GS Notes

1.5 - Geography Notes 09

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Elaborate Notes

Interior of the Earth

The study of the Earth’s interior is primarily based on indirect evidence, as direct observation is limited to shallow depths through mining and drilling projects (e.g., the Kola Superdeep Borehole). The primary sources of information are seismic waves (P-waves and S-waves), whose velocity and path change as they travel through materials of different densities and states. Other sources include the study of meteorites (believed to represent the primitive material of the solar system), gravitational anomalies, and the Earth’s magnetic field.

Chemical Divisions of the Earth’s Interior

This classification, largely based on the work of geologist Eduard Suess in the late 19th century, categorizes the Earth’s layers based on their chemical composition.

  • Crust:

    • The outermost, thinnest, and most brittle layer of the Earth. It constitutes less than 1% of the Earth’s volume.
    • Its composition is dominated by lighter silicate minerals. The boundary between the crust and the mantle is known as the Mohorovičić (Moho) Discontinuity, identified in 1909 by Andrija Mohorovičić.
    • Continental Crust: Predominantly granitic in composition, it is thicker (average 35-45 km, but can exceed 70 km under major mountain ranges like the Himalayas) and less dense (approx. 2.7 g/cm³). It is rich in Silica and Aluminium, hence often termed ‘SiAl’. Its rocks are geologically ancient, with some parts being over 4 billion years old.
    • Oceanic Crust: Primarily basaltic in composition, it is thinner (average 8-10 km) and denser (approx. 3.0 g/cm³). It is rich in Silica and Magnesium, hence termed ‘SiMa’. Due to the process of seafloor spreading, oceanic crust is continuously formed at mid-oceanic ridges and destroyed at subduction zones, making it much younger than continental crust (rarely older than 200 million years).

  • Mantle:

    • Located beneath the crust, extending down to a depth of approximately 2,900 km. It is the most voluminous layer, accounting for about 83% of the Earth’s volume and 68% of its mass.
    • It is composed of silicate rocks rich in iron and magnesium, such as peridotite. While predominantly solid, its high temperatures and pressure allow the rock to deform and flow slowly over geological timescales.
    • This slow, convective movement within the mantle is the primary driving force behind plate tectonics.
    • It is divided into the Upper Mantle and the Lower Mantle, separated by a transition zone marked by the Repetti Discontinuity. The change is due to mineral phase transitions under increasing pressure.

  • Core:

    • The innermost layer, extending from the base of the mantle to the Earth’s center (approx. 6,371 km). It is the densest layer, composed primarily of an iron-nickel alloy, hence termed ‘NiFe’.
    • The boundary between the mantle and the core is the Gutenberg Discontinuity, discovered by Beno Gutenberg in 1913.
    • Outer Core: This layer, extending from 2,900 km to 5,150 km, is in a liquid state. The movement of the molten iron alloy within the outer core, coupled with the Earth’s rotation, generates the Earth’s magnetic field through a process known as the geodynamo effect.
    • Inner Core: Extending from 5,150 km to the center, it is a solid sphere despite having higher temperatures than the outer core. The immense pressure at this depth (over 3.6 million atmospheres) raises the melting point of the iron-nickel alloy above the ambient temperature, forcing it into a solid state. Its existence was confirmed by Danish seismologist Inge Lehmann in 1936, and the boundary separating it from the outer core is named the Lehmann Discontinuity.

Physical/Mechanical Divisions of the Earth’s Interior

This modern classification is based on the mechanical properties and physical state of the layers, which is crucial for understanding dynamic processes like plate tectonics.

  • Lithosphere: (from Greek lithos, “rocky”)

    • This is the rigid, brittle outer layer of the Earth, comprising the entire crust and the uppermost, solid part of the upper mantle.
    • Its thickness is variable, ranging from about 15 km under mid-oceanic ridges to over 200 km under old continental shields, with an average of around 100 km.
    • The lithosphere is fragmented into several large and small tectonic plates that move relative to each other over the underlying, more ductile layer.

  • Asthenosphere: (from Greek asthenēs, “weak”)

    • Located directly beneath the lithosphere, within the upper mantle, typically extending from about 100 km to 400 km depth.
    • It is a highly viscous, mechanically weak, and ductile region. It is not fully molten but is in a semi-solid, plastic state (partially molten).
    • This layer’s plasticity allows the rigid lithospheric plates to “float” and move upon it.
    • It is often referred to as the Low-Velocity Zone (LVZ) because seismic waves (particularly S-waves) slow down as they pass through it, indicating a change in physical state from the rigid lithosphere above. It is also the primary source of magma that erupts at mid-oceanic ridges and volcanoes.

