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40-455: Rock glaciers occur in parts of the Dry Andes, but are lacking in the more southern Wet Andes . In the Dry Andes, ordinary glaciers develop usually at higher altitudes than rock glaciers. Around Aconcagua rock glaciers reach altitudes as a low as 900 m a.s.l. The Principal Cordillera near Santiago may have been subject to significant glaciation as early as 1 million years ago, as indicated by

80-594: A joint ice stream network. Its dendritic glacier arms, i.e. components of valley glaciers, were up to 112.5 kilometres (69.9 mi) long, over 1,250 metres (4,100 ft) thick and spanned a vertical distance of 5,150 metres (16,900 ft). The climatic glacier snowline (ELA) was lowered from the current 4,600 metres (15,100 ft) to 3,200 metres (10,500 ft) during glacial times. Rock glacier Rock glaciers are distinctive geomorphological landforms , consisting either of angular rock debris frozen in interstitial ice, former "true" glaciers overlain by

120-401: A large difference in size between the tiny matrix crystals and the much larger phenocrysts. Porphyries may be aphanites or phanerites , that is, the groundmass may have microscopic crystals as in basalt , or crystals easily distinguishable with the eye, as in granite . Most igneous rocks have some degree of porphyritic texture. This is because most magma from which igneous rock solidifies

160-600: A layer of talus, or something in-between. Rock glaciers are normally found at high latitudes and/or elevations, and may extend outward and downslope from talus cones, glaciers or terminal moraines of glaciers. There are two types of rock glaciers: periglacial glaciers (or talus-derived glaciers), and glacial rock glaciers, such as the Timpanogos Glacier in Utah , which are often found where glaciers once existed. Possible Martian rock glacier features have been identified by

200-420: A single day, apparently due to climate change . Porphyry (geology) Porphyry ( / ˈ p ɔːr f ə r i / POR -fə-ree ) is any of various granites or igneous rocks with coarse-grained crystals such as feldspar or quartz dispersed in a fine-grained silicate -rich, generally aphanitic matrix or groundmass . In its non-geologic, traditional use, the term porphyry usually refers to

240-731: A talus lobe. Periglacial rock glaciers can form from the alternation of rock debris incoming with autumn firn or avalanche snow. Nearby cliffs are in many cases a requirement for the formation of rock glaciers, and as such many rock glaciers form in valleys steepened by glacier erosion . Rock masses of rock glaciers have been found to make up different rock types depending on the local geology. These rock types include andesite , basalt , granite , porphyry , quartzite , and sandstone . Ordinary glaciers can override rock glaciers, acquiring some of its material and properties. Likewise, rock glaciers can originate from debris-rich remnants of glaciers. Rock glaciers move downslope by deformation of

280-420: A very slow rate, in part dependent on the amount of ice present. According to recent studies, rock glaciers positively influence the streams around them. Subject to climate variation, rock glaciers in proximity tend to have a highly synchronous movement pattern over a short time scale; over long term, however, the relationship between rock glacier velocity and climate difference may not be as pronounced, due to

320-488: Is produced by partial melting of a mixture of different minerals. At first the mixed melt slowly cools deep in the crust. The magma begins crystallizing, the highest melting point minerals closest to the overall composition first, in a process called fractional crystallization . This forms phenocrysts , which usually have plenty of room for growth, and form large, well-shaped crystals with characteristic crystal faces ( euhedral crystals). If they are different in density to

360-509: The De Ceremoniis (mid-10th century), who specified them to be respectively of Constantine the Great , Constantius II , Julian , Jovian , Theodosius I , Arcadius , Aelia Eudoxia , Theodosius II , and Marcian . Of these, most still exist in complete or fragmentary form, despite depredations by later Byzantine Emperors, Crusaders , and Ottoman conquerors . Four presently adorn the facade of

400-623: The Basilica of San Lorenzo , in Florence, Italy, for the de' Medici family . Purple porphyry was used lavishly throughout the opulent chapel as well, with a revetment of marbles, inlaid with other colored marbles and semi-precious stone, that covers the walls completely. Envisioned by Cosimo I, Grand Duke of Tuscany (1537–1574), it was initiated by Ferdinand I de' Medici , following a design by Matteo Nigetti that won an informal competition held in 1602 by Don Giovanni de' Medici (a son of Cosimo I), which

