Re: What's an underwater storm like?

Hi, Jeremy.

The term "Underwater Storm" is probably used by newspapers and TV journalists to help readers and listeners visualise the chaos and confusion that takes place in some underwater events. Often, these are caused by fundamental events that are not similar to atmospheric storms (e.g. Earthquakes, landslides etc.), but since water is such a dense medium compared to air, any major disturbance propogates throughout the water layer, causing turbulent eddies and secondary effects. It would certainly feel like a storm to the animals and plants being swirled around by the currents and rained with sand and sediment, like hail on land.

There are several different classes of things that are probably being described this way (I don't know where you came across the term, so I can't be sure which of these is the most appropriate one for your situation, but you can always email me with more details)

Obviously, whenever there is an atmospheric storm, the surface of the sea is whipped up into waves and these crash on the beach, drag on the sea bed, and agitate the water column. At these times, animals flee or burrow down into sand and rocks, and plants are thrahed around. Some strong swimming species will use these times to hunt for injured animals.

The clash of tidal, seasonal, and long-term currents can lead to violent churning of the ocean, but these tend to be recurring features of particular areas. In these zones, sediments are scoured away and sea life is limited to rock-clinging plants and algae, and animals that hide in crevices when the currents are strong.

Earthquakes can raise tsunami that act impact as huge unexpected waves when they arrive in shallow water. Unless the earthquake is near enough for animals to have sensed the vibrations and hidden, they will caught in the open by this large wave. In the 1750 Lisbon earthquake, reports record that the sea withdrew like a giant low tide, leaving fish flopping around on the seabed. People who went out to view this phenomenon, or to collect the fish, were drowned when the tsunami crashed back over the beaches.

Underwater avalanches can raise clouds of sediment and displace water in confused eddies that can damage plants and animals. As the sediment rains down again, communities can be buried. In some circumstances, a density flow (turbidity current) can be set up. If enough sediment is entrained into a turbulent cloud, it becomes denser than normal sea water and will flow downhill, down underwater valleys. This rushing cloud of sediment and water can travel at several hundred km per hour, transporting mud, sand, and even rocks up to about 1 metere in size! The closest analogue in terrestrial environments is a volcanic eruption cloud (a nuee ardente, or pyroclastic flow).

Many fossil assemblages are beleived to have been caused by individual calamities like this. The plants and animals that were busily living their lives were suddenly either buried in choking mud, or swept away and dumped in deep hostile environments. In one famous example, the Burgess Shale of British Columbia, a community of very early animals was swept off a shelf sea into deep anoxic water. They were buried in fine mud and did not decompose due to the lack of oxygen. Over millions of years, they fossilised into finely detailed casts with most of their soft body tissues preserved as shapes and outlines in the rock. Palaeontologists have spent years decoding and interpreting these early animals and fitting them into the picture of life on earth. It wasn't a good day for those animals when the "storm" hit them, but it has preserved a fantastic legacy for scientists.

Nick Hoffman
hoffman.nick.n@bhp.com.au