At Universität Hamburg’s Center for Earth System Research and Sustainability (CEN), my students and I are investigating how salt is deposited in the Earth’s crust, the structure of the surrounding rock, and what processes were responsible for creating and distorting the massive layers. Since 2001, every year a group of geophysics students has travelled to the Baltic Sea to study the Earth’s crust beneath the seabed. We then use the recorded measurements to create cross-sectional images that show us the stratification of the bedrock and in which areas there are cracks, fissures and distortions.
Until now we have assumed that faults and fractures in the rock were the result of tectonic plates colliding several million years ago, making them geologically very old. One day, however, when I was looking at the data with my students, I stopped short. Together with my Ph.D. candidate Mu'ayyad Al Hseinat I took a closer look at the cross-sections. The uppermost and therefore youngest one to two kilometers – in other words, the layers with the most salt deposits – seemed to have been shaped by other processes, since the distortions we found in them didn’t match with the tectonic movements of this time. So which forces were the culprits?
We immediately made plans for further research, and soon thereafter we launched an expedition to the Baltic on board the research ship Maria S. Merian, towing a three-kilometer-long “sock” full of recording equipment behind us. The equipment measured sound waves that we had fired toward the seafloor – since boundary layers, faults and fissures reflect these waves back to the surface, this approach provided us with a detailed image.
After carefully analyzing our results, we were certain: In the past 400,000 years, the glaciers from Scandinavia during the glacial periods kneaded the uppermost one to two kilometers of the Earth’s surface like a giant pie crust – leaving behind immense distortions in the Baltic Sea Basin and the North German Plain. Strata collided, sediments mixed with ice, and valleys and hills were formed. And we can still see the effects today: For example, since the melting of the last great glaciers, northern Scandinavia has continued to rise, no longer burdened by the ice’s tremendous weight.
The same phenomenon affects older layers, which have also partially been pushed upward over millions of years – like the numerous salt domes to be found in northern Germany. As such, we assume that a future glaciation of northern Germany would also produce major changes in the domes.
In the course of the next 100,000 years, northern Germany and the Baltic region as a whole will only remain ice-free for a few tens of thousands of years. That makes it important that we understand the processes that can release the weight of the ice from the Earth’s crust – especially since salt domes are currently being investigated as potential candidates for safely storing radioactive waste for the next million years.
This content was first published as a guest article in the newspaper Hamburger Abendblatt on 10th July 2017
Prof. Dr. Christian Hübscher is a geophysicist at Universität Hamburg’s Center for Earth System Research and Sustainability, where he is currently researching seismic ground motion and geological deformations.