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79-490: Landmarks in this woreda include three megalithic sites with one stele in each; one of these sites is Cheme Hembico. Near the She'a river there is a site with animal engravings; obsidian and ceramic fragments have been observed on the surface of the She'a site. Boloso Sore has 57 kilometers of all-weather roads and 74 kilometers of dry-weather roads, for an average road density of 206 kilometers per 1000 square kilometers. The woreda

158-663: A relative dating system, obsidian hydration dating , to calculate the age of obsidian artifacts . Obsidian artifacts first appeared in the European continent in Central Europe in the Middle Paleolithic and had become common by the Upper Paleolithic , although there are exceptions to this. Obsidian played an important role in the transmission of Neolithic knowledge and experiences . The material

237-486: A change in composition. Solidification into rock occurs either below the surface as intrusive rocks or on the surface as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses . Igneous rocks occur in a wide range of geological settings: shields, platforms, orogens, basins, large igneous provinces, extended crust and oceanic crust. Igneous and metamorphic rocks make up 90–95% of

316-459: A combination of these processes. Other mechanisms, such as melting from a meteorite impact , are less important today, but impacts during the accretion of the Earth led to extensive melting, and the outer several hundred kilometres of our early Earth was probably an ocean of magma. Impacts of large meteorites in the last few hundred million years have been proposed as one mechanism responsible for

395-405: A crystalline basement formed of a great variety of metamorphic and igneous rocks, including granulite and granite. Oceanic crust is composed primarily of basalt and gabbro . Both continental and oceanic crust rest on peridotite of the mantle. Rocks may melt in response to a decrease in pressure, to a change in composition (such as an addition of water), to an increase in temperature, or to

474-556: A culture or place can be of considerable use to reconstruct commerce, production, and distribution, and thereby understand economic, social and political aspects of a civilization. This is the case in Yaxchilán , a Maya city where even warfare implications have been studied linked with obsidian use and its debris. Another example is the archeological recovery at coastal Chumash sites in California, indicating considerable trade with

553-455: A distinguishable type of obsidian allowing archaeologists to use methods such as non-destructive energy dispersive X-ray fluorescence to select minor element compositions from both the artifact and geological sample to trace the origins of a particular artifact. Similar tracing techniques have also allowed obsidian in Greece to be identified as coming from Milos , Nisyros or Gyali , islands in

632-501: A high level of sophistication by the late Neolithic, and was traded as far as Sicily, the southern Po river valley, and Croatia. Obsidian bladelets were used in ritual circumcisions and cutting of umbilical cords of newborns. Anatolian sources of obsidian are known to have been the material used in the Levant and modern-day Iraqi Kurdistan from a time beginning sometime about 12,500 BC. Obsidian artifacts are common at Tell Brak , one of

711-527: A huge mass of analytical data—over 230,000 rock analyses can be accessed on the web through a site sponsored by the U. S. National Science Foundation (see the External Link to EarthChem). The single most important component is silica, SiO 2 , whether occurring as quartz or combined with other oxides as feldspars or other minerals. Both intrusive and volcanic rocks are grouped chemically by total silica content into broad categories. This classification

790-653: A microscope for fine-grained volcanic rock, and may be impossible for glassy volcanic rock. The rock must then be classified chemically. Mineralogical classification of an intrusive rock begins by determining if the rock is ultramafic, a carbonatite, or a lamprophyre . An ultramafic rock contains more than 90% of iron- and magnesium-rich minerals such as hornblende, pyroxene, or olivine, and such rocks have their own classification scheme. Likewise, rocks containing more than 50% carbonate minerals are classified as carbonatites, while lamprophyres are rare ultrapotassic rocks. Both are further classified based on detailed mineralogy. In

869-553: A simplified compositional classification, igneous rock types are categorized into felsic or mafic based on the abundance of silicate minerals in the Bowen's Series. Rocks dominated by quartz, plagioclase, alkali feldspar and muscovite are felsic. Mafic rocks are primarily composed of biotite, hornblende, pyroxene and olivine. Generally, felsic rocks are light colored and mafic rocks are darker colored. For textural classification, igneous rocks that have crystals large enough to be seen by

