A caldera is a large cauldron-like hollow that forms following the evacuation of a magma chamber/reservoir. When large volumes of magma are erupted over a short time, structural support for the crust above the magma chamber is lost; the ground surface collapses downward into the emptied magma chamber, leaving a massive depression at the surface. Although sometimes described as a crater, the feature is a type of sinkhole, as it is formed through subsidence and collapse rather than an explosion or impact. Only seven known caldera-forming collapses have occurred since the start of the 20th century, most at Bárðarbunga volcano in Iceland; the word comes from Spanish caldera, Latin caldaria, meaning "cooking pot". In some texts the English term cauldron is used; the term caldera was introduced into the geological vocabulary by the German geologist Leopold von Buch when he published his memoirs of his 1815 visit to the Canary Islands, where he first saw the Las Cañadas caldera on Tenerife, with Montaña Teide dominating the landscape, the Caldera de Taburiente on La Palma.
A collapse is triggered by the emptying of the magma chamber beneath the volcano, sometimes as the result of a large explosive volcanic eruption, but during effusive eruptions on the flanks of a volcano or in a connected fissure system. If enough magma is ejected, the emptied chamber is unable to support the weight of the volcanic edifice above it. A circular fracture, the "ring fault", develops around the edge of the chamber. Ring fractures serve as feeders for fault intrusions which are known as ring dikes. Secondary volcanic vents may form above the ring fracture; as the magma chamber empties, the center of the volcano within the ring fracture begins to collapse. The collapse may occur as the result of a single cataclysmic eruption, or it may occur in stages as the result of a series of eruptions; the total area that collapses may be thousands of square kilometers. Some calderas are known to host rich ore deposits. One of the world's best-preserved mineralized calderas is the Sturgeon Lake Caldera in northwestern Ontario, which formed during the Neoarchean era about 2,700 million years ago.
If the magma is rich in silica, the caldera is filled in with ignimbrite, tuff and other igneous rocks. Silica-rich magma has a high viscosity, therefore does not flow like basalt; as a result, gases tend to become trapped at high pressure within the magma. When the magma approaches the surface of the Earth, the rapid off-loading of overlying material causes the trapped gases to decompress thus triggering explosive destruction of the magma and spreading volcanic ash over wide areas. Further lava flows may be erupted. If volcanic activity continues, the center of the caldera may be uplifted in the form of a resurgent dome such as is seen at Cerro Galán, Lake Toba, etc. by subsequent intrusion of magma. A silicic or rhyolitic caldera may erupt hundreds or thousands of cubic kilometers of material in a single event. Small caldera-forming eruptions, such as Krakatoa in 1883 or Mount Pinatubo in 1991, may result in significant local destruction and a noticeable drop in temperature around the world.
Large calderas may have greater effects. When Yellowstone Caldera last erupted some 650,000 years ago, it released about 1,000 km3 of material, covering a substantial part of North America in up to two metres of debris. By comparison, when Mount St. Helens erupted in 1980, it released ~1.2 km3 of ejecta. The ecological effects of the eruption of a large caldera can be seen in the record of the Lake Toba eruption in Indonesia. About 74,000 years ago, this Indonesian volcano released about 2,800 cubic kilometres dense-rock equivalent of ejecta; this was the largest known eruption during the ongoing Quaternary period and the largest known explosive eruption during the last 25 million years. In the late 1990s, anthropologist Stanley Ambrose proposed that a volcanic winter induced by this eruption reduced the human population to about 2,000–20,000 individuals, resulting in a population bottleneck. More Lynn Jorde and Henry Harpending proposed that the human species was reduced to 5,000-10,000 people.
