Ocean facts for kids

Quick facts for kids Earth's ocean
Pacific Ocean, largest part of Earth and its ocean, seen from space in 2023
Basin countries	List of countries by length of coastline
Surface area	361,000,000 km2 (139,382,879 sq mi) (70.8% of Earth's surface area)
Average depth	3,688 m (12,100 ft)
Max. depth	11,034 m (36,200.787 ft) (Challenger Deep)
Water volume	1,370,000,000 km3 (328,680,479 cu mi) (97.5% of Earth's water)
Shore length1	Low interval calculation: 356,000 km (221,208 mi) High interval calculation: 1,634,701 km (1,015,756 mi)
Max. temperature	30 °C (86 °F) (max. surface) 20 °C (68 °F) (avg. surface) 4 °C (39 °F) (avg. overall)
Min. temperature	−2 °C (28 °F) (surface) 1 °C (34 °F) (deepest points)
Sections/sub-basins	Main divisions (volume %): Pacific Ocean (50.1%) Atlantic Ocean (23.3%) Indian Ocean (19.8%) Antarctic/Southern Ocean (5.4%) Arctic Ocean (1.4%) Other divisions: Marginal seas
Trenches	List of oceanic trenches
1 Shore length is not a well-defined measure.

The ocean is the huge body of salt water that covers most of our Earth – about 70.8% of its surface! We usually divide this massive ocean into five main parts: the Pacific Ocean, Atlantic Ocean, Indian Ocean, Southern (Antarctic) Ocean, and Arctic Ocean. These big oceans are then split into smaller areas like seas and gulfs.

The ocean holds 97% of all the water on Earth. It's like a giant heater and cooler for our planet, storing a lot of heat. It also plays a super important role in Earth's carbon cycle and water cycle, which helps create all the climate and weather we experience. Without the ocean, life on Earth wouldn't be possible. It's home to most of Earth's animals and tiny living things, and it even produces half of the oxygen we breathe through photosynthesis!

Ocean scientists divide the ocean into different zones based on how deep they are and what conditions are like there. Human activities mostly happen near the coast, which can have big effects on marine life.

What is the Ocean?

Ocean vs. Sea

When we talk about "the ocean," we usually mean the huge, connected body of salt water that covers most of Earth. This includes the five main oceans: the Pacific, Atlantic, Indian, Southern, and Arctic. Often, people use "ocean" and "sea" to mean the same thing.

However, a "sea" is usually a smaller part of the world ocean. It is often partly or fully surrounded by land. Think of the North Sea or the Red Sea. Seas are generally smaller than oceans and are often next to continents. The idea of a "World Ocean" means that all these oceans are actually connected. Water can flow between them, making them one giant body of water. This connection is very important for how the ocean works.

Where Did the Word "Ocean" Come From?

The word "ocean" comes from ancient Greek stories. It comes from "Oceanus," a powerful god in Greek mythology. The ancient Greeks and Romans believed Oceanus was a huge river that circled the entire world.

How Did Our Oceans Form?

The Beginning of Water on Earth

Scientists think that Earth had water from the very start, when our planet was forming. Early Earth was very hot, possibly covered in magma oceans. Over millions of years, volcanic activity and meteorite impacts released a lot of water vapor into the air.

As Earth cooled down, this water vapor turned into liquid water. This is how our first oceans were born! These early oceans might have been much hotter and even looked green because of iron. Rocks found in places like Canada show that water existed on Earth at least 3.8 billion years ago. Some scientists even believe that enough water to fill the oceans has been on Earth since its formation.

Oceans Through Time

The oceans have changed a lot over Earth's long history. Things like plate tectonics (how Earth's big land plates move) and changes in sea level constantly reshape our coastlines. For example, during the last ice age, so much water was frozen in glaciers that ocean levels were about 122 meters (400 feet) lower than they are today. In warmer times, sea levels were much higher.

Exploring the Ocean's Geography

The ocean covers about 70.8% of Earth's surface. It's so big that Earth is often called a "water world"! The ocean's shape isn't perfectly round; it's uneven and covers more of some parts of the globe than others. The total area of seawater is about 361 million square kilometers. There's even a spot in the Pacific Ocean called "Point Nemo" that's the furthest place from any land, about 2,688 kilometers away!

