Ocean Quiz — 50 Questions, Ending in What We Get Wrong

Roughly how much of the Earth's surface is ocean?

Answer About seventy percent

About 71 percent, and that share is uneven: the southern hemisphere is far more ocean than the northern. Most of that water is also deep — the average depth is around 3,700 meters, so the shallow coastal fringe we picture is a thin edge of the whole.

Which ocean is the largest?

Answer The Pacific

The Pacific alone covers more of the surface than all the land on Earth combined. It is also slowly shrinking, because the plates around its rim are being pushed under the continents faster than new floor is made.

What is the continental shelf?

Answer The shallow, gently sloping edge of a continent before the seafloor drops away

The shelf is drowned continent, not ocean floor. It typically runs out to about 200 meters deep before the slope drops away, and during ice ages much of it was dry land where people walked.

Where is the deepest known point in the ocean?

Answer In the Mariana Trench, in the western Pacific

The Challenger Deep, close to 11 kilometers down. Only a handful of crewed descents have reached it — fewer people have been to the bottom of that trench than have walked on the Moon.

What is a mid-ocean ridge?

Answer A long undersea mountain chain where new seafloor is being formed

New seafloor forms along the ridge and spreads outward, which is why the rock gets older the further you go from it. The chain runs for tens of thousands of kilometers and is the longest mountain range on the planet, almost entirely underwater.

What drives the large surface currents?

Answer Wind, deflected into loops by the Earth's rotation

Wind pushes the surface, and the Earth's rotation bends the flow into great loops called gyres. Those loops are also why floating debris collects in a few predictable places.

What is an atoll?

Answer A ring of coral around a lagoon, usually where a volcanic island once stood

Darwin worked out the sequence: a reef grows around a volcanic island, the island slowly sinks, and the reef keeps building upward until only the ring is left. It was a testable prediction, and drilling later confirmed it.

Which of these is a sea rather than an ocean?

Answer The Mediterranean

A sea is smaller and largely enclosed by land. The Mediterranean is nearly closed — it connects to the Atlantic through a strait only about 14 kilometers wide at its narrowest.

What is an estuary?

Answer Where a river meets the sea and fresh water mixes with salt

Fresh and salt water mix here, which makes estuaries brackish and highly variable. That variability is hard to live in, but the shelter and nutrients make them nurseries for a great many species.

Why are the edges of continents often the most productive fishing grounds?

Answer Shallow shelves and upwelling bring nutrients into the sunlit layer

Sunlight reaches the bottom on a shallow shelf, and upwelling brings nutrients from below. Both happen at the edges, which is why a small fraction of the ocean's area produces most of the catch.

Why is the sea salty?

Answer Rivers carry dissolved minerals from rock into the sea, and evaporation leaves them behind

Rivers dissolve minerals from rock and carry them to the sea, and hydrothermal vents add more. Water leaves by evaporation but the salt stays, so it concentrates — sodium and chloride make up most of it.

Why does the ocean look blue?

Answer Water absorbs red light faster than blue, so what returns to your eye is mostly blue

Water absorbs red light within the first few meters and blue far more slowly, so blue is what remains to be scattered back. It is a property of the water itself, not a reflection of the sky.

What causes tides?

Answer The gravitational pull of the Moon and, less strongly, the Sun

The Moon does most of it and the Sun adds to it. When the two line up the range is largest, and when they pull at right angles it is smallest — spring and neap tides, which have nothing to do with the season.

Why does surface water in the tropics not simply mix with the cold water below?

Answer Warm water is less dense and floats on the colder, denser layer

Warm water is less dense and floats, so it sits on the cold water as a distinct layer. The boundary between them is the thermocline, and it acts as a lid that keeps deep nutrients from reaching the sunlit surface.

What is a wave, in terms of what actually travels?

Answer Energy moving through the water while the water itself mostly circles in place

The energy travels; the water mostly does not. Individual water particles move in circles and return close to where they started, which is why a floating object bobs rather than being carried along.

What makes a tsunami different from an ordinary wave?

Answer It is set off by a sudden displacement of the seafloor and involves the whole water column

An ordinary wave is made by wind at the surface. A tsunami is the whole water column displaced at once, so its wavelength can be hundreds of kilometers — in open ocean it may pass under a ship as a barely noticeable swell.

What is upwelling?

Answer Deep, nutrient-rich water rising toward the surface, often along a coast

Wind pushes surface water away from a coast and deeper water rises to replace it, carrying nutrients up into the light. The richest fisheries in the world sit over upwelling zones.

