1.5 - Geography Notes 15

Elaborate Notes

Extrusive Volcanic Landforms

Extrusive or volcanic landforms are created when magma, referred to as lava upon reaching the surface, cools and solidifies. The nature of these landforms is primarily determined by the viscosity, composition, and gas content of the lava.

• Volcanic Cones: These are the most characteristic features of volcanoes, built by the accumulation of erupted materials around a central vent. Their shape and size vary significantly based on the type of eruption.

  • Shield Volcano:
    • Formation: These volcanoes are formed by the effusive eruption of highly fluid, low-viscosity basaltic magma. This fluidity allows the lava to travel great distances from the vent before solidifying, resulting in broad, gently sloping cones that resemble a warrior’s shield.
    • Geological Context: Shield volcanoes are typically associated with mantle plumes that create hotspots. As a tectonic plate moves over a stationary hotspot, a chain of volcanoes can form, such as the Hawaiian-Emperor seamount chain. The concept of hotspots was famously articulated by Canadian geophysicist J. Tuzo Wilson in 1963.
    • Characteristics: They are the largest volcanoes on Earth in terms of volume and area. Eruptions are generally non-explosive, characterized by lava fountains and flows.
    • Examples: Mauna Loa and Mauna Kea in Hawaii are classic examples. Mauna Loa is the world’s largest active shield volcano. Olympus Mons on Mars is the largest known volcano in the solar system and is also a shield volcano.
  • Ash-Cinder Cone (or Scoria Cone):
    • Formation: These are the simplest type of volcanic cone, built from ejected lava fragments called cinders or scoria. During explosive eruptions, gas-charged lava is blown violently into the air, breaking into small fragments that solidify and fall as cinders around the vent.
    • Characteristics: They are characterized by steep, straight sides and a large summit crater. They are dominated by pyroclastic material (ash, cinders, bombs) with very little lava flow. They are relatively small, rarely exceeding 300-500 meters in height.
    • Examples: The Taal Volcano in the Philippines has a complex system that includes cinder cones. Parícutin in Mexico is a famous example that grew out of a cornfield in 1943, offering a unique opportunity for scientists to study the entire life cycle of a cinder cone.
  • Composite Volcano (or Stratovolcano):
    • Formation: These are constructed from alternating layers (strata) of viscous lava flows, volcanic ash, cinders, blocks, and bombs. The lava is typically andesitic or dacitic, which is more viscous and silica-rich than basaltic lava. This high viscosity prevents it from travelling far, thus building up steep-sided, conical structures.
    • Geological Context: They are most commonly found along subduction zones, where an oceanic plate sinks beneath a continental or another oceanic plate, such as the Pacific “Ring of Fire”.
    • Characteristics: These volcanoes are known for their picturesque, conical shape but also for their highly explosive and dangerous eruptions (Plinian eruptions). The alternating layers provide structural strength.
    • Examples: Mount Fuji in Japan is an iconic, near-symmetrical composite volcano. Mount Vesuvius in Italy, infamous for its 79 AD eruption that destroyed Pompeii and Herculaneum (documented by Pliny the Younger), is another example. Krakatoa in Indonesia produced one of the most catastrophic eruptions in modern history in 1883.

• Crater:

  • This is a bowl or funnel-shaped depression typically found at the summit of a volcano, formed by the explosive ejection of material from a central vent. Its diameter is generally less than 1.5 kilometers. When volcanic activity ceases, craters can fill with precipitation or groundwater, forming a crater lake, such as Crater Lake in Oregon, USA (which is technically a caldera lake).

• Caldera:

  • A caldera is a large, basin-shaped depression, much larger than a crater, with a diameter that can range from several kilometers to tens of kilometers. It is formed by the collapse or subsidence of a volcano’s summit following a massive, explosive eruption that empties the underlying magma chamber.
  • Formation Process: A powerful eruption ejects a vast volume of magma, leaving the magma chamber below partially empty. The overlying rock structure, no longer supported, collapses inward, creating the caldera.
  • Examples: Yellowstone Caldera in the USA is a “supervolcano” caldera. Lake Toba in Sumatra, Indonesia, is a massive caldera lake formed after a super-eruption around 74,000 years ago, an event some scholars link to a global volcanic winter.