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948-653: A site in the western Negev in Israel. Eight obsidian artifacts dating to the Chalcolithic Age found at this site were traced to obsidian sources in Anatolia . Neutron activation analysis (NAA) on the obsidian found at this site helped to reveal trade routes and exchange networks previously unknown. Lithic analysis helps to understand pre-Hispanic groups in Mesoamerica . A careful analysis of obsidian in

1027-490: A total population for this woreda of 247,239 of whom 121,152 were men and 126,087 were women; 17,517 or 7.09% of its population were urban dwellers. (This total consists of an estimate for the inhabitants of part of one rural kebele in this woreda, who was not counted; the uncounted inhabitants were estimated to have 2,378 inhabitants, of whom 1,190 were men and 1,188 women.) The largest ethnic group reported in Boloso Sore

1106-459: A type of glass knife made using naturally occurring obsidian instead of manufactured glass. Obsidian is used by some surgeons for scalpel blades, although this is not approved by the US Food and Drug Administration (FDA) for use on humans. Well-crafted obsidian blades, like any glass knife, can have a cutting edge many times sharper than high-quality steel surgical scalpels: the cutting edge of

1185-462: A viscosity similar to thick, cold molasses or even rubber when erupted. Felsic magma, such as rhyolite , is usually erupted at low temperature and is up to 10,000 times as viscous as basalt. Volcanoes with rhyolitic magma commonly erupt explosively, and rhyolitic lava flows are typically of limited extent and have steep margins because the magma is so viscous. Felsic and intermediate magmas that erupt often do so violently, with explosions driven by

1264-549: Is metastable at the Earth's surface (over time the glass devitrifies , becoming fine-grained mineral crystals), obsidian older than Miocene in age is rare. Exceptionally old obsidians include a Cretaceous welded tuff and a partially devitrified Ordovician perlite . This transformation of obsidian is accelerated by the presence of water. Although newly formed obsidian has a low water content, typically less than 1% water by weight, it becomes progressively hydrated when exposed to groundwater , forming perlite . Pure obsidian

1343-463: Is also a Financial affairs sector advisor of the Prime minister. 7°05′N 37°40′E  /  7.083°N 37.667°E  / 7.083; 37.667 Obsidian Obsidian ( / ə b ˈ s ɪ d i . ən , ɒ b -/ əb- SID -ee-ən ob- ) is a naturally occurring volcanic glass formed when lava extruded from a volcano cools rapidly with minimal crystal growth . It

1422-408: Is an igneous rock . Produced from felsic lava, obsidian is rich in the lighter elements such as silicon , oxygen , aluminum, sodium , and potassium . It is commonly found within the margins of rhyolitic lava flows known as obsidian flows. These flows have a high content of silica , giving them a high viscosity . The high viscosity inhibits diffusion of atoms through the lava, which inhibits

1501-584: Is an example. The molten rock, which typically contains suspended crystals and dissolved gases, is called magma . It rises because it is less dense than the rock from which it was extracted. When magma reaches the surface, it is called lava . Eruptions of volcanoes into air are termed subaerial , whereas those occurring underneath the ocean are termed submarine . Black smokers and mid-ocean ridge basalt are examples of submarine volcanic activity. The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting. Extrusive rock

1580-679: Is caused by inclusions of magnetite nanoparticles creating thin-film interference . Colorful, striped obsidian ( rainbow obsidian ) from Mexico contains oriented nanorods of hedenbergite , which cause the rainbow striping effects by thin-film interference . Obsidian is found near volcanoes in locations which have undergone rhyolitic eruptions. It can be found in Argentina, Armenia , Azerbaijan , Australia, Canada, Chile, Georgia , Ecuador , El Salvador , Greece, Guatemala , Hungary , Iceland, Indonesia , Italy, Japan, Kenya , Mexico, New Zealand, Papua New Guinea , Peru, Russia , Scotland,