Why does the ocean take so long to warm up and cool down?

Answer Water stores a great deal of heat for each degree of temperature change

Water has an unusually high heat capacity, so it takes a lot of energy to change its temperature. That is why coastal climates are milder than inland ones, and why the ocean has absorbed most of the extra heat trapped by greenhouse gases.

What is the role of the ocean in carbon dioxide?

Answer It absorbs a large share of the carbon dioxide released into the air

The ocean has taken up roughly a quarter of the carbon dioxide people have released. That has slowed warming in the air, but the gas does not simply vanish — it changes the water's chemistry.

What is ocean acidification?

Answer Seawater becoming less alkaline as it absorbs more carbon dioxide

Dissolved carbon dioxide forms a weak acid, and seawater has become measurably less alkaline since the industrial era. The word acidification describes the direction of the change, not that the sea has become an acid.

What is plankton?

Answer Organisms that drift rather than swim against currents, from microscopic to jellyfish sized

Plankton is defined by behavior rather than by kind: anything that drifts rather than swimming against the current. It includes plants, animals, bacteria, and the young of creatures that will later be strong swimmers.

Where does most of the ocean's food chain begin?

Answer With tiny drifting organisms in the sunlit surface layer

Drifting single-celled organisms in the sunlit layer do the photosynthesis that everything else depends on. They also produce a large share of the oxygen in the air — comparable to what forests contribute.

Is coral an animal, a plant, or a rock?

Answer An animal, which builds a hard skeleton and often hosts algae inside its tissue

A coral is an animal, a relative of jellyfish and anemones. Most reef-building species also house algae inside their tissue, and that partnership is what lets reefs grow fast enough to build islands.

What is coral bleaching?

Answer Corals expelling the algae they depend on when stressed, leaving them pale and vulnerable

Heat stress makes the coral expel the algae living inside it. The white you see is the bare skeleton through transparent tissue — the animal is still alive at that point and can recover if conditions ease.

Whales are what kind of animal?

Answer Mammals, which breathe air and feed their young milk

Mammals: they breathe air, are warm-blooded and nurse their young. Their ancestors were land animals, and whale flippers still contain the bones of a hand.

How do baleen whales feed?

Answer By filtering large volumes of water for small animals such as krill

Baleen is made of keratin, the same material as fingernails, and works as a sieve. The whale takes in an enormous mouthful and pushes the water out, keeping whatever is too big to pass through.

What is the swim bladder in most bony fish for?

Answer Controlling buoyancy so the fish can hold its depth without swimming

The swim bladder holds gas and lets a fish stay at a chosen depth without swimming. Sharks have none, which is why many must keep moving, and why their bodies rely on an oily liver for lift instead.

What does a fish's lateral line do?

Answer Senses movement and pressure changes in the water around it

The lateral line detects movement and pressure changes in the surrounding water. It is how a school turns as one in darkness, and how a fish senses something approaching before it can see it.

Why do many open-ocean animals have dark backs and pale bellies?

Answer It makes them harder to see from above against the deep and from below against the light

This is countershading. Seen from above the dark back blends with the depths; seen from below the pale belly blends with the bright surface. The same pattern appears independently in fish, birds and mammals.

Sea turtles return to lay eggs where?

Answer On beaches, often the region where they themselves hatched

Females return to the region where they hatched, often decades later. The leading explanation is that they imprint on the local magnetic signature as hatchlings and navigate back to it as adults.

How far down does enough sunlight reach for photosynthesis?

Answer Roughly the first two hundred meters, depending on how clear the water is

Roughly the first 200 meters, and much less in cloudy coastal water. Below that there is still light for a while, but not enough to power photosynthesis — which is why almost all the ocean's food originates in a thin surface layer.

What happens to pressure as you descend?

Answer It increases steadily, adding about one atmosphere for every ten meters

About one extra atmosphere for every 10 meters. At the bottom of the deepest trench the pressure is over a thousand times what you feel at sea level, which is roughly an elephant standing on a postage stamp.

What is marine snow?

Answer A steady fall of organic particles from the surface layer to the deep

Marine snow is a constant drift of dead plankton, waste and fragments falling from above. It is the delivery system for the entire deep sea, where nothing grows from sunlight and almost everything eats what falls.

What powers the communities around hydrothermal vents?

Answer Chemical energy from the vent fluids, used by microbes instead of sunlight

Chemosynthesis rather than photosynthesis: bacteria extract energy from the chemicals in the vent fluid. These communities were discovered only in 1977, and they proved an ecosystem can exist with no connection to sunlight at all.

