Earthquake and Volcanoes

The lithospheric plate boundaries are prone to sudden geomorphic movements. Because of the pressure caused by the pushing and pulling of magma in the mantle, the plate boundaries are extremely unstable. Earthquakes and volcanoes are two examples of sudden movements that generate significant deformation in a short period of time. This article will explain the concepts of sudden movement which is an integral part of the geography syllabus. Sudden movement is an important concept for the UPSC exam.

Earthquake

• An earthquake is a vibration or oscillation of the surface of the Earth caused by the elasticity or the isostatic adjustment of the rocks, beneath the surface of the Earth.
• Major earthquakes are usually caused by sudden movements along faults.
• Earthquakes can occur at any time of the year, at any hour of the day or night. It has a jolting effect.
• The sub-surface area along a fault plain, where the seismic waves are initiated, is the focus or hypocenter of an earthquake. The area at the surface directly above the focus is the epicentre.
• The earth's surface vibrates as a result of a quick release of energy.
• Earthquake waves propagate outwards from the epicentre. These earthquake waves wreak havoc on the planet's surface.
• The most damage occurs closest to the epicentre, and the severity of the earthquake weakens as it moves away from it.

Types of Earthquakes:

• Classification on the basics of causative factors:
• Natural earthquake are those which are caused by natural process i.e., due to endogenetic process these are further subdivided into following sub-categories:
• Tectonic Earthquakes: They are caused due to dislocation of rocks block during faulting activities. The most common ones are the tectonic earthquakes. These are generated due to sliding of rocks along a fault plane. Example: 2001 earthquake of Gujarat.
• Volcanic Earthquake: Volcano earthquakes are caused by stress changes in solid rock caused by the injection or withdrawal of magma (molten rock). They are caused due to volcanic eruptions of explosive and fissure types. The intensity and magnitude of the earthquake depends on the intensity and magnitude of volcanic eruptions. They don't predict when the volcano will erupt, but it might happen at any time. Example: severe earthquakes caused by Karakatao volcano 1883.
• Isostatic Earthquake: They are triggered due to sudden disturbance in isostatic balance at regional scale. Generally, earthquake in the active zone of the mountain building are included in this category.
• Plutonic Earthquake: They are deep focus earthquakes whose centres lie at depths from 240 km to 600 km.
• Artificial or Man Induced Earthquakes: They are caused by anthropogenic activities such as underground explosions, mining, large reservoir induced pressure etc.
• Classification on the basis of focus of earthquake:
• Shallow Focus Earthquake: It has its seismic foci located at depths from ground surface to 50 km.
• The majority of earthquakes have a shallow focal point. As a result, they're also known as 'crustal earthquakes.'
• Smaller earthquakes make up the majority of shallow focus earthquakes (usual range of 1 to 5). However, a few are of greater magnitude and can create widespread devastation.
• They happen frequently and seemingly at random. However, because the majority of them are of minor magnitude or occur along submarine ridges, they are rarely felt.
• Intermediate Focus Earthquake: It has it’s foci at depths between 50 km to 250 km.
• Deep Focus Earthquakes: They have centres at depth from 250 km to 700 km. It Wadati–Benioff Zone. It's a subduction zone where earthquakes are widespread due to the interaction of an oceanic crustal plate sliding down against a continental plate.  This zone is home to some of the world's most powerful earthquakes.

Cause and Effect of Earthquake

• Earthquakes occur when excess stored tension in rocks in the earth's interior owing to bending, faulting, or other physical processes is released as kinetic energy through weak zones on the surface.
• Coastal areas may experience uplift or subsidence as a result of these shifts.
• Plates tectonics theory has been accepted as the most possible explanation of the causes of earthquakes. As per this theory the crust or the earth composed of solid moving plates. These plates are constantly moving in relation to each other due to thermal convective currents originating deep within the earth. Thus, all the seismic events take place along the boundaries of these moving plates
• Alterations in contours, river courses, coastline changes, glacier surges, landslides, soil slips, mass wasting, and other effects may occur as a result of earthquakes.

Examples of Earthquake because of Uplift and Subsidence

• A one-metre uplift in coastal areas was produced by an earthquake in Chile in 1822.
• An earthquake in New Zealand in 1885 resulted in a 3 metre rise.
• An earthquake in Japan in 1891 caused up to 6 metres of subsidence.

Volcanoes

• The movement of molten rock (magma) onto or towards the earth's surface through narrow volcanic vents or fissures is known as volcanism.
• When molten magma from the earth's interior erupts through vents and cracks in the crust, accompanied by steam, gases such as hydrogen sulphide, sulphur dioxide, hydrogen chloride, carbon dioxide, etc. and pyroclastic material (cloud of ash, lava fragments carried through the air, and vapour), a volcano is formed.
• A volcano can adopt many different shapes depending on the chemical composition and viscosity of the lava.

Types of Volcanos

• A conical mountain structure arises when cooled lava particles from repeated volcanic eruptions create a cone around the vent. This is a sort of volcano that is located in the centre. Fujiyama (Japan) and Mount Vesuvius are two examples (Italy). In such volcanoes, the magma is viscous, acidic, and silicate.
• The less viscous, acidic, and silicate magma flows out more slowly and silently, forming a broad plateau-like formation with a gradual slope. As a result, a 'shield-shaped' volcano forms, with thin horizontal sheets. Mauna Loa, for example (Hawaii).
• The crater frequently transforms into a lake after the magma eruption has stopped. Caldera is the name for this type of lake. Maharashtra's Lonar and Indonesia's Krakatao are two examples.
• A thin magma can sometimes escape through fractures and fissures in the earth's surface and flow over a large region, eventually generating a layered, undulating, flat surface. Deccan traps, for example (Indian peninsula).

Volcanic Landscape

• Volcanic landforms are formed by the solidification of lava either inside or outside the earth's surface. When lava is unable to reach the earth's surface, it penetrates fissures, where it forms various types of intrusive landforms depending on the shape of the fissure and its location relative to the earth's surface.
• Major intrusive forms include Batholiths, Laccoliths, Lopolith, Phacolith, Sills and Dykes.

Conclusion

• Thus, it can be concluded that the above-mentioned slow movement are important to maintaining the Earth's crust's unique structure.

MCQs

Question. Which of the following are revealed by concept of sea floor spreading?

1. Volcanic eruptions are common along the mid oceanic ridges.
2. The age of the rocks increases as one moves away from the mid oceanic ridges crest.
3. The continental rocks are much younger than the ocean crust rocks.

Select the correct answer using following codes.

1. Only 1
2. Only 1 and 2
3. Only 2 and 3
4. 1,2 and 3

Answer: (b)

Explanation:

• The age of the rocks increases as one moves away from the mid oceanic ridges crest. Volcanic eruptions are common along the mid oceanic ridges. Hence statements 1 and 2 are correct.

• The ocean crust rocks are much younger than the continental rocks. The deep trenches have deep-seated earthquake occurrences. sediments on the ocean floor are thin which shows oceans are younger than continents. Hence statement 3 is incorrect. Therefore (b) is the correct option.

Question. The place where sea-floor spreading occurs is termed as_______.

1. mid-ocean ridges
2. ditches
3. ocean basin
4. crates

Answer: (a)

Explanation:

• Seafloor spreading is a phenomenon that happens at mid-ocean ridges, where new oceanic crust is formed by volcanic activity and then slowly moves away from the ridge. Therefore (a) is the correct option.