Learn › Science
Oceans and Seas: A Kid-Friendly Guide to Earth's Waters
About 71 percent of Earth's surface is covered with salt water, and most of it is in the ocean. Oceans regulate the climate, produce much of the oxygen we breathe through tiny floating plants called phytoplankton, and are home to an enormous variety of life.
The five oceans
| Ocean | Notes |
|---|---|
| Pacific | The largest and deepest ocean. It holds the Mariana Trench, the deepest known place on Earth, nearly 11 kilometers (about 7 miles) deep. |
| Atlantic | Second largest. It separates the Americas from Europe and Africa. |
| Indian | Third largest, mostly in the Southern Hemisphere, and warm. |
| Southern | Surrounds Antarctica. Cold, with strong currents. |
| Arctic | The smallest and shallowest. Much of it is covered by sea ice. |
In reality they are all one connected body of water, sometimes called the "world ocean."
What is a sea?
A sea is generally a smaller part of an ocean that is partly enclosed by land, such as the Mediterranean Sea, the Caribbean Sea, or the Red Sea. A few "seas" are actually salty lakes, like the Dead Sea.
Why is the ocean salty?
Rain slowly wears away rocks on land and carries tiny amounts of minerals into rivers, which flow into the ocean. When ocean water evaporates, the salt stays behind. Ocean water is about 3.5 percent salt, which means a kilogram of seawater holds around 35 grams of salt. Because salt water is denser than fresh water, you float more easily in the sea.
Ocean zones
- Sunlight zone (0 to about 200 meters): bright and warm, with most sea life including fish, dolphins, sea turtles and coral reefs.
- Twilight zone (200 to 1,000 meters): dim light, home to animals like squid and lanternfish that make their own light.
- Midnight zone (1,000 to 4,000 meters): no sunlight, cold and under great pressure. Anglerfish live here.
- The abyss and trenches (below 4,000 meters): near freezing and extremely high pressure, with strange creatures adapted to live there.
Tides and waves
Tides are the daily rise and fall of the sea level, mostly caused by the Moon's gravity pulling on the water. Most coasts have two high tides and two low tides each day. Waves are mainly caused by wind blowing across the water's surface; the energy moves across the water, but the water itself mostly moves in circles.
Ocean currents
Currents are like rivers inside the ocean. The Gulf Stream carries warm water from the Gulf of Mexico across the Atlantic toward Europe, which helps keep parts of western Europe milder than other places at the same latitude. Currents are driven by wind, Earth's rotation and differences in temperature and saltiness.
Ocean life
The blue whale is the largest animal known to have ever lived, and it eats tiny shrimp-like animals called krill. Coral reefs cover less than 1 percent of the ocean floor but support a very large share of marine species. The Great Barrier Reef off Australia is the largest coral reef system in the world.
Protecting the ocean
Plastic waste, overfishing and warming water all harm marine life. Kids can help by using reusable bottles and bags, joining beach cleanups, and learning which seafood is sustainably caught. Even a bucket and gloves on a local beach or riverbank makes a difference.
Simple experiments
- Salt and floating: add salt to a glass of water, one spoon at a time, and see whether an egg floats higher.
- Wave bottle: fill a clear bottle partly with water and a few drops of blue food coloring, add oil, cap it and tilt gently.
- Evaporation: leave a shallow dish of salt water in the sun and watch the salt crystals appear as the water dries.
Practice quiz
- How many oceans are there? Name them.
- Which ocean is the largest?
- About what percent of Earth's surface is covered by water?
- What causes most ocean tides?
- In which zone does no sunlight reach?
- Why is the ocean salty?
- Name the largest animal in the ocean.
Answer key
1) Five: Pacific, Atlantic, Indian, Southern, Arctic 2) Pacific 3) About 71 percent 4) The Moon's gravity 5) The midnight zone (and below) 6) Rivers carry dissolved minerals from rocks into the sea, and salt remains when water evaporates 7) The blue whale
Practice it on BrainBucks
Kids get questions matched to their level, instant feedback, and points for correct answers. Free to try, and parents stay in control of the account.
Try it freeMore guides
Questions or corrections? Email hello.brainbucks@gmail.com.