Lithospheric Plates Float on the Mantle
The mantle is able to convect because of its plasticity, and this property also allows for another very important Earth process known as isostasy. The literal meaning of the word isostasy is “equal standstill,” but the importance behind it is the principle that Earth’s crust is floating on the mantle, like a raft floating in the water, rather than resting on the mantle like a raft sitting on the ground.
The relationship between the crust and the mantle is illustrated in Figure 3.18. On the right is an example of a non-isostatic relationship between a raft and solid concrete. It’s possible to load the raft up with lots of people, and it still won’t sink into the concrete. On the left, the relationship is an isostatic one between two different rafts and a swimming pool full of peanut butter. With only one person on board, the raft floats high in the peanut butter, but with three people, it sinks dangerously low. We’re using peanut butter here, rather than water, because its viscosity (thickness or stiffness) more closely represents the relationship between the crust and the mantle. Although it has about the same density as water, peanut butter is much more viscous (stiffer), and so although the three-person raft will sink into the peanut butter, it will do so quite slowly.
The relationship of Earth’s crust to the mantle is similar to the relationship of the rafts to the peanut butter. The raft with one person on it floats comfortably high. Even with three people on it the raft is less dense than the peanut butter, so it floats, but it floats uncomfortably low for those three people. The crust, with an average density of around 2.6 grams per cubic centimetre (g/cm3), is less dense than the mantle (average density of approximately 3.4 g/cm3 near the surface, but more at depth), and so it is floating on the “plastic” mantle. When more weight is added to the crust through the process of mountain building, it slowly sinks deeper into the mantle and the mantle material that was there is pushed aside (Figure 3.19, left). When that weight is removed by erosion over tens of millions of years, the crust rebounds and the mantle rock flows back (Figure 3.19, right).
Isostasy and Glacial Rebound
The crust and mantle respond in a similar way to glaciation. Thick accumulations of glacial ice add weight to the crust, and as the mantle beneath is squeezed to the sides, the crust subsides. The Greenland ice sheet, for example, is over 2,500 m thick, and the crust beneath the thickest part has been depressed to the point where it is below sea level over a wide area (Figure 3.20a). When the ice eventually melts, the crust and mantle will slowly rebound (Figure 3.20b), but full rebound will likely take more than 10,000 years (3.20c).
Large parts of Canada are still rebounding as a result of the loss of glacial ice over the past 12 ka, as are other parts of the world (Figure 3.21). The highest rate of uplift is in within a large area to the west of Hudson Bay, which is where the Laurentide Ice Sheet was the thickest (over 3,000 m). Ice finally left this region around 8,000 years ago, and the crust is currently rebounding at nearly 2 cm/year. Strong isostatic rebound is also occurring in northern Europe where the Fenno-Scandian Ice Sheet was thickest, and in the eastern part of Antarctica, which also experienced significant ice loss during the Holocene.
There are also extensive areas of subsidence surrounding the former Laurentide and Fenno-Scandian Ice Sheets (Figure 3.21, yellow through red regions). During glaciation, mantle rock flowed away from the areas beneath the main ice sheets and into the surrounding areas, and this material is now slowly flowing back out of the surrounding areas, as illustrated in Figure 3.20b.
How Can the Mantle Be Both Solid and Plastic?
You might be wondering how it is possible that Earth’s mantle is rigid enough to break (like during an earthquake), and yet it convects and flows like a very viscous liquid. The explanation is that the mantle behaves as a non-Newtonian fluid, meaning that it responds differently to stresses depending on how quickly the stress is applied. A good example of this is the behaviour of Silly Putty, which can bounce and will break if you pull on it sharply, but will deform in a liquid manner if stress is applied slowly. In Figure 3.22, Silly Putty was placed over a hole in a glass tabletop, and in response to gravity, it slowly flowed into the hole. The mantle will flow when placed under the slow but steady stress of a growing (or melting) ice sheet.