There is no direct evidence, that either theory is correct, there is no evidence for any other animal decline or extinction in environmentally sensitive species. There is evidence. Eruptions forming larger calderas are known La Garita Caldera in the San Juan Mountains of Colorado, where the 5,000 cubic kilometres Fish Canyon Tuff was blasted out in eruptions about 27.8 million years ago. At some points in geological time, rhyolitic calderas have appeared in distinct clusters; the remnants of such clusters may be found in places such as the San Juan Mountains of Colorado or the Saint Francois Mountain Range of Missouri. Some volcanoes, such as the large shield volcanoes Kīlauea and Mauna Loa on the island of Hawaii, form calderas in a different fashion; the magma feeding these volcanoes is basalt, silica poor. As a result, the magma is much less viscous than the magma of a rhyolitic volcano, the magma chamber is drained by large lava flows rather than by explosive events; the resulting calderas are known as subsidence calderas and can form more than explosive calderas.
For instance, the caldera atop Fernandina Island collapsed
The Edo period or Tokugawa period is the period between 1603 and 1868 in the history of Japan, when Japanese society was under the rule of the Tokugawa shogunate and the country's 300 regional daimyō. The period was characterized by economic growth, strict social order, isolationist foreign policies, a stable population, "no more wars", popular enjoyment of arts and culture; the shogunate was established in Edo on March 24, 1603, by Tokugawa Ieyasu. The period came to an end with the Meiji Restoration on May 1868, after the fall of Edo. A revolution took place from the time of the Kamakura shogunate, which existed with the Tennō's court, to the Tokugawa, when the samurai became the unchallenged rulers in what historian Edwin O. Reischauer called a "centralized feudal" form of shogunate. Instrumental in the rise of the new-existing bakufu was Tokugawa Ieyasu, the main beneficiary of the achievements of Oda Nobunaga and Toyotomi Hideyoshi. Powerful, Ieyasu profited by his transfer to the rich Kantō area.
He maintained two million koku of land, a new headquarters at Edo, a strategically situated castle town, had an additional two million koku of land and thirty-eight vassals under his control. After Hideyoshi's death, Ieyasu moved to seize control from the Toyotomi clan. Ieyasu's victory over the western daimyō at the Battle of Sekigahara gave him control of all Japan, he abolished numerous enemy daimyō houses, reduced others, such as that of the Toyotomi, redistributed the spoils of war to his family and allies. Ieyasu still failed to achieve complete control of the western daimyō, but his assumption of the title of shōgun helped consolidate the alliance system. After further strengthening his power base, Ieyasu installed his son Hidetada as shōgun and himself as retired shōgun in 1605; the Toyotomi were still a significant threat, Ieyasu devoted the next decade to their eradication. In 1615, the Tokugawa army destroyed the Toyotomi stronghold at Osaka; the Tokugawa period brought 250 years of stability to Japan.
The political system evolved into what historians call bakuhan, a combination of the terms bakufu and han to describe the government and society of the period. In the bakuhan, the shōgun had national authority and the daimyō had regional authority; this represented a new unity in the feudal structure, which featured an large bureaucracy to administer the mixture of centralized and decentralized authorities. The Tokugawa became more powerful during their first century of rule: land redistribution gave them nearly seven million koku, control of the most important cities, a land assessment system reaping great revenues; the feudal hierarchy was completed by the various classes of daimyō. Closest to the Tokugawa house were the shinpan, or "related houses", they were twenty-three daimyō on the borders of Tokugawa lands. The shinpan held honorary titles and advisory posts in the bakufu; the second class of the hierarchy were the fudai, or "house daimyō", rewarded with lands close to the Tokugawa holdings for their faithful service.
By the 18th century, 145 fudai controlled the greatest assessed at 250,000 koku. Members of the fudai class staffed most of the major bakufu offices. Ninety-seven han formed the tozama, former opponents or new allies; the tozama were located on the peripheries of the archipelago and collectively controlled nearly ten million koku of productive land. Because the tozama were least trusted of the daimyō, they were the most cautiously managed and generously treated, although they were excluded from central government positions; the Tokugawa shogunate not only consolidated their control over a reunified Japan, they had unprecedented power over the emperor, the court, all daimyō and the religious orders. The emperor was held up as the ultimate source of political sanction for the shōgun, who ostensibly was the vassal of the imperial family; the Tokugawa helped the imperial family recapture its old glory by rebuilding its palaces and granting it new lands. To ensure a close tie between the imperial clan and the Tokugawa family, Ieyasu's granddaughter was made an imperial consort in 1619.
