An earthquake is a sudden movement of the Earth, caused by the abrupt release of strain that has accumulated over a long time. For hundreds of millions of years the forces of plate tectonics have shaped the Earth as the huge plates that form the Earth’s surface slowly move over, under, and past each other. Sometimes the movement is gradual. At other times, the Plates are locked together, unable to release the accumulating energy. When the accumulated energy grows strong enough, the plates break free.
The magnitude or intensity of energy released by an earthquake is measured on the Richter scale. The place of the origin of an earthquake is called focus which is hidden inside the earth. The Place on the ground surface which is perpendicular to the buried focus is called ‘epicentre’. Seismic waves are recorded by an instrument called ‘seismograph’.
Earthquakes can be caused by a variety of things, including meteor impacts and volcanic eruptions, and even sometimes man-made events like mine collapses and underground nuclear tests. But most naturally occurring earthquakes are caused by movement of Pieces of the earth’s surface, which are called tectonic plates.
DIFFERENT THEORIES THAT EXPLAIN THE ORIGIN OF EARTHQUAKES
Faulting and Elastic Rebound Theory: ‘The horizontal and vertical movements caused by endogenetic forces result in the formation of faults and fold which in turn cause isostatic disequilibrium in the crustal rocks which ultimately causes earthquakes of varying magnitudes depending on the nature and magnitude of dislocation of rock blocks caused by faulting and folding. In fact, sudden dislocation of rock blocks caused by both tensile and compressive forces trigger immediate earth tremor due to sudden maladjustment of rock blocks.
According to the theory, the underground rocks are elastic like rubber and expand when stretched and pulled. The stretching and pulling of crustal rocks due to tensile forces is a slow process. The rocks continue to be stretched so long as the tensile forces do not exceed the elasticity of the rocks but as the tensile forces exceed the rocks elasticity they are broken and broken rock blocks try immediately to occupy their previous positions so that they may adjust themselves. All these Processes occur so rapidly that the equilibrium of the concerned crustal surface is suddenly disturbed and hence earth tremors are caused.
Hydrostatic Pressure and Anthropogenic Causes: Certain human activities such as pumping of ground water and oil deep underground mining, blasting of rocks, nuclear explosion storage of huge volume of water causes tremors.The introduction of additional artificial, superincumbent load through the construction of large dams and impounding of enormous volume of water cause disequilibrium of already fragile structures due to faults and fractures.
When volcanoes erupt it is because the molten magma under the crust of the earth is under enormous pressure and to release that pressure it looks for an opening and exerts pressure on the earth’s crust and the plate in turn. A place, which is the seat of an active volcano, is often prone to earthquakes as well. Earthquakes are also caused after a volcanic eruption since the eruption also leads to a disturbance in the position of plates, which either move further or resettle and can result into severe or light tremors.
Plate Tectonic Theory
Finally, in the mid-1960s, researchers in the United States and Great Britain came up with a theory that explained why the Earth shook.
The theory, called Plate tectonics, is that the Earth’s crust, or lithosphere, is comprised of many Plates that slide over a lubricating asthenosphere layer.
Plate tectonics confirms that there are four types of seismic zones. The first follows the line of mid-ocean ridges. Activity is low, and it occurs at very shallow depths’ The Point is that the lithosphere is very thin and weak at these boundaries, so the strain cannot build up enough to cause large earthquakes. Associated with this type of seismicity is the volcanic activity along the axis of the ridges (for example, Iceland, Azores, Tristan da Cunha).
The second type of earthquake associated with plate tectonics is the shallow-focus event unaccompanied by volcanic activity. The, San Andreas fault is a good example, of this, so is the Anatolian fault in Northern Turkey. In these faults, two mature plates are scraping by one another. The friction between the plates can. be so great that very large strains can build up before they are periodically relieved by large earthquakes. Nevertheless, activity does not always occur along the entire length of the fault during any one earthquake. For instance, the 1906 San Francisco event was caused by breakage only along the northern end of the San Andreas fault.
The third type of earthquake is related to the collision of oceanic and continental plates. One plate is thrust or subducted under the other plate so that a deep ocean trench is produced. In the Philippines, ocean trenches are associated with curved volcanic is- land arcs on the landward plate, for example the Java trench. Along the Peru – Chile trench, the Nazca plate is being subducted under the South American plate which responds by crumpling to form the Andes. This type of earthquake can be shallow, intermediate, or deep, according to its location on the downgoing lithospheric slab. Such inclined planes of earthquakes are known as Benioff
The fourth type of seismic zone occurs along the boundaries of continental plates. Typical of this is the broad swath of seismicity from Burma to the Mediterranean, crossing the
Himalayas, Iran, Turkey, to Gilbraltar. Within this zone, shallow earthquakes are associated with high mountain ranges where intense compression is taking place. Intermediate- and deepfocus earthquakes also occur and are known in the Himalayas and in the Caucasus. The interiors of continental plates are very complex, much more so than island arcs. For instance, we do not yet know the full relationship of the AIps or the East African rift system to the broad picture of plate tectonics.
