Earthquakes and volcanoes are two examples of sudden movements that generate significant deformation in a short period of time. Any sudden shaking of the ground generated by seismic waves passing through Earth's rocks is referred to as an earthquake whereas a volcano is a crack in the earth's crust that allows lava, volcanic ash, and gases to escape. Liquid magma with dissolved gases rises through cracks in the Earth's crust beneath a volcano. This article will explain the concepts of earthquakes and volcanoes which is an integral part of the geography syllabus.

Earthquake

• An earthquake is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves.
• Earthquakes can range in size from those that are so weak that they cannot be felt to those violent enough to toss people around and destroy whole cities.
• The seismicity, or seismic activity, of an area refers to the frequency, type, and size of earthquakes experienced over a period of time. The word tremor is also used for non-earthquake seismic rumbling.
• The cause of earthquakes is often due to the movement of tectonic plates beneath the Earth's surface.
• When these plates move past each other, they sometimes get stuck at their edges due to friction.
• When the stress on the edge overcomes the friction, there is an earthquake that releases energy in the form of seismic waves, which causes the ground to shake.
• The location below the Earth's surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface is called the epicenter.
• Earthquakes can occur anywhere, but their most common sites are along tectonic plate boundaries.

Types of Earthquakes

Natural earthquakes are those which are caused by natural processes i.e., due to endogenic processes. These are further subdivided into the following sub-categories:

On the Basis of Causative Factors

1) Tectonic Earthquakes

• They are caused due to dislocation of rock blocks during faulting activities.
• They are the most common ones.
• A tectonic earthquake occurs when two tectonic plates collide at a point called the boundary.
• A convergent plate boundary is formed when two plates push into each other.
• The oceanic Nazca Plate, for example, pushes into and is subducted into the South American Plate off the coast of South America along the Peru-Chile trench.
• The Andes mountains were formed as a result of this action, which raised the South American Plate.
• The Nazca Plate is broken into smaller pieces that are stuck in place for lengthy periods of time before shifting suddenly and causing earthquakes.
• The Mid-Atlantic Ridge, which stretches from the Arctic Ocean to beyond the southern tip of Africa, is an example of a divergent boundary, which happens when two plates shift away from each other, forming a new crust.
• It has produced thousands of kilometers of plate movement over millions of years.
• When plates slide horizontally past one another, they generate a transform boundary that neither destroys nor produces crust.
• Plate movement results in zigzagging plate edges and shallow earthquakes.
• These are also generated due to the sliding of rocks along a fault plane. Example: 2001 earthquake in Gujarat.

2) Volcanic Earthquakes

• Volcano earthquakes are caused by stress changes in the 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 depend on the intensity and magnitude of volcanic eruptions.
• We can't predict when the volcano will erupt, but it might happen at any time. Example: severe earthquakes caused by Karakatao volcano 1883.

3) Isostatic Earthquakes

• Isostatic or isostasy-related earthquakes are those associated with the buoyant behavior of the Earth's crust as it undergoes vertical movements due to changes in surface loads.
• The term "isostasy" refers to the equilibrium that exists between the Earth's lithosphere (the crust and uppermost mantle) and the more fluid asthenosphere beneath it.
• The lithosphere essentially "floats" on the asthenosphere in a manner similar to how an iceberg floats on water.
• They are triggered due to sudden disturbances in isostatic balance at a regional scale.
• If a significant weight, such as a massive ice sheet, is added to or removed from the crust, the crust will respond by sinking or rebounding, respectively.
• A classic example of isostatic adjustment is seen in areas that were once covered by large ice sheets during the last Ice Age.
  • As these ice sheets melted, the previously compressed crust began to rise or "rebound" back to its original position.
  • This process, which can take thousands of years, can cause earthquakes, although they are typically of lower magnitude.
  • This rebound is often referred to as post-glacial or glacio-isostatic rebound.
• Mountains, after being built up due to tectonic forces, can undergo erosion over millions of years. As they erode, the crust can adjust isostatically, leading to earthquakes.

4) Plutonic Earthquakes

• They are deep-focus earthquakes whose centers lie at depths from 240 km to 600 km.
• They occur almost exclusively at convergent boundaries in association with subducted oceanic lithosphere.
• Herbert Hall Turner initially brought preliminary evidence for the presence of deep-focus earthquakes to the attention of the scientific community in 1922.
• Kiyoo Wadati demonstrated the existence of earthquakes deep into the lithosphere in 1928, debunking the myth that earthquakes only occur at shallow focal depths.
• Deep-focus earthquakes produce very few surface waves. Because of their focal depth, earthquakes are less likely to create a seismic wave motion with concentrated energy at the surface.
• Deep-focus earthquake seismic waves only pass through the heterogeneous upper mantle and highly changeable crust once on their way from the focus to the recording station.
• As a result, compared to seismic waves from shallow earthquakes, body waves experience less attenuation and reverberation, resulting in strong body wave peaks.

