It spans from the formation of Earth around 4500 Ma (million years ago) to the evolution of abundant macroscopic hard-shelled animals, which marked the beginning of the Cambrian, the first period of the rock, primarily granite Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry. Granites can be pink to dark gray or even black, depending on their chemistry and mineralogy. Outcrops of granite tend to, in Central Texas Texas ( /ˈtɛksəs/ ) is a state in the South Central United States, nicknamed the Lone Star State. It is bordered by Mexico to the south, New Mexico to the west, Oklahoma to the north, Arkansas to the northeast, and Louisiana to the east. Texas is the second largest U.S. state in both area and population, with an area of 268,820 square miles (696 in the United States The United States of America is a federal constitutional republic comprising fifty states and a federal district. The country is situated mostly in central North America, where its forty-eight contiguous states and Washington, D.C., the capital district, lie between the Pacific and Atlantic Oceans, bordered by Canada to the north and Mexico to the. It is located in the eastern region of the Edwards Plateau The Edwards Plateau is a region of west-central Texas which is bounded by the Balcones Fault to the south and east, the Llano Uplift and the Llano Estacado to the north, and the Pecos River and Chihuahuan Desert to the west. San Angelo, Austin, San Antonio and Del Rio roughly outline the area. Earliest human settlement of this are was by Native, west of the Texas Hill Country The Texas Hill Country is a region of Central Texas, USA, that features rolling, somewhat rugged, hills that consist primarily of limestone but includes the Llano Uplift and the second largest granite outcropping in the United States, Enchanted Rock, which is located 18 miles north of Fredericksburg. The Hill Country also reaches into portions of. The region is characterized by a central exposure of Precambrian granite Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry. Granites can be pink to dark gray or even black, depending on their chemistry and mineralogy. Outcrops of granite tend to (Town Mountain Granite), surrounded by a ring of gneiss Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks. Gneissic rocks are usually medium to coarse foliated and largely recrystallized but do not carry large quantities of micas, chlorite or other platy and schist The schists form a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced. By definition, schist contains more than 50% platy (Valley Spring Gneiss and Packsaddle Schist respectively, also Precambrian), which is in turn surrounded by uplifted lower Paleozoic The Paleozoic or Palaeozoic Era (from the Greek palaios , "old" and zoe (ζωή), "life", meaning "ancient life") is the earliest of three geologic eras of the Phanerozoic eon. The Paleozoic spanned from roughly 542 to 251 million years ago (ICS, 2004), and is subdivided into six geologic periods; from oldest to strata. This area is rimmed by limestone Limestone is a sedimentary rock composed largely of the mineral calcite . The deposition of limestone strata is often a by-product and indicator of biological activity in the geologic record. Calcium (along with nitrogen, phosphorus, and potassium) is a key mineral to plant nutrition: soils overlying limestone bedrock tend to be pre-fertilized ridges.
The Llano Uplift is best known for its large, protruding rock formations such as Enchanted Rock Enchanted Rock is an enormous pink granite pluton located in the Llano Uplift approximately 15 miles north of Fredericksburg, Texas, USA and 15 miles (24 km) south of Llano, Texas. Enchanted Rock State Natural Area, which includes Enchanted Rock and surrounding land, spans the border between Gillespie County and Llano County, south of the Llano. The area includes several major quarries that mine the distinctive pink granite. Furthermore, the area contains the only known deposits of Llanite.
Geology
The area is termed an uplift due to the raised status of the Precambrian rocks in comparison to adjacent, completely buried Precambrian strata, as well as Paleozoic rocks that are buried elsewhere in central Texas. However, the exposures of Precambrian rock are generally located at the lowest surface elevations of the region. This is best pictured by imagining the igneous roots of an ancient mountain range buried entirely beneath newer sedimentary strata, followed by the erosion of the newer strata to the extent that some of the "highest" igneous rocks are exposed.
The origin of the Town Mountain Granite--tightly-dated to late Mesoproterozoic The Mesoproterozoic Era is a geologic era that occurred between 1600 Ma and 1000 Ma times--is a minor mystery, stemming from a debate over whether or not it is related to the Grenville orogeny.[1], [2] What is not in doubt in that the Valley Spring and Packsaddle formations are heavily metamorphosed Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Both mineralogical, chemical and crystallographic changes can occur during this process sedimentary rocks Sedimentary rock is one of the three main rock types . Sedimentary rock is formed by deposition and consolidation of mineral and organic material and from precipitation of minerals from solution. The processes that form sedimentary rock occur at the surface of the Earth and within bodies of water. Rock formed from sediments covers 75-80% of the, originally deposited south of the North American craton Laurentia , like all craton land, was created as continents moved about the surface of the Earth, bumping into other continents and drifting away, and altered by subduction In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. A subduction zone is an area on Earth where two tectonic plates move towards one another and subduction occurs. Rates of subduction are typically or continental A continent is one of several large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents – they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia collision.[3], [4]
The ancient faulting that produced the Llano Uplift is believed to be related to that which produced the Marathon Uplift Marathon is a census-designated place in Brewster County, Texas, United States. The population was 455 at the 2000 census in west Texas. Both may have been formed around the time of the Ouachita orogeny.
See also
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Categories: Geologic provinces of Texas | Landforms of Texas
adam
ue, 16 Jan 2007 22:50:46 GM
i also feel so blessed due to the fact that i am in . llano. county which of course, in my opinion, has more of nature s unique highlights, beauty and history than any other hill country county. we are on the . llano uplift. which basically ...