Why are many deep-sea animals red or black?

Answer Red light does not reach that depth, so red appears black and both colors are effectively invisible

Red light never reaches those depths, so a red animal reflects nothing and appears black. It is camouflage that costs no pigment complexity, which is why it evolved so widely down there.

What is bioluminescence used for in the deep?

Answer Attracting prey, finding mates, and confusing predators, depending on the species

Light is used to lure prey, to signal to mates, and to escape — some animals release a glowing cloud in the way a squid releases ink. Most of the animals in the deep sea produce light of some kind.

Why do some deep-sea fish look distorted when brought to the surface?

Answer Gas-filled structures expand as pressure drops, damaging the animal

The damage is decompression, not the trip itself. Gas-filled organs expand as the pressure falls, which is why deep-sea fish photographed at the surface often look nothing like they do at home.

What is the abyssal plain?

Answer The vast, nearly flat expanse of deep seafloor covering much of the ocean

The abyssal plains are among the flattest surfaces on Earth, buried under sediment that has been settling for millions of years. They cover more of the planet than all the continents together.

Why is the deep sea usually cold, even in the tropics?

Answer Deep water largely originates in polar regions and sinks because it is cold and dense

Deep water sinks in the polar regions and spreads along the bottom, so even under the tropics the water down there began near the poles. It can take a thousand years or more to complete that circuit.

How do researchers reach the deep ocean?

Answer Mainly with remotely operated and autonomous vehicles, with crewed dives being rare

Mostly with remotely operated and autonomous vehicles rather than crewed submersibles, because sending people is far more expensive and risky. Even so, only a small fraction of the seafloor has been mapped in detail.

Why does drinking seawater make thirst worse?

Answer Removing that much salt costs the body more water than the drink supplies

Seawater is saltier than your body fluids, so your kidneys need more fresh water to flush the salt than the drink supplied. You end up in deficit, which is why it accelerates dehydration rather than relieving it.

How does the risk of a shark attack compare with everyday risks?

Answer Attacks are very rare, and the danger is routinely exaggerated in popular accounts

Attacks are extraordinarily rare — a handful of deaths worldwide in a typical year, against thousands of drownings. The risk on the drive to the beach is far higher than the risk in the water.

People often say most of the ocean is unexplored. What does that actually mean?

Answer Much of the seafloor is mapped only at coarse resolution from satellites, not surveyed close up

It means resolution, not blank spaces. Most of the seafloor is mapped from satellite gravity data at kilometer scale; only a minority has been surveyed by ship in the detail we have for the surface of Mars.

Is the ocean silent below the surface?

Answer No; it is full of sound, and sound carries farther in water than in air

It is noisy down there. Whales, snapping shrimp, breaking waves, rain and ship engines all put sound into the water, and sound moves through seawater roughly four to five times faster than through air, which is why it carries so far. The idea of a silent sea comes from the title of a 1956 film, not from any measurement.

Do corals only grow in warm, shallow, sunlit water?

Answer No; cold-water corals build reefs in deep, dark water

Cold-water corals build reefs hundreds of meters down in cold, dark water, including very large ones off Norway. They carry no algae inside them, so they catch food drifting past instead of living on sunlight, and depth stops being an obstacle.

Are tides caused by the Moon alone?

Answer No; the Sun contributes as well, which is why tides are larger at new and full moon

The Sun contributes too, at about half the Moon's effect. When both pull in line the tidal range is at its greatest, and that is what makes spring tides — a name about timing, not about the season.

Is a jellyfish sting best treated with fresh water?

Answer Rinsing with fresh water can make some stings worse, and advice varies by species and region

Fresh water can make it worse, because the change in salinity triggers stinging cells that have not yet fired. Rinsing with seawater is safer, and vinegar helps for some species — which one depends on the animal.

Do fish drink water?

Answer Saltwater fish do, because they constantly lose water to the sea around them

Saltwater fish drink constantly, because the surrounding sea pulls water out of them by osmosis and they must replace it. Freshwater fish have the opposite problem and barely drink at all.

Is the Great Pacific garbage patch a solid floating island?

Answer No; it is mostly small fragments spread through the water, not a surface you could stand on

It is not an island and mostly cannot be seen from a boat. The plastic is largely small fragments spread through the water column, which is exactly what makes it hard to remove and easy for animals to swallow.

Does the sea level rise the same amount everywhere?

Answer No; local land movement, currents and other factors make the change uneven from place to place

No. Land itself rises or sinks in different places, and currents and gravity redistribute water unevenly, so some coastlines see several times the global average and a few see almost none.