440-481: The Mars Orbiter spacecraft. A rock glacier, especially if its origin is unclear, can be considered as a discrete debris accumulation . The two known factors that must be present in order to create rock glaciers are low ice velocity and permafrost . Most glacial rock glaciers are created by the recession of debris covered glaciers. Glacial rock glaciers are often found in cirque basins where rocky debris falls off

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480-646: The Monastery of Santes Creus near Tarragona , reuses a porphyry tub or alveus , which has been conjectured to be originally the sarcophagus of Late Roman Emperor Constans in his mausoleum at Centcelles , a nearby site with a well-preserved 4th-century rotunda . In twelfth- and thirteenth-century Sicily , another group of porphyry sarcophagi were produced from the reign of Roger II onward and used for Royal and then Imperial burials, namely those of King Roger II , King William I , Emperor Henry VI , Empress Constance , and Emperor Frederick II . They are all now in

520-576: The Palermo Cathedral , except William's in Monreale Cathedral . Scholar Rosa Bacile argues that they were carved by a local workshop from porphyry imported from Rome , the latter four plausibly (based on observation of their fluting ) all from a single column shaft that may have been taken from the Baths of Caracalla or the Baths of Diocletian . She notes that these Sicilian porphyry sarcophagi "are

560-889: The QAPF diagram . Rhomb porphyry is found in continental rift areas, including the East African Rift (including Mount Kilimanjaro ), Mount Erebus near the Ross Sea in Antarctica , the Oslo graben in Norway , and south-central British Columbia . To the Romans it was known as Lapis porphyrites . Pliny the Elder 's Natural History (36, 11) affirmed that the "Imperial Porphyry" had been discovered in Egypt during

600-622: The "Porphyra", the official delivery room for use of pregnant Empresses in the Great Palace of Constantinople , giving rise to the phrase "born in the purple". Choosing porphyry as a material was a bold and specific statement for late Imperial Rome. As if it were not enough that porphyry was explicitly for imperial use, the stone's rarity set the emperors apart from their subjects as their superiors. The comparative vividness of porphyry to other stones underscored that these figures were not regular citizens, but many levels above, even gods, and worthy of

640-453: The altars and vases and fountain basins reused in the Renaissance and dispersed as far as Kyiv . The Romans also used "Green Porphyry" ( lapis Lacedaemonius , from Greece, also known today as Serpentine ), and "Black Porphyry" from the same Egyptian quarry. After the fifth century the quarry was lost to sight for many centuries. Byzantium scholar Alexander Vasiliev suggested this was

680-874: The consequence of the Council of Chalcedon in 451 and the subsequent troubles in Egypt . The scientific members of the French Expedition under Napoleon sought it in vain, and it was only when the Eastern Desert was reopened for study under Muhammad Ali that the site was rediscovered by the English Egyptologists James Burton and John Gardner Wilkinson in 1823. Porphyry was extensively used in Byzantine imperial monuments, for example in Hagia Sophia and in

720-587: The development of glacial valleys. Though precipitation increases with the height, there are semiarid conditions in the nearly 7,000 metres (23,000 ft) towering mountains of the Andes. This dry steppe climate is considered to be of the subtropic type at 32-34° S. In the valley bottoms only dwarf shrubs grow. The largest glaciers, e.g. the Plomo glacier and the Horcones glacier, do not reach 10 kilometres (6.2 mi) in length;

760-474: The ice contained within them, causing their surface to resemble those of glaciers. Some rock glaciers can reach lengths of three kilometres (2 mi) and can have terminal embankments 60 m (200 ft) high. Blocks on the surface can be up to 8 m (26 ft) in diameter. Flow features on the surface of rock glaciers may develop from: Their growth and formation is subject to some debate, with three main theories: Rock glaciers may move or creep at