A code of laws was established to regulate the daimyō houses. The code encompassed private conduct, dress, types of weapons and numbers of troops allowed. Although the daimyō were not taxed per se, they were levied for contributions for military and logistical support and for such public works projects as castles, roads and palaces; the various regulations and levies not only strengthened the Tokugawa but depleted the wealth of the daimyō, thus weakening their threat to the central administration. The han, once military-centered domains, became mere local administrative units; the daimyō did have full administrative control over their territory and their complex systems of retainers and commoners. Loyalty was exacted from religious foundations greatly weakened by Nobunaga and Hideyoshi, through a variety of control mechanisms. Like Hideyoshi, Ieyasu encouraged foreign trade but was suspicious of outsiders, he wanted to make Edo a major port, but once he learned that the Europeans favored ports in Kyūshū and that China had rejected his plans for official trade, he moved to control existing trade
A stratovolcano known as a composite volcano, is a conical volcano built up by many layers of hardened lava, tephra and ash. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with a summit crater and periodic intervals of explosive eruptions and effusive eruptions, although some have collapsed summit craters called calderas; the lava flowing from stratovolcanoes cools and hardens before spreading far, due to high viscosity. The magma forming this lava is felsic, having high-to-intermediate levels of silica, with lesser amounts of less-viscous mafic magma. Extensive felsic lava flows have travelled as far as 15 km. Stratovolcanoes are sometimes called "composite volcanoes" because of their composite stratified structure built up from sequential outpourings of erupted materials, they are in contrast to the less common shield volcanoes. Two famous examples of stratovolcanoes are Krakatoa, known for its catastrophic eruption in 1883 and Vesuvius, whose eruption in AD79 caused destruction of Pompeii and Herculaneum in 79 AD.
Both eruptions claimed thousands of lives. In modern times, Mount St. Helens and Mount Pinatubo have erupted catastrophically, with lesser losses of lives; the possible existence of stratovolcanoes on other terrestrial bodies of the Solar System has not been conclusively demonstrated. The one feasible exception are the existence of some isolated massifs on Mars, for example the Zephyria Tholus. Stratovolcanoes are common at subduction zones, forming chains and clusters along plate tectonic boundaries where oceanic crust is drawn under continental crust or another oceanic plate; the magma forming stratovolcanoes rises when water trapped both in hydrated minerals and in the porous basalt rock of the upper oceanic crust is released into mantle rock of the asthenosphere above the sinking oceanic slab. The release of water from hydrated minerals is termed "dewatering", occurs at specific pressures and temperatures for each mineral, as the plate descends to greater depths; the water freed from the rock lowers the melting point of the overlying mantle rock, which undergoes partial melting and rises due to its lighter density relative to the surrounding mantle rock, pools temporarily at the base of the lithosphere.
The magma rises through the crust, incorporating silica-rich crustal rock, leading to a final intermediate composition. When the magma nears the top surface, it pools in a magma chamber within the crust below the stratovolcano. There, the low pressure allows water and other volatiles dissolved in the magma to escape from solution, as occurs when a bottle of carbonated water is opened, releasing CO2. Once a critical volume of magma and gas accumulates, the plug of the volcanic vent is broken, leading to a sudden explosive eruption. In recorded history, explosive eruptions at subduction zone volcanoes have posed the greatest hazard to civilizations. Subduction-zone stratovolcanoes, such as Mount St. Helens, Mount Etna and Mount Pinatubo erupt with explosive force: the magma is too stiff to allow easy escape of volcanic gases; as a consequence, the tremendous internal pressures of the trapped volcanic gases remain and intermingle in the pasty magma. Following the breaching of the vent and the opening of the crater, the magma degasses explosively.
