DK 2020新书 Knowledge Encyclopedia Ocean! 海洋知识百科全书（新增第8本）
Earth’s oceans are almost as old as the planet itself. Formed more than four billion years ago, they have been evolving ever since. Their waters are constantly on the move – flowing in currents around the globe, pulled in a daily cycle of tides, and rising and falling as waves that crash on the shores.
The vast majority of the planet’s water is contained within the oceans, filling these huge basins up to several kilometres deep. Ocean water is salty – in contrast with the fresh water found in rivers and lakes – and varies in temperature around the globe, from the balmy tropics to the freezing poles. Water continually moves between the oceans, air, and land, in the global water cycle.
HOW MUCH WATER?
Seen from space, Earth’s vast expanses of water give it its nickname – the blue planet. More than 1.3 billion cubic km (300 million cubic miles) of salty water covers most of Earth’s surface.
The average ocean basin extends to depths of around 6km (3.7miles) in depth. Compared to this, all other water sources, such as rivers, lakes, and inland seas, only hold very small amounts.
WHY IS THE OCEAN BLUE?
Water is colourless and transparent, so sunlight can penetrate through it. But the sunlight we perceive as white is actually a mixture of all the different colours of the rainbow.
Each colour can penetrate to different depths. Blue light can get much deeper than the others, travelling down to 100m (330ft), which is why the sunlit part of the ocean appears blue. But below this, no light can penetrate, so at deeper depths the ocean looks dark.
WHAT IS WATER?
Water is made up of molecules far too small to see, even with the most powerful microscope. There are more than a thousand million trillion of them in a single drop.
Each molecule of water is made up of a single oxygen atom and two hydrogen atoms joined by strong chemical bonds.
On average ocean water contains about 35g (1.2oz) of salt in every litre (2pints). This salt is made up of a mixture of chemicals. More than 80 per cent of salt is made up of just two types of chemical called sodium and chloride.
These make up sodium chloride – the salt we use for flavouring food. But ocean salt also contains other chemicals, in smaller amounts.
HOT AND COLD SEAS
Ocean temperature varies around the globe. Heat from sunshine is more intense in the areas around the equator, so the ocean’s surface is warmer there and gets colder towards the poles.
But oceans everywhere get cooler with depth too and the water at the ocean floor is always cold, even at the tropics.
THE WATER CYCLE
Since Earth was formed 4.5 billion years ago, the amount of water it contains has stayed about the same. However, this water constantly moves around and constantly changes state – between liquid, gaseous vapour, and solid ice.
This process is called the water cycle and is driven by the Sun. Water evaporates from the ocean surface due to the Sun’s warmth and eventually falls as rain, which then returns to the oceans
More than 80 per cent of the inhabitable areas of our planet is the open ocean – deeper waters away from the shallow coastal seas.
From its sunlit surface to its cold, dark floor, the open ocean is home to some of the most extraordinary animals known to science.
THE OPEN OCEAN
The majority of the ocean’s water stretches beyond the coastal seas of the continental shelves, and reaches down to the deepest parts of the ocean basins – an area called the open ocean.
This is the single biggest habitat on Earth and contains more living things than anywhere else on the planet. But conditions at the surface are very different from those at the bottom.
A drop in the ocean A huge variety of life forms can be found in just a single drop of seawater, from plant-like algae to the ocean’s tiniest herbivores and hunters.
These living things make up the plankton – the community of ocean drifters that are carried along with the currents because they are either too small or too weak to swim against them. Plankton exist everywhere in open water, but are richest near the surface, where sunlight provides the energy for algae to make food.
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