Subjects and lessons to explore.

Law

Earth & Space Science · Foundations

Ocean Systems

9 min read 5 practice questions
9 key termsSourcesReport an error
Want it in plain words first? Jump to Eli explains — the same idea, no jargon.
On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Quick check
  8. Study tools
  9. Sources & references

In 30 seconds

The ocean is Earth's largest feature: saltwater covering about 71 percent of the planet's surface. Seawater averages about 35 parts per thousand salt, mostly chloride and sodium ions, and holds dissolved gases. Sunlight warms a thin surface layer; below it a marks the rapid drop to cold deep water. The Moon's and Sun's gravity drive tides, wind drives surface waves, and light divides the ocean into sunlit, twilight, and midnight zones. The ocean also produces about half of Earth's oxygen and absorbs carbon dioxide from the air, which slowly lowers its pH.

Why this matters

The ocean is not a backdrop; it is an active system that helps make the planet livable. It covers most of Earth's surface, holds nearly all of Earth's water, and stores and moves enormous amounts of heat, which is why it shapes climate everywhere. Roughly half of the oxygen in every breath comes from the ocean, and the sunlit zone supports the majority of commercial fisheries. Understanding how salt, temperature, tides, waves, and light organize the ocean explains why marine life lives where it does, and why the chemistry of seawater — including how it absorbs carbon dioxide — matters for the whole planet.

The college version

Seawater: a solution with a recipe

The ocean is a chemical system. Seawater is water with dissolved substances, and the proportions of those substances are remarkably consistent. Average is about 35 parts per thousand — about 3.5 percent of the weight of seawater comes from dissolved salts. Salinity is not uniform: it is generally low near the equator and the poles and high at mid-latitudes, because it varies with temperature, evaporation, and precipitation. Two ions dominate the dissolved mix: chloride and sodium together make up around 85 percent of all dissolved ions, and magnesium and sulfate account for another 10 percent. The salt arrives from two main sources: rocks on land that are weathered and carried seaward by runoff, and openings in the seafloor. Seawater also holds dissolved gases. Oxygen is produced in the sunlit surface layer by photosynthetic , and carbon dioxide dissolves into the ocean from the atmosphere — a process central to .

Temperature layers: surface, thermocline, deep

Sunlight warms only the top of the ocean, and wind stirs that warmed water into a fairly uniform surface mixed layer. Beneath it lies the thermocline, a transition layer where temperature decreases rapidly with depth, separating the warm surface from the cold water below. The deep ocean is cold because of density: cold, salty water is denser than warm water, so it sinks and fills the deep basins, while less dense warm water stays at the surface. Heat leaking upward from Earth's interior is far too small to warm the deep ocean. The result is a stable stack: warm and bright at the top, a thin zone of rapid temperature change, and a vast, cold, dark deep ocean below. How that deep water travels around the planet belongs to the ocean-circulation sibling lesson; here the focus is the layering itself.

Tides and waves: two kinds of motion

Tides and wind waves both move the sea surface, but they come from different drivers. The Moon's gravity is the main driver of tides, with the Sun adding a smaller effect. They behave as very long-period waves that sweep through the ocean, and where a tidal wave's crest reaches a coast, high occurs; its trough brings low tide. The difference in height between high and low tide is the . As Earth rotates, most coastal locations pass through the raised regions about twice a day, so most coasts see two high tides and two low tides daily — usually unequal ones. Waves, by contrast, are most commonly caused by wind: friction between moving air and the water builds wind-driven surface waves. Each wave has a crest (its highest point), a trough (its lowest point), and a , the distance from one crest to the next. A wave transmits energy across the ocean while the water itself moves mostly in circles — which is why a floating object bobs as waves pass instead of being carried to shore.

Light layers and the life they allow

Light organizes ocean life. The sunlit, or euphotic, zone spans the upper 200 meters; it is where photosynthesis happens, and it holds the vast majority of commercial fisheries. Below it, the twilight, or dysphotic, zone runs from about 200 to 1,000 meters, where light fades rapidly and photosynthesis is no longer possible. Beneath 1,000 meters lies the , in permanent darkness; it includes the midnight zone (1,000 to 4,000 meters), the abyss (4,000 to 6,000 meters), and the hadal zone below 6,000 meters. Plankton — tiny drifting organisms, including photosynthetic phytoplankton — anchor the marine food web in the sunlit waters, while deeper zones are home to animals adapted to dim light, darkness, cold, and crushing pressure. Life exists at every depth, but the energy that fuels almost all of it enters at the sunlit surface.

