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CD 166: Exploring underwater avalanches offshore Morocco


 


Geology in the Agadir Canyon and Basin


The Agadir Canyon and Basin are located just offshore Morocco, several hundred kilometres off the northwestern coast of Africa. This area is classed as a passive margin - there is no tectonic plate boundary, so the land mass grades into continental shelf and down into the deep sea [click here for more on ocean basins]. This means that the earth's crust in this area is very stable - earthquakes are very few, allowing large thicknesses of sediment to build up on the continental shelf.


The Atlas Mountains in Morocco

Much of this sediment is sourced from the erosion of the Atlas Mountains, which reach over 4000 metres altitude in the highest parts. These mountains were formed as a result of the collision between the European plate and the African plate which started some 80 million years ago. Since then, the mountains have been slowly eroded away, with the sediment particles carried down the mountains towards the sea by rivers. Once the rivers reach the sea, the sediment is either deposited on the continental shelf or is transported deeper into the ocean basin via the Agadir Canyon, which is clearly visible on bathymetric maps of the ocean floor in this area.

Some sediment in the Agadir Basin comes from erosion of the Canary Islands. However, because these islands are much younger than the Atlas Mountains, there has been less time for them to erode, so the amount of Canary Islands sediment in the basin is much less than that sourced from the Atlas. There is no submarine canyon connecting the Canary Island to the deep sea basin. This is because there are no major rivers on the islands to carve out a canyon, and also because the islands have not been around for very long, so there has not been time for a canyon to form, even if there was a major river on the islands.


Teide, the largest volcano in the Canary Islands

As sediment builds up on the continental shelf, it becomes unstable from time to time and collapses under its own weight, creating an underwater avalanche of sediment. As the sediment is disturbed, it mixes with water creating a turbidity current which cascades down into the deep sea [click here to find out more about turbidity currents and turbidite deposits].

On the shelf offshore Morocco, these turbidity currents are often funnelled down the Agadir Canyon. The shape of the canyon is strongly influenced by geological features such as salt diapirs on the continental shelf, and seamounts on the basin floor. Salt diapirs are formed when bodies of rock salt, which are less dense and therefore more buoyant than the surrounding rocks, rise up towards the surface causing the seafloor to dome up. Seamounts are underwater volcanoes.

The biggest turbidity currents (those containing the greatest volume of water and sediment) are capable of flowing hundreds of kilometres across the seafloor. In the Agadir area, the biggest flows can travel all the way through the Agadir Basin and out onto the Madeira Abyssal Plain, a distance of about 1500km. To put that distance into perspective, if you set off one of these big turbidity currents at John O'Groats in Scotland, it would flow all the way down to Land's End in Cornwall and on towards France!

As the flow slows down, the sediment is dumped out to form layers on the seafloor. It is often these layers that the marine geologists are looking for when they take samples of the seafloor sediment. By using tiny fossils in the sediment, scientists can work out the age of the turbidity currents. Looking at the composition of the sediment tells them what sort of rock it was eroded from, which can often pinpoint where the sediment originally came from. For example, sediment from the Atlas Mountains is quite different from sediment eroded from the Canary Islands because they are made up of different rocks.




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© Challenger Division for Seafloor Processes
October 2003
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