- Landslides and damming of the rivers in highland regions.
- Causes depression forming lakes. May cause faults, thrusts, folds, etc.
- Formation of cracks or fissures in the epicenter region and sometimes water, mud, gas are ejected from it.
- Causes the raising or lowering of parts of the sea floor e.g. “Sangami bay” in 1923. This causes “tsunamis” or tidal waves.
- May change surface drainage & underground circulation of water like the sudden disappearance of springs in some places.
- Rising and lowering of crustal regions for example in Alaska in 1899-16 m upliftment.
- Devastation of cities, fires, diseases, etc.
Distribution of Earthquakes
The world maps of the distribution of earthquakes prepared by the seismologists on the basis of computer analysis and simulation of 30,000 earthquakes have identified the three main zones of earthquake.
- Circum Pacific Belt or Ring of Fire surrounding the Pacific Ocean. It is a junction of continental and oceanic margins; it is a zone of young folded mountains; it is a zone of active volcanoes thus this belt accounts for the 65 per cent of the total earthquakes of the world.
- Mid-Continental Belt representing Alpine-Himalayan chains of Eurasia and northern Africa and epicenters of east African fault zone. This belt represents the collision or subduction zones of continental plates. About 21 per cent of the total seismic events occur in this belt.
- Mid-Atlantic Belt representing the earthquakes located along the mid-Atlantic Ridge and its offshoots. This belt records moderate and shallow focus earthquakes which are essentially caused due to creation of transform faults and fractures because of divergent movement of plates.
A volcano is an opening or a vent through which heated materials consisting of gases, water, liquid lava and fragments of rocks are discharged. Magma is molten rock within the Earth’s crust. When magma erupts through the earth’s surface it is called lava. Lava can be thick and slow-moving or thin and fast-moving. Rock also comes from volcanoes in other forms, including ash (finely powdered rock that looks like dark smoke coming from the volcano), cinders (bits of fragmented lava) and pumice (light-weight rock that is full of air bubbles and is formed in explosive volcanic eruptions – this type of rock can float on water).
As rock inside the earth melts, its mass remains the same while its volume increases–producing a melt that is less dense than the surrounding rock. This lighter magma then rises towards the surface by virtue of its buoyancy. If the density of the magma between the zone of its generation and the surface is less than that of the surrounding and overlying rocks, the magma reaches the surface and erupts.
Types of Vulcanic eruption
The most common type of volcanic eruption occurs when magma (the term for lava when it is below the Earth’s surface) is released from a volcanic vent. Eruptions can be effusive, where lava flows like a thick, sticky liquid or explosive, where fragmented lava explodes out of a vent. In explosive eruptions, the fragmented rock may be accompanied by ash and gases; in effusive eruptions degassing is common but ash is usually not.
Volcanologists classify eruptions into several different types.
In a Hawaiian eruption fluid basaltic lava is thrown into the air in jets from a vent or line of vents (a fissure) at the summit or on the flank of a volcano. The jets can last for hours or even days, a phenomenon known as fire fountaining. The spatter created by bits of hot lava falling out of the fountain can melt together and form lava flows, or build hills called spatter cones. Lava flows may also come from vents at the same time as fountaining occurs, or during periods where fountaining has paused. Because these flows are very fluid, they can travel miles from their source before they cool and harden.
Strombolian eruptions are distinct bursts of fluid lava (usually basalt or basaltic andesite) from the mouth of a magma-filled summit conduit. The explosions usually occur every few minutes at regular or irregular intervals. The explosions of lava, which can reach heights of hundreds of meters, are caused by the bursting of Large bubbles of gas, which travel upward in the magma-filled conduit until they reach the open air.
This kind of eruption can create a variety of forms of eruptive products: spatter, or hardened globs of glassy lava; scoria, which are hardened chunks of bubbly lava; lava bombs or chunks of lava a few cm to a few m in size; ash; and small lava flows (which form when hot spatter melts together and flows downslope). Products of an explosive eruption are often collectively called tephra.
Strombolian eruptions are often associated with small lava lakes, which can build up in the conduits of volcanoes. They are one of the least violent of the explosive eruptions. although, they can still be very dangerous if bombs or lava flows reach inhabited areas. Strombolian eruptions are named for the volcano that makes up the Italian island of Stromboli, which has several erupting summit vents. These eruptions are particularly spectacular at night, when the lava glows brightly.
A Vulcanian eruption is a short, violent, relatively small explosion of viscous magma (usually andesite, dacite, or rhyolite). This type of eruption results from the fragmentation and explosion of a plug of lava in a volcanic conduit or from the rupture of a lava dome (viscous lava that piles up over a vent). Vulcanian eruptions create powerful explosions in which material can travel faster than 350 meters per second (800 mph) and rise several kilometers into the air. They produce tephra, ash clouds, and pyroclastic density currents (clouds of hot ash, gas and rock that flow almost like fluids).
