Can unconfined ice shelves provide buttressing via hoop stresses?
Wearing, Kingslake and Worster (2020)
Around the coastline of Antarctica, the ice sheet goes afloat as large ice shelves, covering vast areas as large as France. With the current unprecedented levels of climate change, the drive to constrain the dynamics and stability of ice sheets has increased. To be able to predict the impact climate change will have on the Earth’s ice sheets in Greenland and the Antarctic, an understanding of the past and present processes that control ice sheet mass balance is required. Buttressing is an important control on the stability of marine terminating glaciers and ice streams across Antarctica and so is an important factor to consider. The following key questions are addressed by Wearing, Kingslake and Worster (2020):
- Do unconfined Antarctic ice shelves provide buttressing?
- Can hoop stresses allow ice shelves to regrow or advance beyond lateral confinements?
Ice Shelves
Ice shelves are floating sheets of ice which extend from grounded ice sheets. Ice mass flows downstream from the grounded ice sheet under the force of gravity, feeding ice shelves by tributary glaciers or ice streams (Figure 1). The grounding line is the point at which an ice sheet becomes afloat, becoming an ice shelf (Figure 1 and 2). Whilst ice shelves are often located in confined embayments, ice tongues are unconfined ice shelves.
Buttressing
Buttressing of ice shelves is the resistance created by the ice shelf to the flow of the ice sheet upstream from the grounding line (Figure 2). The force of buttressing is a result of the viscous deformation of the ice shelf, which in turn is dependent on a series of factors including:
- The shape and lateral confinement of the ice shelf embayment
- Location of “pinning points” in the ice shelf (points of localised grounding within an ice shelf)
- Ice shelf thickness, extent, rheology and structural integrity
Ice shelf thinning, especially near the grounding line and shear margins of the shelf, are known to dramatically reduce buttressing effects. This not only reduces the volume of the ice shelf, but it can also lead to a rapid increase in velocity of the upstream grounded ice sheet, causing thinning and ablation.
What are hoop stresses?
Hoop stresses are the viscous resistance to spreading perpendicular to the direction of divergent ice flow in an unconfined ice shelf. The strength of buttressing from hoop stresses is raised by higher effective viscosity and greater divergence.
However, many ice shelves across Antarctica have effective viscosities that are lowered as a result of widespread fracturing of the ice mass. Widespread fracturing results in low ice shelf strength, making it unable to generate large hoop stresses. This results in low buttressing forces resisting the flow of the grounded ice sheet.
However, when an ice shelf is surrounded by sea ice, this is thought to help provide stability and prevent collapse, which may allow the advance of the ice shelf front (Figure 3). Sea ice is formed from the freezing of sea water, as opposed to an ice shelf which has a terrestrial source. The presence of surrounding sea ice gives an ice shelve greater viscosity, which in turn generates an increased buttressing force. Furthermore, sea ice can help decrease further ice shelf fracturing and damage.
Conclusions
Wearing, Kingslake and Worster (2020) conclude that theoretically, unconfined ice shelves can provide buttressing via hoop stresses. In reality however, the extensive fracturing and damaged ice of unconfined ice shelves make the effective viscosity too low to exert hoop stresses large enough to provide sufficient buttressing to Antarctic ice sheets. The high extensional stresses required to support buttressing cannot be sustained by damaged ice. However, research found that the presence of sea ice can support unconfined ice shelves, helping to keep the consolidated and reduce fracturing of the ice. Buttressing is an essential process to take into account when considering the dynamics and stability of ice sheets and ice shelves.
