A division of geological time. The latest epoch following the Pleistocene epoch of the Quaternary Period of the Cenozoic Era. It is the period from about 11,700 years ago to the present. It is also called the present epoch. The strata formed during this period are called the Holocene. The Holocene was once called the Alluvial Epoch. The terms alluvium and the corresponding diluvial epoch originate from the fact that glacial deposits left on hills and other places were called diluvial epochs, and deposits that filled valleys that had been eroded by them were called alluvial epochs in the old days in northern Germany. However, it became clear that the lower half of the alluvial epoch was the Pleistocene epoch, and this name is no longer used internationally as a division of time. At the end of the Pleistocene epoch, the climate warmed with repeated cold and warm periods, but the Holocene began when the last sudden cold event (the Younger Dryas cold period) ended and long-term, large-scale warming began. The Holocene is a period characterized by rapid warming and sea level rise. Humans learned to farm around the beginning of this period, and the warm climate led to a rapid population growth and the development of civilization. At the beginning of the Holocene, sea levels near Japan were about 100 meters lower than today, but with the rapid rise in temperature, they rose rapidly, exceeding the current level by 3 meters around 7,000 years ago. This warmer period is called the climatic optimum or hypsithermal. Sea levels then dropped slightly to their current level. As the sea level rose, seawater invaded the lowlands that had previously been rivers on land, and marine strata were deposited on top of the river sediments. This Holocene transgression is named the "Yurakucho Transgression" after the place in Tokyo where these strata and their interrelationships were first described. This transgression allowed the sea to invade the lowlands of the Kanto Plain, forming deep inlets along the former rivers. Shell mounds from the Early Jomon to Early Jomon periods are found in various places along the coast of this inlet, and for this reason this transgression is also called the "Jomon Transgression." During the warmest period, subtropical shellfish such as the cockle, whose distribution is currently limited to western Kyushu, spread as far as the southern Tohoku region. A small coral reef also formed near the tip of the Boso Peninsula. [Kiyotaka Chinzai August 19, 2015] [Reference items] | | | |Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend |
地質時代の区分の一つ。新生代第四紀の更新世から続く時代で、もっとも新しい世(せい)。約1万1700年前から現在までの期間。現世ともいう。この時代に形成された地層は完新統とよばれている。完新世はかつては沖積世といった。沖積層および対応する洪積層という語は、古く北ドイツなどで、丘陵などの上に残した氷河堆積(たいせき)物を洪積層、それを侵食した谷を埋める積物を沖積層とよんだことに由来する。しかし沖積層はその下半部が時代的には更新世であることが明らかになって、この名称は時代区分の名称としては国際的に使用されなくなっている。更新世末期には寒冷期と温暖期を繰り返しながら温暖化するが、最後の急激な寒冷事件(ヤンガー・ドリアス寒冷期)が終り、長期的、大規模な温暖化が始まったときを完新世の始まりとする。完新世は急速な温暖化と海面上昇を特徴とする時代である。人類はこの時代の始めごろから農耕を覚え、温暖な気候のもと人口が急増し、文明が発達した。 日本付近の海面高度は完新世の始めには現在より100メートル程低い位置にあったが、著しい温暖化とともに急速に上昇し、7000年前ごろには現在のレベルを超えてプラス3メートルまで上昇した。この現在より温暖な時期を気候最温暖期あるいはヒプシサーマルとよんでいる。その後海面はやや低下して現在の位置に至った。海面が上昇して、それまで陸上の河川であった低地に海水が侵入し、河川の堆積物の上に海の地層が堆積した。完新世のこの海進は、これらの地層とその相互関係が最初に記載された東京の地名をとって「有楽町海進」と命名されている。この海進により関東平野の低地に海が侵入し、かつての河川沿いに奥深い入江が形成された。この入江の沿岸には縄文早期から前期の貝塚が各所に形成されていて、このためこの海進は「縄文海進」ともよばれている。気候最温暖期には現在は九州西部に主要な分布の限界があるハイガイなどの亜熱帯の貝類が東北地方南部部にまで進出していた。房総半島の先端付近には小規模ながらサンゴ礁も形成された。 [鎮西清高 2015年8月19日] [参照項目] | | | |出典 小学館 日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について 情報 | 凡例 |
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