  • Mesosphere:

    • This term refers to the rest of the mantle, below the asthenosphere, extending to the core-mantle boundary.
    • Unlike the asthenosphere, the mesosphere (or lower mantle) is more rigid and solid due to the immense overlying pressure, which counteracts the effect of high temperature. However, it still undergoes slow convection over geological time.

  • Barrysphere:

    • An older term for the Earth’s core (both outer and inner). It is derived from the Greek word barys, meaning “heavy,” reflecting its high density.

Composition of the Earth

The elemental composition varies significantly between the whole Earth and its crust due to planetary differentiation, a process where heavier elements sank to the center during the Earth’s early molten state.

  • Elemental Composition of the Entire Earth (by mass):

    • Iron (Fe): ~35%
    • Oxygen (O): ~30%
    • Silicon (Si): ~15%
    • Magnesium (Mg): ~13%
    • (Other significant elements include Nickel, Sulphur, and Calcium)

  • Elemental Composition of the Earth’s Crust (by mass):

    • Oxygen (O): 46.60% (predominantly in silicate minerals)
    • Silicon (Si): 27.72%
    • Aluminium (Al): 8.13%
    • Iron (Fe): 5.00%
    • Calcium (Ca): 3.63%
    • (Followed by Sodium, Potassium, and Magnesium)

Discontinuities in the Interior of the Earth

These are transitional boundaries where the physical properties of the Earth’s layers, such as density and seismic wave velocity, change abruptly.

  1. Conrad Discontinuity: Named after seismologist Victor Conrad (1925). A boundary within the continental crust that separates the upper, felsic (granitic) layer from the lower, mafic (basaltic) layer. It is not a globally consistent feature and is often absent or poorly defined.
  2. Mohorovičić (Moho) Discontinuity: Named after Andrija Mohorovičić (1909). Marks the boundary between the crust (both continental and oceanic) and the mantle. Seismic P-waves accelerate significantly as they cross this boundary into the denser mantle material.
  3. Repetti Discontinuity: Named after geophysicist William C. Repetti. It is a transition zone at a depth of around 660 km, separating the upper mantle from the lower mantle. This boundary is associated with mineral phase changes under increasing pressure.
  4. Gutenberg Discontinuity: Named after Beno Gutenberg (1913). It is the core-mantle boundary (CMB) at a depth of approximately 2,900 km. It is a very sharp boundary characterized by a drastic drop in P-wave velocity and the complete stoppage of S-waves, which cannot travel through the liquid outer core.
  5. Lehmann Discontinuity: Named after Inge Lehmann (1936). It is the boundary between the liquid outer core and the solid inner core at a depth of about 5,150 km. It was discovered through the observation of faint P-wave reflections.

Types of Rocks

A rock is a naturally occurring solid aggregate of one or more minerals or mineraloids. The study of rocks is called petrology.

Igneous Rocks

  • Derived from the Latin word ‘ignis’ meaning ‘fire’. They are formed from the cooling and solidification of molten rock material (magma or lava).
  • They are known as primary rocks as they are the original source of material for other rock types. Their texture depends on the rate of cooling.
  • Intrusive (or Plutonic) Igneous Rocks: Formed when magma cools and solidifies slowly beneath the Earth’s surface. The slow cooling allows for the growth of large, visible mineral crystals (a coarse-grained or phaneritic texture). These rocks are generally very hard and resistant.
    • Examples: Granite, Diorite, Gabbro, Peridotite.
  • Extrusive (or Volcanic) Igneous Rocks: Formed when lava cools and solidifies rapidly on or above the Earth’s surface. The rapid cooling results in small, microscopic crystals (a fine-grained or aphanitic texture) or no crystals at all (a glassy texture, like obsidian).
    • Examples: Basalt, Andesite, Rhyolite, Pumice.
  • Chemical Classification: Based on silica (SiO₂) content.
    • Acidic (or Felsic): High silica content (>66%). They are typically light in colour, less dense, and have high viscosity. Example: Granite.
    • Basic (or Mafic): Low silica content (45-52%). Rich in magnesium and iron, they are dark in colour, denser, and have low viscosity (more fluid). Example: Basalt.
    • The sequence of mineral crystallization from magma is described by Bowen’s Reaction Series, developed by Norman L. Bowen in the early 20th century.

