Understanding Earth’s Dynamics: Folds, Faults, Volcanoes, and Orogenesis

Posted on Jan 31, 2025 in Geology

Understanding Earth’s Dynamics

Folds

Folds are continuous deformations produced by compressive stresses. Bending forms a flat terrain.

Elements of a Fold

  • Hinge: The area of maximum curvature of the fold, joining points that are the axes.
  • Direction: The bending angle with the axis in a north-south direction.
  • Axial Plane: Contains the hinge lines, dividing the fold into flanks or limbs.
  • Cartography: The angle between the surfaces of each of the sides with a horizontal plane.
  • Core: The innermost and tightest part of the fold.

Types of Folds

Folds may be classified based on the shape of their profile:

  • Antiform: Convex upward.
  • Synform: Concave upward.

They can also be classified based on axial symmetry:

  • Symmetric: Symmetry with respect to the axial plane.
  • Asymmetric: No symmetry with respect to the axial plane.

Faults

Faults are discontinuous deformations where movement occurs between blocks separated by a fracture.

Elements of a Fault

  • Fault plane: The flat surface that divides the land into two blocks: the hanging wall (moves up) and the footwall (sinks below).
  • Fault direction: The angle between a horizontal line on the fault plane and the north-south direction.
  • Cartography: The angle between the fault plane and an imaginary horizontal plane.
  • Fault displacement: The average distance on the fault plane between two points that were originally attached.

Types of Faults

  • Normal fault: Caused by extensional efforts, resulting in thinning and expansion in the field.
  • Reverse fault: Caused by shortening and compressive stress, resulting in thickening.
  • Strike-slip fault: Caused by shear stress, with blocks moving horizontally parallel to the fault’s direction.

Volcanoes

How Volcanoes Arise

Volcanoes occur when magma, formed by the Earth’s internal energy, reaches the surface through cracks. These cracks can be at divergent or convergent plate tectonic boundaries.

Anatomy of a Volcano

  • Magma chamber: Where magma is stored.
  • Crater: The opening at the top of the volcano.
  • Conduit: Connects the magma chamber to the crater.

Volcanic Products

  • Gases: Water vapor, SO2, CO2, H2S, etc. Gases play an important role in volcanic pressure.
  • Liquids: Lavas, which can be acidic or basic. Acidic lavas tend to trap gases, leading to more explosive eruptions.
  • Solids: Volcanic bombs and ash, formed from solidified lava fragments.

Orogenesis

Orogenesis is the set of processes that create large mountain ranges (cordilleras). Two main types are recognized:

Andean-Type Orogenesis

Formed by the subduction of an oceanic plate beneath a continental margin. This process is characterized by high seismic activity and numerous volcanoes.

  1. Sedimentation: Sediments are deposited on the margin between the plates.
  2. Folding: Subduction causes folding. Deep areas are subjected to high pressures and temperatures, while other areas melt.
  3. Uplift: When subduction ceases, the range is pushed up by isostatic rebound. This stage creates new reliefs.

Collision-Type Orogenesis

These are large cordilleras caused by continent-continent collisions. The collision causes the ocean that was between them to disappear.

  1. Sedimentation: Sediments accumulate between the two approaching continents as the intervening ocean subducts.
  2. Folding: The approach of the continental blocks creates pressures that fold the sediments.
  3. Obduction: Subduction of the oceanic crust ends with obduction. High pressures and temperatures cause rocks to metamorphose.

Island Arcs

Island arcs are formed by ocean-ocean collisions, creating mountain ranges and oceanic trenches.

Formation of Island Arcs

  • Location: They develop through the accumulation of volcanic products, related to the fusion of deep areas. Comprehensive efforts cause fractures and earthquakes.
  • Geological activity: Most are in the Pacific Ocean. The origin is not yet entirely clear. An asymmetric trench is formed on the convex side, and the bottom is filled with sediment from the explosion of other rocks.

Deformation Structures

  • Joints: Fractures with no relative movement between the separated blocks. They can be caused by various factors and are defined by two elements: direction and dip.
  • Faults: Structures formed by compressive stress that exceed the rock’s strength, causing a fracture.
  • Thrust faults: Reverse faults with a nearly horizontal plane and a large displacement. The allochthonous block (hanging wall) moves over the autochthonous block (footwall). Erosion of the thrust sheet can create tectonic windows, and isolated remnants are called klippes.

Types of Volcanic Eruptions

  • Shield volcanoes: Huge volcanic cones with very fluid lava, forming a crater-shaped basin. Eruptions are typically not very explosive.
  • Stratovolcanoes: Characterized by alternating effusive and explosive eruptions.
  • Volcanic domes: Formed by viscous, acidic lava that solidifies in the conduit, creating a cap. This can lead to high pressures and very violent, devastating eruptions.