Glacial isostatic adjustment: the marshmallow effect


The “marshmallow test” refers to a well-known psychological study: researchers at Stanford offered children the choice between one marshmallow immediately or two marshmallows after a fifteen-minute wait. They were seeking insights into how children develop the ability to forgo an immediate reward in order to receive a greater reward later. When I tell you a story from my childhood, you will understand why the “marshmallow effect” has become my personal nickname for delayed gratification on a geologic time scale: glacial isostatic adjustment (GIA). 

I grew up in Juneau, Alaska, between the Juneau Icefield and Glacier Bay National Park. When I was in elementary school, my Dad taught my class about glacial isostatic adjustment with a memorable demonstration. He passed out marshmallows (“it only works with Kraft brand Jet-Puffed”) and asked us to squash them and put them in our pockets. Like the children in the Stanford marshmallow experiment, we waited several agonizing minutes. After what seemed an eternity, we took the marshmallows out. To our surprise and delight, they rebounded to their former puffiness. This graphic – and delicious -- analogy helped us understand why parts of southeast Alaska are rising several centimeters per year.

Unlike marshmallows, glacial ice is dense and heavy. The weight of continental ice sheets or extensive mountain glaciers depresses the Earth’s crust (squashing the marshmallow) and forces the underlying mantle to flow out of the way. The areas directly underneath the ice sink, surrounded by an area that rises: the forebulge. Removing the weight of the ice (taking the marshmallow out of the pocket) reverses this process: the areas that were depressed rise, while the forebulge collapses. While the ensuing effect is sometimes called “post-glacial rebound,” the term “glacial isostatic adjustment” is more precise, reflecting that some areas subside while others rise. The crust and mantle may take 10,000 years or more to reach equilibrium—even longer than the interminable wait to eat both marshmallows. (UNAVCO)

scienceImage1.pngThe general process of GIA. Top: heavy ice loads Earth’s surface. Bottom: once the ice is removed, some areas rebound, while others collapse. (UNAVCO, 2020)

The glaciers and icefields of southeast Alaska have lost mass rapidly over the last 250 years, since the end of the Little Ice Age. This unloading of the crust and mantle results in rapid regional uplift, exceeding thirty millimeters a year in some areas (Hu and Freymueller, 2019). This may not sound like much, but to geologists accustomed to incremental changes that occur over millions and even billions of years, it is dramatic.  

While GIA is unusually pronounced in southeast Alaska, it occurs all over the world. In the European Alps, researchers attribute uplift rates of several millimeters per year to GIA resulting from deglaciation following the Last Glacial Maximum (LGM) about 20,000 years ago (Mey et al., 2016).

 3 LGM alpsThe European Alps were covered in kilometers of ice during the LGM (Adapted from Mey et al., 2016).


4 uplift alps Mey and others’ (2016) observed and modelled GIA uplift rates in the European Alps. Note the relatively high rates in Switzerland (red and orange circles).

How do they know that the Earth’s surface is rising? Networks of antennae for the Global Navigation Satellite System (GNSS) measure the movement. In Switzerland, the Automated GNSS Network for Switzerland (AGNES) maintains stations in picturesque locales like the Jungfraujoch Observatory. 

5 jungfraujoch observatoryAGNES stations like the one at the Jungfraujoch Observatory measure rates of uplift (Samuel Zeller).

GIA is not merely a fascinating scientific phenomenon; it also has surprising effects on day-to-day life. In Alaska, coastal private property ownership extends to the mean high tide line. The high uplift rates mean that waterfront properties are growing in size significantly. For example, several relatively new golf courses were underwater not long ago, and my Dad’s backyard in Juneau gets bigger every year. Uplift rates are also higher than predicted sea level increases, so rising sea levels are less of a concern in southeast Alaska than in many other coastal areas. In the European Alps, both melting of the LGM ice cap and ensuing GIA influence sea level variation in the Mediterranean. 

The marshmallow test correlated self-control in preschool with success later in life, though behavioral psychologists now debate its significance and replicability. As a geologist, I find the marshmallow effect rests on much safer scientific ground. Though GIA rates are rapid, current predictions still require many lifetimes to be borne out. In the meantime, why not enjoy a marshmallow—or two? 

Hu, Y., & Freymueller, J. T. (2019). Geodetic observations of time‐variable glacial isostatic adjustment in Southeast Alaska and its implications for Earth rheology. Journal of Geophysical Research: Solid Earth, 124, 9870– 9889.

Mey, J., Scherler, D., Wickert, A., Egholm, D., Tesauro, M., Schildgen, T., & Strecker, M. (2016). Glacial isostatic uplift of the European Alps. Nature Communications, 7, 1-9.

Stocchi, P., Spada, G., & Cianetti, S. (2005). Isostatic rebound following the Alpine deglaciation: impact on the sea level variations and vertical movements in the Mediterranean region. Geophysical Journal International, 162, 137–147,

UNAVCO, 2020, GPS Spotlight: Station CHUR: (accessed October 2020).

Cover image by Flickr user John Morgan

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