Sedimentary Rocks

  • Derived from the Latin word ‘sedimentum’ meaning ‘settling’. They are formed from the accumulation, compaction, and cementation of sediments over long periods.
  • The process of rock formation is called lithification. It involves:
    1. Weathering & Erosion: Breakdown of pre-existing rocks (igneous, metamorphic, or other sedimentary).
    2. Transportation: Sediments are moved by agents like water, wind, ice, or gravity.
    3. Deposition: Sediments settle out of the transporting medium, typically in layers or strata.
    4. Compaction: The weight of overlying sediments squeezes out water and air, reducing pore space.
    5. Cementation: Dissolved minerals (like calcite, silica, or iron oxides) precipitate in the pore spaces, binding the sediment grains together.
  • Key Characteristics:
    • They are layered or stratified.
    • They are the only rock type to contain fossils, which are the preserved remains of ancient life.
    • They are often porous and permeable.
  • Classification based on formation:
    • Mechanically Formed: Formed from the consolidation of rock fragments. Examples: Sandstone (from sand), Shale (from clay/mud), Conglomerate (from rounded pebbles).
    • Chemically Formed: Formed by precipitation of minerals from water. Examples: Limestone (from calcium carbonate), Rock Salt, Gypsum.
    • Organically Formed: Formed from the accumulation of organic matter (plant or animal remains). Examples: Coal (from plant debris), Chalk and certain types of Limestone (from shells of marine organisms).

Metamorphic Rocks

  • Derived from Greek words ‘meta’ (change) and ‘morphe’ (form). They are formed when pre-existing rocks (igneous, sedimentary, or even other metamorphic rocks) are subjected to intense heat, pressure, or chemical action.
  • This process, metamorphism, changes the rock’s mineralogy, texture, and chemical composition without melting it.
  • Agents of Metamorphism:
    • Thermal (Contact) Metamorphism: Occurs when rocks come into contact with hot magma or lava, causing them to “bake”.
    • Dynamic (Regional) Metamorphism: Occurs over large areas due to the immense pressure and heat associated with tectonic plate collisions and mountain-building.
  • Key Textural Features:
    • Foliation: A parallel alignment of mineral grains or structural bands within the rock, creating a layered or banded appearance. It is caused by directed pressure. Examples: Slate, Schist, Gneiss.
    • Lineation: A linear arrangement of minerals within the rock.
  • Examples of Metamorphism:
    • Granite (Igneous) → Gneiss
    • Limestone (Sedimentary) → Marble
    • Sandstone (Sedimentary) → Quartzite
    • Shale (Sedimentary) → Slate → Phyllite → Schist (with increasing metamorphism)
    • Basalt (Igneous) → Amphibolite
    • Coal (Sedimentary) → Anthracite → Graphite → Diamond (under extreme conditions)

The Rock Cycle

  • The rock cycle is a fundamental concept in geology, first conceptualized by James Hutton in the late 18th century, which describes the dynamic transitions among the three main rock types.
  • It illustrates how geological processes continuously transform rocks from one form to another.
    • Magma cools to form igneous rocks.
    • Igneous rocks are weathered and eroded into sediments, which lithify to form sedimentary rocks.
    • Both igneous and sedimentary rocks can be subjected to heat and pressure to become metamorphic rocks.
    • Metamorphic rocks can be weathered into sediments or melted back into magma.
  • Any rock exposed on the surface (an outcrop) is subject to weathering and can begin the cycle anew. The cycle is driven by energy from both the Earth’s interior (heat) and the sun (driving weathering processes).

Prelims Pointers

  • Chemical Layers of Earth: Crust (SiAl, SiMa), Mantle, Core (NiFe).
  • Physical Layers of Earth: Lithosphere, Asthenosphere, Mesosphere, Barrysphere (Core).
  • Asthenosphere: Semi-solid/plastic layer in the upper mantle; source of magma; also called the Low-Velocity Zone (LVZ).
  • Lithosphere: Comprises the crust and the solid upper part of the mantle; divided into tectonic plates.
  • Earth’s Volume Distribution: Mantle (~83%), Core (~16%), Crust (~1%).
  • Earth’s Mass Distribution: Mantle (~68%), Core (~31%), Crust (<1%).
  • Most Abundant Elements (Whole Earth): 1. Iron (35%), 2. Oxygen (30%), 3. Silicon (15%).
  • Most Abundant Elements (Earth’s Crust): 1. Oxygen (46.6%), 2. Silicon (27.7%), 3. Aluminium (8.13%), 4. Iron (5%).
  • Continental vs. Oceanic Crust:
    • Continental: Thicker, less dense, granitic (SiAl), older.
    • Oceanic: Thinner, denser, basaltic (SiMa), younger.
  • Earth’s Core: Outer core is liquid (generates magnetic field); Inner core is solid due to immense pressure.
  • Seismic Discontinuities:
    1. Conrad: Within Crust (Upper/Lower).
    2. Mohorovičić (Moho): Between Crust and Mantle.
    3. Repetti: Between Upper and Lower Mantle.
    4. Gutenberg: Between Mantle and Core.
    5. Lehmann: Between Outer and Inner Core.
  • Primary Rocks: Igneous rocks are called primary rocks.
  • Intrusive Igneous Rocks (Plutonic): Formed inside the earth, slow cooling, large crystals (e.g., Granite).
  • Extrusive Igneous Rocks (Volcanic): Formed on the surface, fast cooling, small crystals (e.g., Basalt).
  • Acidic vs. Basic Igneous Rocks:
    • Acidic (Felsic): High silica (>66%), light colour (e.g., Granite).
    • Basic (Mafic): Low silica (<52%), dark colour (e.g., Basalt).
  • Sedimentary Rocks: Formed by lithification (compaction and cementation) of sediments; characterized by layers (strata); contain fossils.
  • Metamorphic Rocks: Formed by changes in temperature and/or pressure on existing rocks.
  • Foliation: Parallel alignment of minerals in metamorphic rocks (e.g., Gneiss, Schist).
  • Rock Metamorphosis Examples:
    • Granite → Gneiss
    • Limestone → Marble
    • Sandstone → Quartzite
    • Shale → Slate
    • Coal → Graphite/Diamond