1659-544: Is culturally homogeneous with Welayta -speaking people, though a small but active minority of Protestant and Catholic converts formed a sub-culture. A survey of land reform in the area which became Boloso Sore was performed in 1980-1981 under the direction of Dessalegn Rahmato . It found that Bolosso is "the beneficiary of many years of infrastructural and extension service by the Wollaita Agricultural Development Unit ". Further, although

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1738-453: Is cut: in one direction it is jet black, while in another it is glistening gray. " Apache tears " are small rounded obsidian nuggets often embedded within a grayish-white perlite matrix . Plinths for audio turntables have been made of obsidian since the 1970s, such as the grayish-black SH-10B3 plinth by Technics . Igneous rock Igneous rock ( igneous from Latin igneus  'fiery'), or magmatic rock ,

1817-728: Is distinguishable from the other two on the TAS diagram, being higher in total alkali oxides for a given silica content, but the tholeiitic and calc-alkaline series occupy approximately the same part of the TAS diagram. They are distinguished by comparing total alkali with iron and magnesium content. These three magma series occur in a range of plate tectonic settings. Tholeiitic magma series rocks are found, for example, at mid-ocean ridges, back-arc basins , oceanic islands formed by hotspots, island arcs and continental large igneous provinces . All three series are found in relatively close proximity to each other at subduction zones where their distribution

1896-595: Is expressed differently for major and minor elements and for trace elements. Contents of major and minor elements are conventionally expressed as weight percent oxides (e.g., 51% SiO 2 , and 1.50% TiO 2 ). Abundances of trace elements are conventionally expressed as parts per million by weight (e.g., 420 ppm Ni, and 5.1 ppm Sm). The term "trace element" is typically used for elements present in most rocks at abundances less than 100 ppm or so, but some trace elements may be present in some rocks at abundances exceeding 1,000 ppm. The diversity of rock compositions has been defined by

1975-506: Is formed by the cooling of molten magma on the earth's surface. The magma, which is brought to the surface through fissures or volcanic eruptions , rapidly solidifies. Hence such rocks are fine-grained ( aphanitic ) or even glassy. Basalt is the most common extrusive igneous rock and forms lava flows, lava sheets and lava plateaus. Some kinds of basalt solidify to form long polygonal columns . The Giant's Causeway in Antrim, Northern Ireland

2054-436: Is most often used to classify plutonic rocks. Chemical classifications are preferred to classify volcanic rocks, with phenocryst species used as a prefix, e.g. "olivine-bearing picrite" or "orthoclase-phyric rhyolite". The IUGS recommends classifying igneous rocks by their mineral composition whenever possible. This is straightforward for coarse-grained intrusive igneous rock, but may require examination of thin sections under

2133-409: Is one of the three main rock types , the others being sedimentary and metamorphic . Igneous rocks are formed through the cooling and solidification of magma or lava . The magma can be derived from partial melts of existing rocks in either a planet 's mantle or crust . Typically, the melting is caused by one or more of three processes: an increase in temperature, a decrease in pressure , or

2212-496: Is produced in the following proportions: The behaviour of lava depends upon its viscosity , which is determined by temperature, composition, and crystal content. High-temperature magma, most of which is basaltic in composition, behaves in a manner similar to thick oil and, as it cools, treacle . Long, thin basalt flows with pahoehoe surfaces are common. Intermediate composition magma, such as andesite , tends to form cinder cones of intermingled ash , tuff and lava, and may have

2291-402: Is related to depth and the age of the subduction zone. The tholeiitic magma series is well represented above young subduction zones formed by magma from relatively shallow depth. The calc-alkaline and alkaline series are seen in mature subduction zones, and are related to magma of greater depths. Andesite and basaltic andesite are the most abundant volcanic rock in island arc which is indicative of

