THEY would be a surfer’s dream – if they could ever be ridden. Even the monster rollers off Hawaii cannot match in size and strength the waves that course the bottom of the ocean.
How do they form? Underwater waves start where ocean tides push water over obstacles on the seafloor such as mountains and ridges. This water, which is the densest layer because it is the coldest and the most saline, is then deflected upwards into water that is less dense. It then slows and is ultimately dragged back down by gravity until it overshoots into deeper, denser water – and is promptly bounced back up again. This oscillation creates a great wave that can continue across the ocean floor for thousands of kilometres.
How big do they get? Their peak height often exceeds 50 metres and can reach 100 metres. And they look different to surface waves. Their wavelengths – the distance between the crests – can be anything from hundreds of metres to tens of kilometres, and their period – the time it takes for successive peaks to pass a given point – varies from tens of minutes to several hours. All this makes them appear to move in slow motion.
Advertisement
What effects do they have on the seafloor? Considerable, according to David Cacchione of the US Geological Survey and Lincoln Pratson of Duke University, North Carolina, who have found that these underwater waves are strong enough to prevent deposition of sediments and even erode them to such an extent that they control the evolution of continental slopes. Cacchione has even been down in a submarine to watch the resulting mud-storm. Tell that to a surfer intent on catching the ultimate wave.