• Flood Basalt Province (or Plateau Basalt):

  • This feature results from a series of effusive eruptions where highly fluid basaltic lava flows out from long fissures (fissure eruptions) rather than a central vent. These flows spread over vast areas, covering the original topography and building up thick, extensive plateaus.
  • Geological Significance: These events, known as Large Igneous Provinces (LIPs), are often linked to the initial stages of continental rifting. The Deccan Traps in India, formed around 66 million years ago, is a prime example. Its formation coincides with the Cretaceous–Paleogene extinction event, leading to scientific debate about its role in the demise of the dinosaurs. The Columbia River Plateau in the northwestern United States is another major flood basalt province.

• Hydrothermal Features:

  • Hot Springs / Thermal Springs: These are springs produced by the emergence of geothermally heated groundwater from the Earth’s crust. Water percolates deep into the ground, where it is heated by contact with hot rocks associated with magma chambers or geothermal gradients. The heated, less dense water then rises back to the surface. It often contains a high concentration of dissolved minerals (like sulfur, calcium, lithium), which are believed to have therapeutic properties. Examples are widespread, including those in Manikaran (Himachal Pradesh, India), Rajgir (Bihar, India), and the Blue Lagoon in Iceland.
  • Geysers: A geyser is a rare kind of hot spring that is under pressure and erupts, sending a turbulent column of hot water and steam into the air. The process involves a unique underground plumbing system of fractures and conduits. Water at the bottom is superheated by geothermal energy but is kept from boiling by the pressure of the cooler water above. As it heats, some water turns to steam, which expands and pushes the water column upwards, reducing the pressure on the water below, causing it to flash-boil and erupt explosively.
    • Examples: “Old Faithful” in Yellowstone National Park, USA, is the world’s most famous geyser. Other significant geyser fields are found in Iceland (the origin of the word “geyser” from ‘Geysir’) and New Zealand.
  • Fumaroles: These are vents in the Earth’s surface from which steam and volcanic gases (like sulfur dioxide, carbon dioxide, hydrogen sulfide) are emitted. They are found in areas of active or recent volcanism where magma is close to the surface. The Valley of Ten Thousand Smokes in Alaska, created after the 1912 Novarupta eruption, is a famous example.
  • Mud Volcano: A mud volcano is a landform created by the eruption of mud, water, and gases. They are not typically associated with magmatic activity. Instead, they form in regions where pressurized underground gases, often methane from decomposing organic matter, force their way to the surface through layers of wet clay and silt. The erupted material forms a cone of mud.
    • Examples: Baratang Island in the Andaman and Nicobar Islands, India, is known for its mud volcanoes. Azerbaijan has the highest concentration of mud volcanoes in the world.

Intrusive Volcanic Landforms (or Plutonic Rocks)

These landforms are formed when magma cools and solidifies beneath the Earth’s surface. They are only visible after the overlying rocks have been removed by processes of erosion and denudation.

• Batholiths:

  • These are the largest intrusive igneous bodies, covering areas often greater than 100 square kilometers. They are massive, irregular, and dome-shaped, forming the core of major mountain ranges. They are formed from felsic or intermediate rock types, such as granite. The Sierra Nevada Batholith in California is a classic example.

• Laccoliths:

  • These are mushroom-shaped or lens-shaped intrusions where viscous magma pushes the overlying strata of rock upwards into a dome. They are typically fed by a conduit or dyke from below, maintaining a connection to the magma source during formation. The Henry Mountains in Utah, USA, were studied by geologist G.K. Gilbert in the 1870s, who first described and named this feature.

