World Oceans Day

Many are aware of the oceans’ vital statistics: over 70 per cent of our planet is covered in water, and over 95 per cent of the water on Earth is contained within the oceans. This water sustains life, from the highest predator to the smallest bug, as it cycles through our ecosystems and atmosphere. But just how did it all get there? 

Ocean evolution

Earth’s surface is constantly changing. Over millions of years tectonic plates have shifted to form the land masses and oceans we recognise today

There are a few theories about how the water on Earth came to be, including many different influencing factors.

ocean fact 2

Earth’s oceans contain a surprising amount of precious minerals

The first is the inside-out model, suggesting that once the Earth formed water existed in a bond with other minerals, then came to the surface as a result of volcanic activity. Another idea is that water was present as vapour, which condensed as Earth cooled. A third theory is the outside-in approach, stating that some of our water came from outer space as ice contained within asteroids or comets. Regardless of which is true, the first permanent ocean on Earth is thought to have formed between 4.3 and 3.8 billion years ago.

The metamorphosis into the map we know today is the result of tectonic activity. Earth’s crust floats on a layer of molten rock, known as the mantle. The crust is separated into several plates that, thanks to convection currents, are constantly moving against one another, spreading apart or disappearing underneath one another. At areas where the plates are pulling away, magma from beneath the crust wells up in the gap created by the separating plates. This cools, hardens and creates new layers of rock, which is how some oceans are growing at a surprisingly fast rate – sometimes more than 15 centimetres (5.9 inches) per year.

The water bodies of our five main oceans are all interlinked, cycling through a series of currents. There are two types: surface and deep-water currents. Surface currents are whipped up by wind and governed by landforms and the Coriolis effect – a force that exists because of the Earth’s spin on its axis. This force also has an effect on the water bodies within oceans and produces large-scale whirls of water that circulate around major ocean basins, known as gyres.

The deep-water variety of currents, meanwhile, is mainly a result of water temperature and salinity at the depths of ocean basins, known as thermohaline circulation. Salty, chilly water from the poles is dense, so it sinks to the bottom and slips along the seafloor. Water from the North Pole flows south, through the Atlantic down to the Southern Ocean. It then heads into the Indian and Pacific Oceans, where it heats up as it encounters warmer waters. Heated water that is less salty is less dense, so it rises but sinks again once it eventually reaches the poles and cools. It’s estimated that it takes 1,000 years for water to complete this global ocean conveyor belt.

Some oceans are growing at a surprisingly fast rate – sometimes more than 15 centimetres per year

The circulation of water transports oxygen and nutrients around the oceans. It also carries vast amounts of moisture and heat around our planet, which affects our climate.

Without the currents to regulate the uneven distribution of solar radiation that reaches Earth’s surface, the climate would be far more extreme.

ocean statistics

Seas by numbers

One major climactic process produced by ocean current activity is El Niño – the appearance of warm surface water off the coast of Peru every few years in December. This warming process is actually a by-product of a much larger ocean process known as the El Niño Southern Oscillation (ENSO), which is a natural climate phenomenon concerning oceans and atmospheres, the effects of which can have widespread implications for global weather.

The Sun-lit surface of the oceans is known as the epipelagic zone, where as on land photosynthesis forms the basis of the food chain. Plankton blooms take advantage of the light and provide the dinner of choice for many marine species. The smaller fish provide food for larger hunters and so the transfer of energy is passed down the oceans’ layers.

In the mesopelagic zone light gradually tails off to twilight and photosynthesis is no longer possible. Beneath this is the blackness of the bathypelagic zone, some 1,000 metres (3,300 feet) below the surface. Finally, deeper still, the crushing depths of the abyssopelagic zone are found at the very bottom of the ocean basins.

The effects of El Niño can have widespread implications for global weather

The waters of ocean trenches are known as the hadalpelagic zone – an unforgiving realm of inky darkness, freezing waters and crippling pressure, yet a surprising array of life survives there against the odds. At the deepest point – the Challenger Deep in the Mariana Trench – the pressure is more than 1.2 tons per square centimetre (7.7 tons per square inch), which is the equivalent of one person trying to hold up 50 jumbo jets!

Due to the tectonic activity of these regions, ocean trenches and areas around them are punctuated with large vent chimneys that spew out chemical-rich water from within Earth’s core. Vent communities are teeming with unique species, thanks to the presence of bacteria that use chemosynthesis to form the basis of the food chain. The bacteria use hydrogen sulphide in the vent water, along with oxygen and carbon dioxide, to create sugars, providing sustenance for smaller vent-dwellers. This gets transferred up the food web and soon the vent is jumping with biodiversity.

It’s said we know more about outer space than we do about our oceans on Earth. It’s not hard to see why, when the average depth of the oceans is more than 3.7 kilometres (2.3 miles). As well as sending ROVs (remotely operated vehicles) and subs down to investigate the depths, oceanographers have developed many different methods for sampling this watery world.

Hydrothermal vents are teeming with unique species

For instance, the seafloor can be mapped using sound waves, which travel at 1,500 metres (4,921 feet) per second in water. Echo sounders, backscatter and sound velocity profilers can all be used to accurately detect the depth, shape and composition of the seabed.

Other methods for sampling the ocean include salinity and temperature profiling, while lowering instruments to depths and then returning them to the surface can build up a more detailed picture. Samples can also be taken using dredges, core samples and trawls to collect organic matter.

Satellites in orbit are equipped with various sensors to relay many different ocean variables back to Earth for analysis. For example, sea surface temperature, air-sea interactions, ocean waves, currents and sea-ice patterns can all be viewed and monitored from afar. There are also monitoring systems that use buoys moored in the seas that constantly measure ocean movements. This kind of technology is especially useful for applications such as advance warnings for tsunamis.

There’s so much left to discover about our oceans and with so much technology at our fingertips, it’s impossible not to wonder at what else is out there. How many more breakthroughs are lurking just beneath the waves, waiting to be found? 


Article extract from “Earth’s Amazing Oceans” by Ella Carter in How It Works issue 59, available now from the Imagine Shop


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