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Unit 2 - Notes

GEO295

Unit 2: Advanced Concepts of Geomorphology

1. Plate Tectonics

The theory of Plate Tectonics is the unifying framework of modern geology, explaining the movement of the Earth's lithosphere and the formation of major landforms. It superseded the hypothesis of Continental Drift (Alfred Wegener) by providing a physical mechanism for movement.

Core Concepts

  • The Lithosphere: The rigid outer shell of the Earth, comprising the crust and the uppermost mantle. It is broken into tectonic plates.
  • The Asthenosphere: The semi-fluid, plastic layer of the upper mantle below the lithosphere. Plates "float" and move upon this layer.
  • Driving Mechanisms:
    • Mantle Convection: Heat from the core generates convection currents in the mantle, dragging plates along.
    • Ridge Push: Newly formed, elevated lithosphere at mid-ocean ridges slides down due to gravity, pushing the plate.
    • Slab Pull: Cold, dense oceanic lithosphere sinks into the mantle at subduction zones, pulling the rest of the plate behind it (considered the strongest force).

Plate Boundaries

The interactions between plates occur at their boundaries, resulting in distinct geomorphic features.

A. Divergent Boundaries (Constructive Margins)

Plates move apart. Magma rises from the mantle to fill the gap, creating new crust.

  • Oceanic: Mid-Ocean Ridges (e.g., Mid-Atlantic Ridge). Features include rift valleys, transform faults, and basaltic volcanism.
  • Continental: Rift Valleys (e.g., East African Rift Valley). The crust stretches, thins, and fractures, creating grabens and horsts.

B. Convergent Boundaries (Destructive Margins)

Plates move toward each other. The nature of the boundary depends on the type of crust involved.

  • Oceanic-Continental: The denser oceanic plate subducts under the continental plate.
    • Features: Oceanic trenches, volcanic arcs (e.g., The Andes), Benioff zones (earthquake foci).
  • Oceanic-Oceanic: The older, colder (denser) oceanic plate subducts.
    • Features: Island arcs (e.g., Japan, Philippines), deep trenches (e.g., Mariana Trench).
  • Continental-Continental: Neither plate subducts due to buoyancy. The crust buckles and thickens.
    • Features: Fold mountains (e.g., The Himalayas). No volcanism, but intense seismicity.

C. Transform Boundaries (Conservative Margins)

Plates slide laterally past one another. Lithosphere is neither created nor destroyed.

  • Features: Strike-slip faults (e.g., San Andreas Fault). Intense shallow earthquakes, but no volcanic activity.

2. Recent Views on Mountain Building (Orogeny)

Orogeny refers to the process of mountain formation, particularly involving folding and thrusting.

Geosynclinal Theory (Classical View)

Historically, geologists believed mountains formed in geosynclines—massive, sediment-filled depressions.

  1. Lithogenesis: Sediment accumulates, causing the basin to subside.
  2. Orogenesis: Lateral compression squeezes sediments into folds.
  3. Gliptogenesis: Erosion sculpts the uplifted mass.

Plate Tectonic Theory of Orogeny (Modern View)

Modern geomorphology views mountain building as a direct result of plate convergence.

  1. Cordilleran Type (Ocean-Continent Collision):

    • Accretionary wedges form as sediments are scraped off the subducting plate.
    • Magmatism creates a batholithic core (volcanic roots).
    • Examples: Rockies, Andes.

  2. Alpine/Himalayan Type (Continent-Continent Collision):

    • Total closure of an ocean basin (e.g., the Tethys Sea closed to form the Himalayas).
    • Since continental crust cannot subduct deeply, it shortens, folds, and stacks via thrust faults (nappes).
    • Result: Extreme crustal thickening and high elevation.

  3. Island Arc Type (Ocean-Ocean Collision):

    • Volcanic activity builds mountains rising from the ocean floor.

Isostasy

The concept of gravitational equilibrium between the Earth's crust and mantle.

  • Airy Hypothesis: Mountains have deep low-density "roots" floating in the higher-density mantle (like an iceberg). Higher mountains have deeper roots.
  • Pratt Hypothesis: Different topographic heights differ in density; higher mountains are less dense than lower ocean basins, but all float at the same base level.

3. Vulcanicity

Vulcanicity (or volcanism) encompasses all processes by which molten rock (magma), gases, and pyroclastic materials are forced into the Earth's crust or ejected onto the surface.

Classification by Location

  1. Extrusive (Volcanic): Magma reaches the surface as lava.
  2. Intrusive (Plutonic): Magma cools and solidifies beneath the surface.

Intrusive Landforms

  • Batholiths: Massive, deep-seated igneous bodies (usually granite) forming the roots of mountains. Exposed by erosion (e.g., Sierra Nevada).
  • Laccoliths: Mushroom-shaped bodies that dome the overlying strata.
  • Sills: Horizontal sheets of magma intruded between sedimentary layers.
  • Dykes: Vertical wall-like structures cutting across rock layers.
  • Phacoliths: Lens-shaped masses found in the anticlines or synclines of folded strata.

Extrusive Landforms (Volcanoes)

The shape depends on lava viscosity (silica content).

  1. Shield Volcanoes: Formed by low-viscosity (fluid) basaltic lava. Broad, gentle slopes (e.g., Mauna Loa, Hawaii).
  2. Composite Cones (Stratovolcanoes): Formed by high-viscosity andesitic/rhyolitic lava and pyroclastics. Steep, conical, explosive (e.g., Mt. Fuji, Mt. St. Helens).
  3. Cinder Cones: Small, steep piles of pyroclastic debris (scoria).
  4. Calderas: Massive depressions formed when a volcano collapses into its emptied magma chamber (e.g., Crater Lake, USA).