• Lopoliths:

  • This is a large, saucer-shaped (concave upwards) intrusion. It is formed when magma intrudes into a pre-existing structural basin or when the weight of the intruding magma causes the underlying rock layers to sag. The Bushveld Igneous Complex in South Africa is the world’s largest lopolith and a major source of platinum group metals.

• Sills:

  • A sill is a tabular sheet of igneous rock that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. They are concordant intrusions, meaning they lie parallel to the bedding planes of the surrounding rock. The Great Whin Sill in Northumberland, UK, is a famous example upon which part of Hadrian’s Wall was built.

• Dykes:

  • A dyke is a vertical or near-vertical sheet-like intrusion of igneous rock that cuts across the bedding planes of the host rock. They are discordant intrusions. They often form as magma fills pre-existing fractures. They can occur in swarms, radiating from a volcanic center. The Mackenzie dyke swarm in Canada is one of the largest on Earth.

Geomagnetism

Geomagnetism refers to the Earth’s magnetic field, which extends from the Earth’s interior out into space, where it meets the solar wind.

• Origin (Geodynamo Theory): The Earth’s magnetic field is generated in its liquid outer core. The outer core is primarily composed of molten iron and nickel. Three conditions are believed to be necessary for its generation: an electrically conductive fluid medium (molten iron), kinetic energy provided by planetary rotation, and an internal energy source to drive convective motion in the fluid. Heat flowing from the solid inner core creates convection currents in the liquid outer core. The combination of this convective motion and the Earth’s rotation (Coriolis effect) causes the moving conductive material to generate and sustain the magnetic field, a process known as the geodynamo.

• Palaeomagnetism and Polar Wandering:

  • Palaeomagnetism: This is the study of the record of the Earth’s magnetic field in rocks, sediment, or archeological materials. Certain magnetic minerals in rocks (like magnetite) align themselves with the Earth’s magnetic field at the time of their formation.
  • Fossil Magnetism (Thermoremanent Magnetization): When igneous rocks like basalt cool from magma, their ferromagnetic minerals (e.g., iron oxides) become magnetized in the direction of the local geomagnetic field at that specific time. This “fossilized” magnetism becomes locked in as the rock solidifies below the Curie point (the temperature at which magnetic materials lose their permanent magnetism).
  • Polar Wandering: When scientists in the mid-20th century, like S. K. Runcorn, started mapping the paleomagnetic data from rocks of different ages on a single continent (e.g., Europe), it appeared that the magnetic pole had “wandered” over time. However, when they compared these apparent polar wander paths from different continents (e.g., Europe and North America), the paths were different. The most logical explanation was not that the poles had moved, but that the continents themselves had moved relative to a fixed pole. This provided strong quantitative evidence for Alfred Wegener’s earlier theory of Continental Drift.

• Magnetic Field Reversal:

  • Studies of the ocean floor in the 1950s and 1960s revealed a pattern of magnetic “stripes” of normal and reversed polarity, symmetric on either side of mid-oceanic ridges. This discovery was central to the Vine-Matthews-Morley hypothesis (1963).
  • Mechanism: As new oceanic crust is formed at mid-oceanic ridges, it records the Earth’s magnetic polarity at that time. The symmetrical pattern of alternating polarity stripes provided conclusive proof of seafloor spreading. It is now understood that the Earth’s magnetic field has reversed its polarity numerous times throughout its history.
  • Cause: The reversal is thought to be an inherent property of the geodynamo process, possibly triggered by chaotic changes in the convection currents within the outer core.
  • Frequency: The reversals are not periodic, but on average, they occur every 200,000 to 300,000 years. The last major reversal, the Brunhes-Matuyama reversal, occurred about 780,000 years ago.

