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 span of time before the current Phanerozoic Eon, and is divided into several eons of the geologic time scale. It spans from the formation of Earth around 4500 Ma (million years ago) to the beginning of the Cambrian Period, when macroscopic hard-shelled animals first appeared in abundance about 542 Ma rock, primarily granite Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granites usually have a medium to coarse grained texture. Occasionally some individual crystals (phenocrysts) are larger than the groundmass in which case the texture is known as porphyritic. A granitic rock with a porphyritic texture is sometimes known as a porphyry, in Central Texas in the United States ^ b. English is the de facto language of American government and the sole language spoken at home by 80% of Americans age five and older. Spanish is the second most commonly spoken language. 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, west of the Texas Hill Country The Texas Hill Country is a vernacular term applied to a region of Central Texas, that features tall rugged hills that consist of thin layers of soil lying ontop of either limestone or granite. It also includes the Llano Uplift and the second largest granite monadnock in the United States, Enchanted Rock, which is located 18 miles north 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. Granites usually have a medium to coarse grained texture. Occasionally some individual crystals (phenocrysts) are larger than the groundmass in which case the texture is known as porphyritic. A granitic rock with a porphyritic texture is sometimes known as a porphyry (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 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 . Like most other sedimentary rocks, limestones are composed of grains; however, most grains in limestone are skeletal fragments of marine organisms such as coral or foraminifera. Other carbonate grains comprising limestones are ooids, peloids, intraclasts, and extraclasts 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 rock formation located in the Llano Uplift approximately 15 miles (24 km) 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,. The area includes several major quarries that mine the distinctive pink granite. Furthermore, the area contains the only known deposits of Llanite Llanite is a hypabyssally intruded rhyolite with phenocrysts of microcline and euhedral quartz. The quartz crystals are blue hexagonal bipyramids. The unusual blue coloration of the quartz is thought to be due to ilmenite inclusions . It is named after Llano County, Texas, the only place where it is found. A dike of llanite outcrops on Texas State.
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 The Grenville Orogeny was a long-lived Mesoproterozoic mountain-building event associated with the assembly of the supercontinent Rodinia. Its record is a prominent orogenic belt which spans a significant portion of the North American continent, from Newfoundland to Mexico, as well as to Scotland. Orogenic crust of mid-late Mesoproterozoic age is.[1] 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. Mineralogical, chemical and crystallographic changes can occur during this process sedimentary rocks Sedimentary rock is a type of rock that is formed by sedimentation of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution. Particles that form a sedimentary rock by accumulating, 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.[2]
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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