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EARTHQUAKES AND VOLCANOES
EARTHQUAKE
The study of seismic waves provides a complete picture of the layered interior.
An earthquake in simple words is the shaking of the earth. It is a natural event. It is caused due to release of energy, which generates waves that travel in all directions.
The release of energy occurs along a Fault (A fault is a sharp break in the crustal rocks).
Rocks along a fault tend to move in opposite directions.
As the overlying rock strata press them, the friction locks them together.
However, their tendency to move apart at some point in time overcomes the
As a result, the blocks get deformed, and eventually, they slide past one another abruptly.
This causes a release of energy and the energy waves travel in all directions.
The point where the energy is released is called the focus of an earthquake, alternatively, it is called the hypocenter.
The energy waves traveling in different directions reach The point on the surface, nearest to the focus is called the epicenter. It is the first one to experience the waves. It is a point directly above the focus.
EARTHQUAKES – ORIGIN BASED ON PLATE TECTONICS
As per the theory of plate tectonics the crust or the earth is composed of solid and moving plates having either the continental crust or oceanic crust or even both continental- oceanic crusts.
The earth’s crust consists of 6 major plates (Eurasian plate, American plate, African plate, Indian plate, Pacific plate, and Antarctic plate) and 20 minor plates. These plates are constantly moving in relation to each other due to thermal convective currents originating deep within the earth.
Thus, all the tectonic events take place along the boundaries of these moving plates.
TYPES OF PLATE BOUNDARIES
Constructive plates boundaries,
Destructive plate boundaries and
Conservative plate
Constructive plate boundaries represent the trailing ends of divergent plates which move in opposite directions from the mid-oceanic ridges,
Destructive plate boundaries are those where two convergent plates collide against each other and the heavier plate boundary is subducted below the relatively lighter plate boundary.
Conservative plate boundaries are those where two plates slip past each other without any collision.
MAGNITUDE OF EARTHQUAKES ON DIFFERENT PLATE BOUNDARIES
Major tectonic events associated with these plate boundaries are ruptures and faults along the constructive plate boundaries, faulting and folding along the destructive plate boundaries, and transform faults along the conservative plate boundaries.
All sorts of disequilibrium are caused due to different types of plate motions and consequently, earthquakes of varying magnitudes are caused
Moderate earthquakes are caused along the constructive plate boundaries because the rate of rupture of the crust and consequent movement of plates away from the mid-oceanic ridges is rather slow and the rate of upwelling of lavas due to fissure flow is also slow.
Consequently, shallow focus earthquakes are caused along the constructive plate boundaries or say along the mid-oceanic ridges.
Earthquakes of high magnitude and deep focus are caused along the convergent or destructive plate boundaries because of the collision of two convergent plates and the consequent subduction of one plate boundaries.
Here mountain building, faulting, and violent volcanic eruptions (central explosive type of eruptions) cause severe and disastrous earthquakes having the focus at the depth up to 700 km.
DISTRIBUTION OF EARTHQUAKE
The earthquakes of the Mid- Continental Belt along the Alpine- Himalayan Chains are caused due to the collision of Eurasian plates and African and Indian plates.
The earthquakes of the western marginal areas of North and South America are caused because of the subduction of the Pacific plate beneath the American plate and the resultant tectonic forces whereas the earthquakes of the eastern margins of Asia are originated because of the subduction of the Pacific plate under the Asiatic plate.
Similarly, the subduction of African plate below the European plate and the subduction of the Indian plate under the Asiatic plate cause earthquakes of the mid-continental belt.
The creation of transform faults along the conservative plate boundaries explains the occurrence of severe earthquakes in California (USA).
Here one part of California moves north-eastward while the other part moves south-westward along the fault plane and thus is formed transform fault which causes earthquakes.
EARTHQUAKE WAVES
All-natural earthquakes take place in the lithosphere.
It is sufficient to note here that the lithosphere refers to the portion of depth up to 200 km from the surface of the earth.
An instrument called ‘seismograph’ records the waves reaching the surface.
Earthquake waves are basically of two types — body waves and surface waves.
BODY WAVES
Body waves are generated due to the release of energy at the focus and move in all directions traveling through the body of the earth. Hence, they are known as body waves.
