VOLCANOES AND EARTHQUAKES, Plate tectonics and Volcanoes, hotspot, Distribution of volcanoes, Causes of Earthquakes, Plate Tectonic Theory, volcanic eruption, Distribution of Earthquakes Geography notes UPSC, Geography optional Subject notes UPSC, Geography notes IAS, Geography optional Subject notes IAS
VOLCANOES AND EARTHQUAKES
A volcano is a vent or opening usually circular in form through which heated materials consisting of gases, water, liquid lava and fragments of rocks are ejected from the highly heated interior to the surface of the earth. 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).
Volcanic eruptions are closely associated with several interconnected processes such as
- The gradual increase in temperature with increasing depth at a rate of 1°c per 32m due to heat generated by degeneration of radioactive elements inside the earth.
- origin of magma because of lowering of melting point caused by reduction in pressure of overlying rocks due to fractures caused by splitting of plates
- origin of gases and water vapour due to heating of water
- ascent of magma due to pressure from gases and vapour
- Occurrence of volcanic eruption. These eruptions are closely associated with plate boundaries.
Volcanoes are classified under different schemes:
- Classification on the basis of periodicity of eruptions:
(a) Active volcano e.g. Etna, Stromboli, Pinatubo etc.
(b) Dormant volcano e.g. Vesuvius, Barren island volcano (Andaman)
(c) Extinct volcano e.g. where no indication of future eruption is estimated.
- Classification on the basis of the mode of eruption:
(A) Volcanic of central Eruption type– Eruption occurs through a central pipe and small opening are rapid and violent. Such volcanoes are very destructive and disastrous. It is divided into 5 sub types as follow:-
(a) Hawaiian Types: Such Volcanoes erupt quietly due to less viscous Laves and non-violent natures of gases. They emit long glossy threads of red molten Lava known as peel’s hair e.g. Hawaiian Island.
(b) Strombolian Type: The eruptions are almost rhythmic or nearly continuous in nature but sometimes they are interrupted by long intervals. Ex- Stromboli volcano of Lipari island.
(c) Vulcanian Type: Such volcanoes erupt with great force and intensity The Lava is so viscous and pasty that these are quickly solidified e.g. Mt. Vulcano of Lipari Island of Mediterranean Sea.
(d) Peleean Type: Most violent and most explosive type of volcanoes, named as Nuee Ardente, meaning thus by glowing cloud e.g. Pelee volcano of Martinique Island in the Caribbean Sea, Krakatau volcano between Java and Sumatra in Sunda strait.
(e) Visuvian type: Extremely violent and enormous volume of gases and ashes forms which clouds like cauliflower. This is also called Plinian type.
(B) Fissure eruption or quiet eruption type e.g. Lava flow or flood, mud flow and fumaroles. Large quantities of lava quietly well up from fissure and spread out over the surrounding countryside. Successive lava flows results in the growth of a lava platform which may be extensive to be called a plateau like “Deccan” “Columbia snake plateau”, Drakensberg mountains”, “Victoria and Kimberley” districts of Australia, “ Java island”.
Plate tectonics and Volcanoes
Divergent plate boundaries
At the mid-oceanic ridges, two tectonic plates diverge from one another. New oceanic crust is being formed by hot molten rock slowly cooling and solidifying. The crust is very thin at mid-oceanic ridges due to the pull of the tectonic plates. The release of pressure leads to partial melting of the mantle causing volcanism and creating new oceanic crust. Most divergent plate boundaries are at the bottom of the oceans, therefore most volcanic activity is submarine, forming new seafloor. Black smokers or deep sea vents are an example of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed, for example, Iceland.
Convergent plate boundaries
Subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide. In this case, the oceanic plate subducts or submerges under the continental plate forming a deep ocean trench just offshore. Water released from the subducting plate lowers the melting temperature of the overlying mantle wedge, creating magma. This magma tends to be very viscous due to its high silica content so that it often does not reach the surface and cools at depth. When it does reach the surface, a volcano is formed. Typical examples for this kind of volcano are Mount Etna and the volcanoes in the Pacific Ring of Fire.