2370-487: Is summarized in the following table: The percentage of alkali metal oxides ( Na 2 O plus K 2 O ) is second only to silica in its importance for chemically classifying volcanic rock. The silica and alkali metal oxide percentages are used to place volcanic rock on the TAS diagram , which is sufficient to immediately classify most volcanic rocks. Rocks in some fields, such as the trachyandesite field, are further classified by

2449-406: Is usually dark in appearance, though the color varies depending on the impurities present. Iron and other transition elements may give the obsidian a dark brown to black color. Most black obsidians contain nanoinclusions of magnetite , an iron oxide . Very few samples of obsidian are nearly colorless. In some stones, the inclusion of small, white, radially clustered crystals ( spherulites ) of

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2528-428: Is usually dark in color, similar to mafic rocks such as basalt , the composition of obsidian is extremely felsic. Obsidian consists mainly of SiO 2 ( silicon dioxide ), usually 70% by weight or more; the remainder consists of variable amounts of other oxides, mostly oxides of aluminum, iron, potassium, sodium and calcium. Crystalline rocks with a similar composition include granite and rhyolite . Because obsidian

2607-651: The Aegean Sea . Obsidian cores and blades were traded great distances inland from the coast. In Chile obsidian tools from Chaitén Volcano have been found as far away as in Chan-Chan 400 km (250 mi) north of the volcano, and also in sites 400 km south of it. The Lapita culture , active across a large area of the Pacific Ocean around 1000 BC, made widespread use of obsidian tools and engaged in long distance obsidian trading. The complexity of

2686-701: The Canary Islands , Turkey and the United States. Obsidian flows which are so large that they can be hiked on are found within the calderas of Newberry Volcano (Big Obsidian Flow, 700 acres) and Medicine Lake Volcano in the Cascade Range of western North America, and at Inyo Craters east of the Sierra Nevada in California. Yellowstone National Park has a mountainside containing obsidian located between Mammoth Hot Springs and

2765-702: The Göllü Dağ volcano were the most important sources in central Anatolia , one of the more important source areas in the prehistoric Near East . The first known archaeological evidence of usage was in Kariandusi (Kenya) and other sites of the Acheulian age (beginning 1.5 million years BP) dated 700,000 BC, although only very few objects have been found at these sites relative to the Neolithic. Manufacture of obsidian bladelets at Lipari had reached

2844-479: The IUGS , this is often impractical, and chemical classification is done instead using the TAS classification . Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body. The classification of the many types of igneous rocks can provide important information about the conditions under which they formed. Two important variables used for

2923-535: The Norris Geyser Basin , and deposits can be found in many other western U.S. states including Arizona , Colorado , New Mexico , Texas , Utah , and Washington , Oregon and Idaho . There are only four major deposit areas in the central Mediterranean: Lipari , Pantelleria , Palmarola and Monte Arci ( Sardinia ). Ancient sources in the Aegean were Milos and Gyali . Acıgöl town and

3002-453: The convection of solid mantle, it will cool slightly as it expands in an adiabatic process , but the cooling is only about 0.3 °C per kilometre. Experimental studies of appropriate peridotite samples document that the solidus temperatures increase by 3 °C to 4 °C per kilometre. If the rock rises far enough, it will begin to melt. Melt droplets can coalesce into larger volumes and be intruded upwards. This process of melting from

3081-473: The 1640s and is derived either from French granit or Italian granito , meaning simply "granulate rock". The term rhyolite was introduced in 1860 by the German traveler and geologist Ferdinand von Richthofen The naming of new rock types accelerated in the 19th century and peaked in the early 20th century. Much of the early classification of igneous rocks was based on the geological age and occurrence of

3160-434: The 1960s. However, the concept of normative mineralogy has endured, and the work of Cross and his coinvestigators inspired a flurry of new classification schemes. Among these was the classification scheme of M.A. Peacock, which divided igneous rocks into four series: the alkalic, the alkali-calcic, the calc-alkali, and the calcic series. His definition of the alkali series, and the term calc-alkali, continue in use as part of