Vulcanian eruptions may be repetitive and go on for days, months, or years, or they may precede even larger explosive eruptions. They are named for the Italian island, of Vulcano, where a small volcano that experienced this type of explosive eruption was thought to be the vent above the forge of the Roman smith god Vulcan.
The largest and most violent of all the types of volcanic eruptions are Plinian eruptions. They are caused by the fragmentation of gassy magma, and are usually associated with very viscous magmas (dacite and rhyolite). They release enormous amounts of energy and create eruption columns of gas and ash that can rise up to 50 km (35 miles) high at speeds of hundreds of meters per second. Ash from an eruption column can drift or be blown hundreds or thousands of miles away from the volcano. The eruption columns are usually shaped like a mushroom (similar to a nuclear explosion) or an Italian pine tree; plinian eruptions are extremely destructive, and can even obliterate the entire top of a mountain, as occurred at Mount St. Helens in 1980. They can produce falls of ash, scoria and lava bombs miles from the volcano, and pyroclastic density currents that raze forests, strip soil from bedrock and obliterate anything in their paths. These eruptions are often climactic, and a volcano with a magma chamber emptied by a large plinian eruption may subsequently enter a period of inactivity.
Volcanic Belts of the World
1. The Circum-Pacific Belt: This is the most important belt of volcanoes. This is the also called Ring of Fire. The belt extends through the
Andes of South America, Central America, Mexico, the Cascade Mountains of Western United States, the Aleutain Islands, Kamchatka, the Kuril Isles, Japan, the Philippines, Celebes, New Guinea, the Solomon Islands, New Caledonia and New Zealand.
This belt has 80 active volcanoes. The Circum-Pacific belt meets the mid- continental belt in the East Indies. This belt is characterised by high volcanic cones and volcanic mountains. The volcanoes of the AIeutian Island, Hawaii Island and Japan are found in Chains.
Cotapaxi is the highest volcanic mountain (6035m) in the world. Other important volcanoes found in this belt are Fuziyama, Shasta, Rainer and Hood.
In Alaska there is a Valley of Ten Thousand Smokes. It may be pointed out that in this belt volcanii eruptions occur because of the subduction of the Pacific plate below the Asiatic plate.
In Equador, South America, there are about 22 volcanoes out of which
15 are more than 11450 metres above the sea level. Besides, other high volcanic mountains are St. Helens (Washington, U.S.A.), Kilavea (Hawaii Island U.S.A.), Mt. Taal, Pinatubo and Mayon (Philippines). It may be mentioned that the volcanoes of Hawaii Island are situated in the intra- plate region.
- The Mid-Continental Belt: This belt has various volcanoes of the Alpine mountain chain Mediterranean Sea (Stromboli, Vesuvius, Etna etc.), Volcanoes of the Aegean Sea. Mt. Ararat, Elburz and Hindukush are also included in this belt.
It is interesting to note that there are several volcanic free zones found along the Alps and the Himalayas. The Rift Valleys of Africa have volcanoes such as Kilimanjaro, Elgon, Birunga and Rungwe, etc.
In the region where the boundaries of Persia, Afghanistan, and Baluchistan meet, there are several volcanic cones of large size, and one or two of them emit steam and other gases. This region has also a few extinct volcanoes.
- The Mid- Atlantic Belt: As the name indicates, this belt includes the volcanoes of the Mid-Atlantic Ridge. The volcanoes associated with the Atlantic Ocean are located either on swells or ridges rising from the sea floor, or on or near the edge of the continent where it slopes abruptly into the deep oceanic basins. However, in each case, the volcanoes are associated with zones of crystal movement.
The volcanoes formed along the Mid-Atlantic Ridge actually represent the splitting zone of the American plate moving towards west and the Eurasian plate moving towards east.
In the splitting zone stated above there is constant upwelling of magmas. Thus, it is a zone of crustal weakness. Volcanoes in this belt are generally of fissure-eruption type. Volcanoes of Lesser Antilles, Azores, St. Helens, etc. are included in this belt.
HOT – SPOT VOLCANOES
About five per cent of volcanoes are not near the margins of tectonic plates. They are over especially hot places in the Earth’s interior called HOT SPOTS.
HOT SPOTS are created by mantle plumes – hot currents that rise all the way from the core through the mantle. When mantle plume come up under the crust, they burn their way through to become hot-spot volcanoes.
Famous hot-spot volcanoes include the Hawaiian island volcanoes and Reunion Island in the Indian Ocean.
The movement of the Pacific plate over the Hawaiian hot spot created a chain of old volcanoes 6000 km long. It starts with THE MEIJI SEAMOLINT under the sea north of Japan, and ends with THE HAWAIIAN ISLANDS.