Hotspots are not usually located on the edges of tectonic plates, above mantle plumes, where the convection of the Earth’s mantle creates a column of hot material that rises until it reaches the crust, which tends to be thinner than in other areas of the Earth. The temperature of the plume causes the crust to melt and form pipes, which can vent magma. Because the tectonic plates move whereas the mantle plume remains in the same place, each volcano becomes dormant after a while and a new volcano is then formed as the plate shifts over the hotspot. The Hawaiian Islands are thought to be formed in such a manner, as well as the Snake River Plain, with the Yellowstone Caldera of the North American plate currently above the hot spot. Another example is India’s Deccan plateau which is the result of lava outflow from the Reunion hotspot.
Topography produced by volcanoes:
- Cinder or ash cone: They are of low height and are formed of volcanic dust, ashes and pyroclastic matter. Its formation takes place due to accumulation of finer particles around the volcanoes vent.
- Shield volcanoes: So named for their broad, shield-like profiles, are formed by the eruption of low-viscosity lava that can flow a great distance from a vent, but not generally explode catastrophically. Since low-viscosity magma is typically low in silica, shield volcanoes are more common in oceanic settings. The Hawaiian volcanic chain is a series of shield cones, and they are common in Iceland as well.
- Composite cones: They are formed due to the accumulation of different layers of various volcanic materials.
- Parasite cones: When lava comes out of the minor pipes coming out of the main central pipe, parasite cones are formed.
- Basic lava cone: It has less quantity of silica in its lava.
- Acidic lava cone: It has more silica in its lava.
- Lava domes: These are formed due to accumulation of solidified lavas around the volcanic vents.
- Lava plugs: They are formed due to plugging of volcanic pipes and vents when volcano becomes extinct.
- Craters: The depression formed at the mouth of a volcanic vent is called a crater. When it is filled with water, it becomes a ‘crater lake’ e.g. Lake Lonar in Maharashtra.
- Calderas: Generally enlarged form of craters is called caldera. It is formed due to subsidence of a crater.
- Geysers: They are intermittent hot springs that from time to time spout steam and hot water from their craters.
- Fumaroles: It is a vent through which there is emission of gases and water vapour.
- Cryptodomes: These are formed when viscous lava forces its way up and causes a bulge. The 1980 eruption of Mount St. Helens was an example. Lava under great pressure forced a bulge in the mountain, which was unstable and slid down the north side.
- A supervolcano is a large volcano that usually has a large caldera and can potentially produce devastation on an enormous, sometimes continental, scale. Such eruptions can cause severe cooling of global temperatures for many years afterwards because of the huge volumes of sulphur and ash erupted. They are the most dangerous type of volcano. Examples include Yellowstone Caldera in Yellowstone National Park and Valles Caldera in New Mexico (both western United States), Lake Taupo in New Zealand, Lake Toba in Sumatra and Ngorogoro Crater in Tanzania. Supervolcanoes are hard to identify centuries later, given the enormous areas they cover. Large igneous provinces are also considered supervolcanoes because of the vast amount of basalt lava erupted, but are non-explosive.
Distribution of volcanoes
Volcanoes are unevenly distributed over the earth and vast areas have no active volcanoes at all. There are no volcanoes in Australia. In Asia, they are largely concentrated in circum-pacific region and Africa has a few of them. Thus, the pacific belt is truly known as the “ring of fire” because of the largest number of active volcanoes along the coasts of America and Asia around this region. Iceland, Sicily and Japan are the biggest volcanic islands in the world. Most of the volcanoes in the world occur along linear belts or lines of weakness marked by intense folding and faulting.
An earthquake is a vibration or oscillation of the surface of the earth caused by a transient disturbance of the elastic or gravitational equilibrium of the rocks at or beneath the surface. The magnitude or intensity of energy released by an earthquake is measured by 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’. Isoseismal lines join places which experience the earthquake at the same time.
Causes of Earthquakes:
If seen broadly we can say that earthquakes are caused due to two major reasons. The first reason is the eruption of volcanoes, which are sudden. Volcanoes are seats of inner disturbance and can affect the plates which are the second cause of earthquakes. Earthquakes are caused due to disturbance in the movement of plates, which again can be caused due to various reasons like under-crust waves or cracks in the plates.
