QA – history of the area

Tags: Carol Cotterill

We had a question come through from Birdie about the geology we are seeing up here, and so I thought I’d write a little about the history of the area, and in the process try to answer Birdie’s question! So here goes with the rocks……

One of the questions was how this area compares to Iceland, and whether the degree of volcanic activity is the same. With Iceland you have some of the youngest rocks in the world, with new crust forming constantly at the mid-ocean spreading ridge. This normally runs through the deep Atlantic basin, but at Iceland it comes onto land, effectively splitting the existing land apart as new land is created. There is associated magmatic activity, with vast reserves of geothermal energy that heat Reykjavik and other towns.

We have a different situation here along the western Greenland coast with some of the oldest rocks. The majority of the rocks we are seeing surrounding Disko Bay, and to the north of the Nugssuaq Peninsula are a Gneiss, re-worked in the early Proterozoic, so approximately 2100 – 1750 million years ago! The rocks were initially accreted crust, formed approximately 3100 – 2500 million years ago. A period of increased volcanic activity and heat caused these rocks to alter their mineral composition, through melting and re-working, forming the Gneiss which is a high-grade banded metamorphic rock. Disko Island itself, and the outermost half of the Nugssuaq Peninsula are formed of Picrite, an olivine rich basaltic rock from an extrusive volcanic period during the Paleocene (approximately 66 – 57 million years ago).

So although there is very conclusive evidence of past volcanic activity, there are no active magma streams present on Greenland at the moment that could be affecting the glaciers and icecap.

When we run the survey profiles offshore we are imaging different layers of sediments, much like taking a slice through a cake that has sponge, filling and sponge again. Where you get different sediments lying next to each other, there is a change in the speed that sound will travel through them, forming what we call an acoustic impedance boundary. This boundary reflects some of the sound back to us on the ship which is heard by the hydrophones (marine microphones) we pull behind the ship. We can therefore differentiate between bedrock, glacial till (deposited by the glaciers in moraines), and finer sediments, be they silts or muds or volcanic sands.

However, although we can clearly see these different layers, we cannot instantly tell exactly what sediments they are without sampling them to ground-truth our records, which will hopefully happen next year. But we do get a very clear image of where we have ancient channels that have been gouged out by the ice streams and glaciers of the past, and what thickness of sediments have infilled these channels, and whether or not these sediments are undisturbed, or have been subject to later periods of erosion and re-deposition……it’s a fascinating multi-layered story!!!

Hope this answers your questions Birdie, but if you have any other ones please email them in and I will try to answer them for you!

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