4 weeks 1 day
Submitted by Carol Larson on Wed, 07/10/2013 - 11:07
Alaska uses a coastal water transportation system of tugboats and barges loaded with containers to distribute goods from coastal town to town. We have seen several passing by in the last few days as they take advantage of the good weather to do this.
Talk about traffic flow! We are in a rich area of glacial transportation.
How has all this highly sought after sediment that we have been extracting traveled so far from the coast? It’s got a lot to do with elevation, temperature, gravity, and precipitation.
The highest coastal mountain range in the world awaits our vision but we can see on the map that the St Elias Mountains rise from the sea to elevations of over 6000 metres. Elevation is one of the criteria for glacial formation. This explains why of all the continents; Australia is the only one that doesn’t have glaciers. Puts them at the bottom of the glacial heap!
Glaciers form when annual snowfall exceeds annual snowmelt. Temperature plays a huge role here too in glacial formation and movement.
Alaska has over 100,000 glaciers, which are always on the move- melting, expanding, contracting, scraping, gouging and starting a process of sediment transport and deposition. Sometimes they can be heard “growling” in the process!
Glaciers are agents of erosion and are driven by gravity. The natural flow is downward where rain or melt water moves through the best channels possible winding down from the higher elevations.
During the spring when there is much precipitation and in the summer when the sun melts the ice, the glacial rivers then bring huge amounts of sediment from the mountains to the coast. From here the sediment filled waters mix in the sea and are further transported by wave action, storms and ocean currents.
The sea floor matches the land for topography with basins, undersea mountains and valleys. The flowing sediments become trapped in channels, submarine fans or are carried off the continental shelf into the deeper sea. We have spent the last month hovering above the Surveyor Fan looking at what the Bering, Malaspina, and other glaciers in this area have brought down in the flow.
The last stop is when ice breaks off tidewater glaciers and often carries rocks that have shifted from the mountains with it. These rock hitchhikers are called IRD or ice rafted debris. Sometimes these are found in the core samples and are also called drop stones or lone stones.
Hitchhikers have the best stories to tell, if only these lone stones could talk! But our scientists have amazing ways of extracting their information.
Photo credit-NASA Image by Robert Simmon