Spring tides, the largest tides of the month, happen when what occurs?

The Sun, Earth and Moon fall roughly into line, at new and full moon

The Moon sits at right angles to the Sun as seen from Earth

The Moon reaches the most distant point of its orbit

The Sun crosses the equator at an equinox

The Gulf Stream carries warm water in which direction?

North-east from the Gulf of Mexico and across the North Atlantic

South from the Arctic along the coast of Canada

West from Africa towards the coast of Brazil

South from the North Sea into the Mediterranean

Coastal upwelling brings what to the surface?

Cold water carrying nutrients up from below

Warm surface water pushed in from further offshore

Fresh water from springs beneath the sea floor

Sediment stirred up from the mouths of rivers

In which regions does surface water become dense enough to sink and feed the deep ocean?

The North Atlantic and the seas around Antarctica

The tropics on either side of the equator

The centres of the great ocean gyres

The shallow water over continental shelves

How would you best describe the mid-ocean ridge system?

A chain of undersea volcanic mountains running through every ocean basin where plates pull apart

A trough where one plate slides down beneath another

A belt of volcanoes around the rim of the Pacific

A bank of sediment piled at the outer edge of the continental shelf

Deep ocean trenches form where what happens?

One tectonic plate bends downwards and descends beneath another

Two plates grind past each other sideways

The sea floor collapses into an emptied magma chamber

Strong currents scour a channel through soft sediment

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Oceans & Marine Life Hard ⚡ 10 Questions ⏱️ ~8 Mins +100 XP

Currents, Trenches and the Sea Floor

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Drain the oceans and you would be looking at the biggest mountain range on Earth, plains flatter than anywhere on land, and trenches deeper than the Himalayas are tall. These ten questions cover that landscape and the water circulating over it.

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About this topic

The sea is not a still bath with weather on top of it. Water is dragged across the surface by wind, deflected by the planet's rotation, pulled about twice a day by the moon and sun, and turned over slowly from top to bottom by differences in temperature and salt. Each of those runs on its own timescale, from hours to a thousand years. Underneath is a landscape that was almost entirely unmapped a century ago. The questions here cover both halves: the tides and currents that move the water, and the ridges, shelves, trenches and plains that decide where it can go.

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Currents, Trenches and the Sea Floor: all 10 questions and answers

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1Spring tides, the largest tides of the month, happen when what occurs?
- A The Moon sits at right angles to the Sun as seen from Earth
- B The Moon reaches the most distant point of its orbit
- C The Sun crosses the equator at an equinox
- D The Sun, Earth and Moon fall roughly into line, at new and full moon Correct
Answer: D. The Sun, Earth and Moon fall roughly into line, at new and full moon

Why: With the Sun and Moon pulling along the same axis their effects add together, producing the biggest range between high and low water. The name has nothing to do with the season and comes from the older sense of water springing up.
2Which body of water is known for the greatest tidal range in the world?
- A Chesapeake Bay
- B The Gulf of Carpentaria
- C The Bay of Fundy Correct
- D The Bay of Biscay
Answer: C. The Bay of Fundy

Why: The difference between low and high water at the head of the bay can reach around 16 metres. Its length and funnel shape happen to match the natural rhythm of the tide, so each incoming tide reinforces the last.
3The Gulf Stream carries warm water in which direction?
- A South from the North Sea into the Mediterranean
- B North-east from the Gulf of Mexico and across the North Atlantic Correct
- C South from the Arctic along the coast of Canada
- D West from Africa towards the coast of Brazil
Answer: B. North-east from the Gulf of Mexico and across the North Atlantic

Why: It is one of the strongest currents in the ocean and moves an enormous quantity of heat towards north-west Europe. Benjamin Franklin had it charted in the 1770s after noticing that mail ships took longer westbound than east.
4The huge circular current systems that dominate each ocean basin are called what?
- A Gyres Correct
- B Eddies
- C Fronts
- D Meridians
Answer: A. Gyres

Why: There are five major gyres, turning clockwise in the northern hemisphere and anticlockwise in the southern because of the Earth's rotation. Floating debris that drifts into the middle of one tends to stay there.
5Coastal upwelling brings what to the surface?
- A Warm surface water pushed in from further offshore
- B Fresh water from springs beneath the sea floor
- C Sediment stirred up from the mouths of rivers
- D Cold water carrying nutrients up from below Correct
Answer: D. Cold water carrying nutrients up from below

Why: When wind drags surface water away from a coast, deeper water rises to replace it and brings nutrients with it. This is why some of the richest fisheries in the world sit off the driest coasts, including Peru and Namibia.
6In which regions does surface water become dense enough to sink and feed the deep ocean?
- A The centres of the great ocean gyres
- B The shallow water over continental shelves
- C The North Atlantic and the seas around Antarctica Correct
- D The tropics on either side of the equator
Answer: C. The North Atlantic and the seas around Antarctica

Why: Water there is cold, and freezing sea ice leaves salt behind in what remains, so it becomes heavy enough to sink to the bottom. A parcel of water can take something like a thousand years to complete a full circuit of the deep ocean and return to the surface.
7How would you best describe the mid-ocean ridge system?
- A A bank of sediment piled at the outer edge of the continental shelf
- B A chain of undersea volcanic mountains running through every ocean basin where plates pull apart Correct
- C A trough where one plate slides down beneath another
- D A belt of volcanoes around the rim of the Pacific
Answer: B. A chain of undersea volcanic mountains running through every ocean basin where plates pull apart

Why: At around 65,000 kilometres it is comfortably the longest mountain range on the planet, and almost all of it is underwater. New sea floor is manufactured along it continuously, which is why no ocean crust anywhere is much older than 200 million years.
8Deep ocean trenches form where what happens?
- A One tectonic plate bends downwards and descends beneath another Correct
- B Two plates grind past each other sideways
- C The sea floor collapses into an emptied magma chamber
- D Strong currents scour a channel through soft sediment
Answer: A. One tectonic plate bends downwards and descends beneath another

Why: The bend in the descending plate pulls the sea floor into a long, narrow trough, which is why trenches are thousands of kilometres long but only tens of kilometres wide. Almost all of the deepest ones lie around the edges of the Pacific.
9What is the gently sloping, relatively shallow sea floor bordering a continent called?
- A The abyssal plain
- B The mid-ocean ridge
- C The submarine canyon
- D The continental shelf Correct
Answer: D. The continental shelf

Why: The shelf is rarely deeper than about 200 metres before the sea floor drops away more steeply. Much of it was dry land during ice ages, when so much water was locked up in ice sheets that sea level stood far lower.
10The vast, remarkably flat expanses covering much of the deep ocean floor are known as what?
- A Guyots
- B Seamount chains
- C Abyssal plains Correct
- D Continental slopes
Answer: C. Abyssal plains

Why: They are among the flattest surfaces anywhere on Earth, sloping by only a metre or so over several kilometres. The volcanic rock beneath them is rough, but millions of years of fine sediment have buried it.

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