The magma and gases blast out with full force. Since 1600 CE, nearly 300,000 people have been killed by volcanic eruptions. Most deaths were caused by pyroclastic flows and lahars, deadly hazards that accompany explosive eruptions of subduction-zone stratovolcanoes. Pyroclastic flows are swift, avalanche-like, ground-sweeping, incandescent mixtures of hot volcanic debris, fine ash, fragmented lava and superheated gases that can travel at speeds in excess of 160 km/h. Around 30,000 people were killed by pyroclastic flows during the 1902 eruption of Mount Pelée on the island of Martinique in the Caribbean. In March to April 1982, three explosive eruptions of El Chichón in the State of Chiapas in southeastern Mexico, caused the worst volcanic disaster in that country's history. Villages within 8 km of the volcano were destroyed by pyroclastic flows, killing more than 2,000 people. Two Decade Volcanoes that erupted in 1991 provide examples of stratovolcano hazards. On June 15, Mount Pinatubo spewed an ash cloud 40 km into the air and produced huge pyroclastic surges and lahar floods that devastated a large area around the volcano.
Pinatubo, located in Central Luzon just 90 km west-northwest from Manila, had been dormant for 6 centuries before the 1991 eruption, which ranks as one of the largest eruptions in the 20th century. In 1991, Japan's Unzen Volcano, located on the island of Kyushu about 40 km east of Nagasaki, awakened from its 200-year slumber to produce a new lava dome at its summit. Beginning in June, repeated collapse of this erupting dome generated ash flows that swept down the mountain's slopes at speeds as high as 200 km/h. Unzen is one of more than 75 active volcanoes in Japan; the eruption of Mount Vesuvius in 79 smothered the nearby ancient cities of Pompeii and Herculaneum with thick deposits of pyroclastic surges and lava flows. Although death toll is estimated between 13,000 and 26,000 remains, the exact number still remains unknown. Vesuvius is recognized as one of the most dangerous volcanoes, due to its
Oki District, Shimane
Oki is a district located in Shimane Prefecture, Japan. As of 2003, the district has an estimated population of 24,500 and a density of 70.79 persons per km². The total area is 346.10 km². The Oki District encompasses the Oki Islands called Oki Province. Ama Chibu Nishinoshima Okinoshima - formed from the merger of Fuse, Saigō and Tsuma on October 1, 2004
The Shōgun was the military dictator of Japan during the period from 1185 to 1868. The shogunate was their government. In most of this period, the shōguns were the de facto rulers of the country, although nominally they were appointed by the Emperor as a ceremonial formality; the shōguns held absolute power over territories through military means. An unusual situation occurred in the Kamakura period upon the death of the first shōgun, whereby the Hōjō clan's hereditary titles of shikken and tokusō dominated the shogunate as dictatorial positions, collectively known as the Regent Rule; the shōguns during this 134-year period met the same fate as the Emperor and were reduced to figurehead status until a coup d'état in 1333, when the shōgun was restored to power in the name of the Emperor. Shōgun is the short form of Sei-i Taishōgun, the individual governing the country at various times in the history of Japan, ending when Tokugawa Yoshinobu relinquished the office to Emperor Meiji in 1867; the tent symbolized the field commander but denoted that such an office was meant to be temporary.
The shōgun's officials were collectively the bakufu, were those who carried out the actual duties of administration, while the imperial court retained only nominal authority. In this context, the office of the shōgun had a status equivalent to that of a viceroy or governor-general, but in reality, shōguns dictated orders to everyone including the reigning Emperor. In contemporary terms, the role of the shōgun was equivalent to that of a generalissimo; the title of Sei-i Taishōgun was given to military commanders during the early Heian period for the duration of military campaigns against the Emishi, who resisted the governance of the Kyoto-based imperial court. Ōtomo no Otomaro was the first Sei-i Taishōgun. The most famous of these shōguns was Sakanoue no Tamuramaro. In the Heian period, one more shōgun was appointed. Minamoto no Yoshinaka was named sei-i taishōgun during the Genpei War, only to be killed shortly thereafter by Minamoto no Yoshitsune. In the early 11th century, daimyō protected by samurai came to dominate internal Japanese politics.