3239-520: The Earth's surface. Intrusive igneous rocks that form at depth within the crust are termed plutonic (or abyssal ) rocks and are usually coarse-grained. Intrusive igneous rocks that form near the surface are termed subvolcanic or hypabyssal rocks and they are usually much finer-grained, often resembling volcanic rock. Hypabyssal rocks are less common than plutonic or volcanic rocks and often form dikes, sills, laccoliths, lopoliths , or phacoliths . Extrusive igneous rock, also known as volcanic rock,

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3318-681: The Roman writer Pliny the Elder includes a few sentences about a volcanic glass called obsidian ( lapis obsidianus ), discovered in Ethiopia by Obsidius, a Roman explorer. Obsidian is formed from quickly cooled lava , which is the parent material . Extrusive formation of obsidian may occur when felsic lava cools rapidly at the edges of a felsic lava flow or volcanic dome, or when lava cools during sudden contact with water or air. Intrusive formation of obsidian may occur when felsic lava cools along

3397-679: The area had been self-sufficient in food in years past, there were clear signs of gradual impoverishment. In a 1971 survey about 30% of Bolosso households did not use ox-drawn ploughs for cultivation; ten years later, 52% of the peasants interviewed did not own oxen. Prime Minister Tamirat Layne on 1 June 1994 began touring villages affected by famine in Boloso Sore. In this woreda alone, over 5,000 people had so far died of famine or malaria, 60% of them being children. In March 1996, Boloso Sore experienced several hail storms in eight rural kebeles , of which two reported severe damage. The woreda Disaster Prevention and Preparation Committee ameliorated

3476-450: The basic TAS classification include: In older terminology, silica oversaturated rocks were called silicic or acidic where the SiO 2 was greater than 66% and the family term quartzolite was applied to the most silicic. A normative feldspathoid classifies a rock as silica-undersaturated; an example is nephelinite . Magmas are further divided into three series: The alkaline series

3555-514: The blade is only about three nanometers thick. All metal knives have a jagged, irregular blade when viewed under a strong enough microscope ; however, obsidian blades are still smooth, even when examined under an electron microscope . One study found that obsidian incisions produced fewer inflammatory cells and less granulation tissue in a group of rats after seven days but the differences disappeared after twenty-one days. Don Crabtree has produced surgical obsidian blades and written articles on

3634-568: The calc-alkaline magmas. Some island arcs have distributed volcanic series as can be seen in the Japanese island arc system where the volcanic rocks change from tholeiite—calc-alkaline—alkaline with increasing distance from the trench. Some igneous rock names date to before the modern era of geology. For example, basalt as a description of a particular composition of lava-derived rock dates to Georgius Agricola in 1546 in his work De Natura Fossilium . The word granite goes back at least to

3713-429: The chemical composition of an igneous rock was its most fundamental characteristic, it should be elevated to prime position. Geological occurrence, structure, mineralogical constitution—the hitherto accepted criteria for the discrimination of rock species—were relegated to the background. The completed rock analysis is first to be interpreted in terms of the rock-forming minerals which might be expected to be formed when

3792-625: The classification of igneous rocks are particle size, which largely depends on the cooling history, and the mineral composition of the rock. Feldspars , quartz or feldspathoids , olivines , pyroxenes , amphiboles , and micas are all important minerals in the formation of almost all igneous rocks, and they are basic to the classification of these rocks. All other minerals present are regarded as nonessential in almost all igneous rocks and are called accessory minerals . Types of igneous rocks with other essential minerals are very rare, but include carbonatites , which contain essential carbonates . In

3871-405: The crust of a planet. Bodies of intrusive rock are known as intrusions and are surrounded by pre-existing rock (called country rock ). The country rock is an excellent thermal insulator , so the magma cools slowly, and intrusive rocks are coarse-grained ( phaneritic ). The mineral grains in such rocks can generally be identified with the naked eye. Intrusions can be classified according to

