Why Is the Sea Salty?
海はなぜしょっぱいの？
Why is the sea salty? The salt comes from rocks on land. Rain falls on the rocks and breaks them down very slowly. Tiny bits of salt go into the rain water. Rivers carry this water to the sea. The sun heats the sea, and some of the water goes up into the air. But the salt does not go up. It stays in the sea. This has happened for millions of years. That is why the sea is salty.
Have you ever wondered why the sea is salty but rivers are not? Most of the salt in the ocean comes from rocks on land. Rainwater is slightly acidic because it contains a little carbon dioxide from the air. When rain falls on rocks, it slowly breaks them down and releases salts and other minerals. Rivers then carry these into the ocean. River water contains only a tiny amount of salt, so it does not taste salty. In the ocean, however, the sun heats the water and it evaporates, but the salt stays behind. Over millions of years, the salt has built up. Some salt also comes from hot openings in the seafloor. Today, about 3.5 percent of the weight of seawater is dissolved salt.
Anyone who has swallowed a mouthful of seawater knows that it is far too salty to drink. But where does all that salt come from? According to the U.S. Geological Survey, most of it originally came from rocks on land. Rain absorbs a small amount of carbon dioxide from the air, which makes it slightly acidic. Over time, this weak acid wears away rocks and releases dissolved substances called ions. Streams and rivers carry these ions to the ocean. Living things use some of them, but many remain in the water, and their concentration has gradually increased over enormous periods of time. Rivers taste fresh because the amount of salt they carry at any moment is tiny; only in the ocean, where water constantly evaporates and leaves the salt behind, does it become concentrated. A second source lies beneath the sea itself. At hydrothermal vents, seawater seeps into cracks in the ocean floor, is heated by magma and comes back out carrying dissolved metals and minerals. Underwater volcanoes also add minerals. Today, dissolved salts make up about 3.5 percent of the weight of seawater, and most of them are sodium and chloride, the two ingredients of ordinary table salt. The amount varies from place to place: water near the equator and the poles tends to be less salty, while the middle latitudes are saltier. The total is huge. The USGS estimates that if all the salt in the ocean were spread evenly over Earth's land, it would form a layer more than 160 meters thick.
Level 2 の和訳を見る
海はしょっぱいのに、川はしょっぱくないのはなぜだろうと思ったことはありますか？海の塩のほとんどは、陸の岩から来ています。雨水は空気中の二酸化炭素を少し含んでいるため、わずかに酸性です。雨が岩に降ると、岩を少しずつくずし、塩分やほかのミネラルを溶かし出します。それを川が海へ運ぶのです。川の水に含まれる塩はごくわずかなので、しょっぱくは感じません。ところが海では、太陽が水を温めて水は蒸発しますが、塩はあとに残ります。何百万年ものあいだに、塩がたまってきたのです。海底にある熱い噴出口から来る塩もあります。現在、海水の重さのおよそ3.5％が、溶けている塩分です。
キーワード
塩からい、しょっぱい
（少しずつ）くずす、分解する
運ぶ
酸性の
放出する、溶かし出す
蒸発する
たまる、蓄積する
溶けている
熱水噴出孔
濃度
質問に英語で答えよう
Q1. Where does most of the salt in the ocean come from?
Most of it comes from rocks on land. Rain slowly breaks down the rocks, and rivers carry the salts to the sea.
Q2. Why don't rivers taste salty?
Because river water contains only a tiny amount of salt. The salt becomes concentrated only in the ocean, where water evaporates and the salt stays behind.
Q3. What is a second source of salt in the ocean?
Hydrothermal vents on the seafloor, where seawater is heated by magma and comes back out carrying minerals. Underwater volcanoes also add minerals.
意見を言ってみよう（独り言・日記でOK）
- Before reading this article, why did you think the sea was salty?
- Seawater can be turned into drinking water, but it is expensive. Do you think more countries should do this? Why or why not?
事実の確認に使った資料：oceanservice.noaa.gov・www.usgs.gov（英文は原田英語の書きおろしです）