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Unit 1 - Notes

GEO295

Unit 1: Basic Concepts of Geomorphology

1. Factors Controlling Landform Development

Landforms are the surface features of the Earth, produced by the interaction of various physical forces. The development of landforms is complex and is generally governed by the famous "Trio of Davis" (W.M. Davis): Structure, Process, and Stage, along with time and relief.

A. Geological Structure

Structure refers to the lithological and structural characteristics of the rocks.

  • Lithology: The nature of the rock (hard vs. soft, permeable vs. impermeable).
    • Resistant rocks (e.g., granite, quartzite) often form highlands or ridges.
    • Weaker rocks (e.g., shale, clay) erode easily to form valleys or lowlands.
  • Arrangement: The disposition of rock strata.
    • Horizontal strata: Lead to mesas, buttes, and plateaus.
    • Folded strata: Lead to anticlines (ridges) and synclines (valleys).
    • Faulted strata: Lead to rift valleys and block mountains (horsts).
  • Permeability: Determines surface runoff versus infiltration, influencing drainage density and erosion rates.

B. Geomorphic Processes

These are the physical and chemical actions that modify the earth's surface (detailed in Section 2).

  • The intensity of processes varies by climate (e.g., glacial processes in high latitudes, aeolian processes in arid regions).

C. Stage (Time)

Landforms evolve through time. Davis proposed a cycle of erosion:

  1. Youth: High relief, steep gradients, vigorous vertical erosion (V-shaped valleys).
  2. Maturity: Lateral erosion dominates, valleys widen, relief decreases.
  3. Old Age: Landscape is reduced to a base level, forming a Peneplain with scattered remnant hills (Monadnocks).

2. Endogenetic and Exogenetic Forces

The Earth’s surface is shaped by the interplay of two opposing forces.

A. Endogenetic Forces (Internal Forces)

Originating from within the Earth, these forces build landforms and create relief inequalities. They are driven by radioactivity, rotational friction, and primordial heat.

1. Diastrophic Forces (Slow Movements):
These operate over long geological periods.

  • Epeirogenic (Continent Building): Vertical movements causing uplift or subsidence of large landmasses.
    • Uplift: Raised beaches, coastal plains.
    • Subsidence: Submerged forests, rias.
  • Orogenic (Mountain Building): Horizontal movements causing tangential forces.
    • Tensional Forces: Create faults, fractures, rift valleys, and block mountains.
    • Compressional Forces: Create folds (fold mountains like the Himalayas).

2. Sudden Movements:

  • Volcanism: Movement of magma onto or toward the surface. Forms volcanic cones, lava plateaus, and intrusive features (dykes, sills, batholiths).
  • Earthquakes: Sudden release of strain energy causing ground shaking, landslides, and faulting.

B. Exogenetic Forces (External Forces)

Originating from the atmosphere (sun, wind, rain), these forces aim to smooth out the relief created by endogenetic forces. This is known as Gradation.

  • Degradation (Erosion): Lowering of land surfaces.
    • Weathering: In-situ disintegration and decomposition of rocks (Physical, Chemical, Biological).
    • Mass Wasting: Downslope movement of material due to gravity (landslides, creep, mudflow).
    • Erosion: Removal of material by kinetic agents (Rivers, Glaciers, Wind, Waves).
  • Aggradation (Deposition): Filling up of depressions.
    • Deposition of sediments transported by erosional agents (deltas, sand dunes, moraines).

3. Physical Conditions of the Earth's Interior

Our understanding of the interior is derived primarily from Seismology (study of earthquake waves).

A. Sources of Information

  • Direct Sources: Deep ocean drilling (limited depth), volcanic eruptions.
  • Indirect Sources: Density studies, temperature/pressure gradients, meteors, gravitation, and magnetic fields.
  • Seismic Waves:
    • P-waves (Primary): Longitudinal, fastest, travel through solids, liquids, and gases.
    • S-waves (Secondary): Transverse, slower, cannot travel through liquids.
    • L-waves (Surface): Cause the most damage but are confined to the surface.

B. Structure of the Earth

The Earth is chemically divided into Crust, Mantle, and Core.

1. The Crust (Lithosphere surface)

  • Continental Crust (Sial): Rich in Silica and Aluminum. Lower density (~2.7 g/cm³), granitic, thicker (30-50 km).
  • Oceanic Crust (Sima): Rich in Silica and Magnesium. Higher density (~3.0 g/cm³), basaltic, thinner (5-10 km).
  • Conrad Discontinuity: Boundary between upper and lower crust.

2. The Mantle (Mesosphere)

  • Extends from the base of the crust to 2,900 km.
  • Mohorovicic Discontinuity (Moho): Boundary between Crust and Mantle.
  • Asthenosphere: The upper portion of the mantle (up to 400 km). It is in a semi-molten, plastic state. This is the source of magma and the layer on which tectonic plates slide.
  • Composition: Rich in olivine and pyroxene (Ultramafic).
  • Repetti Discontinuity: separates upper and lower mantle.

3. The Core (Barysphere)

  • Gutenberg Discontinuity: Boundary between Mantle and Core (2,900 km).
  • Outer Core: Liquid state (proven by the S-wave shadow zone). Composed of Nickel and Iron (NiFe). Generates Earth's magnetic field.
  • Inner Core: Solid state due to extreme pressure.
  • Lehmann Discontinuity: Separates Outer and Inner Core.

4. Geosynclines

Historically, the "Geosynclinal Theory" was the precursor to Plate Tectonics for explaining mountain building.

