Understanding Global Tectonic Plates and Earthquakes
Global Tectonic Plates: This theory states that the lithosphere moves and explains what causes these movements and what their consequences are:
- The lithosphere is divided into a set of rigid pieces called lithospheric plates. Most plates contain continental and oceanic lithosphere.
- The limits or boundaries of lithospheric plates can be of three types: dorsal (where new oceanic lithosphere is generated), subduction zones (where lithosphere is destroyed), and transform faults (which are not created or destroyed, but a plate moves laterally with respect to another).
- The movements of lithospheric plates are caused by heat within the Earth, aided by gravitational potential energy.
- The oceanic lithosphere is continually renewed, while the continental lithosphere has a more permanent character.
Dorsal and Deep Ocean Ridges: The Atlantic Ocean is traversed from north to south by an undersea rising above the surrounding plains and emerges in Iceland: the mid-ocean ridge. The dorsal branches off into the Indian Ocean and the Pacific. The Atlantic Ridge has a central groove, bounded on both sides by normal faults, called rift.
- Distribution and Ages of the Sediment: Sediment is not evenly distributed. There is sediment in the ridges. The more surface area current and their length increases with depth. The age of the oldest in each area is similar to that of basalts beneath them. In the ridges, rocks are present and their age increases with distance from them.
- Magnetic Stripe: The process occurs during the cooling of magma, and once completed, the direction of magnetization of minerals is permanent and will indicate the direction that the magnetic field had when the rock was formed. This allows use as fossil compasses. The magnetism in rocks is called paleomagnetism. Its study has allowed us to know that the Earth’s magnetic field has reversed many times, switching the positions of the north and south magnetic poles.
Thermal Subsidence: The lithosphere cools by moving away from the ridge, becoming thicker and denser, causing it to sink. This sinking of the ocean floor is called thermal subsidence. It explains why, in the dorsal, the crust is thin and can be achieved under very high temperatures. The contact with ocean water quickly cools the newly formed crust, thus reducing its volume. In the sections near the ridge, the lithosphere is made only of oceanic crust, but as it ages, it cools and the uppermost layer of the mantle attaches to the base of the crust.
Subduction Zones: The process is called subduction, by which the lithosphere is introduced into the deep Earth. Subduction zones are within the boundaries of two lithospheric plates that have a converging motion, so they are also called convergent margins. Following the subducting oceanic lithosphere is destroyed at a rate that balances, on a global scale, the amount of lithosphere generated at the ridges. There are three cases of convergence:
- Oceanic-Continental Convergence: The continental lithosphere is lighter and thicker than the oceanic. For this reason, in a convergence with another oceanic plate, it is the latter that is inserted under the mainland. The oceanic lithosphere transports sediments in its upper part, and most of them do not subduct. These sediments are stacked and deformed, causing what is called the accretion prism. Between the prism and sediment accretion that have not yet been stacked, an elongated groove forms, called the oceanic trench. Sometimes fragments of subducted oceanic lithosphere do not ride on the continent and are attached to it (the process called obduction). The most common case occurs when obduction of the continent collides with volcanic islands. The displacement of a plate from the other is not continuous but occurs in fits and starts, resulting in earthquakes. Subduction zones have the highest seismic activity on the planet. Earthquakes, taking into account the depth of the seismic focus, are classified into the following types: shallow, intermediate, and deep. The subducting oceanic lithosphere is cold and contains certain amounts of water. The friction with the continental lithosphere increases the temperature, and the water lowers the melting point of minerals. This allows for the production of partial melting of the silica-rich minerals, which melt at lower temperatures. They originate volcanic magmas that feed the Andes.
- Ocean-Ocean Convergence: Its characteristics are: the lithosphere subducting at an angle of inclination, the coupling between the two plates is weak, favoring subduction of ocean sediments. As a result, there is no accretion prism, and it has very deep trenches (Mariana Islands), and the associated magmatism causes an island arc.
- Continental-Continental Convergence: If the subducting plate has an oceanic and a continental stage behind it, once it has entered all the oceanic lithosphere, the meeting of continents occurs. Because the continental lithosphere is light enough to not subduct, this is a collision. Following the continental collision, one continent straddles the other. This type of convergence has caused mountain ranges like the Himalayas and the Alps.
Earthquakes and Seismic Waves: The seismic method is the procedure that has provided more information about Earth’s internal structure and is based on the study of earthquakes and how traveling waves arise. Earthquakes are vibrations of the ground generated by the sudden release of energy stored in rocks that are subjected to stress. They arise when large masses of rock fracture or further displace and fractured rocks. These fractures are called faults.
