Ocean Currents and Tides
Grade 6 · Science · Study Guide
Learn how ocean currents shape Earth's climate and how the Moon's gravity creates tides.
The ocean is never still. It moves constantly in patterns called currents, and it rises and falls every day with the tides. Both have enormous impacts on life on Earth — from climate and weather to the fishing industry and coastal ecosystems.
Ocean currents are large, flowing movements of water. Surface currents are driven mainly by wind and can extend hundreds of meters deep. The Coriolis effect (caused by Earth's rotation) deflects currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, creating large circular patterns called gyres. The Gulf Stream is a powerful surface current that carries warm tropical water north along the eastern US coast and across to Europe, keeping places like Britain warmer than they would otherwise be at that latitude.
Deep-water currents are driven by differences in water density caused by temperature and salinity — this is called thermohaline circulation (thermo = heat, haline = salt). Cold, salty water is denser and sinks near the poles, while warm, less-dense water flows in from the tropics at the surface. This creates a slow global conveyor belt that mixes ocean water over thousands of years.
Tides are the rhythmic rise and fall of ocean water caused primarily by the gravitational pull of the Moon, with a smaller contribution from the Sun. The Moon's gravity pulls ocean water toward it, creating a high tide on the side of Earth nearest the Moon. At the same time, the Earth's rotation creates another bulge on the opposite side. Most coastal areas experience two high tides and two low tides each day. During a new or full moon, the Sun and Moon align, creating especially strong spring tides. When they are at right angles, weaker neap tides occur.
| Tide Type | Moon Phase | Sun-Moon Alignment | Tidal Range |
|---|---|---|---|
| Spring tide | New or Full Moon | Sun, Moon, Earth aligned | Largest (highest high, lowest low) |
| Neap tide | First or Third Quarter | Sun and Moon at 90° | Smallest (moderate high and low) |
Key terms
- Ocean current
- A continuous, directed movement of ocean water driven by wind, temperature, or density differences.
- Surface current
- A shallow ocean current driven primarily by wind patterns.
- Coriolis effect
- The deflection of moving air or water caused by Earth's rotation; deflects to the right in the Northern Hemisphere.
- Gyre
- A large circular ocean current caused by the interaction of wind and the Coriolis effect.
- Thermohaline circulation
- Deep ocean circulation driven by differences in temperature and salinity (density).
- Tide
- The periodic rise and fall of sea level caused mainly by the Moon's gravitational pull.
- Spring tide
- An especially strong tide occurring during new or full moon when the Sun and Moon align.
- Neap tide
- A weaker tide occurring when the Sun and Moon are at right angles to each other.
Flashcards
- Ocean current
- A continuous movement of ocean water driven by wind, heat, or density differences.
- Gyre
- Large circular ocean current system formed by wind + Coriolis effect.
- Thermohaline circulation
- Deep ocean 'conveyor belt' driven by density differences from temperature and salinity.
- Tide
- Daily rise and fall of sea level caused mainly by the Moon's gravitational pull.
- Spring tide
- Strong tide during new/full moon when Sun and Moon align; widest tidal range.
- Neap tide
- Weak tide during quarter moon phases when Sun and Moon are at right angles.
Practice questions
What primarily causes surface ocean currents?
- The Moon's gravity
- Wind patterns
- Differences in water temperature
- Earth's magnetic field
Show answer
Answer: Wind patterns
Global wind patterns drag water along the ocean surface, creating surface currents. The Coriolis effect then deflects them into circular gyres.
Tides are caused mainly by the Moon's gravitational pull on Earth's oceans.
Show answer
Answer: True
The Moon's gravity pulls ocean water toward it, creating a tidal bulge. The Sun also contributes but less strongly since it's much farther away.
Extra-strong tides that occur during new or full moons are called ___ tides.
Show answer
Answer: spring
Spring tides have nothing to do with the season — they occur when the Sun, Moon, and Earth align (new or full moon), combining their gravitational forces.
The Gulf Stream is an example of which type of ocean current?
- Deep thermohaline current
- Surface current
- Tidal current
- Upwelling current
Show answer
Answer: Surface current
The Gulf Stream is one of Earth's most powerful surface currents. It flows from the Gulf of Mexico northward along the US East Coast and then across the Atlantic, warming Western Europe.
The Coriolis effect deflects ocean currents to the LEFT in the Northern Hemisphere.
Show answer
Answer: False
The Coriolis effect deflects moving water and air to the RIGHT in the Northern Hemisphere and to the left in the Southern Hemisphere, due to Earth's rotation.
Fun facts
- The Bay of Fundy in Canada has the world's highest tides — up to 16 meters between high and low tide, enough to strand large ships on the seafloor.
- The Gulf Stream carries about 30 times more water than all the rivers on Earth combined.
- Thermohaline circulation takes about 1,000 years to circulate water around the entire globe — the deep water rising in the Pacific today may have sunk in the North Atlantic 1,000 years ago.
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