Plate Tectonic Theory
The outer layer of the earth is divided into many sections known as plates, which are floating on the molten magma beneath the earth’s crust. Now the movement of these plates is determined by the convection current in the molten magma. Therefore after intervals there are plates that get submerged in the molten magma and there are plates that rise upwards and at times even new crust is formed from the molten magma which in turn forms a new plate until it connects itself with the already existing ones. At times these plates and can be pushed up to form mountains and hills and the movement is so slow that it is really hard to comprehend that there is any movement at all. The movement and the results come out to be visible suddenly. Now these plates are the bases on which the continents stand and when these plates move the continents also move. Most of the earthquakes occur on the edges of the plates where a plate is under, on or across another plate. This movement disrupts the balance and position of all plates, which leads to tremors, which are called earthquakes.
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.
The excessive exploitation of earth’s resources for our own benefits like building dams to store large volumes of water (earthquake at Koyna Nagar Township) and blasting rocks and mountains to build bridges and roads is also the reason behind such natural disruptions.
(1) Landslides and damming of the rivers in highland regions.
(2) Causes depression forming lakes. May cause faults, thrusts, folds, etc
(3) Formation of cracks or fissures in the epicenter region and some- times water, mud, gas are ejected from it.
(4) Causes the raising or lowering of parts of the sea floor e.g. “Sangami bay” in 1923. This causes “tsunamis” or tidal waves.
(5) May change surface drainage & underground circulation of water like the sudden disappearance of springs in some places.
(6) Rising and lowering of crustal regions for example in Alaska in 1899-16 m upliftment.
(7) Devastation of cities, fires, diseases, etc. Classification of earthquakes
Earthquakes are classified on the basis of causative factors:
- Natural earthquakes: They are caused by endogenetic forces:
- Volcanic earthquake: they are caused due to volcanic eruptions. E.g. Earthquakes caused by explosion of Krakatau volcano in 1883 and Etna volcano in 1968.
II.Tectonic earthquake: they are caused due to dislocation of rock blocks during faulting e.g. 1906 earthquake of California and 1923 earthquake of Sangami bay, Japan etc.
III. Isostatic earthquake: they are triggered due to sudden disturbance in the isostatic balance at regional scale due to imbalance in geological processes e.g. near active mountain building zones.
IV.Plutonic earthquakes: these are deep focus earthquakes generally located between 240 and 670 km deep.
- Artificial earthquakes: They are caused due to man-made activities like pumping water and mineral oil underground, blasting of rocks, nuclear explosion, storage of huge volume of water in reservoirs etc. Examples of earthquake due to construction of huge dams include Koyna earthquake of 1967 and Hoover dam earthquake of 1936.
Distribution of Earthquakes
About 68% of all the earthquakes are observed in the vast region of the pacific ocean known as the “ring of fire” and is closely linked with the region of crustal dislocations and volcanic eruptions. Chile, California, Alaska, Japan, Philippines, New Zealand constitute the ring of fire.
Around 21% of the earthquakes occur in the mid-word mountain belt extending parallel to the equator from Mexico across Atlantic Ocean, the Mediterranean Sea from Alpine-Caucasus ranges to the Caspian, Himalayan Mountains and the adjoining belts: the earthquakes in India are at present mainly confined to the Himalayan regions and its foot hills. They are also felt in the Ganga valley.
Points to Remember
- The largest crater known to have formed by a meteorite is Coon Butte or Barringer crater in USA. Lonar lake of Maharashtra is the largest meteoric Crater Lake in India.
- Earth’s rotational velocity at equator is 1690 km/hr.
- Elastic rebound theory explains anthropogenic earthquake.
- Earth’s magnetic north and South Pole are located on Prince of Wales Island in Canada and South Victoria island in Antarctica, respectively.
- “Homoseismal line” is the line joining places that experience earthquake at the same time.
- The only active volcano in India is Barren Island in Andaman-Nicobar islands.
- Seismic waves that cause maximum destruction is ‘L’ or long waves.
- Magnitude of earthquake is measured on Richter scale.