5) Artificial or Man Induced Earthquakes

• They are caused by anthropogenic activities such as underground explosions, mining, large reservoir-induced pressure, etc.
• Example: The enormous load exerted by dam reservoirs resulted in the strongest ever induced earthquake.
• The most notable fluid-induced earthquake in India happened in 1967 near Koyna, Maharashtra, and was ascribed to seismic activity caused by the Koyna dam's impoundment.
• Earthquakes in Oklahoma's tectonically quiet region have also been connected to oil and gas drilling activity.
• Such areas of man-made earthquake activity are thought to exceed the degree of seismic activity in hotspots like southern California.

On the Basis of Focus of the Earthquake

1) Shallow Focus Earthquakes

• It has its seismic foci located at depths from the 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.
• Shallow focus earthquakes are particularly common at divergent boundaries, such as mid-ocean ridges, and transform boundaries, like the San Andreas Fault in California.

2) Intermediate Focus Earthquakes

• Intermediate focus earthquakes are seismic events that originate at depths ranging from 70 km to 300 km below the Earth's surface.
• These earthquakes predominantly occur in subduction zones where one tectonic plate is being forced under another. As the subducting plate descends into the mantle, it can cause earthquakes at varying depths, including the intermediate range.
• While shallow focus earthquakes are the most common, intermediate focus events also occur frequently, especially in regions with active subduction zones.
• Examples include the Pacific "Ring of Fire", which encircles the Pacific Ocean and features many subduction zones.
• The intensity of shaking at the Earth's surface from an intermediate focus earthquake can be less than that of a similarly sized shallow focus earthquake.
• This is because the seismic waves have a longer distance to travel before reaching the surface, which can cause them to dissipate some of their energy.
• Examples: The 2011 earthquake off the Pacific coast of Tōhoku, Japan, which led to the devastating tsunami and Fukushima Daiichi nuclear disaster, was an intermediate focus earthquake. Another example is the 1976 Tangshan earthquake in China.

3) Deep Focus Earthquakes

• Deep focus earthquakes are seismic events that originate at depths ranging from 300 km to about 700 km beneath the Earth's surface. It is also known as Wadati–Benioff Zone.
• They stand out because, at such profound depths, the Earth's materials are expected to deform ductilely rather than fracture and slip, which is how earthquakes typically occur.
• Like intermediate focus earthquakes, deep focus earthquakes predominantly occur in subduction zones, where one tectonic plate is diving beneath another into the mantle.
• The occurrence of earthquakes at these depths is somewhat enigmatic because temperatures and pressures are so high that rocks are expected to deform plastically rather than breaking in a brittle fashion.
• Several mechanisms, such as phase transitions (changes in the crystal structure of minerals), have been proposed to explain these events.
• Deep focus earthquakes are particularly common in a few subduction zones around the Pacific "Ring of Fire."
• Regions such as beneath the Fiji-Tonga area in the South Pacific and beneath the Sea of Okhotsk off the Russian Far East are known for these types of earthquakes.
• Like intermediate focus earthquakes, the intensity of shaking from a deep focus earthquake at the Earth's surface can be less than that of a similarly sized shallow focus earthquake. They can, however, still be of high magnitude and potentially damaging.
• Examples: One of the most significant deep focus earthquakes in recorded history was the 2013 Sea of Okhotsk earthquake, with a depth of approximately 609 km and a magnitude of 8.3.
• Because deep focus earthquakes occur at depths inaccessible for direct study, their investigation offers valuable insights into the Earth's deep interior, the properties of materials under extreme conditions, and the complex interactions in subduction zones.

Causes and Effects 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 of the earth is 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 due to Uplift and Subsidence

• A one-meter uplift in coastal areas was produced by an earthquake in Chile in 1822.
• An earthquake in New Zealand in 1885 resulted in a 3-meter rise.
• An earthquake in Japan in 1891 caused up to 6 meters 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.
• A volcano is formed 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 can adopt many different shapes depending on the chemical composition and viscosity of the lava.

Types of Volcanoes

On the Basis of their Eruption

• Active: An active volcano is one that has recently erupted or is on the verge of erupting again.
• Dormant: A dormant volcano is one that hasn't erupted in a long time but is still capable of doing so in the future.
• Extinct: An extinct volcano is one that last erupted thousands of years ago and no longer has the potential to erupt.

On the Basis of their Shape

Cinder Cones

• Cinder cones are round or oval cones made up of tiny lava pieces that have been blown up from a single vent.
• Cinder cones are formed by the accumulation of largely small fragments of scoria and pyroclastics around the vent.
• The majority of cinder cones only erupt once.
• Cinder cones can arise as side vents on bigger volcanoes or as isolated cinder cones.
• Examples: Parícutin in Mexico and Sunset Crater in the USA.

Composite Volcano

• Composite volcanoes are steep-sided volcanoes made up of multiple layers of volcanic rocks, most of which are made up of high-viscosity lava, ash, and rock debris.
• These volcanoes are towering conical mountains made up of lava flows and other ejecta layered in alternate layers, hence the name strata.
• Cinder, ash, and lava make up composite volcanoes.
• Cinders and ash build up on top of one another, lava flows over the ash, cools and hardens, and the cycle continues.
• Examples: Mount St. Helens in the USA, Mount Fuji in Japan, and Mount Pinatubo in the Philippines.

