Understanding Plate Tectonics and the Dynamics of the Earth
The Internal Engine of the Earth
The internal dynamics of the Earth depend on two factors: internal energy in the form of heat, and the force of gravity. The internal temperature of the Earth increases as the distance from the surface increases; this is called the geothermal gradient. In the crust, the average geothermal gradient is 3 *C per 100 m. In volcanic areas, it can be as high as 10 *C per 100 m estimated according to extrapolations based on lab experiments and on seismic data.
Heat Flow in the Geosphere
Heat travels from the hot interior of the Earth up to the surface. The amount of heat energy that reaches the surface is called the heat flow. This heat may be transmitted by conduction, but rocks have low conductivity. As a result, transmission takes place very slowly. The real engines driving the internal dynamics of the Earth are convection currents. Hot materials, which are less dense, This continuous flow is generated by high temperature variations between the lithosphere and the D” layer.
Vertical Movements of the Lithosphere
The rigid lithosphere ‘floats’ on top of the asthenosphere. The state of gravitational equilibrium between the two layers is called isostasy. This equilibrium is altered by dynamics of internal and external origin. Blocks or sections of the lithosphere move up or down to re-establish subsidence. Ice accumulation during glaciation or sediment accumulation in sedimentary basins can cause subsidence. In contrast, both ice thaw and erosion reduce the weight on top of the blocks. This results in uplift, which means the lithosphere rises.
Horizontal Movements of the Lithosphere
Until the beginning of the 20th century, most scientists believed that the continents had always been fixed in the same positions. The most complete and significant was proposed by Alfred Wegener in 1912: the theory of continental drift. It explained numerous phenomena observed in fields such as palaeontology, palaeoclimatology. Wegener believed that the continents could move. He theorized that 300 million years ago they were joined together in a single supercontinent he called Pangea. Then the supercontinent gradually began to break up, different parts moved horizontally across the seabed like icebergs in water. Eventually they formed the current continents. Wegener presented considerable evidence to support his theory. However, he was unable to name a force strong enough to move the continents. He suggested that the rotation of the Earth could have been responsible. The evidences are Geographical evidence, Palaeoclimatic Evidence, and Palaeontological evidence.
Plate Tectonics
Plate tectonics is a comprehensive theory of geological processes. The concept of plates was developed by Canadian geologist, John T. Wilson, in 1965. It was based on his study of the global distribution of earthquakes and volcanoes. There are two main types of plates: oceanic plates, which consist entirely of oceanic lithosphere, and mixed plates, which consist of a mixture of continental lithosphere and oceanic lithosphere.
Principles of Plate Tectonics
The lithosphere is divided into plates. These plates are separated by unstable boundaries characterized by intense seismic and volcanic activity. All the plates fit together. Oceanic lithosphere is thinner and denser than continental lithosphere. It is generated continuously at mid-ocean. Gravity and the internal heat of the Earth generate convection currents. These currents move the tectonic plates with respect to one another, changing the position of the continents. Tectonic plates interact. This interaction creates large relief features and related phenomena, such as earthquakes and tsunamis. Some evidences are maps of the sea floor, measure of the plates moving, and the direction of the plate.
Relative Movements of Plates
The place where two plates meet is called a boundary. Each type of plate boundary moves and interacts differently, and each creates specific geological structures. Various geological phenomena occur at these boundaries. For example, lithosphere can be created, destroyed, or conserved. Other intense phenomena, such as earthquakes, volcanic activity, and subsidence also occur. The type of boundary depends on the relative movements. There are three types:
Boundary type Events Geological structures generated
Convergent Plates collide Oceanic lithosphere is destroyed Orogens, orogenic belts: mountain ranges, etc.
Divergent Oceanic lithosphere is created Mid-ocean ridges
Transform Plates slide past each other Transform faults