Why the ocean matters — and how its chemistry is changing

Roughly 71 percent of Earth's surface is ocean, and the ocean holds about 97 percent of the planet's water. It absorbs and releases heat slowly, and sea surface temperature provides fundamental information about the global climate system — the ocean and atmosphere exchange energy and water continuously, a coupling explored in the atmosphere-and-oceans sibling topic. The ocean also supplies about half of Earth's oxygen, produced mostly by plankton, and its sunlit zone supports the majority of the world's commercial fisheries. One more chemical process deserves attention. When carbon dioxide from the atmosphere dissolves into seawater, a series of reactions raises the concentration of hydrogen ions, making the water more acidic and reducing carbonate ions; the ocean absorbs about 30 percent of the CO2 released into the atmosphere. This long-term lowering of pH is ocean acidification. Rising atmospheric CO2 is part of the climate-change story, a sibling topic; here the point is the chemistry: more dissolved CO2 means lower pH.

Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Start with a glass of seawater. It is about 96.5 percent water and 3.5 percent dissolved salt, mostly chloride and sodium. Now imagine the whole ocean. Sunlight warms only the top; below a thin warm layer, a thermocline drops quickly to cold deep water, because cold, salty water is dense and sinks. Two quiet rhythms move the sea. The Moon and the Sun pull on the ocean with their gravity, raising and lowering the water about twice a day in most places — that is the tide. Wind rubs the surface and makes waves; the wave shape travels, but the water mostly stays in place. Light organizes life: sunlit water teems with plankton, the twilight zone is dim, and the midnight zone is dark and cold. The ocean gives us about half of our oxygen, and when it absorbs carbon dioxide, the water slowly becomes more acidic.

Picture it like this

Picture the ocean as a giant bathtub on a train. The train's motion is the tide: the Moon and Sun tug the whole tub gently, and the water sloshes up and down the sides about twice a day. The tub's surface is the wave zone: blow across it and ripples travel end to end, while the water itself barely moves. Shine a flashlight down into the tub and the beam fades with depth — bright near the surface, dimmer halfway down, gone at the bottom — just like the ocean's sunlit, twilight, and midnight zones.

Where the picture stops working

The bathtub has walls, but the ocean has no container: tides are a stretching of the whole ocean by gravity, not a slosh against fixed sides, and their timing follows the Moon's orbit, not any schedule. A flashlight beam fades smoothly, while real ocean light zones have rough boundaries that shift with water clarity and seasons. And no bathtub holds a food web, produces oxygen, or absorbs carbon dioxide.

Worked example

A steady wind blows across open ocean. Friction between the moving air and the surface water builds small ripples that grow into wind-driven surface waves, each with a crest, a trough, and a wavelength — the distance from one crest to the next. A gull bobbing on the surface rises and falls in small circles as each wave passes: the wave's energy moves on across the water, but the water the gull rides does not travel with it. Meanwhile the Moon's gravity raises the tide, and in a place with two daily tides the water climbs for about six hours, peaks at high tide, then falls to low tide roughly six hours later. The wind made the waves; the Moon and Sun make the tide.

Key takeaway

The ocean is one interconnected physical, chemical, and biological system: salt and dissolved gases set its chemistry, temperature and light stack it into layers, tides and waves move it, and its zones support life from the sunlit surface to the midnight zone — while the whole ocean shapes climate, oxygen, and food.

Quick check

3 questions here, of 5 in this lesson’s practice set. Answers stay hidden until you check.

Question 1 of 3foundational

What is the average salinity of Earth's ocean water?

Choose an answer, then check it.
Question 2 of 3intermediate

Which pair of ions makes up about 85 percent of the dissolved salts in seawater?

Choose an answer, then check it.
Question 3 of 3intermediate

In the open ocean, where does water temperature fall most rapidly with depth?

Choose an answer, then check it.
Practice all 5

Keep learning

Ready to build on this? Continue to the next lesson.