3950-402: The different types of extrusive igneous rocks than between different types of intrusive igneous rocks. Generally, the mineral constituents of fine-grained extrusive igneous rocks can only be determined by examination of thin sections of the rock under a microscope , so only an approximate classification can usually be made in the field . Although classification by mineral makeup is preferred by

4029-657: The disaster by distributing seeds provided by the NGO Redd Barna to 4,205 households in five kebeles. Based on the 2019 population projection conducted by the CSA, this woreda has a total population of 279,218, of whom 144,236 are women and 134,982 are men ; 31,408 or 15.87% of its population are urban dwellers. The majority of the inhabitants were Protestants , with 56.83% of the population reporting that belief, 35.07% practiced Ethiopian Orthodox Christianity , and 5.99% were Catholic . The 1994 national census reported

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4108-502: The distant site of Casa Diablo Hot Springs in the Sierra Nevada . Pre-Columbian Mesoamericans' use of obsidian was extensive and sophisticated; including carved and worked obsidian for tools and decorative objects. Mesoamericans also made a type of sword with obsidian blades mounted in a wooden body. Called a macuahuitl , the weapon could inflict terrible injuries, combining the sharp cutting edge of an obsidian blade with

4187-501: The earliest Mesopotamian urban centers, dating to the late fifth millennium BC. Obsidian was valued in Stone Age cultures because, like flint , it could be fractured to produce sharp blades or arrowheads in a process called knapping . Like all glass and some other naturally occurring rocks, obsidian breaks with a characteristic conchoidal fracture . It was also polished to create early mirrors . Modern archaeologists have developed

4266-632: The eastern Mediterranean and southern Red Sea regions. Obsidian scalpels older than 2100 BC have been found in a Bronze Age settlement in Turkey. In the eastern Mediterranean area the material was used to make tools, mirrors and decorative objects. The use of obsidian tools was present in Japan near areas of volcanic activity. Obsidian was mined during the Jōmon period . Obsidian has also been found in Gilat ,

4345-411: The edges of a dike . Tektites were once thought by many to be obsidian produced by lunar volcanic eruptions, though few scientists now adhere to this hypothesis . Obsidian is mineral-like, but not a true mineral because, as a glass, it is not crystalline ; in addition, its composition is too variable to be classified as a mineral. It is sometimes classified as a mineraloid . Though obsidian

4424-484: The extensive basalt magmatism of several large igneous provinces. Decompression melting occurs because of a decrease in pressure. The solidus temperatures of most rocks (the temperatures below which they are completely solid) increase with increasing pressure in the absence of water. Peridotite at depth in the Earth's mantle may be hotter than its solidus temperature at some shallower level. If such rock rises during

4503-409: The first step ( nucleation ) in the formation of mineral crystals . Together with rapid cooling, this results in a natural glass forming from the lava. Obsidian is hard, brittle , and amorphous ; it therefore fractures with sharp edges. In the past, it was used to manufacture cutting and piercing tools, and it has been used experimentally as surgical scalpel blades. The Natural History by

4582-486: The great majority of cases, the rock has a more typical mineral composition, with significant quartz, feldspars, or feldspathoids. Classification is based on the percentages of quartz, alkali feldspar, plagioclase, and feldspathoid out of the total fraction of the rock composed of these minerals, ignoring all other minerals present. These percentages place the rock somewhere on the QAPF diagram , which often immediately determines

4661-607: The island outcrops of the central Mediterranean . Through trade, these artifacts ended up in lands thousands of kilometers away from the original source; this indicates that they were a highly valued commodity. John Dee had a mirror, made of obsidian, which was brought from Mexico to Europe between 1527 and 1530 after Hernando Cortés's conquest of the region. In the Ubaid in the 5th millennium BC , blades were manufactured from obsidian extracted from outcrops located in modern-day Turkey . Ancient Egyptians used obsidian imported from

