Earth’s Structure, Plate Tectonics, and Earthquakes
Earth’s Structure
The materials that make up the planet are arranged in layers according to their densities: atmosphere, hydrosphere, crust, upper and lower mantle, and outer and inner core.
Plates
A plate is a rigid structure, called the lithosphere, which contains rock materials of the upper mantle and crust, both continental crust and the oceanic crust. These lithospheric plates have a thickness of about 100 km and float on the mantle.
The continuous movement of the plates is due to the existence of convection currents between the core, mantle, and lithosphere. The energy that causes convection currents is supposed to be the Earth’s internal heat, which is the engine that drives the plates.
Ocean Ridges
The ocean ridges are undersea mountains that surround the globe. In their central ridge, they have a slot called a rift and are constantly expelling magma ascending currents driven by convection.
Subduction Zones
Subduction zones are sinks located in the depths of the oceans, where the oceanic lithosphere sinks continuously and forms elongated oceanic trenches. On their way back into the deep, the rocky material of the oceanic lithosphere describes an inclined plane, called the Benioff plane.
The transform faults are the earthquake desfarres appearing in areas under different thrusts.
Earthquakes
In subduction zones, in the ocean ridges, and faults, rock masses collide and cause earthquakes. When affecting the continents, they are called earthquakes, and if affecting the ocean floor, they are called tsunamis or tidal waves.
Volcanoes
In ocean ridges, in subduction zones, and hot spots, magma escapes through the cracks and forms volcanic mountains. The formation of thrust plates crushes sinking sediments accumulated in the subduction zones, which folds and fractures and then rises to form the great mountain ranges.
As a result of the creation of oceanic lithosphere on both sides of the ridges, oceans are becoming larger.
Magma
Magma is a hot and fluid mixture formed by molten rock materials, mainly of the type of silicates, usually also containing solid particles of minerals and rock fragments in suspension and various dissolved gases.
Volcanic Eruptions
Volcanoes are formed when magma from the mantle rises to the surface, cools, and gives rise to gas flares, solid products called pyroclastic, and molten rock, called lava. In subduction zones, the eruptions are not usually so calm, and volcanoes are shaped like giant cones, which can consist of accumulations of solid fragments of lava without cohesion, called pyroclastics.
A volcanic eruption is a set of phenomena that take place when the magma reaches the surface. When the magma from the mantle is close to the surface, it forms a watertight sac called the magma chamber. The gases dissolved in the magma are no longer pregnant, which increases the pressure in the chamber and pushes the magma. It rises through one or more tubes called chimneys and out the crater.
The main volcanic hazards are lava flows, pyroclastics rain, cloud formation, burning, explosions, lahars, the sliding of the slopes of the volcanic cone, toxic and asphyxiating gases, earthquakes, and tsunamis.
Measures of Prediction
When volcanoes show signs of activity, such as animals killed by toxic fumes, acidification of water from springs and lakes, etc., it can be a sign of an impending eruption. These signs can also be detected with certain instruments.
Prevention
The best way to prevent volcanic eruptions is not to live near volcanic activity. Another way is to have evacuation plans, and houses should be built with pitched roofs to better withstand eruptions.
Measures of Correction
Sometimes it is possible to divert the path of a lava flow into a vacuum by controlled blasting of rocks with explosives.
Earthquakes
An earthquake is ground shaking caused by the fracturing of rocks at depth. The place inside the Earth where the earthquake originates is called the focus. The surface area located directly above the focus is the epicenter.
The fracturing of rock masses generates vibrations that extend from the focus in all directions in the form of seismic waves. There are three types of seismic waves:
P Waves
These are the first to reach a given point, being the fastest, and do not cause major damage.
S Waves
S waves are closely followed by P waves, arriving somewhat later, causing major damage.
L Waves
L waves are transverse waves. They are the last to arrive and spread over the surface from the epicenter. They produce two types of motion: vertical motion, like a secondary wave, along with a movement going from one place to another. These are the most destructive.
Earthquakes are detected by seismographs, which are devices that record on a graph, called the seismogram, the time of arrival of different groups of seismic waves and the amplitude of their vibrations.
The Richter scale measures the magnitude of an earthquake, which is a measure of the energy released by the earthquake. The Richter magnitude theoretically has no limits and can reach all values. The MSK scale (Medvedev, Sponheuer, Karnik) measures the intensity of an earthquake, which is an estimate of the effects produced by the earthquake on people, objects, etc.
The risk of a given area is the probability of a given magnitude earthquake occurring in that area.
Measures of Prediction
There is no way to predict the exact time and place of an earthquake. The only thing that is certain is that where one occurs, another can occur.
Prevention
Prevention consists in developing seismic hazard maps in order to develop evacuation plans and civil protection.
Measures of Correction
Measures of correction are the most difficult to establish, as there are no measures to prevent the development of an earthquake.
Mountain Formation
Mountains were formed in subduction zones, by the folding of large accumulations of sediment. In the subduction zones, the oceanic lithosphere sinks and leads to a large trench. Sediments from erosion of the continent are accumulated in the trench and arranged in horizontal layers, called strata.
As it sinks, the lithospheric plate compresses and pushes sideways the layers, which may form folds, which are called anticlines and synclines. When the rock strata fracture, faults originate. After millions of years, the folded and fractured rock strata emerge and form a ridge where a pit was before.
Hesperian Massif
The Hesperian Massif is the eroded remnants of an early Variscan cordillera.
Cordillera Peripheral
The Cordillera Peripheral is younger and is arranged in the periphery of the Hesperian Massif.
Depressions or Pits
These are subsidence of the crust that are filled with sediments: depressions in the Douro, Tagus, the Ebro, and Guadalquivir.