Volcanic gas - Kazan gas (English spelling)

Japanese: 火山ガス - かざんガス（英語表記）volcanic gas

Gas that erupts from volcanoes. It is also called fume, and a place where volcanic gas constantly erupts is called a fumarole. It originates from volatile components contained in magma, but before it erupts due to volcanic activity, its composition changes due to reactions with surrounding rocks, dissolution in groundwater or hot water pools, and mixing with groundwater. The main component is water (water vapor), and in many volcanoes in island arcs (arc-shaped archipelagoes) such as the Japanese archipelago, it accounts for approximately 90% by volume. Other components include carbon dioxide, sulfur dioxide (sulfur oxide), hydrogen sulfide, hydrogen, helium, nitrogen, hydrogen chloride, hydrogen fluoride, and mercury, but the proportions of the components vary greatly depending on the volcano. For example, the average chemical composition of Mt. Usu is 99.2% water, 0.43% carbon dioxide, 0.067% sulfur dioxide, 0.017% hydrogen sulfide, etc., while Mt. Kilauea on the island of Hawaii is 70.75% water, 14.07% carbon dioxide, 6.40% sulfur dioxide, 5.45% nitrogen, 1.92% sulfur trioxide, 0.40% carbon monoxide, 0.33% hydrogen, 0.18% argon, 0.10% sulfur, 0.05% chlorine, etc. Also, even within a single volcano, the composition of volcanic gases can change significantly depending on the location and time of eruptions. Among volcanic gases, sulfur dioxide (colorless, pungent odor) and hydrogen sulfide (colorless, rotten egg odor) are toxic and have adverse effects on the human body. The island-wide evacuation measures implemented on Miyakejima from 2000 to 2005 were largely influenced by the generation of toxic volcanic gases. The Japan Meteorological Agency issues information on volcanic activity, such as eruption warnings and forecasts, ash fall forecasts (→ Volcanic Ash), and volcanic gas forecasts. In addition, when a large amount of volcanic gas containing sulfur enters the stratosphere due to a large volcanic eruption, the amount of solar radiation reaching the ground is reduced by the produced sulfuric acid aerosols, which can temporarily lower the global temperature (→ Volcanic Winter).

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

火山から噴出する気体。噴気ともいい，火山ガスが絶えず噴出する場所を噴気孔という。マグマに含まれる揮発性成分を起源とするが，火山作用によって噴出するまでの間に，周辺の岩石との反応，地下水や熱水だまりへの溶解，地下水との混合などの影響を受けて組成変化する。主成分は水（水蒸気）で，日本列島など島弧（弧状列島）の多くの火山では体積比はおよそ 90%以上を占める。ほかに二酸化炭素，二酸化硫黄（酸化硫黄），硫化水素，水素，ヘリウム，窒素，塩化水素，フッ化水素，水銀などが含まれるが，成分の割合は火山によって大きく異なる。たとえば有珠山の平均的化学成分は水 99.2%，二酸化炭素 0.43%，二酸化硫黄 0.067%，硫化水素 0.017%等で，ハワイ島のキラウエア山では水 70.75%，二酸化炭素 14.07%，二酸化硫黄 6.40%，窒素 5.45%，三酸化硫黄 1.92%，一酸化炭素 0.40%，水素 0.33%，アルゴン 0.18%，硫黄 0.10%，塩素 0.05%等である。また一つの火山でも，噴出する場所や時期によって火山ガスの成分が著しく変化することもある。火山ガスのうち，二酸化硫黄（無色・刺激臭），硫化水素（無色・腐卵臭）などは有毒で，人体に悪影響を及ぼす。2000～05年に三宅島で実施された全島避難措置は，有毒な火山ガスの発生が大きく影響した。気象庁は火山活動に関する情報として，噴火警報・予報，降灰予報（→火山灰）のなどほかに火山ガス予報を発表している。また，大規模な火山噴火に伴い硫黄分を含んだ大量の火山ガスが成層圏に入ると，生成された硫酸のエアロゾルにより地上に到達する日射量が減少し，一時的に世界の気温を低下させることもある（→火山の冬）。

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