LESSON 2
How Did the Ocean Form, and Why is it Salty?
Introduction:
A Planet of Water
Earth is often called the "Blue Planet" because over 70% of its surface is covered in water. But how did this vast ocean come to exist? And why is it salty when most freshwater lakes and rivers are not?
1. The Birth of the Ocean: Where Did All the Water Come From?
Earth wasn’t always covered in water. In fact, when our planet first formed about 4.5 billion years ago, it was a fiery, molten world with no oceans, no lakes—just extreme heat and volcanic activity.
The Two Leading Theories
Theory 1: Water from Within (Volcanic Outgassing)
Billions of years ago, Earth was an unrecognisable planet—a molten sphere of rock with volcanoes constantly erupting across its surface. These violent eruptions didn’t just spew lava; they also released huge amounts of gas, including water vapour (H₂O), into the early atmosphere.
As Earth gradually cooled, this steam condensed into clouds. Eventually, it rained. And rained. And rained—for millions of years. The constant downpour began to fill the planet’s craters and basins, slowly forming the first ancient oceans.
Imagine
A lifeless planet cloaked in steam, where the very first raindrops fell onto boiling rock—a world slowly turning blue.
Theory 2: Water from Space (Comet and Asteroid Bombardment)
Now for the space twist: around the same time, Earth was being bombarded by icy comets and water-rich asteroids. These celestial bodies are loaded with frozen water and dust. With every impact, more water was delivered to our young planet—topping up its oceans from the stars.
Scientists studying the composition of ocean water and extraterrestrial ice have found clues that some of Earth’s water may indeed have an alien origin.
Fun Fact
The water in your cup of tea might include molecules that originated in a comet from billions of kilometres away!
The Reality?
Scientists now believe both processes contributed: volcanic activity released large amounts of water vapour, while ice-bearing comets and asteroids added more water over time.
2. Why is the Ocean Salty?
If the ocean started with fresh rainwater or icy comets, why is it salty?
It all comes down to chemistry—and the way water interacts with rocks.
The Role of Rocks and Rivers
When rain falls, it picks up carbon dioxide from the atmosphere, becoming slightly acidic. As this water flows over rocks and soil, it breaks them down, carrying tiny amounts of dissolved minerals into rivers and streams.
Eventually, these minerals—especially sodium and chloride, the ingredients in common salt—are carried into the sea. Over millions of years, these minerals have built up, giving seawater its salty taste.
Fun Fact
If you could extract all the salt from the ocean and spread it evenly across the world’s land surface, it would form a layer 150 metres deep—about as tall as the London Eye!
Salt Balance: Why Doesn’t the Ocean Keep Getting Saltier?
Great question! While rivers are always adding salt to the ocean, several natural processes remove it:
- Some salt becomes trapped in seabed sediments.
- Marine animals like corals and plankton use minerals to build their shells—these eventually sink and lock salt away.
- Seawater also seeps into the ocean floor at hydrothermal vents, where minerals are exchanged with volcanic rock.
Did you know?
Seawater has remained at roughly 3.5% salt (or 35 parts per thousand) for millions of years, meaning the ocean has a natural balance of adding and removing salt.
3. Why Ocean Chemistry Matters for Life
The chemistry of seawater isn’t just about salt—it affects everything that lives in the ocean.
How Salinity Shapes Marine Life
The ocean's saltiness isn’t just a quirky detail—it’s essential for life.
Most marine creatures are finely tuned to the salt concentration of seawater. A sudden change can disrupt osmosis, the process cells use to balance water and salts, which can be deadly.
Fun Fact
Salmon are osmoregulation experts. They migrate between rivers and the ocean, adjusting their internal chemistry to survive the shift in salinity.
The Ocean’s Role in the Carbon Cycle
Seawater doesn’t just hold salt—it also stores gases like oxygen and carbon dioxide.
In fact, the ocean plays a vital role in the global carbon cycle. It absorbs CO₂ from the atmosphere, helping to regulate Earth's climate. But there’s a catch: too much CO₂ leads to ocean acidification, which threatens shell-forming creatures like corals and some plankton.
Did you know?
The ocean absorbs about 25% of the CO₂ humans release into the atmosphere each year—acting as a giant climate buffer.
Conclusion
Why Understanding Ocean Chemistry is Essential
Earth’s oceans are ancient, formed through volcanic activity and impacts from space over billions of years. Their salinity comes from the slow breakdown of rocks, carried by rivers into the sea.
Key Takeaways:
Earth’s first oceans formed from a combination of volcanic steam and water-rich asteroids.
Salt comes from rock erosion, carried by rivers into the ocean.
The ocean has a natural balance of adding and removing salt, keeping its chemistry stable.
Salinity and ocean chemistry directly affect marine life, ocean currents, and the global carbon cycle.
NEXT LESSON
Ocean Basins and Features: Trenches, Ridges, and Plate Tectonics
The seafloor isn’t flat—it’s shaped by powerful forces that create deep trenches, ridges, and shifting plates. We’ll explore how these features form, how they shape marine life, and what drives earthquakes, tsunamis, and deep-sea vents.
START WITH THE BASICS
