Earth’s Surface: Relief and Geological Processes

Posted on Oct 26, 2024 in Geology

1. Relief: Earth’s Surface Forms

Descriptive Classification (Appearance)

  • Mountain: Isolated relief with a large gap between its base and top.
  • Hill: Isolated relief with little gap.
  • Gorge/Throat: Narrow valley bottom with steep walls.
  • Plateau: Isolated relief with a flat top.
  • Terraces: Phased relief.

Interpretative Classification (Process)

  • Mountain: Relief that stands out from surrounding areas. A line of mountains is called a range. An erogenous mountain range is one formed by tectonic uplift.
  • Monadnock/Mesa: Isolated relief whose top represents the past height of the surrounding area.
  • Terraces: Fragmented and tiered area formed by river erosion.
  • Anthropogenic Modeling: Relief produced by human activity.

2. Major Land Reliefs

  • Oceanic Crust: Composed primarily of basalt, forms ocean basins.
  • Continental Crust: Formed by granite and metamorphic rocks, thicker and less dense than oceanic crust.

3. External Geological Processes: Weathering and Erosion

Weathering

The destruction of rocks by water, air, and living beings.

Types of Weathering

  • Mechanical: Division of rock into fragments (gelifraction, thermoclasty, decompression).
  • Chemical: Alteration of rocks by chemical reactions.
    • Water (Hydration): Water becomes part of the rock’s molecular structure (e.g., clays).
    • O2 (Oxidation): Rocks containing iron change color.
    • CO2 (Carbonation): Water with CO2 dissolves rocks like limestone.
    • Chemical Attack: Substances produced by living organisms (e.g., bird droppings, plant acids).
  • Biological: Alteration of rocks by living things.

Erosion

Weathering of rocks by water, wind, ice, or particles carried by these agents.

  • Debris Evacuation: Rock fragments accumulate at the foot of escarpments.
  • Characteristic Landforms: Erosion creates landforms that reveal the geological agent.
  • Peneplains: Large, virtually flat areas resulting from long-term erosion.
  • Redistribution of Continental Mass: Erosion, transport, and deposition cause weight loss in mountains and weight gain in lower areas.

4. External Geological Processes: Transport and Sedimentation

Transport

Movement of eroded fragments.

Sedimentation

Accumulation of transported materials.

Transport Parameters

  • Energy: Ability to mobilize sediments (high energy moves large clasts).
  • Clast Selection: Related to energy.
  • Sediment Maturation: Changes in composition, shape, and size during transport.

Forms of Transport

  • In Contact with the Bottom: Rolling or saltation.
  • Without Touching the Bottom: Suspension, flotation, or dissolved.

Sedimentation

Accumulation of transported material.

  • Uplifting Process: Erosion in high areas.
  • Subsidence Process: Sediment accumulation in sedimentary basins.

Sediment Accumulation

  • Decantation: Clasts fall when the current stops.
  • Kinetic Accretion: Clasts accumulate against an obstacle.

Sedimentary Structures

  • Stratification: Layering of several centimeters or meters.
  • Lamination: Layering of a few millimeters.
  • Settling Down: Sorting by clast size.
  • Cross-Bedding: Intersecting layers formed by currents.

5. Caves and Sedimentary Environments

Caves

Extensive sedimentary areas with subsidence.

Sedimentary Environments

Continental

  • Fluvial: Rivers.
  • Torrential: Flash floods and streams.
  • Glacial: Glaciers.
  • Aeolian: Wind.
  • Karst: Water dissolving rocks.

Marine

  • Coastal: Waves.
  • Reef: Coral reefs and other organisms.
  • Continental Shelf: Large areas with various environments.
  • Continental Slope: Fine materials.
  • Turbidite: Abyssal plain.

Transitional

  • Beaches: Sand and gravel.
  • Deltas: River sediment and waves.
  • Estuaries: River mouths with tides.

6. Fluvial Model

  • V-Shaped Valleys: High erosive capacity.
  • Flat-Bottomed Valleys: Decreased erosive capacity, meandering.
  • Peneplains: End result of fluvial erosion.
  • Monadnocks/Mesas: Remnants of old plains.
  • Terraces: Staggered reliefs representing valley enlargements.

Fluvial Transport and Sedimentation

Rivers reduce clast size and alter minerals. High energy in upper course, lower energy in middle and lower courses.

7. Torrential Model

Ephemeral streams and flash floods in arid climates.

  • Gullies: Deep ruts.
  • Ravines: Deep, V-shaped valleys.
  • Ramblas/Arroyos: Wide, dry channels.

Torrential Transport and Sedimentation

High transport capacity, fast, short, unselective. Sedimentation occurs violently.

  • Alluvial Fans: Small accumulations.
  • Bajadas: Large accumulations.
  • Pediments: Merged alluvial fans.

8. Wind Model (Aeolian)

Principal agent in hot deserts.

  • Deflation: Wind removes smaller materials, leaving stony desert (reg).
  • Wind Abrasion: Sandblasting of rocks.

Wind Transport and Sedimentation

Large volumes of fine material transported long distances.

  • Loess Deposits: Accumulations of windblown dust.
  • Dunes: Crescent-shaped sand accumulations.

9. Coastal Model

Coastal Erosion

  • Cliff Retreat: Wave action erodes cliff base, causing falls.
  • Crushing and Washing: Rock fragments are shredded, finer materials carried away.

Coastal Transport and Sedimentation

Sand transported by littoral currents.

  • Beaches: Sand or gravel accumulations on gentle slopes.
  • Spits and Bars: Sandy cords perpendicular or parallel to the coast.

10. Glacial Model

Erosion and transport by ice masses.

  • U-Shaped Valleys: Broad, deep valleys.
  • Overdeepenings: Cavities produced by ice flow.

Glacial Transport and Sedimentation

Sedimentation in till (unsorted glacial deposits). Moraines are common.

11. Karst Model

Surface

  • Dissolution: Rainwater creates grooves (lapiés).
  • Collapse: Sinkholes (dolines) formed by collapsed cavities.

Underground

  • Galleries: Horizontal tunnels.
  • Shafts: Vertical ducts.
  • Chambers: Large cavities.

Karst Sedimentation

  • Decalcification Clay Accumulation: Clay residue from dissolved limestone.
  • Precipitation of Soluble Salts: Calcium carbonate precipitates as tufa, stalagmites, stalactites, and travertine.

12. Determinants of Modeling

Climate

  • Hot/Arid: No glaciers or rivers.
  • Temperate/Rainy: Abundant vegetation.
  • Polar/High Mountain: Water as ice.

Tectonic and Structural

Tectonics influence agent activity (e.g., rivers follow faults).

Lithology

Rock type influences modeling (e.g., soft rocks erode easily).

Anthropic

Human activity alters relief and agent action.