How the Ocean is Divided

Even though it's one big connected body of water, we divide the World Ocean into five main parts for easier study. These are the Pacific, Atlantic, Indian, Arctic, and Southern (Antarctic) oceans. The Southern Ocean was officially recognized more recently, around the early 2000s.

Here's a look at the five main oceans, from largest to smallest:

Ocean Basins and Underwater Mountains

The ocean fills huge dips in Earth's surface called oceanic basins. The bottom of the ocean isn't flat! It has amazing features like giant underwater mountain ranges called mid-oceanic ridges. These ridges form where Earth's tectonic plates are pulling apart, and new magma pushes up from below.

The global mid-oceanic ridge system is the longest mountain range in the world, stretching for about 65,000 kilometers (40,000 miles). That's much longer than any mountain range on land! In other places, one ocean plate can slide under another, creating deep trenches. As of 2024, scientists have mapped just over 26% of the ocean floor in high detail. There's still so much more to explore!

Where Land Meets Sea: The Coast

Lighthouse at the coast of Ocean County, New Jersey, U.S., facing the Atlantic Ocean at sunrise

The place where land and sea meet is called the coast. The area between the lowest and highest tides is known as the shore. A beach is where sand or pebbles gather on the shore.

Waves constantly shape our coastlines. Gentle waves tend to build up beaches, while strong storm waves can wear them away. Over time, waves can even carve out cliffs and platforms. Rivers also bring sediment to the sea, forming deltas at their mouths. People build things like seawalls and breakwaters to protect coastal areas from flooding and erosion.

Physical Properties of the Ocean

Ocean Color

Ocean chlorophyll concentration is a proxy for phytoplankton biomass. In this map, blue colors represent lower chlorophyll and reds represent higher chlorophyll. Satellite-measured chlorophyll is estimated based on ocean color by how green the color of the water appears from space.

The color of the ocean can change depending on what's in the water. For example, tiny plant-like organisms called phytoplankton can make the water look greener. Scientists use satellites to measure ocean color, which helps them understand how much phytoplankton is present.

Water Cycle, Weather, and Rainfall

The ocean is a major driver of Earth's water cycle.

The ocean is a huge part of Earth's water cycle. Water evaporates from the ocean, goes into the atmosphere, and then falls back as rain on land and sea. About 90% of all rainfall comes from ocean evaporation. This process greatly affects our climate and weather. The ocean also influences big weather events like tropical cyclones (hurricanes and typhoons).

Waves and Swell

Ocean surface currents

Wind waves are the up and down movements of the ocean surface. When wind blows over water, it creates ripples and then larger waves. The size of a wave depends on how strong the wind is and how far it blows over the water. Sometimes, very large and unexpected "rogue waves" can form, reaching heights of over 25 meters (82 feet)!

When waves get close to land, they slow down and get taller before they "break" and crash onto the beach. Earthquakes or volcanic eruptions can cause very dangerous giant waves called tsunamis.

Sea Level and Surface

The ocean's surface is an important reference point, especially for measuring sea level. The surface of the ocean is also where the water and air meet, allowing gases and tiny particles to move between them. This exchange helps support life in the ocean and affects our atmosphere.

Tides

High tide and low tide in the Bay of Fundy, Canada

Tides are the regular rise and fall of ocean water levels. They are mainly caused by the the Moon's gravity pulling on Earth's water. The Sun's gravity and Earth's rotation also play a part.

During each tidal cycle, the water rises to its highest point (high tide) and then falls to its lowest point (low tide). The difference between high and low tide is called the tidal range. When the Sun and Moon line up (during a full or new moon), we get higher "spring tides." In the open ocean, tidal ranges are usually less than 1 meter, but in some coastal areas, like the Bay of Fundy in Canada, they can be over 10 meters!

Ocean Depth

The average depth of the oceans is about 3,688 meters (12,099 feet). Nearly half of the world's ocean waters are over 3,000 meters (9,800 feet) deep. The "deep ocean" (below 200 meters) covers about 66% of Earth's surface.

The deepest part of the ocean is the Challenger Deep in the Mariana Trench, located in the Pacific Ocean. It's about 10,971 meters (36,000 feet) deep! To give you an idea, that's deeper than Mount Everest is tall.

