کتاب بهرام

Why Are Seas Salty, but Rivers Sweet?

🔹 Introduction

When you sit by a river and dip your hand in the water, you feel the coolness and sweetness of the fresh water. But as soon as you step into the sea, a different taste touches your tongue: saltiness! The question is: why is there this difference? Why are all rivers and springs sweet, but seas and oceans salty?

This simple question opens a door to a grand story in geology, the water cycle, and even the history of our planet.

🔹 Where Does Water Come From?

The waters on Earth mainly come from two large sources:

1. Rain and snow that fall from the clouds to the Earth's surface.
2. Melting ice and springs that originate from underground reservoirs.

These waters are initially almost pure and do not have any saltiness. However, when they flow through the earth's soil and rocks, the story changes.

🔹 Water’s Journey Through the Earth

When rain falls to the earth, it comes into contact with rocks and soil. This water is a "weak acid" because it has dissolved a small amount of carbon dioxide from the air. As a result, it can slowly dissolve minerals like sodium, calcium, potassium, and chlorine from the rocks.

When these waters gather and form rivers:

• Rivers remain sweet because the amount of salt in them is very low compared to the volume of water.
• River water is constantly moving, so salt doesn't accumulate much.

🔹 Why Do Seas Become Salty?

Here is where the main difference becomes evident:

1. Accumulation of Salt Over Millions of Years
   All rivers eventually flow into seas and oceans. Each river carries a small amount of salt, but over millions of years, this process has delivered an enormous amount of salt to the seas.

2. Evaporation of Water Without Salt Leaving
   When seawater evaporates, only the water molecules rise and form clouds. The salt and minerals stay behind in the sea. This continuous cycle has increased the salinity of the seas.

3. Geological Activities
   The ocean floor's cracks (hydrothermal vents) introduce fresh minerals, particularly chloride and sodium ions, into the water. This process plays a big role in maintaining the sea's salinity.

🔹 Sea Salinity Is Not the Same Everywhere

The Atlantic Ocean is saltier than the Pacific Ocean due to higher evaporation rates.

The Caspian Sea and the Aral Sea become much saltier because rivers flow into them, but they have no outlets to the sea, so the salt accumulates.

The Dead Sea is one of the saltiest places on Earth; floating on its water effortlessly is one of nature’s wonders.

🔹 What If Seas Were Not Salty?

• Marine life would be very different. Many creatures depend on specific salinity levels.
• The Earth's chemical and mineral cycles would be disrupted.
• The Earth might struggle to supply essential minerals for life.

🔹 Why Are Rivers Still Sweet?

• River water is continuously flowing, so salt doesn’t accumulate much.
• Fresh rainfall constantly adds new fresh water.
• Rivers evaporate much less compared to seas, so salt doesn’t stay behind.

🔹 A Historical Look

At the beginning of Earth’s formation, acid rains fell on the hot crust and created the first seas. The process of washing rocks and transporting salt has continued for billions of years, and today Earth's oceans have an average salinity of 3.5%.

It’s interesting to note that if all the salt from the seas were removed and spread across the land, it would form a 150-meter-thick layer covering the entire continental surface!

🔹 Conclusion

Rivers are sweet because their continuous flow and replenishment prevent salt accumulation.

Seas are salty because the salt, resulting from millions of years of washing the Earth, has accumulated in them, and evaporation removes the water but not the salt.

This simple difference holds the secret to the survival of different ecosystems on land and at sea.

Persian translation of this article