• Aurora:

  • The Earth’s magnetic field creates a protective bubble around the planet called the magnetosphere, which deflects most of the charged particles from the solar wind.
  • However, near the magnetic poles, the field lines are nearly vertical and enter the Earth’s atmosphere. Some charged particles (electrons and protons) from the solar wind are guided along these lines and funnelled into the upper atmosphere (ionosphere).
  • These energetic particles collide with atoms and molecules of gases in the atmosphere (primarily oxygen and nitrogen), exciting them to higher energy states. When these atoms and molecules return to their normal state, they release this excess energy as photons of light, creating the dazzling, colourful displays known as auroras.
  • Names: In the Northern Hemisphere, they are called Aurora Borealis (Northern Lights). In the Southern Hemisphere, they are called Aurora Australis (Southern Lights). The colour depends on the gas being excited (oxygen produces green and red light, while nitrogen produces blue and purplish-red light).

Earthquakes

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.

• Causes of Earthquakes:
  • Tectonic Plate Movements: The vast majority of earthquakes are caused by the movement of tectonic plates. According to the Elastic Rebound Theory, developed by H.F. Reid after the 1906 San Francisco earthquake, stress builds up along faults (fractures in the Earth’s crust) as plates try to move past each other. When the stress exceeds the strength of the rocks, the rocks suddenly fracture and slip, releasing the stored strain energy in the form of seismic waves.
  • Mining: Mining operations, especially large-scale underground or open-pit mining, can induce seismicity. The removal of large amounts of rock can alter the stress balance in the surrounding crust, leading to small-to-moderate earthquakes, often referred to as rock bursts or mining-induced tremors.
  • Reservoir-Induced Seismicity (RIS): The impoundment of large volumes of water in a reservoir can trigger earthquakes. The weight of the water increases the load on the underlying crust, and more importantly, the water can seep into pre-existing faults, increasing pore fluid pressure. This increased pressure can lubricate the fault, reducing the friction that holds it in place and allowing it to slip. The 1967 Koyna earthquake (Magnitude 6.3) in Maharashtra, India, is a classic and debated example of RIS.

Prelims Pointers

• Shield Volcano: Formed by fluid basaltic magma; gentle slopes; largest volcanoes; associated with hotspots. Example: Mauna Loa (Hawaii).
• Cinder Cone: Built from ejected cinders/scoria; steep sides; small in size; pyroclastic dominated. Example: Parícutin (Mexico).
• Composite Volcano (Stratovolcano): Alternating layers of lava and ash; steep-sided and conical; viscous andesitic lava; explosive eruptions; found at subduction zones. Example: Mount Fuji (Japan), Mount Vesuvius (Italy).
• Crater: Funnel-shaped depression at the mouth of a volcano.
• Caldera: Large depression formed by the collapse of a volcano after a massive eruption. Example: Yellowstone Caldera (USA), Lake Toba (Indonesia).
• Flood Basalt Province: Extensive plateau built by fluid basaltic lava from fissure eruptions. Example: Deccan Traps (India), Columbia Plateau (USA).
• Hot Spring: Natural spring of geothermally heated groundwater. Example: Manikaran (India), Blue Lagoon (Iceland).
• Geyser: A hot spring that intermittently ejects a column of hot water and steam. Example: Old Faithful (USA).
• Fumarole: A vent emitting steam and volcanic gases. Example: Valley of Ten Thousand Smokes (Alaska).
• Mud Volcano: Eruption of mud, water, and gases; not typically related to magmatic activity. Example: Baratang Island (Andaman & Nicobar).
• Batholith: Largest intrusive body; forms the core of mountain ranges.
• Laccolith: Mushroom-shaped intrusion that domes overlying strata.
• Lopolith: Saucer-shaped (concave) intrusion.
• Sill: Horizontal, tabular intrusion parallel to bedding planes (concordant).
• Dyke: Vertical, wall-like intrusion cutting across bedding planes (discordant).
• Geomagnetism: The Earth’s magnetic field, generated by the geodynamo in the liquid outer core.
• Palaeomagnetism: The study of the record of the Earth’s magnetic field in rocks.
• Polar Wandering: Apparent movement of magnetic poles over time, which provided key evidence for continental drift.
• Magnetic Reversal: The periodic flipping of the Earth’s magnetic north and south poles. Evidence is found in magnetic stripes on the ocean floor.
• Aurora Borealis: Northern Lights, caused by solar wind particles interacting with the atmosphere near the North Pole.
• Aurora Australis: Southern Lights, occurring near the South Pole.
• Reservoir-Induced Seismicity (RIS): Earthquakes triggered by the filling of large reservoirs. Example: Koyna Dam (India).