Two of the most powerful families – the Taira and Minamoto – fought for control over the declining imperial court. The Taira family seized control from 1160 to 1185, but was defeated by the Minamoto in the Battle of Dan-no-ura. Minamoto no Yoritomo seized power from the central government and aristocracy and established a feudal system based in Kamakura in which the private military, the samurai, gained some political powers while the Emperor and the aristocracy remained the de jure rulers. In 1192, Yoritomo was awarded the title of Sei-i Taishōgun by the Emperor and the political system he developed with a succession of shōguns as the head became known as a shogunate. Yoritomo's wife's family, the Hōjō, seized power from the Kamakura shōguns; when Yoritomo's sons and heirs were assassinated, the shōgun himself became a hereditary figurehead. Real power rested with the Hōjō regents; the Kamakura shogunate lasted for 150 years, from 1192 to 1333. In 1274 and 1281, the Mongol Empire launched invasions against Japan.
An attempt by Emperor Go-Daigo to restore imperial rule in the Kenmu Restoration in 1331 was unsuccessful, but weakened the shogunate and led to its eventual downfall. The end of the Kamakura shogunate came when Kamakura fell in 1333, the Hōjō Regency was destroyed. Two imperial families – the senior Northern Court and the junior Southern Court – had a claim to the throne; the problem was solved with the intercession of the Kamakura shogunate, who had the two lines alternate. This lasted until 1331, when Emperor Go-Daigo tried to overthrow the shogunate to stop the alternation; as a result, Daigo was exiled. Around 1334 -- 1336, Ashikaga Takauji helped; the fight against the shogunate left the Emperor with too many people claiming a limited supply of land. Takauji turned against the Emperor when the discontent about the distribution of land grew great enough. In 1336 Daigo was banished again, in favor of a new Emperor. During the Kenmu Restoration, after the fall of the Kamakura shogunate in 1333, another short-lived shōgun arose.
Prince Moriyoshi, son of Go-Daigo, was awarded the title of Sei-i Taishōgun. However, Prince Moriyoshi was put under house arrest and, in 1335, killed by Ashikaga Tadayoshi. In 1338, Ashikaga Takauji, like Minamoto no Yoritomo, a descendant of the Minamoto princes, was awarded the title of sei-i taishōgun and established the Ashikaga shogunate, which lasted until 1573; the Ashikaga had their headquarters in the Muromachi district of Kyoto, the time during which they ruled is known as the Muromachi period. While the title of Shōgun went into abeyance due to technical reasons, Oda Nobunaga and his successor, Toyotomi Hideyoshi, who obtained the position of Imperial Regent, gained far greater power than any of their predecessors had. Hideyoshi is considered by many historians to be among Japan's greatest rulers. Tokugawa Ieyasu seized power and established a government at Edo in 1600, he received the title sei-i taishōgun in 1603, after he forged a family tree to show he was of Minamoto descent.
The Tokugawa shogunate lasted until 1867, when Tokugawa Yoshinobu resigned as shōgun and abdicated his authority to Emperor Meiji. Ieyasu set a precedent in 1605 when he retired as shōgun in favour of his son Tokugawa Hidetada, though he maintained power from b
The Jōmon period is the time in Japanese prehistory, traditionally dated between c. 14,000–300 BCE refined to about 1000 BCE, during which Japan was inhabited by a hunter-gatherer culture, which reached a considerable degree of sedentism and cultural complexity. The name "cord-marked" was first applied by the American scholar Edward S. Morse, who discovered sherds of pottery in 1877 and subsequently translated it into Japanese as jōmon; the pottery style characteristic of the first phases of Jōmon culture was decorated by impressing cords into the surface of wet clay and is accepted to be among the oldest in East Asia and the world. The Jōmon period was rich in tools and jewellery made from bone, stone and antler, it is compared to pre-Columbian cultures of the North American Pacific Northwest and to the Valdivia culture in Ecuador because in these settings cultural complexity developed within a hunting-gathering context with limited use of horticulture. The long 14,000 years, Jōmon period is conventionally divided into a number of phases: Incipient, Early, Middle and Final, with the phases getting progressively shorter.