800-585: The ice thickness is not very significant. During glacial times however, c. 20,000 years ago, the glaciers were over ten times longer. On the east-side of this section of the Mendoza Andes they flowed down to 2,060 metres (6,760 ft) and on the west-side to c. 1,220 metres (4,000 ft). The massifs of Cerro Aconcagua 6,962 metres (22,841 ft), Cerro Tupungato 6,550 metres (21,490 ft) and Nevado Juncal 6,110 metres (20,050 ft) are situated deca-kilometres away from each other and were connected by

840-559: The influences of topographic factors and lack of ice or debris budget within the glacier body. Rock glaciers in the Chilean Andes help supply the water for much of Chile, including the capital of Santiago. Mining operations in the high mountains have led to the degradation and destruction of more than two rock glaciers. Several copper mines dump their waste rock onto rock glaciers, which results in faster melting and higher velocity movement of these rock glaciers. The dumping of waste rock on

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880-441: The main building of the İstanbul Archaeology Museums , including one whose rounded shape led Alexander Vasiliev to suggest attribution to Emperor Julian on the basis of Constantine Porphyrogenitus's description. Vasiliev conjectures that the nine imperial sarcophagi, including one which carries a crux ansata or Egyptian cross , were carved in Egypt before shipment to Constantinople. The imperial porphyry sarcophagi tradition

920-456: The phenocrysts, as they crowd each other out. The significance of porphyritic texture as an indication that magma forms through different stages of cooling was first recognized by the Canadian geologist, Norman L. Bowen , in 1928. Porphyritic texture is particularly common in andesite , with the most prominent phenocrysts typically composed of plagioclase feldspar . Plagioclase has almost

960-431: The phenocrysts. The crystallization of the phenocrysts during fractional crystallization changes the composition of the remaining liquid magma, moving it closer to the eutectic point , with a mixed composition of minerals. As the temperature continues to decrease, this point is reached, and the rock is entirely solidified. The simultaneous crystallization of the remaining minerals produces the finer-grained matrix surrounding

1000-514: The purple-red form of this stone, valued for its appearance, but other colours of decorative porphyry are also used such as "green", "black" and "grey". The term porphyry is from the Ancient Greek πορφύρα ( porphyra ), meaning " purple ". Purple was the colour of royalty, and the Roman "imperial porphyry" was a deep purple igneous rock with large crystals of plagioclase . Some authors claimed

1040-712: The reign of Tiberius; an inscription recently discovered and dated from AD 18 mentions the Roman Caius Cominius Leugas as the finder of this new quarry. Ancient Egyptians used other decorative porphyritic stones of a very close composition and appearance, but apparently remained unaware of the presence of the Roman grade although it was located in their own country. It was also sometimes used in Minoan art , and as early as 1850 BC on Crete in Minoan Knossos there were large column bases made of porphyry. It

1080-499: The remaining melt, these phenocrysts usually settle out of solution, eventually creating cumulates ; however if the partially crystallized magma is then erupted to the surface as a lava, the remainder of the melt is quickly cooled around the phenocrysts and crystallizes much more rapidly to form a very fine-grained or glassy matrix. Porphyry can also form even from magma that completely solidifies while still underground. The groundmass will be visibly crystalline, though not as large as

1120-429: The respect they expected. Porphyry made the emperors unapproachable in terms of power and nature, belonging to another world, the world of the mighty gods, present for a short time on earth. Porphyry also stood in for the physical purple robes Roman emperors wore to show status, because of its purple colouring. Similar to porphyry, purple fabric was extremely difficult to make, as what we now call Tyrian purple required

1160-574: The rock glaciers may lead to their destabilization. In 2004, protesting irrigation farmers and environmentalists changed rules so new mining projects can no longer damage or alter rock glaciers in Chile. Parts of the only road into Denali National Park and Preserve in Alaska are built on a rock glacier known as "Pretty Rocks". In late summer 2021 the road had to be closed due to accelerating rockslides in that area, sometimes sliding up to 10 inches (250 mm) in

1200-454: The rock was the hardest known in antiquity. Thus porphyry was prized for monuments and building projects in Imperial Rome and thereafter. Subsequently, the name was given to any igneous rocks with large crystals. The adjective porphyritic now refers to a certain texture of igneous rock regardless of its chemical and mineralogical composition or its color. Its chief characteristic is