4740-433: The larger crystals, called phenocrysts, grow to considerable size before the main mass of the magma crystallizes as finer-grained, uniform material called groundmass. Grain size in igneous rocks results from cooling time so porphyritic rocks are created when the magma has two distinct phases of cooling. Igneous rocks are classified on the basis of texture and composition. Texture refers to the size, shape, and arrangement of

4819-399: The magma crystallizes, e.g., quartz feldspars, olivine , akermannite, Feldspathoids , magnetite , corundum , and so on, and the rocks are divided into groups strictly according to the relative proportion of these minerals to one another. This new classification scheme created a sensation, but was criticized for its lack of utility in fieldwork, and the classification scheme was abandoned by

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4898-457: The majority of minerals will be visible to the naked eye or at least using a hand lens, magnifying glass or microscope. Plutonic rocks also tend to be less texturally varied and less prone to showing distinctive structural fabrics. Textural terms can be used to differentiate different intrusive phases of large plutons, for instance porphyritic margins to large intrusive bodies, porphyry stocks and subvolcanic dikes . Mineralogical classification

4977-409: The mineral cristobalite in the black glass produce a blotchy or snowflake pattern ( snowflake obsidian ). Obsidian may contain patterns of gas bubbles remaining from the lava flow, aligned along layers created as the molten rock was flowing before being cooled. These bubbles can produce interesting effects such as a golden sheen ( sheen obsidian ). An iridescent , rainbow -like sheen ( fire obsidian )

5056-697: The mineral grains or crystals of which the rock is composed. Texture is an important criterion for the naming of volcanic rocks. The texture of volcanic rocks, including the size, shape, orientation, and distribution of mineral grains and the intergrain relationships, will determine whether the rock is termed a tuff , a pyroclastic lava or a simple lava . However, the texture is only a subordinate part of classifying volcanic rocks, as most often there needs to be chemical information gleaned from rocks with extremely fine-grained groundmass or from airfall tuffs, which may be formed from volcanic ash. Textural criteria are less critical in classifying intrusive rocks where

5135-415: The naked eye are called phaneritic ; those with crystals too small to be seen are called aphanitic . Generally speaking, phaneritic implies an intrusive origin or plutonic, indicating slow cooling; aphanitic are extrusive or volcanic, indicating rapid cooling. An igneous rock with larger, clearly discernible crystals embedded in a finer-grained matrix is termed porphyry . Porphyritic texture develops when

5214-554: The production technique for these tools, and the care taken in their storage, may indicate that beyond their practical use they were associated with prestige or high status. Obsidian was also used on Rapa Nui (Easter Island) for edged tools such as Mataia and the pupils of the eyes of their Moai (statues), which were encircled by rings of bird bone. Obsidian was used to inscribe the Rongorongo glyphs . Obsidian can be used to make extremely sharp knives, and obsidian blades are

5293-579: The ragged cut of a serrated weapon. The polearm version of this weapon was called tepoztopilli . Obsidian mirrors were used by some Aztec priests to conjure visions and make prophecies. They were connected with Tezcatlipoca , god of obsidian and sorcery, whose name can be translated from the Nahuatl language as 'Smoking Mirror'. Indigenous people traded obsidian throughout the Americas. Each volcano and in some cases each volcanic eruption produces

5372-441: The ratio of potassium to sodium (so that potassic trachyandesites are latites and sodic trachyandesites are benmoreites). Some of the more mafic fields are further subdivided or defined by normative mineralogy , in which an idealized mineral composition is calculated for the rock based on its chemical composition. For example, basanite is distinguished from tephrite by having a high normative olivine content. Other refinements to

5451-403: The release of dissolved gases—typically water vapour, but also carbon dioxide . Explosively erupted pyroclastic material is called tephra and includes tuff , agglomerate and ignimbrite . Fine volcanic ash is also erupted and forms ash tuff deposits, which can often cover vast areas. Because volcanic rocks are mostly fine-grained or glassy, it is much more difficult to distinguish between

