Rock Formation and Transformation Processes

Posted on Nov 19, 2024 in Geology

1.3 Magmatism and Plate Tectonics

Magmatism is a geological process linked to plate tectonics.

Construction at Edges: 80% of terrestrial magmatism occurs due to reduced pressure and increased temperature, producing basaltic magma that consolidates into plutonic gabbro.

Destructive Edges: 12% of high-temperature magma forms due to water presence. Magmas originate superficially (basaltic, leading to volcanic rocks) or deeper (high viscosity, forming plutonic granites) and intermediate (volcanic andesites and plutonic diorites).

Inside Plates: Terrestrial magmatism leads to intraplate volcanism due to mantle hot spots.

2.1.1 Plutonic Rocks

  • Crystallize slowly within the Earth, ensuring good crystallization.
  • Have a grainy texture and pegmatites.
  • Examples: syenite, granite, granodiorite, diorite, gabbro, peridotite.

2.1.2 Volcanic Rocks

  • Crystallize quickly on the Earth’s surface, forming non-crystalline rocks.
  • Create volcanic glass (glassy texture) and microcrystalline structures.
  • Examples: rhyolite, trachyte, basalt, andesite, obsidian, pumice.

2.1.3 Filonian Rocks

  • Result from rapid magma cooling near the surface.
  • Form dykes or veins.
  • Examples: aplite, diabase, granite porphyry.

3 Metamorphism

Physicochemical changes in rocks without losing their solid state, occurring when pressure and temperature conditions change, resulting in metamorphic rocks.

3.1 Factors Involved

  • Increased temperature
  • Pressure
  • Fluids or volatiles

3.2 Metamorphic Processes

  • Rock changes due to metamorphic agents.
  • Brecciation or rupture (under directed pressure, forming cataclastic texture).
  • Recrystallization (mineral particles form larger crystals above 300°C).
  • Formation of oriented structures (minerals align perpendicular to compression).
  • Dehydration and decarbonation (minerals lose H2O and CO2, facilitating chemical reactions).
  • Mineralogical adjustments or new mineral formation (minerals react under specific pressure and temperature conditions).

3.3 Metamorphic Facies

  • Metamorphism occurs across a wide pressure and temperature range.
  • Classified by intensity: very low grade (similar to diagenesis) to high grade (similar to magmatism).
  • Metamorphic facies define the pressure and temperature conditions for rock formation.

3.4 Types of Metamorphism

  • Dynamic or Pressure: Occurs in large faults (increased pressure, temperature), causing vitrification, mineral orientation, and mylonite formation.
  • Thermal or Contact: Increased temperature with pressure, rocks contact hot magma, forming a contact aureole with partial (shale) or total (cornubianite) recrystallization.
  • Regional: Occurs in large crust areas (increased pressure, temperature), from very low to high grade, forming oriented structures and completely changing the rock, including burial metamorphism in oceanic trenches.

4 Metamorphic Rocks

Genesis influenced by original rock nature, metamorphism type, and degree of metamorphism. Differ in texture and mineral composition. Classified as silicate or carbonate rocks. Examples: slate, schist, gneiss, migmatite.

1 Geodynamic Processes

1.1 Weathering

Rock alteration process caused by water, air, ice, or tectonic stress.

Physical Weathering: No chemical composition change, caused by tectonic stress (fractures, folding, rifting), gelifraction (ice breaking rocks), thermoclastia (disintegration due to temperature changes), and bioclastia (organisms breaking rocks).

Chemical Weathering: Composition change caused by solution (water dissolving minerals), oxidation (electron loss/gain), carbonation (CO2 in water reacting with limestone), and hydrolysis (internal structure breakdown by water).

1.2 Erosion

Mobilization of materials by water, ice, or air due to weathering.

1.3 Transportation

Transfer of dispersed materials by wind (rolling, saltation, suspension), river water (rolling, saltation, suspension), seawater (waves, currents, tides), glacial ice, and gravity (sliding). Transport forms include solution, flotation, rolling, dragging, suspension, and saltation.

1.4 Sedimentation

Settling of materials in sedimentary basins. Patterns include deposition (larger materials), settling (suspended materials), and precipitation (dissolved minerals).

3 Sedimentary Environments

  • Glaciers: Ice deposits leave disordered, poorly sorted materials forming moraines (lateral, central, frontal, bottom).
  • Estuaries: River mouth deposits where river energy is insufficient to carry sediments into the sea.
  • Platform: Accumulation of calcareous clay and other mainland materials.
  • Slope Side: Sediment falls by gravity and accumulates at the slope’s edge.
  • Seabed: Primarily marine sediment provenance.
  • Deltas: River-transported materials deposited at the mouth where sea energy is insufficient to carry them.
  • River: Mainly detrital sediments, with potential chemical and organic sediments.
  • Karstic: Carbonation in carbonate lands, forming caves and galleries, with carbonate precipitation forming speleothems (stalactites, stalagmites).
  • Coral: Calcareous deposits from polyp skeletons.