Mains Insights

GS Paper I (Physical Geography):

  1. Interrelation between Earth’s Interior and Surface Features: The structure of the Earth’s interior is not a static concept but the very engine of surface dynamics.

    • Cause-Effect: The heat from the core drives convection currents in the plastic asthenosphere. This movement is the primary mechanism for plate tectonics (as per the theory proposed by scholars like Harry Hess and Robert Dietz in the 1960s). This, in turn, causes the formation of major landforms like fold mountains (Himalayas), rift valleys (East African Rift Valley), volcanic arcs (Andes), and mid-oceanic ridges. A question on the formation of continents and oceans is fundamentally a question about the Earth’s interior.
    • Debate/Historiography: The understanding of the Earth’s interior has evolved significantly. Early theories were speculative. The chemical classification (SiAl, SiMa, NiFe) by Suess was a major step but is now considered secondary to the physical/mechanical classification (Lithosphere, Asthenosphere) which better explains the processes of geodynamics, not just the composition.

  2. The Rock Cycle as a Geomorphic Agent:

    • The rock cycle is central to understanding geomorphology and landscape evolution. The type of rock determines its resistance to weathering and erosion, directly influencing the resulting landforms.
    • Example: Hard, crystalline igneous rocks like granite often form resistant uplands and tors, while soft sedimentary rocks like shale are easily eroded to form valleys and lowlands. Carbonate rocks like limestone lead to the development of unique Karst topography through chemical weathering.
    • Analysis: The interaction between endogenic (internal) forces that create rocks (e.g., volcanism) and exogenic (external) forces that destroy them (weathering, erosion) is the essence of the rock cycle and landform development.

  3. Economic Significance:

    • A thorough understanding of rock types and their formation is crucial for resource mapping.
    • Sedimentary Rocks: Host the world’s most significant fossil fuel reserves (coal, oil, natural gas) as well as aquifers for groundwater.
    • Igneous and Metamorphic Rocks: Are the primary sources of metallic minerals (e.g., iron, copper, gold) found in veins and lodes. Metamorphic rocks like marble and slate are valuable building materials. Diamonds are formed under the extreme metamorphic conditions deep within the mantle.

GS Paper III (Disaster Management & Environment):

  1. Hazard Zonation and Prediction:

    • Knowledge of the lithosphere-asthenosphere interaction is fundamental to understanding earthquakes and volcanoes. Most seismic and volcanic activity is concentrated along the boundaries of the lithospheric plates.
    • Mapping these boundaries and understanding the processes at the Moho and within the asthenosphere allows for better seismic hazard zonation and contributes to early warning systems.
    • The composition of magma (acidic vs. basic) determines the nature of a volcanic eruption. Viscous, acidic magmas lead to explosive eruptions (e.g., stratovolcanoes), posing a greater immediate hazard than the effusive eruptions of fluid, basic lavas (e.g., shield volcanoes).

  2. Infrastructure and Environmental Management:

    • The properties of rocks (hardness, porosity, jointing) are critical considerations in civil engineering projects like dam construction, tunneling, and road building. The failure to account for rock type can lead to disasters like landslides and dam failure.
    • The study of sedimentary rocks is essential for groundwater management, as their porosity and permeability determine the characteristics of aquifers. It is also vital for understanding the subsurface sequestration of carbon dioxide.

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