Gneiss is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures and pressures than schist. Gneiss nearly always shows a banded texture characterized by alternating darker and lighter colored bands and without a distinct cleavage.
Schist is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes or plates. This texture reflects a high content of platy minerals, such as micas, talc, chlorite, or graphite. These are often interleaved with more granular minerals, such as feldspar or quartz.
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than 150 to 200 °C and, often, elevated pressure of 100 megapascals (1,000 bar) or more, causing profound physical or chemical changes. During this process, the rock remains mostly in the solid state, but gradually recrystallizes to a new texture or mineral composition. The protolith may be an igneous, sedimentary, or existing metamorphic rock.
Migmatite is a composite rock found in medium and high-grade metamorphic environments, commonly within Precambrian cratonic blocks. It consists of two or more constituents often layered repetitively: one layer is an older metamorphic rock that was reconstituted subsequently by partial melting ("neosome"), while the alternate layer has a pegmatitic, aplitic, granitic or generally plutonic appearance ("paleosome"). Commonly, migmatites occur below deformed metamorphic rocks that represent the base of eroded mountain chains.
Amphibolite is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (flaky) structure. The small flakes of black and white in the rock often give it a salt-and-pepper appearance.
Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. These properties are caused by fine grained non-aligned crystals with platy or prismatic habits, characteristic of metamorphism at high temperature but without accompanying deformation. The term is derived from the German word Hornfels, meaning "hornstone", because of its exceptional toughness and texture both reminiscent of animal horns. These rocks were referred to by miners in northern England as whetstones.
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 Labrador to Mexico, as well as to Scotland.
The Spruce Pine Mining District is a swath of the valley of the North Toe River in the Blue Ridge Mountains of northwestern North Carolina. The area is mined for its mica, kaolin, quartz and feldspar. Spruce Pine district is one of the largest suppliers of high-purity quartz, which is used in the manufacture of silicon for integrated circuits. The district is named after the town of Spruce Pine, which is located in the middle of the region and is the hub of major mining activity there. The district is approximately 25 miles long and 5 miles wide.
The Narryer Gneiss Terrane is a geological complex in Western Australia that is composed of a tectonically interleaved and polydeformed mixture of granite, mafic intrusions and metasedimentary rocks in excess of 3.3 billion years old, with the majority of the Narryer Gneiss Terrane in excess of 3.6 billion years old. The rocks have experienced multiple metamorphic events at amphibolite or granulite conditions, resulting in often complete destruction of original igneous or sedimentary (protolith) textures. Importantly, it contains the oldest known samples of the Earth's crust: samples of zircon from the Jack Hills portion of the Narryer Gneiss have been radiometrically dated at 4.4 billion years old, although the majority of zircon crystals are about 3.6-3.8 billion years old.
The Gascoyne Complex is a terrane of Proterozoic granite and metamorphic rock in the central-western part of Western Australia. The complex outcrops at the exposed western end of the Capricorn Orogen, a 1,000 km-long arcuate belt of folded, faulted and metamorphosed rocks between two Archean cratons; the Pilbara craton to the north and the Yilgarn craton to the south. The Gascoyne Complex is thought to record the collision of these two different Archean continental fragments during the Capricorn Orogeny at 1830–1780 Ma.
The Musgrave Block is an east-west trending belt of Proterozoic granulite-gneiss basement rocks approximately 500 kilometres (310 mi) long. The Musgrave Block extends from western South Australia into Western Australia.
The Lizard complex, Cornwall is generally accepted to represent a preserved example of an exposed ophiolite complex in the United Kingdom. The rocks found in The Lizard area are analogous to those found in such famous areas as the Troodos Mountains, Cyprus and the Semail Ophiolite, Oman.
A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures. The assemblage is typical of what is formed in conditions corresponding to an area on the two dimensional graph of temperature vs. pressure. Rocks which contain certain minerals can therefore be linked to certain tectonic settings, times and places in the geological history of the area. The boundaries between facies are wide because they are gradational and approximate. The area on the graph corresponding to rock formation at the lowest values of temperature and pressure is the range of formation of sedimentary rocks, as opposed to metamorphic rocks, in a process called diagenesis.
The Vishnu Basement Rocks is the name recommended for all Early Proterozoic crystalline rocks exposed in the Grand Canyon region. They form the crystalline basement rocks that underlie the Bass Limestone of the Unkar Group of the Grand Canyon Supergroup and the Tapeats Sandstone of the Tonto Group. These basement rocks have also been called either the Vishnu Complex or Vishnu Metamorphic Complex. These Early Proterozoic crystalline rocks consist of metamorphic rocks that are collectively known as the Granite Gorge Metamorphic Suite; sections of the Vishnu Basement Rocks contain Early Paleoproterozoic granite, granitic pegmatite, aplite, and granodiorite that have intruded these metamorphic rocks, and also, intrusive Early Paleoproterozoic ultramafic rocks.
The geology of Sierra Leone is primarily very ancient Precambrian Archean and Proterozoic crystalline igneous and metamorphic basement rock, in many cases more than 2.5 billion years old. Throughout Earth history, Sierra Leone was impacted by major tectonic and climatic events, such as the Leonean, Liberian and Pan-African orogeny mountain building events, the Neoproterozoic Snowball Earth and millions of years of weathering, which has produced thick layers of regolith across much of the country's surface.
The geology of Uganda extends back to the Archean and Proterozoic eons of the Precambrian, and much of the country is underlain by gneiss, argillite and other metamorphic rocks that are sometimes over 2.5 billion years old. Sedimentary rocks and new igneous and metamorphic units formed throughout the Proterozoic and the region was partially affected by the Pan-African orogeny and Snowball Earth events. Through the Mesozoic and Cenozoic, ancient basement rock has weathered into water-bearing saprolite and the region has experienced periods of volcanism and rift valley formation. The East Africa Rift gives rise to thick, more geologically recent sediment sequences and the country's numerous lakes. Uganda has extensive natural resources, particularly gold.
The geology of Belarus began to form more than 2.5 billion years ago in the Precambrian, although many overlying sedimentary units deposited during the Paleozoic and the current Quaternary. Belarus is located in the eastern European plain. From east to west it covers about 650 kilometers while from north to south it covers about 560 kilometers, and the total area is about 207,600 square kilometers. It borders Poland in the north, Lithuania in the northwest, Latvia and Russia in the north, and Ukraine in the south. Belarus has a planar topography with a height of about 160 m above sea level. The highest elevation at 346 meters above sea level is Mt. Dzerzhinskaya, and the lowest point at the height of 80 m is in the Neman River valley.
The geology of Ecuador includes ancient Precambrian basement rock and a complex tectonic assembly of new sections of crust from formerly separate landmasses, often uplifted as the Andes or transformed into basins.
The Ammonoosuc Volcanics is a rock unit in parts of New Hampshire and Vermont in the United States. This unit is named for the Ammonoosuc River that runs through the portion of New Hampshire that houses the Ammonoosuc Volcanics.
Gore Mountain Garnet, found in the Adirondack Mountains in New York, contains the world's largest garnets. The rock that holds these garnets, garnet amphibolite, is sometimes referred to as 'black ore' or 'dark ore.' This rock formation formed during metamorphism during the Ottawan phase of the Grenvillian orogeny, and extremely high temperatures combined with introduction of fluids is what most likely contributed to the unusual size of the megacrystic garnets.