Practice this lesson
Study tools & related lessonsYou’ll learn to · Common mistakes · Easily confused · Key vocabulary · Related

You’ll learn to

  • Describe the composition of seawater, including average salinity, the main dissolved ions, and dissolved gases.
  • Explain the temperature structure of the open ocean: the surface mixed layer, the thermocline, and cold deep water.
  • Distinguish tides, driven by the Moon's and Sun's gravity, from wind-driven surface waves, and identify crest, trough, and wavelength.
  • Describe the light-based life zones of the ocean from the sunlit surface to the deep sea.
  • Explain why the ocean matters for climate, oxygen production, and food.
  • Explain the basic chemistry of ocean acidification: carbon dioxide dissolving in seawater lowers pH.

Common mistakes

  • Waves carry water from far out at sea onto the beach.

    Waves transmit energy, not water. Surface water moves mostly in circular orbits, so a floating object bobs as each wave passes instead of riding in from offshore.

  • The Moon causes tides, and the Sun has nothing to do with them.

    Tides are caused by the gravitational pull of both the Moon and the Sun; the Moon's pull is the stronger driver, but the Sun's pull also shapes the tides.

  • The deep ocean is warm because it sits above Earth's hot interior.

    Cold, salty water is denser than warm water, so it sinks and keeps the deep ocean cold. Heat leaking from Earth's interior is negligible at the seafloor.

  • Ocean acidification happens only where pollution is dumped into the sea.

    Acidification is a global chemical response: seawater absorbs carbon dioxide from the atmosphere, reactions raise hydrogen-ion levels, and pH falls over time across the whole ocean.

  • Salinity is the same everywhere in the ocean.

    Salinity varies with temperature, evaporation, and precipitation — generally lower near the equator and poles and higher at mid-latitudes. About 35 parts per thousand is the average, not a universal value.

Easily confused

Tide vs. Wind-driven wave

Tides are very long-period waves caused by the gravitational pull of the Moon and the Sun, rising and falling about twice a day; wind waves are surface waves built by friction between wind and water, passing in seconds to minutes.

Sunlit (euphotic) zone vs. Midnight zone

The sunlit zone is the upper 200 meters, where photosynthesis occurs and most commercial fisheries live; the midnight zone (1,000 to 4,000 meters) is in permanent darkness with no photosynthesis.

Surface mixed layer vs. Deep water

Surface water is warm, wind-stirred, and less dense; deep water is cold and dense, having sunk from the surface, with the thermocline between them.

High tide vs. Low tide

High tide occurs where the crest of the tidal wave reaches a coast and low tide at its trough; the height difference between them is the tidal range.

Key vocabulary

Salinity
The concentration of dissolved salts in seawater, usually measured in parts per thousand; average ocean salinity is about 35 parts per thousand.
Thermocline
A transition layer of ocean water where temperature drops rapidly with depth, separating warmer surface water from colder deep water.
Tide
The regular rise and fall of sea level along coasts, caused mainly by the gravitational pull of the Moon and the Sun.
Tidal range
The difference in height between a high tide and the following low tide at a location.
Wavelength
The distance from one wave crest to the next crest, the full length of one wave.
Euphotic zone
The sunlit upper layer of the ocean, roughly the top 200 meters, where photosynthesis can occur.
Aphotic zone
The deep, dark part of the ocean below about 1,000 meters, where sunlight does not reach.
Plankton
Tiny drifting organisms carried by ocean tides and currents, including photosynthetic phytoplankton that support marine food webs.
Ocean acidification
The long-term lowering of the ocean's pH, caused mainly by seawater absorbing carbon dioxide from the atmosphere.

Sources & references

  1. Why is the ocean salty? — NOAA National Ocean Service
  2. What is a thermocline? — NOAA National Ocean Service
  3. Why does the ocean get colder at depth? — NOAA National Ocean Service
  4. What are tides? — NOAA National Ocean Service
  5. How many high tides are there per day? — NOAA National Ocean Service
  6. Why does the ocean have waves? — NOAA National Ocean Service
  7. What is a rogue wave? — NOAA National Ocean Service
  8. How far does light travel in the ocean? — NOAA National Ocean Service
  9. How much oxygen comes from the ocean? — NOAA National Ocean Service
  10. What are plankton? — NOAA National Ocean Service
  11. What is Ocean Acidification? — NOAA National Ocean Service
  12. Why do scientists measure sea surface temperature? — NOAA National Ocean Service
  13. How much water is in the ocean? — NOAA National Ocean Service

EliExplains lessons are original prose written from the open, credible references above. See Copyright & Licensing.

Researched 2026-08-21

Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.