Ever wondered why a dip in the ocean leaves your skin feeling sticky and your mouth craving fresh water? It’s not magic, it’s science. And the story of ocean salinity is deeper than it seems.
A build-up of salt on the shore of the Dead Sea in Jordan © Source: Zaid Saadallah/ Shutterstock
Let’s first define salinity. Salinity is the measure of dissolved salts in water. It is usually expressed in parts per thousand (ppt). On average, sea water has a salinity of about 35 ppt. This implies that for every 1,000 grams of seawater, 35 grams are salts.
But where does all this salt come from? The answer lies in the Earth’s structure. For millions of years, rainwater which is slightly acidic has been breaking down rocks on land. This is called weathering. It deposits minerals like sodium and chloride into rivers, which eventually carry them into the oceans. Once in the ocean, these salts don’t just vanish. They accumulate, making sea water salty.
Volcanic eruptions under the seafloor and hydrothermal vents also contribute to sea salinity by releasing minerals directly into the ocean. So, while rivers constantly bring in fresh water, they also carry a steady supply of salt, and because oceans don’t have an outlet like rivers do, that salt stays locked in, slowly increasing water salinity over time.
How Much of Earth is Water and Why Are Oceans Salty but Lakes Aren’t?
Let’s zoom out. About 71% of the Earth is covered by water, but all of it’s not saltwater. Oceans hold 97% of the Earth’s water, and it’s this massive volume that contains the bulk of the world’s salt.
But here’s a twist: not all water bodies are equally salty. Rivers, lakes, and glaciers contain freshwater with low salinity. So why don’t they get salty too? The key difference is flow. Rivers and lakes are part of an open system—water comes in and goes out, carrying minerals along the way. Oceans, on the other hand, are like a giant sink with no drain. The salts get in, but they don’t get out, and over time, they’ve turned the oceans into a salty soup.
This also explains why sea salinity can vary slightly around the world. Near the equator, high evaporation increases salinity. In polar regions, melting ice dilutes seawater, lowering it. Salinity isn’t just a number—it’s a living, shifting part of Earth’s climate system.
More Than Just Salt: Why Salinity Matters
Sure, salty oceans can spoil your beach picnic snacks. However, water salinity is crucial for Earth’s ecosystems. It influences ocean currents, marine life, and even global climate patterns. Some creatures are specially adapted to thrive in high salinity, while others do well only in freshwater.
Next time someone asks, “Why are oceans salty?”, you’ll know it’s a story of time, rocks, rivers, and Earth’s remarkable balance. The next time you taste the ocean, remember you’re tasting a billion-year-old process that still shapes our planet today.
Key Takeaways:
- What percentage of the Earth is covered by water? → ~71%
- What is sea salinity? → Average of 35 ppt (parts per thousand)
- Why is the ocean salty? → Rock weathering, river runoff, seafloor activity
- Why aren’t rivers and lakes salty? → They have outlets; oceans don’t.
From raindrops to rocks, and rivers to the vast seas, salinity is the signature of Earth’s long, winding water story.