The place of land where they originate inside the earthquake is called the hypocenter, and the site of the land area closest to the epicenter is located directly above the hypocenter.
The vibration generated at the hypocenter is propagated as seismic waves that go in all directions. These waves do not cause the material to move but merely vibrate, without leaving the site.
Types of Seismic Waves:
- P Waves (Primary): They are named because they travel faster and arrive first. They are longitudinal waves, meaning earthquake particles vibrate in the direction of wave propagation. They travel through all media.
- S Waves (Secondary): These waves are transverse, meaning the particles vibrate in a direction perpendicular to the propagation of waves. They travel more slowly than P waves and only through solid media.
- L Waves (Longitudinal): These are surface waves that propagate from the quake’s epicenter, with greater intensity and a shorter drive. These waves are responsible for disasters.
Seismographs: These are very sensitive instruments whose function is to record and measure the magnitude of an earthquake through the drawing of graphs called seismograms.
Directorate of Seismic Waves: The speed at which seismic waves propagate depends on the characteristics of the materials they travel through. Each variation in the propagation speed causes a change in the direction of travel of the wave.
- · Wavefront: It is the surface separating the material disturbed by the passage of the wave and that has not yet been hit by it.
- · Seismic Rays: These are each of the spokes emanating from the origin of the disturbance. The beam follows a straight path, but if it passes from one medium to another, it changes its direction (refraction), a process similar to what happens to light.
If the seismic wave passes through media that is spreading with increasing speed, its trajectory will be curved. If its speed is progressively smaller, it will also have a curved path but in the opposite direction. The shadow zones are places where no seismic waves are received.
Information Provided by the Earthquake: The discontinuities are abrupt changes produced in the propagation velocity of seismic waves inside the Earth. This speed of propagation of waves undergoes gradual changes due to two factors:
- The composition of the materials.
- The physical state of these materials.
Because of these factors, seismic discontinuities are used to differentiate the layers in which the deep Earth is divided.
Main Breaks:
- Moho (Crust) – 70 km: In the areas closest to the surface, P and S waves travel at speeds less than at the Moho, where their speeds increase. The crust is used to differentiate the mantle.
- Repetti Discontinuity (Upper Mantle) – 700 km: There is an increase and decrease in P and S waves.
- Gutenberg Discontinuity (Lower Mantle) – 2900 km: P waves decrease, and S waves cease to propagate.
- Lehman Discontinuity: (Inner Core) – 5150 km: P waves increase their speed.
A Land Layered Structure: Our planet is roughly divided into concentric layers. According to the criteria used to distinguish layers of other areas of the Earth, they can be classified into:
- · Geochemical Units: If the criterion used is the chemical composition of the materials that compose it. Distinct areas: crust, mantle, and core.
- · Dynamic Units: If the criterion used is the mechanical performance that presents each terrestrial hinterland. This distinguishes: lithosphere, asthenosphere, mesosphere, and outer and inner core.
Units of Geochemistry:
Crust: The thin outer layer of the Earth. It extends from the surface to the Moho discontinuity, showing large lateral differences in thickness and composition. It is divided into:
- · Continental Crust: From 25 to 75 km thick. It is very heterogeneous and consists of dense rocks. In the lower half, it is dominated by metamorphic rocks.
- · Oceanic Crust: It is much thinner, with a thickness between 5 and 10 km, structured in three levels. The rocks of this layer are denser and younger than those of the continental crust.
Mantle: Extends from the base of the crust to a depth of 2900 km. It constitutes 83% of the volume of the Earth.
Core: The central area of the planet, located below the Gutenberg discontinuity.
Dynamics Units:
- · Lithosphere: The outer shell and rigid. Includes some of the crust and upper mantle. Under the ocean, the oceanic lithosphere is 50 to 100 km thick, while the continents, the continental lithosphere, is 100 to 200 km.
- · Asthenosphere: The plastic layer that lies beneath the lithosphere and reaches the discontinuity of 670 km deep. This is a portion of the mantle. The materials are in solid state and are subject to convection currents.
- · Mesosphere: Includes the rest of the mantle. The rocks are also subject to convection currents due to temperature and density. At the base of this plate is the layer D (double prime). This is a discontinuous and irregular layer with thickness from 0 to 300 km.
- · Inner Core: It extends to a depth of 5150 km and is in a liquid state.
- · Outer Core: As the core releases heat, it solidifies the iron which is composite.