The Mariana Trench in the Pacific has been mapped (mentioned in an earlier piece) with ultrabeam sounding technology and scientists, subsequently, have measured the depth – roughly 11000m. However, it is the discovery of four bridges spanning the trench that is most surprising and they are thought to be caused by one plate descending into the hole and becoming snagged when the sea plate subducting has sea mounts. There are lots of sea mounts on the Pacific plate and are as much as 2500m high. Sea mounts are thought to form as carbonate reefs close to the mid ocean ridges and are carried along on top of the ocean plate on the sea floor. Hence, the bridges appear to be sea mounts jammed up against the two plates at the subduction zone – which is what the Mariana Trench is thought to be. Geologists are now thinking in terms of sea mounts creating jerks at plate boundaries, a novel idea as it allows stress to build up and thereby when the sea mount moves down, eventually, there is a rapid movement of the plates one side of the trench. This accounts for so many earthquakes in the region – and the Japanese tsunami last year. However, the expedition also illustrates that scientists know very little about the ocean depths.
Put into your search engine, or simply google, 'What lies beneath: Exploring the Ocean depths' which is the title of an article originally published by New Scientist – but see also Down to Earth 78 Feb 2012.
Meanwhile, Iceland is on volcano watch – the pregnant lady, or Katla, is the subject of monitoring after a sub glacial explosion that caused some flooding last year. There were some 500 tremors in October alone. Katla last erupted in 1918, causing ice melt and flooding.