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Oceans and Water
Factfile |
 Oceans and Water |
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The Water Cycle |
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We have two types of water on Earth: salty seawater in the oceans and seas and fresh water in lakes, rivers, ponds and the frozen ice sheets that cover Antarctica and Greenland.
Ice Sheet - BAS
Ice sheets or Continental glaciers are made of snow, which fall in the middle of a landmass and gets squashed into dense ice. When it becomes heavy enough, the ice sheet starts to spread out slowly outwards, until it reaches the coast. There it forms an ice shelf, a thick, frozen ledge that floats on the sea. The Antarctic ice sheet is the largest single mass of ice on Earth. It covers an area of almost 14 million km 2 and contains 30 million km 3 of ice. Around 90 per cent of the fresh water on the Earth's surface is held in the ice sheet, an amount equivalent to 70 m of water in the world's oceans.
The amount of water in our oceans and seas stays about the same - but how does that happen? After all, seawater has been around for millions and millions of years, why haven't we run out of it? The amount of water in the sea actually stays about the same because of the Hydrological Cycle or Water Cycle and here is how it works:
The Sun is the power source behind the Water Cycle. As the Sun heats up rivers, lakes, oceans and even puddles, the heat causes the surface of the water to evaporate. This means the water has turned from a liquid to a vapour, (water in a gas form) and it is invisible in the air. The vapour is then lifted up into the atmosphere on warm air currents.
As the vapour reaches the cooler layers of the atmosphere, it cools down. As it does so, it condenses, or turns back to into tiny droplets of water, forming the clouds, mist or haze that covers our planet. (Our ever-changing cloud patterns are a result of old clouds constantly re-evaporating and forming new ones).
Clouds - Melinda Kolk
Once formed, these clouds might get blown on land (clouds protect our Earth from the heat of the Sun). As the clouds grow in size, the clouds will become too heavy with droplets. Eventually, these then fall back to Earth as Precipitation. (Depending on other conditions at this time, Precipitation can fall as rain, snow or hail). Once on land, the Water Cycle can start again.
Animals and plants also contribute to the Water Cycle. Animals and plants loose water in the form of water vapour through their pores. This process is called Transpiration. Think of your breath on a very cold day. The steam that you blow out is in fact water vapour. Once you have blown it out, it will start the process of returning to the atmosphere (and so start the Water Cycle process again).
Each time a drop of water goes through the Water Cycle, it is purified or cleaned and animals and plants can use it again and again (and again!). However, this also means that at some point in the past, the water in your next glass of water could have been a gas, solid or liquid many times over. Just think, it is quite possible that a dinosaur could have swum the water you are about to drink!
So what is it about seawater that makes it seawater?
Seawater has many minerals, chemical compounds and gases dissolved within it. Would you believe that about 9 million tons of gold is dissolved in the world's oceans? There are also large qualities of potassium, magnesium and calcium salts. Creatures such as crabs, mussels and barnacles depend upon seawater for calcium carbonate to make their shells. These soft-bodied creatures take calcium carbonate from seawater as they grow, and the resulting shell helps to protect them from predators.
The main mineral dissolved in seawater is sodium chloride or common salt, the same stuff that you put on your food. Salt in the ocean has come from the rocks and soil on land. Rain washes the salt into rivers, which in turn carry it to the sea. Some salt also comes from undersea volcanoes. It is a process that has been happening for millions of years. In some very hot countries, people trap the tide when it comes in. They then wait for the Sun to dry up the water, before collecting the salt that has been left behind.
The saltiness of seawater is called its salinity and a water's salinity level is measured as p.s.u. (practical salinity units). This is achieved by measuring the amount of dissolved salt there is in 1,000 parts of water. In the case of ordinary seawater, it has a salinity level of 35 p.s.u and fresh drinking water has a salinity level of less than 1 p.s.u. Some seawater, like the water found in the Dead Sea has a very high salinity level, which is why swimmers can float in it.
US Virgin Islands - C M Cross
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