Ocean Zones

The major oceanic zones, based on depth and biophysical conditions

Scientists divide the ocean into different zones based on how much light they get and how deep they are.

Zones by Light

This diagram offers a basic illustration of the depth at which different colors of light penetrate ocean waters.

• Photic Zone: This is the top layer, usually up to about 200 meters (660 feet) deep. It's where sunlight can reach, allowing photosynthesis to happen. Tiny plant-like organisms called phytoplankton live here and produce most of the ocean's food. This zone is full of life!
• Mesopelagic Zone: Also called the twilight zone, this area has very little light. Photosynthesis can't happen here.
• Aphotic Zone: This is the deep ocean, where no sunlight reaches at all. It's cold and completely dark. Life here relies on food sinking from above or special energy sources like hydrothermal vents.

Zones by Depth and Temperature

The deep, dark aphotic zone is further divided:

• Mesopelagic: The top part of the deep ocean.
• Bathypelagic: Deeper, with temperatures between 4°C and 10°C.
• Abyssopelagic: Covers the vast abyssal plains.
• Hadalpelagic: The deepest zone, found in the ocean trenches.

There are also zones based on how close they are to the seafloor:

• Bathyal Zone: Covers the continental slope.
• Abyssal Zone: Covers the deep abyssal plains.
• Hadal Zone: Found in the deepest trenches.

Ocean Temperature

Ocean temperatures depend on how much sunlight hits the surface. In warm areas near the equator, surface temperatures can reach over 30°C (86°F). Near the poles, where sea ice forms, the water can be as cold as -2°C (28°F).

Deep ocean water, however, is very cold everywhere, usually between -2°C and 5°C (28°F and 41°F). This cold water moves around the globe in deep currents.

Sea Ice

Annual mean sea surface salinity in Practical Salinity Units (psu) from the World Ocean Atlas

Saltwater freezes at about -1.8°C (28.8°F). When sea ice forms, it's less dense than water, so it floats. Sea ice covers about 12% of the world's oceans. As ice forms, it pushes out most of the salt, making the remaining seawater saltier and denser.

Ocean Currents and Global Climate

A map of the global thermohaline circulation; blue represents deep-water currents, whereas red represents surface currents.

Ocean currents are like giant rivers of water flowing through the ocean. They are caused by wind, Earth's rotation (Coriolis effect), and differences in water temperature and saltiness.

Types of Ocean Currents

• Tidal currents are caused by the Moon's and Sun's gravity.
• Surface currents are mainly driven by wind and waves. Major surface currents include the Gulf Stream and the Antarctic Circumpolar Current.
• Deep-water currents are part of the thermohaline circulation. Cold, dense water sinks near the poles and moves slowly along the ocean floor, eventually rising in warmer areas.

How Currents Affect Climate

Ocean currents move huge amounts of water and heat around the world, which greatly affects our climate. For example, the Gulf Stream carries warm water from the equator to northern Europe, making its climate milder.

Changes in these currents, like the Atlantic meridional overturning circulation (AMOC), can have big impacts on global climate. Scientists observed that the AMOC has weakened since the preindustrial era. Projections from 2021 suggested it would likely weaken further over the 21st century, which could cause significant changes to global climate, especially in the North Atlantic.

Chemical Properties of the Ocean

Salinity

Salinity is how much salt is dissolved in seawater. The average ocean salinity is about 3.47%. Salinity makes seawater denser and affects its freezing and boiling points.

Ocean water is saltier where there is more evaporation (like in tropical regions) and less salty where there is more rain or melting ice (like near the poles). When sea ice forms, it leaves behind saltier water. Observations between 1950 and 2019 showed that salty areas became saltier, and less salty areas became fresher, likely due to climate change.

Dissolved Gases

Ocean water contains many dissolved gases, like oxygen, carbon dioxide, and nitrogen. These gases move between the ocean surface and the atmosphere. Colder water can hold more dissolved gases than warmer water.

Oxygen, Photosynthesis, and Carbon Cycling

Diagram of the ocean carbon cycle showing the relative size of stocks (storage) and fluxes

In the surface ocean, phytoplankton use sunlight to perform photosynthesis. This process releases oxygen and uses carbon dioxide. When these tiny organisms die, they sink, and bacteria break them down. This uses up oxygen and releases carbon dioxide in the deep ocean.