5530-473: The rock must be classified chemically. There are relatively few minerals that are important in the formation of common igneous rocks, because the magma from which the minerals crystallize is rich in only certain elements: silicon , oxygen , aluminium, sodium , potassium , calcium , iron, and magnesium . These are the elements that combine to form the silicate minerals , which account for over ninety percent of all igneous rocks. The chemistry of igneous rocks

5609-424: The rock type. In a few cases, such as the diorite-gabbro-anorthite field, additional mineralogical criteria must be applied to determine the final classification. Where the mineralogy of an volcanic rock can be determined, it is classified using the same procedure, but with a modified QAPF diagram whose fields correspond to volcanic rock types. When it is impractical to classify a volcanic rock by mineralogy,

5688-467: The rocks. However, in 1902, the American petrologists Charles Whitman Cross , Joseph P. Iddings , Louis V. Pirsson , and Henry Stephens Washington proposed that all existing classifications of igneous rocks should be discarded and replaced by a "quantitative" classification based on chemical analysis. They showed how vague, and often unscientific, much of the existing terminology was and argued that as

5767-435: The shape and size of the intrusive body and its relation to the bedding of the country rock into which it intrudes. Typical intrusive bodies are batholiths , stocks , laccoliths , sills and dikes . Common intrusive rocks are granite , gabbro , or diorite . The central cores of major mountain ranges consist of intrusive igneous rocks. When exposed by erosion, these cores (called batholiths ) may occupy huge areas of

5846-436: The subject. Obsidian scalpels may be purchased for surgical use on research animals . The major disadvantage of obsidian blades is their brittleness compared to those made of metal, thus limiting the surgical applications for obsidian blades to a variety of specialized uses where this is not a concern. Obsidian is also used for ornamental purposes and as a gemstone . It presents a different appearance depending on how it

5925-475: The top 16 kilometres (9.9 mi) of the Earth's crust by volume. Igneous rocks form about 15% of the Earth's current land surface. Most of the Earth's oceanic crust is made of igneous rock. Igneous rocks are also geologically important because: Igneous rocks can be either intrusive ( plutonic and hypabyssal) or extrusive ( volcanic ). Intrusive igneous rocks make up the majority of igneous rocks and are formed from magma that cools and solidifies within

6004-586: The widely used Irvine-Barager classification, along with W.Q. Kennedy's tholeiitic series. By 1958, there were some 12 separate classification schemes and at least 1637 rock type names in use. In that year, Albert Streckeisen wrote a review article on igneous rock classification that ultimately led to the formation of the IUGG Subcommission of the Systematics of Igneous Rocks. By 1989 a single system of classification had been agreed upon, which

6083-467: Was further revised in 2005. The number of recommended rock names was reduced to 316. These included a number of new names promulgated by the Subcommission. The Earth's crust averages about 35 kilometres (22 mi) thick under the continents , but averages only some 7–10 kilometres (4.3–6.2 mi) beneath the oceans . The continental crust is composed primarily of sedimentary rocks resting on

6162-609: Was mainly used for production of chipped tools which were very sharp due to its nature. Artifacts made of obsidian can be found in many Neolithic cultures across Europe. The source of obsidian for cultures inhabiting the territory of and around Greece was the island of Milos ; the Starčevo–Körös–Criș culture obtained obsidian from sources in Hungary and Slovakia, while the Cardium -Impresso cultural complex acquired obsidian from

6241-624: Was the Welayta (98.42%), and Welayta was the dominant first language, spoken by 98.6% of the inhabitants. The majority of the inhabitants practiced Ethiopian Orthodox Christianity , with 60.23% of the population reporting they professed that belief, while 33.2% were Protestants , and 4.91% were Roman Catholic . 1. H/E Haile Mariam Desalegn - Former prime Minister of Ethiopia (2012-2018) 2. Teklewold Atnafu - Former governor of National Bank of Ethiopia since 2000s to 6 February 2020. Currently serving as chair person of Commercial Bank of Ethiopia. He

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