The science behind the science...

Treasure at the bottom of the ocean?

Did you know that some of the most important metal deposits in the world started life as an accumulation of mineral debris around hydrothermal vents?


Hydrothermal vents spew out superheated fluids rich in metals and minerals into the sea. As the fluids react with the cold seawater, particles of metal-bearing minerals fall out of solution and settle out on the seafloor, rather like ash settling from a smokey chimney. Over the years, the mineral deposits build up into a thick layer.

Some of the largest metal deposits in the world are thought to have been created by hydrothermal activity, with enormous metal sulphide deposits being the direct result of hot water stripping out elements from volcanic rocks. Most of the hydrothermal ore deposits now being exploited are millions of years old, and represent the fossilised remains of ancient hydrothermal vent systems.

Mineral deposits formed near mid-ocean ridges fall into a category of mineral deposits known as volcanogenic massive sulphide (or VMS for short) deposits. This name is used becasue the heat source for the water is volcanic, and the metals in the deposit are combined with sulphur to form metal sulphides. The photo on the right shows open cast mining of a VMS-type deposit in northern Spain. The most common metals found in this sort of deposit are iron, copper, lead and zinc. As well as the metal deposits formed around the vent itself, hydrothermal fluids also deposit minerals in the pipes feeding the vent, forming mineral veins.


However, not all hydrothermal vents occur on mid-ocean ridges. In fact, hydrothermal activity can occur wherever water becomes heated up and circulates through rocks. Active hydrothermal vents are a huge tourist attraction - for example, Old Faithful and the numerous other geysers and hot springs in Yellowstone National Park, USA (pictured left) and the hot springs in Iceland.

Geysers (from the Old Norse word 'geysa', meaning to rush forth) form where hot water (hydrothermal solutions) become trapped underground while continuing to be heated. The pressure builds up until the water forces its way out, resulting in an explosive eruption of scalding water and steam.

Hot springs occur when heated water bubbles up to the surface and forms a pool. The Roman baths at Bath in the UK (pictured right) are fed by hydrothermal solutions passing up a fault from hundreds of metres below the surface. If you have ever tasted the water from the springs at Bath you'll know that it tastes slightly strange - this is because of the sulphur and other minerals dissolved in the water.

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Go to:

Hydrothermal vents: introduction
Mineral water with a difference
Treasure at the bottom of the ocean?
Oasis on the ridge

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