SURFACE WAVES
The body waves interact with the surface rocks and generate a new set of waves called surface waves. These waves move along the surface.
The velocity of waves changes as they travel through materials with different densities.
The denser the material, the higher is the velocity.
Their direction also changes as they reflect or refract when coming across materials with different densities.
The surface waves are the last to report on seismographs.
These waves are more destructive.
They cause the displacement of rocks, and hence, the collapse of structures occurs.
TYPES OF BODY WAVES
There are two types of body waves. They are called P and S- waves.
P-W AVES
P-waves move faster and are the first to arrive at the these are also called ‘primary waves’.
The P-waves are similar to sound waves.
This characteristic of the S-waves is quite important, as it has helped scientists to understand the structure of the interior of the earth. They travel through gaseous, liquid, and solid.
S-W AVES
S-waves arrive at the surface with some time lag. These are called secondary waves.
An important fact about S-waves is that they can travel only through solid materials.
This characteristic of the S-waves is quite important, as it has helped scientists to understand the structure of the interior of the earth.
Long Period waves or Lwaves:
These waves generally affect only the surface of the earth and die out at a smaller depth. T. These waves cover the longest distances of all the seismic waves. Though their speed is lower than P and S waves these are most violent and destructive.
PROPAGATION OF EARTHQUAKE WAVES
Different types of earthquake waves travel in different manners.
As they move or propagate, they cause vibration in the body of the rocks through which they pass.
P-waves vibrate parallel to the direction of the wave. This exerts pressure on the material in the direction of the propagation.
As a result, it creates density differences in the material leading to stretching and squeezing of the material.
The other three waves vibrate perpendicular to the direction of propagation.
The direction of vibrations of S- waves are perpendicular to the wave direction in the vertical plane.
Hence, they create troughs and crests in the material through which they pass.
EMERGENCE OF SHADOW ZONE
Earthquake waves get recorded in seismographs located at far-off locations.
However, there exist some specific areas where the waves are not reported. Such a zone is called the ‘shadow zone’
The study of different events reveals that for each earthquake, there exists an altogether different shadow zone.
It was observed that seismographs located at any distance within 105° from the epicenter, recorded the arrival of both P and S-waves.
However, the seismographs located beyond 145° from the epicenter; record the arrival of P- waves, but not that of S-waves.
Thus, a zone between 105° and 145° from the epicenter was identified as the shadow zone for both types of waves.
The entire zone beyond 105° does not receive S-waves.
The shadow zone of the S-wave is much larger than that of the P- waves.
The shadow zone of P-waves appears as a band around the earth between 105° and 145° away from the epicenter.
The shadow zone of S-waves is not only larger in extent but is also a little over 40 percent of the earth's surface.
TYPES OF EARTHQUAKES
The most common ones are tectonic earthquakes. These are generated due to the sliding of rocks along a fault plane.
A special class of tectonic earthquakes is sometimes recognized as volcanic earthquakes. However, these are confined to areas of active volcanoes.
In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors. These are called collapse earthquakes.
Ground shaking may also occur due to the explosion of chemical or nuclear devices. Such tremors are called explosion earthquakes
The earthquakes that occur in the areas of large reservoirs are referred to as reservoir-induced earthquakes.
MEASURING EARTHQUAKES
The earthquake events are scaled either according to the magnitude or intensity of the shock.
The magnitude scale is known as the Richter
The magnitude relates to the energy released during the quake.
The magnitude is expressed in absolute numbers, 0-10.
The intensity scale is named after Mercalli, an Italian seismologist.
The intensity scale takes into account the visible damage caused by the event.
The range of intensity scale is from 1-12.
VULCANICITY AND VOLCANOES
The term vulcanicity covers all those processes in which molten rock material or magma rises into the crust or is poured out on its surface, there to solidify as crystalline or semi-crystalline rocks.
VOLCANO
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
Distribution of Volcanoes
World’s active volcanoes are found along the constructive plate margins or divergent plate margins (along the mid-oceanic ridges where two plates move in opposite directions) whereas 80 percent of volcanoes are associated with the destructive or convergent plate boundaries (where two plates collide).
