This article is part of our collection on the 'Golden Globe Ocean Race' where former Open University student Antoine Cousot is circumnavigating the globe for 30,000 miles, alone. Our academics are championing this by getting you to explore information, science and ideas about the ocean with their explainer articles.
Mountains and Valleys
Why are there mountain ranges and valleys under the sea?
Until the late 1930s the only way to measure the depth of the ocean was to use a line with a weight on the end. For example, in 1521 Ferdinand Magellan stopped his ships in the Pacific Ocean during the first circumnavigation of the globe and lowered such a line. After paying out all the line they had he was sure that the weight had not touched the bottom, but how deep was it? He knew the ocean was deeper than his 400 fathoms of rope (~730 m) - so he concluded that his ship was over the greatest depths in the oceans!
The development and use of a technique called SONAR (SOund Navigation And Ranging) revolutionised the investigation of the oceans' depths. Originally developed for hunting submarines, in sonar a ship sends out a pulse of sound (a 'ping'), which is reflected by the target and the reflected sound wave is detected. It was soon realised that if they were 'loud' enough the underwater pings could detect the sea floor. If you measure the time it takes for a reflected ping to be heard and know the speed of sound in water, then you can derive the water depth. Using sonar, ships could record continuous depth measurements without stopping.
The diagram below shows the bathymetry (i.e. depth measurement) across the North Atlantic Ocean along a latitude close to 39° N. At the point −30° W is the Mid-Atlantic Ridge (MAR) – a colossal mountain range under the ocean. The variability in underwater depth is astonishing!
These ridges, the mountain ranges under the sea, are the most active volcanic features on the globe and new ocean crust is being formed there every day.
These ridges and trenches are part of the tectonic system of our planet.
The oceans also contain very deep valleys, known as trenches, where ocean crust is being destroyed. You may have heard of the famous Mariana Trench, about 11 km (nearly 7 miles!) deep in the ocean. The famous film maker James Cameron travelled to the deepest place on Earth!
The average swimming pool deep end is about 5m deep. How many times deeper is the Mariana Trench?
The Mariana Trench is 11 km, which is 11000 metres. So the trench is 11000 m / 5 m = 2200 times deeper!
Next to these underwater trenches further volcanic activity often occurs on the land as the crust that is being destroyed at the bottom of the trenches heats up and melts. This process produces some of the world’s most famous volcanoes such as Mount Fuji on the so-called ‘Pacific Ring of Fire’.
Take a closer look at the trenches and the majestic ‘Ring of Fire’!
These days SONAR is complemented by other techniques, including satellite measurements, to determine the bathymetry of the ocean.
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