A. Definition

A geosyncline is a long, narrow, shallow, and subsiding water-filled depression (trough) bordered by rigid landmasses (cratons/forelands), where immense sedimentation occurs. They are often called the "Cradles of Mountains."

B. Stages of Geosynclinal History

  1. Lithogenesis (Sedimentation): Rivers erode the forelands and deposit sediments into the geosyncline. The floor of the geosyncline subsides under the weight of the sediments, allowing for kilometers of accumulation.
  2. Orogenesis (Folding): Compressive forces from the moving forelands squeeze the sediments. The sediments buckle and fold, rising up to form fold mountains.
  3. Gliptogenesis (Erosion): The newly formed mountains undergo weathering and erosion, sculpting the final landscape.

C. Example: The Tethys Sea

The Himalayas were formed from the Tethys Geosyncline. The forelands were Angaraland (Eurasian Plate) and Gondwanaland (Indian Plate). As these landmasses converged, the Tethys sediments were folded to form the Himalayas.

5. Continental Drift

Proposed by Alfred Wegener in 1912 in his book The Origin of Continents and Oceans.

A. Core Hypothesis

  • About 250 million years ago (Carboniferous period), all continents were united in a single supercontinent called Pangaea.
  • Pangaea was surrounded by a superocean called Panthalassa.
  • Pangaea broke into Laurasia (North) and Gondwanaland (South), separated by the Tethys Sea.
  • The fragments drifted to their current positions.

B. Forces Responsible (According to Wegener)

  • Pole-fleeing Force (Polflucht): Centrifugal force due to Earth's rotation, moving continents toward the equator.
  • Tidal Force: Attraction of the Moon and Sun, causing westward drift.
  • (Note: These forces were later proven insufficient by modern science, leading to Plate Tectonics).

C. Evidence Supporting Drift

  1. Jig-Saw Fit: The bulge of Brazil fits perfectly into the Gulf of Guinea (Africa).
  2. Geological Matching: The Appalachian mountains (USA) align with the Caledonian mountains (Scotland/Scandinavia).
  3. Fossil Evidence: Mesosaurus (small reptile) fossils found only in South Africa and Brazil. Glossopteris flora found across India, Australia, Antarctica, and Africa.
  4. Paleoclimatic Evidence: Tillite deposits (glacial debris) found in tropical lands (India, Africa) indicate they were once near the South Pole.
  5. Placer Deposits: Gold deposits in Ghana verify the source from Brazil, despite no gold-bearing veins in Ghana itself.

6. Isostasy

Isostasy (Greek: isos = equal, stasis = standstill) refers to the mechanical stability or equilibrium between the upstanding parts of the earth (mountains, plateaus) and the low-lying parts (basins) on a rotating earth.

A. The Concept

The lighter crust floats on the denser, plastic mantle (asthenosphere) much like icebergs float in water. To maintain balance, higher features must have deeper "roots."

B. Major Theories

1. Airy’s Theory (Roots of Mountains)

  • Principle: Uniform density, varying thickness.
  • Analogy: Wooden blocks of same density but different heights floating in water. The taller blocks are submerged deeper.
  • Conclusion: Mountains are made of the same density material as plains but are thicker. Therefore, mountains have deep "roots" of light sialic material extending into the mantle to support their height.

2. Pratt’s Theory (Compensation Level)

  • Principle: Varying density, uniform depth.
  • Analogy: Bars of different metals (lead, iron, zinc) floating in mercury. They sink to the same depth (Level of Compensation) but rise to different heights based on their lightness.
  • Conclusion: There is a "Level of Compensation" at a certain depth. Above this line, density varies: higher columns (mountains) have lower density, and lower columns (ocean floors) have higher density.

C. Global Isostatic Adjustment

  • Glacial Isostasy: When ice sheets melt (e.g., Scandinavia), the crust rebounds (uplifts) because the weight is removed.

7. Fundamentals of Geomagnetism

A. Earth as a Magnet

The Earth behaves like a giant bar magnet with a North and South pole. The magnetic field creates the magnetosphere, shielding Earth from solar wind.

  • Geographic Poles: Defined by the axis of rotation.
  • Magnetic Poles: Defined by the point where magnetic field lines enter vertically. They are not coincident with geographic poles and wander over time.
  • Declination: The angle between magnetic north and true (geographic) north.
  • Inclination (Dip): The angle the magnetic field lines make with the horizontal. (0° at Magnetic Equator, 90° at Magnetic Poles).

B. Dynamo Theory (Origin)

The magnetic field is generated by the Geodynamo mechanism.

  • The Outer Core consists of molten Iron and Nickel.
  • Convection currents in this fluid, combined with the Coriolis force from Earth's rotation, create electric currents.
  • These spiraling electric currents generate the magnetic field.

C. Paleomagnetism

The study of the record of the Earth's magnetic field in rocks.

  • Curie Point: When magma cools below this temperature, magnetic minerals (magnetite) align themselves with the Earth's current magnetic field and "lock in" that orientation.
  • Sea-Floor Spreading Evidence: Magnetic surveys of the ocean floor revealed Magnetic Striping. Parallel stripes of normal and reversed polarity exist symmetrically on both sides of mid-ocean ridges. This proved that new crust is created at ridges and pushes old crust away, validating Plate Tectonics.

D. Magnetic Reversals

The Earth's magnetic field is not static. Periodically (every few hundred thousand years), the polarity flips (North becomes South). The last reversal was the Brunhes-Matuyama reversal (~780,000 years ago).