Besides, some volcanoes are also found in intraplate regions g. volcanoes of the Hawaii Island, fault zones of East Africa, etc.
Two plates move in opposite directions from the mid-oceanic ridges due to thermal convective currents which are originated in the mantle below the crust (plates). This splitting and lateral spreading of plates create fractures and faults (transform faults) which cause pressure release and lowering of melting point and thus materials of the upper mantle lying below the mid-oceanic ridges are melted and move upward as magmas under the impact of the enormous volume of accumulated gases and vapor. This rise of magmas along the mid-oceanic ridges (constructive or divergent plate boundaries) causes fissure eruptions of volcanoes and there is constant upwelling of lavas.
CLASSIFICATION ON THE BASIS OF PERIODICITY OF ERUPTIONS
Volcanoes are divided into 3 types on the basis of period of eruption and interval period between two eruptions of a volcano e.g.
- active volcanoes,
- dormant volcanoes and
- extinct volcanoes
ACTIVE VOLCANOES
Active volcanoes are those which constantly eject volcanic lavas, gases, ashes, and fragmental material.
It is estimated that there are more than 500 volcanoes in the world. Etna and Stromboli of the Mediterranean Sea are the most significant examples of this category.
Most of the active volcanoes are found along the mid-oceanic ridges representing divergent plate margins (constructive plate margins) and convergent plate margins (destructive plate margins represented by eastern and western margins of the Pacific Ocean).
DORMANT VOLCANOES
Dormant volcanoes are those which become quiet after their eruptions for some time and there are no indications for future eruptions but suddenly they erupt very violently and cause enormous damage to human health and wealth.
EXTINCT VOLCANOES
The volcanoes are considered extinct when there are no indications of future
They become explosive if somehow water gets into the vent; otherwise, they are characterized by low- explosivity. The upcoming lava moves in the form of a fountain and throws out the cone at the top of the vent and develops into a cinder. The crater is filled up with water and Jakes are formed. It may be pointed out that no volcano can be declared permanently dead as no one knows, what is happening below the ground surface.
CLASSIFICATION OF VOLCANOES BASED ON NATURE OF ERUPTION
Volcanoes are classified on the basis of the nature of eruption and the form developed at the surface.
Major types of volcanoes are as follows:
SHIELD VOLCANOES
Barring the basalt flows, the shield volcanoes are the largest of all the volcanoes on the earth.
The Hawaiian volcanoes are the most famous examples. These volcanoes are mostly made up of basalt, a type of lava that is very fluid when erupted.
For this reason, these volcanoes do not sleep.
They become explosive if somehow water gets into the vent; otherwise, they are characterized by low explosivity.
The upcoming lava moves in the form of a fountain and throws out the cone at the top of the vent and develops into a cinder cone.
COMPOSITE VOLCANOES
These volcanoes are characterized by eruptions of cooler and more viscous lavas than basalt. These volcanoes often result in explosive eruptions. Along with lava, large quantities of pyroclastic material and ashes find their way to the ground. This material accumulates in the vicinity of the vent openings leading to the formation of layers, and this makes the mounts appear as composite volcanoes.
CALDERA
These are the most explosive of the earth’s volcanoes. They are usually so explosive that when they erupt they tend to collapse on themselves rather than building any tall structure. The collapsed depressions are called calderas.
Their explosiveness indicates that the magma chamber supplying the lava is not only huge but is also in close vicinity.
FLOOD BASALT PROVINCES
These volcanoes outpour highly fluid lava that flows for long-distance.
Some parts of the world are covered by thousands of km of thick basalt lava flows.
There can be a series of flows with some flows attaining a thickness of more than 50 m. Individual flows may extend for hundreds of Km.
The Deccan Traps from India, presently covering most of the Maharashtra plateau, are a much larger flood basalt province.
It is believed that initially, the trap formations covered a much larger area than the present.
MID-OCEAN RIDGE VOLCANOES
These volcanoes occur in oceanic areas. There is a system of mid-ocean ridges more than 70,000 km long that stretches through all the ocean basins.
The central portion of this ridge experiences frequent eruptions.
Schematic of volcano injection of aerosols and gases
