When you swim in a lake, the water is fresh; when you dive into the ocean, the water is salty. But why is that? Let’s find out.
The water that flows into the ocean originates in freshwater streams and rivers. These bodies of water do have salt in them. It dissolves from land rocks. This is due to the fact that rain is slightly acidic.
Carbonic acid is formed when it reacts with carbon dioxide in the air. When rain falls on rock, the acid dissolves tiny bit of salt and minerals. These runoffs end up in streams, lakes, and rivers. But those bodies of water don’t appear to be salty—certainly not like the ocean.
Rain may add trace amounts of salt to streams, rivers, and lakes, but it also adds more freshwater. Rain on a regular basis prevents these waters from becoming salty.
Freshwater carries salts and minerals with it when it flows into the ocean. Seafloor vents contribute more salts and minerals. Water seeps into cracks in the Earth’s crust, deep in the ocean. It is heated by magma there. Hot water dissolves rock salts and minerals.
Seawater then flows through seafloor vents, carrying those dissolved elements up into the ocean’s waters. Many of those salts and minerals are utilized by marine life. Organisms, for example, remove iron, zinc, and copper from water.
Sodium and chloride, the main ingredients in table salt, are not. Because they are left behind, salt levels in the ocean have accumulated over time. This is why seawater is salty.
People, animals, and other objects become more buoyant in seawater as their density increases.
The saltiness of seawater (called salinity by scientists) varies across the oceans. It is typically lower near the equator and poles. However, salinity rises in the spaces between.
Some seas, such as the Mediterranean, are saltier than others. Some lakes, like Mono Lake in California and the Caspian Sea in Asia, are even saltier.
Salts are left behind as water evaporates from these landlocked bodies of water. Salt levels continue to rise over time. Many of these salty lakes are located in dry areas with little rain and high daytime temperatures.