Mains Insights

GS Paper I (Geography) & GS Paper III (Environment & Disaster Management)

1. Plate Tectonics as a Unifying Theory:

   • Interlinkage: The distribution of volcanoes and earthquakes is not random. The majority of active composite volcanoes and major earthquakes are concentrated along plate boundaries, particularly the “Pacific Ring of Fire.” This pattern provides fundamental evidence for the theory of Plate Tectonics.
   • Analytical Point: Understanding plate boundaries (convergent, divergent, transform) is crucial for predicting and mitigating the risks associated with seismic and volcanic hazards. For instance, subduction zones (convergent boundaries) are associated with the most explosive volcanoes (composite) and the most powerful earthquakes (megathrust earthquakes).

2. Volcanism and its Impact on Climate and Civilization:

   • Cause-Effect (Climate): Large volcanic eruptions, especially from composite volcanoes or flood basalt provinces, can inject massive amounts of sulfur dioxide (SO₂) into the stratosphere. This SO₂ forms sulfate aerosols that reflect solar radiation, leading to a net cooling effect on a global scale (a “volcanic winter”). The 1815 eruption of Mount Tambora led to the “Year Without a Summer” in 1816. The Deccan Traps eruptions are debated as a contributing factor to the K-Pg mass extinction.
   • Historical Impact: The eruption of Vesuvius in 79 AD provides a stark historical case study of societal vulnerability, preserving the Roman cities of Pompeii and Herculaneum and offering invaluable archaeological insights. While destructive, volcanic soils (andosols) are also extremely fertile, which explains why many civilizations have thrived in the shadow of active volcanoes despite the risks.

3. Paleomagnetism: The Keystone of Plate Tectonic Theory:

   • Historiographical Viewpoint: Before the 1960s, Alfred Wegener’s theory of Continental Drift (1915) was largely dismissed due to the lack of a plausible mechanism. The discovery of magnetic stripes on the ocean floor and the concept of seafloor spreading (Hess, 1962; Vine-Matthews-Morley, 1963), underpinned by paleomagnetic data, provided the “smoking gun” evidence that transformed Wegener’s idea into the robust theory of Plate Tectonics. This represents a major paradigm shift in Earth sciences.

4. Anthropogenically Induced Seismicity:

   • Development vs. Environment Debate: The phenomenon of Reservoir-Induced Seismicity (RIS) highlights a critical conflict in development. Large dams, often hailed as “temples of modern India,” can have unforeseen and dangerous geological consequences. The Koyna Dam case study is central to this debate in India.
   • Policy Implication (GS Paper III): This necessitates rigorous Environmental Impact Assessment (EIA) and Geological/Seismic Hazard Assessment before the construction of large infrastructure projects. It underscores the need to move beyond a purely engineering-focused approach to one that integrates geological and environmental sciences into developmental planning. Other human activities like fracking and deep-well injection can also induce earthquakes, raising complex regulatory challenges.

5. Geomagnetism: A Planetary Shield and Navigational Tool:

   • Significance: The Earth’s magnetosphere is vital for life, protecting the atmosphere from being stripped away by the solar wind and shielding surface life from harmful cosmic radiation. The potential consequences of a prolonged magnetic field collapse during a reversal are a subject of scientific research, with possible impacts on power grids, satellite communications, and increased radiation exposure.
   • Application: Understanding geomagnetism has practical applications, from traditional compass navigation to modern geophysical prospecting for minerals. The study of paleomagnetism helps in reconstructing past continental configurations and understanding long-term geological processes.