The fact that this entire period is given the same name by archaeologists should not be taken to mean that there was not considerable regional and temporal diversity. Dating of the Jōmon sub-phases is based upon ceramic typology, to a lesser extent radiocarbon dating. Traces of Paleolithic culture stone tools, occur in Japan from around 30,000 BCE onwards; the earliest "Incipient Jōmon" phase began while Japan was still linked to continental Asia as a narrow peninsula. As the glaciers melted following the end of the last glacial period, sea levels rose, separating the Japanese archipelago from the Asian mainland. In addition, a continuous chain of islands encompasses Luzon, Taiwan and Kyushu, allowing for continuous contact between the Jōmon and maritime Southeast Asia. Within the archipelago, the vegetation was transformed by the end of the Ice Age. In southwestern Honshu and Kyushu, broadleaf evergreen trees dominated the forests, whereas broadleaf deciduous trees and conifers were common in northeastern Honshu and southern Hokkaido.
Many native tree species, such as beeches, buckeyes and oaks produced edible nuts and acorns. These provided abundant sources of food for animals. In the northeast, the plentiful marine life carried south by the Oyashio Current salmon, was another major food source. Settlements along both the Sea of Japan and the Pacific Ocean subsisted on immense amounts of shellfish, leaving distinctive middens that are now prized sources of information for archaeologists. Other food sources meriting special mention include Sika deer, wild boar, wild plants such as yam-like tubers, freshwater fish. Supported by the productive deciduous forests and an abundance of seafood, the population was concentrated in central and northern Honshu, but Jōmon sites range from Hokkaido to the Ryukyu Islands; the earliest pottery in Japan was made before the start of the Incipient Jōmon period. Small fragments, dated to 14,500 BCE, were found at the Odai Yamamoto I site in 1998. Pottery of the same age was subsequently found at other sites such as Kamikuroiwa and Fukui Cave.
Archaeologist Junko Habu claims "he majority of Japanese scholars believed, still believe, that pottery production was first invented in mainland Asia and subsequently introduced into the Japanese archipelago." This seems to be confirmed by recent archaeology. As of now, the earliest pottery vessels in the world date back to 20,000 BP and were discovered in Xianren Cave in Jiangxi, China; the pottery may have been used as cookware. Other early pottery vessels include those excavated from the Yuchanyan Cave in southern China, dated from 16,000 BCE, at present it appears that pottery emerged at the same time in Japan, in the Amur River basin of the Russian Far East; the first Jōmon pottery is characterized by the cord-marking that gives the period its name and has now been found in large numbers of sites. The pottery of the period has been classified by archaeologists into some 70 styles, with many more local varieties of the styles; the antiquity of Jōmon pottery was first identified after World War II, through radiocarbon dating methods.
The earliest vessels were smallish round-bottomed bowls 10–50 cm high that are assumed to have been used for boiling food and storing it beforehand. They belonged to hunter-gatherers and the size of the vessels may have been limited by a need for portability; as bowls increase in size, this is taken to be a sign of an settled pattern of living. These types continued to develop, with elaborate patterns of decoration, undulating rims, flat bottoms so that they could stand on a surface; the manufacture of pottery implies some form of sedentary life because pottery is heavy and fragile and thus unusable for hunter-gatherers. However, this doe
Daisen-Oki National Park
Daisen-Oki National Park is a national park in the Chūgoku region, Honshū, spans the prefectures of Okayama and Tottori. Mount Daisen is the focus of the park, which includes the volcanic mountains and plains of Hiruzen, Mount Kenashi, Mount Sanbe, Mount Hōbutsu; the Izumo Plain region of the park is home to the oldest Shinto shrine in the Izumo-taisha. The Oki Islands are an important component of the park; the park was established in 1936 as Daisen National Park, but was expanded and renamed in 1961 to include the Oki Islands and Shimane Prefecture areas. List of national parks of Japan Introducing places of interest: Daisen-Oki National Park Daisen-Oki National Park Daisen-Oki National Park