Shield Volcano

• Shield volcanoes have long, gradual slopes formed by basaltic lava flows and are fashioned like a bowl or shield in the middle.
• These are generated by the eruption of low-viscosity lava that can travel a long way from the vent.
• They don't usually blow out in a big way.
• Shield volcanoes are more prevalent in marine than continental settings because low-viscosity magma is often low in silica.
• Shield cones are found throughout the Hawaiian volcanic system, and they are also frequent in Iceland.
• Examples: Mauna Loa and Kilauea in Hawaii.

Lava Domes

• Lava domes arise when erupting lava becomes too thick to flow and stacks up near the volcanic vent, forming a steep-sided mound.
• Slow outbursts of exceedingly viscous lava form them.
• They can sometimes be found within the crater of an earlier volcanic eruption.
• They can erupt violently and explosively, just like a composite volcano, although the lava rarely flows far from the erupting vent.
• Example: Mount St. Helens has a lava dome in its crater.

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

Sudden movements such as earthquakes and volcanoes create widespread devastation on the planet's surface. However, it can be concluded that these earthquakes and volcanoes are important to maintaining the Earth's crust's unique structure.

FAQs

Question: What causes earthquakes?

Question: What are the different types of volcanoes?

Question: What is the difference between the focus and the epicenter of an earthquake?

Question: What are tectonic plates?

Question: How are earthquakes measured?

MCQs

1. Which of the following is the point inside the Earth where an earthquake originates?

a) Epicenter

b) Focus

c) Fault

d) Tremor

2. Which of the following is a shield volcano?

a) Mount St. Helens

b) Mauna Loa

c) Mount Vesuvius

d) Krakatoa

3. Which scale is used to measure the magnitude of an earthquake?

a) Richter scale

b) Mercalli scale

c) Volcanic Explosivity Index (VEI)

d) Beaufort scale

4. What type of plate boundary causes the most destructive earthquakes?

a) Divergent boundary

b) Transform boundary

c) Convergent boundary

d) Tensional boundary

5. Which of the following is an example of a composite volcano?

a) Mount Fuji

b) Kilauea

c) Mount Mauna Loa

d) Mount Elgon

GS Mains Questions and Model Answers

Q1: Discuss the role of plate tectonics in the occurrence of earthquakes and volcanic eruptions.

Answer: Plate tectonics is the primary driving force behind earthquakes and volcanic eruptions. The Earth's lithosphere is divided into tectonic plates that move over the asthenosphere. Earthquakes occur at plate boundaries due to the release of stress when plates interact, while volcanic eruptions happen when magma escapes from beneath the Earth's crust, typically at divergent and convergent plate boundaries. The Pacific Ring of Fire is an example of a tectonically active region with frequent seismic and volcanic activity.

Q2: Analyze the impacts of earthquakes on human settlements and the measures that can be taken to mitigate these impacts.

Answer: Earthquakes can cause widespread destruction to human settlements, including building collapses, infrastructure damage, landslides, and tsunamis. To mitigate these impacts, it is crucial to implement earthquake-resistant building designs, early warning systems, and efficient disaster response plans. Public awareness and preparedness, along with proper land-use planning in seismic zones, can significantly reduce the loss of life and property.

Q3: Explain how volcanic activity influences the global climate.

Answer: Volcanic eruptions can inject large quantities of ash and sulfur dioxide into the atmosphere, which can reflect sunlight and lead to temporary global cooling. The 1991 eruption of Mount Pinatubo, for example, caused a significant reduction in global temperatures. Volcanic gases like CO₂ can also contribute to long-term climate change. However, the immediate impact of volcanic activity on climate is often cooling due to the blockage of solar radiation by volcanic aerosols.

Previous Year Questions on Earthquakes and Volcanoes

1. UPSC CSE Prelims 2018:

Question: Which one of the following volcanoes is known as a composite volcano?

A. Kilauea

B. Mount St. Helens

C. Mauna Loa

D. Mount Elgon

Answer: B

Explanation: Mount St. Helens is a composite volcano, known for its explosive eruptions and the alternating layers of ash, pumice, and lava flows that define its structure.

2. UPSC CSE Mains 2016 (GS Paper 1):

Question: Explain the distribution of earthquakes and volcanoes with reference to plate tectonics.

Answer: Earthquakes and volcanoes are primarily distributed along plate boundaries. Earthquakes occur at all three types of plate boundaries: convergent, divergent, and transform. Volcanoes are mostly found at convergent (subduction zones) and divergent boundaries (mid-ocean ridges). For example, the Pacific Ring of Fire is a region with frequent earthquakes and volcanic eruptions due to the movement of several tectonic plates.

3. UPSC CSE Prelims 2017:

Question: Earthquake waves are measured by which of the following instruments?

A. Hydrometer

B. Barometer

C. Seismometer

D. Anemometer

Answer: C

Explanation: A seismometer is an instrument used to measure and record seismic waves generated by earthquakes. It detects ground motion and provides data for determining the earthquake's magnitude and epicenter.