Coal seam

Coal beds are sedimentary rocks made of coal. The remains of trees that flourished on Earth tens of millions of years ago or more accumulated and became covered with sediment, forming rocks together with the sediment. Trees are organic matter, and are compounds of elements such as oxygen, hydrogen, carbon, sulfur, phosphorus, and nitrogen, but coal beds are made almost entirely of carbon. Also, due to various geological changes, faults and folds are always present within the coal beds. On the other hand, it seems that the trees flourished and accumulated several times during the same period, and the sedimentary layers often contain several layers, or even a dozen or more layers of coal. For this reason, these sedimentary rock layers are called "coal-bearing layers."

Most of the world's coal-bearing seams contain layers of gymnosperms dating back to the Carboniferous period of the Paleozoic era in geological time. However, in Japan and around the world (for example, the Fushun coal mine in China) there are also layers of coalified pine trees from the Tertiary period of the Cenozoic era. Because they are newer in age, there are many layers of subbituminous coal and lignite, but there are also many layers of high-quality bituminous coal, such as Japanese coal and Fushun coal.

[Toshiro Isobe]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

石炭でできている堆積(たいせき)岩をいう。数千万年あるいはそれ以上古い時代に地球上に繁茂していた樹木の遺骸(いがい)が堆積して土砂をかぶり、その土砂とともに岩石になったものが炭層である。樹木は有機物であり、酸素、水素、炭素、硫黄(いおう)、リン、窒素等々の元素の化合物であるが、炭層はほとんど炭素分のみで形成されている。また、各種の地質変動を受け、炭層内にも断層褶曲(しゅうきょく)等がつねに介在している。一方、同じ時代に何回か繁茂・堆積を繰り返していたらしく、堆積層中には数枚、十数枚以上の炭層を含むことが多い。そこで、この堆積岩層を「夾炭層(きょうたんそう)」という。

　世界の夾炭層はほとんど、地質時代でいえば古生代の石炭紀前後の裸子植物類の炭層を含んでいる。しかし、日本のほか世界各地（たとえば中国の撫順(ぶじゅん)炭鉱）にも新生代第三紀の松柏(しょうはく)類が石炭化したものもある。年代的には新しいので、亜瀝青(あれきせい)炭、亜炭層も多いが、日本炭および撫順炭のように良質の瀝青炭層も少なくない。

［磯部俊郎］

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