Earth’s Tectonic System and Isostasy
Tectonic System
The tectonic dynamic system is closely tied to the internal structure of the Earth and the physical characteristics of the planet.
This system is responsible for the major features of the land surface, such as continents and ocean basins. In turn, on the continents, major structural features are shields, platforms, mountains or fold mountains, and on the ocean floor are mid-ocean ridges, abyssal plains, seamounts, trenches, and continental margins.
The tectonic system is the dynamic system that controls the horizontal movements of the crust (continental drift and expansion of the ocean floor) and the life cycles of the oceans (their “birth,” evolution, and “death”).
The tectonic dynamic system is directly associated with the theory of plate tectonics. This theory states that the outer part of the solid Earth (lithosphere) is broken and divided into a series of plates or lithospheric units. These units are mechanically rigid and independent, moving freely relative to one another. Their respective mutual contacts manifest most of the major geological activity of the system. It is a unifying theory that explains the evolution of continents and ocean basins, the genesis of folded mountain ranges, and the genesis and distribution of mineral deposits, etc.
Lithospheric plates (or lithosphere), which consist of the crust and upper mantle, are about 100 km thick and slide over the warmer and more plastic mantle.
Where lithosphere plates move away from one another, the remaining space is occupied by hot material that comes from the mantle, creating new lithosphere. The major features that testify to the occurrence of this process are: continental rifts, ridges, and new ocean basins.
Where the movement of lithospheric plates is convergent (one collides with another), one slides under the other and plunges into the mantle. The major structural features that testify to the occurrence of this process are folded mountain ranges, with thickening of the crust, volcanic arcs, and deep ocean trenches.
The energy required for the occurrence of geological processes related to the tectonic system comes from inside the Earth. Its heat source is relic, mechanical, and radioactive, which is transferred by conduction and convection.
All geological processes are called endogenous to the tectonic system, and on a human time scale, the most representative are seismicity and volcanism.
Gravity and Isostasy
Gravity plays a fundamental role in the dynamics of the Earth. In the hydrological system, the flow or movement of ice (glaciers), the running surface water (rivers), and groundwater (aquifers and water table) are regulated by gravitational force. In the tectonic system, uplifts and subsidence of the crust (vertical movements of the land area), combined with horizontal movements of the plates, are often controlled by gravity.
Gravity is also closely linked to differentiation within the Earth (the planet’s internal structure), as it is the force that controls the rheological or limit contact between the lithosphere and the asthenosphere. The lithosphere, being colder, more rigid, and lightweight, floats on the asthenosphere, which is hotter, softer (plastic), and denser.
Isostasy theory argues that the ongoing uprisings and subsidence affecting the crust, in response to the force of gravity, tend to maintain a balance or equilibrium of gravity throughout the system.
All isostatic adjustments, this trend towards a point of gravitational balance between the crust and mantle, cause the former to move the mantle based on their volume and density, and the force that controls these settings is gravity.
Any change in the Earth’s surface by the addition or removal of material causes isostatic adjustment (high mountainous terrain has deep roots in the mantle).
Isostasy is linked to all processes that modify the distribution of materials on the surface, as any type of surface loading and unloading produces vertical movements of the lithosphere.
The fundamental concept of isostasy is used to study the major features of the crust, such as continents, ocean basins, and mountain ranges, and, therefore, to understand the response of the crust to erosion, sedimentation, and glaciation. This means that isostatic adjustment is involved in almost all exogenous processes.