Continents, like icebergs and rafts, also follow the laws of buoyancy. Imagine an iceberg floating in the ocean—the more its top melts, the higher it rises. Similarly, if you load a raft, it sinks deeper into the water. Now, think of continents experiencing these buoyant forces, but at a much slower pace due to the thick, slow-moving layer beneath them called the asthenosphere. This viscosity creates a sort of geological dance, resulting in the fascinating processes we observe in our world today. Let’s explore a few examples of these dynamic geological phenomena.
- When massive ice sheets melt, the Earth’s surface can sink or rise. In places that had ice sheets during the last ice age, the land is now rising because the heavy ice is gone, reducing the load on the Earth’s outer shell (lithosphere).
- We can see evidence of this in areas like the Baltic and Canada, where the land is still rising because of the slow “rebound” from the last ice age. Former sea-cliffs and wave-cut platforms, now found far above sea level, provide clues.
- Isostatic uplift also compensates for mountain erosion. When material erodes from mountains, the land rebounds upward. This is particularly noticeable at the edges of plateaus, where drainage patterns lead to more prominent erosion.
- In plateau regions with surrounding mountain ranges (like the Himalayas and Kunlun Mountains around the Tibetan Plateau), isostatic uplift can raise the edges higher than before, resulting in ridge tops at a considerably higher elevation than the plateau itself.
- Over time, the thick crustal root beneath mountains can become denser and sink into the mantle through processes like convection or delamination. Once detached, the rise of the asthenosphere leads to more mountain building. This is thought to explain the late Cenozoic uplift of the Sierra Nevada in California.
- Seismic data in the Sierra Nevada region shows crust-mantle interactions during the supposed sinking of the dense crustal root. In the southern Sierra Nevada, dense matter flows into a mantle drip, creating a V-shaped cone of crust being dragged down, causing the disappearance of the Mohorovicic discontinuity in seismic images. A similar seismic “hole,” the Redding anomaly, is observed in the northern Sierra Nevada, suggesting lithospheric foundering there as well.
In conclusion, isostasy is yet another example of a deceptively simple idea in physics that provides crucial and sweeping explanatory power for other sciences.
Read Also: What Is Isostasy?