1240-419: The same density as basaltic magma, so plagioclase phenocrysts are likely to remain suspended in the magma rather than settling out. Rhomb porphyry is a volcanic rock with gray-white large porphyritic rhombus -shaped phenocrysts of feldspar (commonly anorthoclase ) embedded in a very fine-grained red-brown matrix . The composition of rhomb porphyry places it in the trachyte – latite classification of

Dry Andes - Misplaced Pages Continue

1280-581: The steep sides and accumulates on ice glaciers. As the glaciers shrink, their composition changes as they become increasingly covered with debris. Eventually, the glacial ice is replaced by ice cored rocks. With the exception of ice-cored rock glaciers, rock glaciers are a periglacial process. This means that they are a nonglacial landform associated with cold climates, particularly with various aspects of frozen ground. Periglacial rock glaciers require permafrost instead of glacial ice in order to form. Instead, they are caused by continuous freezing occurring within

1320-442: The use of rare sea snails to make the dye. The colour itself reminded the public how to behave in the presence of the emperors, with respect bordering on worship for the self-proclaimed god-kings. A uniquely prestigious use of porphyry was its choice as material for imperial sarcophagi in the 4th and early 5th centuries. That tradition appears to have been started with Diocletian 's porphyry sarcophagus in his mausoleum , which

1360-519: The very first examples of medieval free-standing secular tombs in the West, and therefore play a unique role within the history of Italian sepulchral art (earlier and later tombs are adjacent to, and dependent on walls)." Six grand porphyry sarcophagi are featured along the walls of the octagonal Cappella dei Principi (Chapel of the Princes) that was built as one of two chapels in the architectural complex of

1400-540: Was altered somewhat during execution by Buontalenti . The tomb of Napoleon at Les Invalides in Paris , designed by architect Louis Visconti , is centered on the deceased emperor's sarcophagus that often has been described as made of red porphyry although this is incorrect. Napoleon's sarcophagus is made of quartzite , however, its pedestal is made of green andesite porphyry from Vosges . The sarcophagus of Arthur Wellesley, 1st Duke of Wellington at St Paul's Cathedral

1440-635: Was called "Imperial" as the mines, as elsewhere in the empire, were owned by the emperor. The red porphyry all came from the Gabal Abu Dukhan quarry (or Mons Porphyrites ) in the Eastern Desert of Egypt , from 600 million-year-old andesite of the Arabian-Nubian Shield . The road from the quarry westward to Qena (Roman Maximianopolis) on the Nile, which Ptolemy put on his second-century map,

1480-401: Was completed in 1858. and was made from a single piece of Cornish porphyry, of a type called luxullianite , which was found in a field near Lostwithiel . In countries where many automobiles have studded winter tires such as Sweden, Finland, and Norway, it is common that highways are paved with asphalt made of porphyry aggregate to make the wearing course withstand the extreme wear from

1520-835: Was destroyed when the building was repurposed as a church but of which probable fragments are at the Archaeological Museum in Split, Croatia . The oldest and best-preserved ones are now conserved at the Vatican Museums and known as the Sarcophagi of Helena and Constantina . Nine other imperial porphyry sarcophagi were long held in the Church of the Holy Apostles in Constantinople . They were described by Constantine VII Porphyrogenitus in

1560-567: Was emulated by Ostrogothic King Theodoric the Great (454-526), whose mausoleum in Ravenna still contains a porphyry tub that was used as his sarcophagus. Similarly Charles the Bald , King of West Francia and Roman Emperor , was buried at Saint-Denis in a porphyry tub which may be the same one known as " Dagobert 's tub" ( cuve de Dagobert ), now in the Louvre . The tomb of Peter III of Aragon , in

1600-711: Was first described by Strabo , and it is to this day known as the Via Porphyrites , the Porphyry Road, its track marked by the hydreumata , or watering wells that made it viable in this utterly dry landscape. It was used for all the red porphyry columns in Rome, the togas on busts of emperors , the panels in the revetment of the Pantheon , the Column of Constantine in Istanbul as well as

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