The Earth is a dynamic planet, where the surface we inhabit is constantly changing due to geological processes that, although often imperceptible, have a profound impact on the landscape and the life around it. Among these processes, isostasy and subsidence play a key role in the shape and balance of the Earth's crust.
In this article we are going to show you What is isostasy and subsidence?, how they originate and why they are essential to understanding the dynamics of our planet.
What is isostasy?
Isostasy is the state of gravitational equilibrium that the outer layer of the geosphere, comprising the crust and the adjacent mantle, attempts to achieve. This condition accounts for variations in altitude, exemplified by the distinctions between oceans and continents, which serve to compensate for density differences present in different regions. Vertical motions, known as epirogenic motions, are involved in this process and are based on Archimedes' principle, which was articulated as a principle in the late 19th century.
Disturbances in isostatic equilibrium can occur due to tectonic activity or melting of the polar ice caps. Isostasy plays a crucial role in shaping the Earth's relief. The continental crust exhibits a lower density compared to the mantle and oceanic crust. When folding occurs within the continental crust, it leads to the accumulation of substantial amounts of material in designated areas.
After this period of uplift, erosion begins, resulting in the eventual deposition of materials beyond the mountain range, which in turn causes a reduction in weight and volumeTo compensate for this loss, the roots of the crust ascend, leaving behind materials that have undergone significant metamorphic processes on the surface.
What is subsidence?
The term describes the gradual lowering of a surface, usually referring to the lithosphere, that occurs due to the relative motions of tectonic plates, encompassing both convergence and divergence of these plates. At a more localized level, Subsidence may result from the settling of land within sedimentary basins, a process that is often accelerated by human activities such as oil extraction or groundwater depletion.
Furthermore, the cessation of volcanic activity in specific areas surrounding volcanoes, as seen in atolls, contributes to this phenomenon. As subsidence occurs, a rise in sea level occurs. In contrast, uplift refers to the increase in elevation of the Earth's solid surface, leading to a subsequent decrease in sea level.
Subsidence in various scientific fields
In meteorology, the term subsidence denotes the downward movement of cold air from the upper layers of the atmosphere, a phenomenon that can be accurately attributed to the increase in density of air as it cools.
In the field of oceanography, Subsidence refers to the sinking of ocean waters within the continental coastal zone, which is typically found along the eastern coasts of continents. Here, ocean currents, including equatorial and other currents, pick up significant volumes of water, which are then carried in the opposite direction (from west to east) due to the influence of the seabed, following the Earth's rotational motion.
A similar process occurs in the atmosphere. In the intertropical zone, the equatorial current is generated as an inertial response to the Earth's rotation. The compensation of water moving in the opposite direction to the Earth's rotation occurs not only on the ocean floor within the equatorial zone, but also on the surface. This phenomenon manifests itself both on a smaller scale, giving rise to the formation of equatorial countercurrents, and on a much larger scale, exemplified by the west-east currents.
Subsidence occurs at areas of tectonic plate convergence, which refers to the collision between two plates moving in opposite directions. This interaction leads to the uplift of one plate, usually the continental plate, while the other plate, usually the oceanic plate, sinks into the mantle. Evidently, subsidence takes place on the submarine plate, creating what is known as an oceanic trench; in this context, the sinking is called subduction.
A notable example is the South American trench located on the Pacific coast (Peru-Chile Trench), which reaches depths of over 8.000 meters below sea level. The Mariana Trench holds the record for the deepest oceanic trench, with a depth of just over 11.000 meters.
Differences between isostasy and subsidence
Although isostasy and subsidence are related to changes in the Earth's surface, they are distinct processes both in their origin and in their implications.
Isostasy is a theoretical concept that describes the gravitational balance between the different masses of the Earth's crust floating on top of the denser mantle. It is similar to how an iceberg floats on water: the denser or bulkier parts of the crust, such as mountains, They sink deeper into the mantle, while lighter areas, such as plains, remain higher. This balance is constantly being adjusted in response to changes in mass, such as erosion, sediment deposition or glacial melting. Isostasy acts over millions of years, contributing to the shaping of the Earth's relief.
On the other hand, subsidence is a localized phenomenon that occurs when a portion of the land descends progressively. This sinking can have natural causes, such as the compaction of sediments, the emptying of magma chambers or the movement of tectonic plates, but it can also be induced by human activities, such as the extraction of groundwater, oil or gas. Unlike isostasy, which seeks large-scale equilibrium, subsidence is typically a faster process and can lead to immediate problems such as flooding, damage to infrastructure and changes to ecosystems.
I hope that with this information you can learn more about what isostasy and subsidence are in the field of geology.