Volcanic Products

  • Lava: Acidic (high silica, viscous) vs. Basic (low silica, fluid).
  • Tephra/Pyroclasts: Ash, lapilli, and volcanic bombs.
  • Gases: Water vapor, CO2, sulfur dioxide.

4. Earthquakes and Tsunamis

Earthquakes

Sudden shaking of the ground caused by the release of energy stored in the crust.

  • Elastic Rebound Theory (H.F. Reid): Rocks accumulate strain energy due to tectonic stress. When stress exceeds the rock's strength, it ruptures (faults), snapping back to an unstrained shape and releasing waves.
  • Focus (Hypocenter): The point of origin within the crust.
  • Epicenter: The point on the surface directly above the focus.

Seismic Waves

  1. Body Waves (Travel through Earth's interior):
    • P-Waves (Primary): Longitudinal (push-pull). Fastest. Travel through solids and liquids.
    • S-Waves (Secondary): Transverse (shear). Slower. Travel only through solids (stopped by the outer core).
  2. Surface Waves (Travel along the surface; most destructive):
    • Love Waves: Side-to-side horizontal motion.
    • Rayleigh Waves: Rolling, elliptical motion (like ocean waves).

Measurement

  • Richter Scale (Magnitude): Logarithmic scale measuring energy released.
  • Mercalli Scale (Intensity): Observational scale measuring damage and human perception.

Tsunamis

Large ocean waves caused by the displacement of a large volume of water.

  • Causes: Undersea earthquakes (subduction zones), volcanic eruptions, submarine landslides, or meteorite impacts.
  • Mechanics:
    1. Generation: Vertical displacement of the seafloor lifts the water column.
    2. Propagation: In deep water, waves have low amplitude (<1m) but very long wavelengths (>100km) and high speeds (up to 800 km/h).
    3. Shoaling (Run-up): As waves approach shallow water, speed decreases due to friction, wavelength shortens, and amplitude (height) increases drastically, causing coastal inundation.

5. Concepts of Geomorphic Cycles and Landscape Development

Theories attempting to explain how landscapes evolve over time.

A. The Davisian Cycle of Erosion (W.M. Davis, 1899)

Davis proposed that landscapes evolve through a time-dependent series of stages.

  • Formula: Landscape = (Structure, Process, Stage).
    • Structure: Rock composition and geological arrangement.
    • Process: Agents of denudation (rivers, wind, glaciers).
    • Stage: The time duration and phase of development.

The Stages:

  1. Youth: Rapid uplift followed by vertical incision. V-shaped valleys, waterfalls, steep gradients. Broad, flat interfluves (remnants of original uplift).
  2. Maturity: Vertical erosion decreases; lateral erosion increases. Valleys widen, floodplains form, interfluves become rounded divides.
  3. Old Age: Relief is reduced to a featureless plain near sea level called a Peneplain. Rivers meander sluggishly. Resistant rock remnants stand as Monadnocks.
  • Critique: Oversimplified; assumes rapid uplift then tectonic stability (unrealistic); overemphasis on time.

B. Penck’s Model of Slope Replacement (Walther Penck)

Rejected the time-bound "stages" of Davis. Proposed that landforms are a function of the ratio between the rate of uplift and the rate of erosion.

  • Slope Retreat: Slopes do not flatten (as Davis suggested) but retreat parallel to themselves (Parallel Retreat).
  • Treppen Concept: Landscapes resemble staircases (piedmont benchlands) due to phases of uplift.

C. King’s Pediplanation Cycle (L.C. King)

Applicable primarily to semi-arid/savanna regions (derived from studies in Africa).

  • Process: Parallel retreat of scarps (steep slopes).
  • Pediment: A gently inclined bedrock surface carved at the foot of a retreating scarp.
  • Pediplain: The coalescence of multiple pediments creates a multi-concave surface called a Pediplain (distinct from Davis's convex-concave Peneplain).

D. Dynamic Equilibrium (J.T. Hack)

Rejected cyclic evolution. Suggested that landscapes adjust to balance the resisting force of rocks with the energy of erosional processes.

  • Once equilibrium is reached, the landform shape remains constant as it lowers, provided forcing factors (climate/tectonics) don't change. Time is not a variable causing change in shape.

6. Erosion Surfaces

An erosion surface is a recognizable, extensive, flat or nearly flat land surface formed by erosion agents.

Types of Erosion Surfaces

  1. Peneplain:

    • Associated with: W.M. Davis / Humid temperate climates.
    • Formation: Downwearing of interfluves over long periods of stability.
    • Features: Low relief, deep soil cover, covered in sediment.

  2. Pediplain:

    • Associated with: L.C. King / Semi-arid and arid climates.
    • Formation: Formed by the coalescence of pediments via scarp retreat.
    • Features: Thin veneer of gravel, exposed bedrock, abrupt transition from plain to inselbergs (steep hills).

  3. Etchplain:

    • Associated with: Tropical humid environments (Deep weathering).
    • Formation: Two-stage process.
      1. Deep chemical weathering (hydrolysis) rots the bedrock beneath the soil creates a "saprolite" layer.
      2. Climate change or uplift strips the regolith (soil), exposing the uneven "etch surface" or "weathering front" beneath.
    • Features: Tors, castle koppies, and bornhardts.

  4. Panplain:

    • Associated with: Lateral erosion by rivers (Crickmay).
    • Formation: Formed by the coalescence of floodplains.

Identification of Erosion Surfaces

In complex landscapes, old erosion surfaces may be identified by:

  • Accordance of Summit Levels: Hilltops at roughly the same elevation suggesting a dissected plateau.
  • Benchlands: Steps on valley sides indicating pauses in uplift (rejuvenation).
  • Unconformities: Erosional boundaries in the geological strata.