How melting ice moves mountains, quite literally
Hurricane Sandy leaves devastation in its wake as it strikes the Eastern United States.
We’ve all heard speculations, some scientific and some not at all, that melting ice in Antarctica will eventually lead to calamities: the submergence of New York, oscillating ocean temperatures, and tragic loss of biodiversity. These conclusions are logical on the surface. When water, previously tied up inside massive glaciers, begins to return to the ocean, sea levels will indisputably rise. Or will they?
According to research teams of both the present and past, changing water levels are comparatively less ostensible compared to the immense geological changes caused by melting glacial structures. In the 18th century, Swedish geologists were dumbfounded when they could not find evidence supporting a rise in sea level after the last ice age, which officially ended 7,000 years ago1 . In fact, geochemical evidence pointed out that sea level has fallen - how could that be? This shocking discovery first hinted that a phenomenon called post-glacial rebound was affecting the Earth’s crust. This phenomenon is illustrated in Figure 1 and is also referred to as glacial isostatic adjustment.
Figure 1: The immense weight of ice presses down on the crust, causing the formation of a central depression and noticeable bulges around the ice mass. But when the ice sheet melts, a process often lasting hundreds of years, the pressure on the crust is relieved. Therefore, the crust slowly begins to rebound back to its original shape.
The Swedish geologists then realized that though the total amount of water in the ocean has augmented, the increase is overshadowed by the changes in the surrounding land. Due to post-glacial rebound, the land has risen well above sea level, which consequently appears to have diminished. Because these geologists were observing the effects of deglaciation thousands of years after the actual melting of the ice, the crust was naturally beginning to return to its initial position at a rate of about 10 mm per year.
In addition to vertical motion, recent studies in Antarctica have shown that horizontal motion is not unexpected in areas where mantle compositions differ. The mantle closest to the Earth’s crust is rigid, and predominantly solid, but in fact behaves like a viscous fluid in geological time (extremely long periods of time). Because it is difficult to measure viscosity within short periods of time, and the viscosity of the mantle changes with changing conditions, rheology is used to measure the composition of the mantle. Unlike viscosity measurements, rheology takes into account other factors such as change in rate of deformity when pressure is applied. Over the past few years, researchers at the Ohio State University have studied the rheology of Antarctica. They sent seismic waves into the mantle and discovered that West Antarctica has much softer, warmer mantle rock than East Antarctica2 . At the American Geophysical Union conference this fall, the team announced that West Antarctica was actually being pushed horizontally, a few inches a year, by the rigid mantle of East Antarctica. The soft rock, which is supposed to regain shape according to post-glacial rebound, is instead being obstructed and pushed sideways by harder rock masses. This find has led to another unprecedented yet integral piece to deglaciation, and shows that its geological effects reach far further than expected. It seems that this phenomenon is far from the brink of subsiding. The European Space Agency’s Cryosat spacecraft has reported that West Antarctica, only a tiny section of Antarctica’s entire ice mass, dumps 150 cubic km of water into the ocean each year (a 15% increase from last year)3 . On a similar note, University of Chicago geologist Alison Banwell concluded that the drainage of a single pool of liquid actually leads to more fractures in the ice, lubricating the bed of the ice sheet and increasing the rate of water flow. Water is also gravitationally attracted to these ice sheets. When the ice melts, the attraction is reduced, causing the sea level rise to be more prominent in areas geographically distal in the ice sheet. For example, the melting of the West Antarctica Ice Sheet would increase North America's sea level 30% more than the global mean sea level value (Mitrovica et al., 2009).
Other geological effects are less noticeable, but no less tangible. The components of the mantle (mostly oxygen but also containing Si, Mg, Fe, and Al) are significantly heavier than ice, which refocus the gravitational field of the earth. This small shift was detected by gravimeters on the GRACE and LAGEOS missions, and causes a noticeable flux in vertical datum, a reference commonly used by humans to survey land and construct buildings4 . Furthermore, as ice melts and glacial rebound occurs, not only does the center of gravity change, but the Earth also becomes less oblate.
Figure 2: As land mass returns to the poles when ice melts, the earth’s oblate shape, caused mostly by its gravitational pull, will be reduced.
Melting ice also accounts for earthquakes that happen away from plate-to-plate boundaries. Glaciers are heavy – the average glacier imposes about 30 MPa of stress on crustal rock (a whopping 4351 pounds per square inch). The relief of vertical stress not only causes the land to bounce back, but also shifts tectonic plates. A post-glacial fault may be formed when stress changes significantly over a short period of time, leading to the breakage of brittle crust. The rift formed as a result of deglaciation may have been key in the New Madrid, MO earthquake of 1811. Conversely, according to the well-known Mohr-Coulomb theory, large glacial loads may also stop earthquakes from happening due to the sheer weight of their presence.
Unfortunately, melting ice also has intensified legal squabble over land ownership. After a glacier melts and the surface area of land increases, who has the legal right to ownership? For some countries in which glaciers are quite common, such as Finland, a rule has already been established. The new land will go to the owner of the water source, such as a sea, lake, or ocean bordering the deglaciated land.
Though it seems that the global warming crisis is slowly disfiguring the Earth, there are other natural, albeit weaker forces, that are working to protect the planet. According to Leela Frankcombe, a geophysicist at the Centre of Excellence of Climate System Science in Sydney, Australia, the widening ozone hole on top of Antarctica has caused Antarctic winds to speed up and creep upward5 . The wind lowers temperatures and helps to counteract rising sea levels and melting polar ice. But the ozone hole also significantly increases the amount of solar and thermal radiation absorbed by the planet, placing jet stream and cloud cover directly above the South Pole. In the recent Geophysical Research Letters publication, atmospheric scientist Kevin Grise of Columbia University revealed that this positioning hinders the reflection potential of cloud cover, as the Pole itself does not receive much light6 . Ultimately, deglaciation is a balancing act. Whether a larger proportion of positive or negative effects will be unleashed by geological alterations is unknown; only time will tell.
- De Geer, Gerard. "Om Skandinaviens niråförändringar under qvartär-perioden." GFF 12.2 (1890): 61-110 [↩]
- "East Antarctica Is Sliding Sideways: Ice Loss On West Antarctica Affecting Mantle Flow Below." ScienceDaily. ScienceDaily, 11 Dec. 2013. Web. 05 Jan. 2014. [↩]
- Amos, Jonathan. "Esa's Cryosat Mission Detects Continued West Antarctic Ice Loss." BBC News. BBC, 12 Nov. 2013. Web. 05 Jan. 2014. [↩]
- "GFZ Potsdam, Department 1: The GRACE Mission". Archived from the original on 2008-05-08. Retrieved 2014-01-04. [↩]
- "Sea Level Changes Forced by Southern Ocean Winds." - Frankcombe. N.p., n.d. Web. 05 Jan. 2014. [↩]
- "The Ozone Hole Indirect Effect: Cloud-radiative Anomalies Accompanying the Poleward Shift of the Eddy-driven Jet in the Southern Hemisphere." - Grise. N.p., n.d. Web. 05 Jan. 2014. [↩]