The ocean is a major "carbon sink," meaning it absorbs a lot of carbon dioxide from the atmosphere. This helps regulate Earth's climate. Some coastal habitats like mangroves and saltmarshes are especially good at storing carbon, a process called "Blue carbon."

Some parts of the deep ocean have very low oxygen levels, called oxygen minimum zones. These areas are expected to get worse due to climate change.

pH and Ocean Acidification

The pH value of the ocean surface is currently around 8.05 to 8.08, which is slightly alkaline. However, between 1950 and 2020, the average pH of the ocean surface dropped from about 8.15 to 8.05. This process is called ocean acidification.

It happens because the oceans absorb extra carbon dioxide from the atmosphere, mainly from human activities like burning fossil fuels. This carbon dioxide reacts with water to form carbonic acid, which makes the ocean more acidic (lowers its pH). This change can harm marine life, especially creatures with shells or skeletons made of calcium carbonate.

Marine Life

Some representative ocean animals (not drawn to scale) within their approximate depth-defined ecological habitats. Marine microorganisms also exist on the surfaces and within the tissues and organs of the diverse life inhabiting the ocean, across all ocean habitats. The animals rooted to or living on the ocean floor are not pelagic but are benthic animals.

Life in the ocean started billions of years before life on land! The ocean is home to an incredible variety of plants and animals, from the surface to the deepest trenches. The amount of light and the depth greatly affect what kind of life can be found in each area.

The ocean is home to most animal groups, including sponges, corals, jellyfish, octopus, squid, lobsters, crabs, fish, sharks, and giant whales. Many land animals, like seabirds and sea turtles, also spend most of their lives in the ocean.

Besides animals, the ocean has many types of plants (like seagrass), algae (like kelp and tiny phytoplankton), and countless bacteria, archaea, and fungi. These tiny organisms play vital roles in the ocean's ecosystem.

Human Uses of the Oceans

Global map of all exclusive economic zones

Humans have always used the ocean for many things. We use it for travel and exploration, shipping goods around the world, and getting food like fish and shrimp. Many of the world's products are transported by large container ships across the oceans.

The ocean also provides energy through tidal power, wave power, and offshore wind power. There are also large deposits of petroleum (oil and natural gas) beneath the ocean floor, which are extracted using offshore platforms.

The idea of "freedom of the seas" means that everyone should be free to navigate the oceans. This concept is now part of international law, managed by organizations like the International Maritime Organization (IMO).

Threats to Our Oceans

Global cumulative human impact on the ocean

Human activities are causing many problems for marine life and ocean habitats.

Climate Change Effects

The ocean is warming up, absorbing extra heat from the atmosphere. This ocean warming affects marine life and can lead to sea level rise as water expands and glaciers melt. As mentioned earlier, ocean acidification is also a major problem, making the water more acidic and harming marine creatures.

Marine Pollution

Marine pollution is a big threat. It includes all the trash, chemicals, and other harmful substances that end up in the ocean. Plastic pollution, especially tiny pieces called microplastics, is a huge problem. These plastics can harm marine animals that mistake them for food. Pollution can also come from oil spills, sewage, and runoff from land.

Overfishing

Overfishing happens when people catch too many fish, faster than the fish populations can reproduce. This can lead to a decrease in fish numbers and harm the entire ocean ecosystem. In 2017, about 34% of the world's fish stocks were considered overfished.

Protecting Our Oceans

Protecting the ocean is super important because we depend on it for so much. One way to protect marine life is by creating marine protected areas (MPAs). These are special places where fishing and other activities are limited to help ecosystems recover.

Other efforts include promoting sustainable seafood choices and developing projects to clean up ocean pollution, like removing plastic. In March 2023, a new High Seas Treaty was signed. This agreement makes it possible to create protected areas in international waters, aiming to protect 30% of the oceans by 2030. This treaty helps hold countries accountable for their impact on the oceans.

See also

In Spanish: Océano para niños

• European Atlas of the Seas
• Land and water hemispheres
• List of seas
• Ocean world
• List of ocean worlds
• Planetary oceanography
• World Ocean Atlas
• World Oceans Day