Igneous and Sedimentary Rock Classification

Igneous Rock Textures and Classification

Basic Textures of Igneous Rocks

According to the degree of crystallization:

• Holocrystalline: The rock is composed entirely of crystals.
• Hypocrystalline: The rock contains crystals within a glassy matrix.
• Vitreous: The rock is presented as an amorphous mass with the appearance of glass.

According to the size of the crystals: Classified as coarse, medium, or fine.

According to the relationship of crystal sizes:

• Homometric: Equal-sized crystals.
• Heterometric: Crystals of different sizes.
• Porphyritic: Features large crystals, known as phenocrysts, set in a finer matrix.

Plutonic and Volcanic Rock Types

Plutonic Rocks

Ordered from acidic to basic (clearer to darker):

• Granite: Composed of quartz, feldspar, and mica.
• Syenite: Typically grayed.
• Gabbro: Dark-colored.
• Peridotite: Very dark.

Volcanic Rocks

Primarily hypocrystalline. Ordered from basic to acidic:

• Basalt: Dark, containing phenocrysts.
• Andesite: Gray.
• Trachyte: Clear.
• Rhyolite: White.
• Vitreous Rocks: Obsidian and pumice.

Subvolcanic (Filolianas) Rocks

Formed by cooling in reefs or fissures. If the magma cooled halfway, it forms Aplite and Granitic Porphyry (composed of quartz, feldspar, and mica).

Intraplate Volcanism and the Wilson Cycle

Intraplate Volcanism

Originating from the D'' layer, thermal plumes emerge to create a hot spot. As the lithospheric plate moves, it drills through the plate, potentially forming island arcs. O_o

The Wilson Cycle

1. Rifting: Buildup of heat beneath a surface causes a rift and continental fragmentation.
2. Separation: Seafloor extension and separation of the continents.
3. Subduction: Formation of subduction zones and the approximation of the continents.
4. Collision: Continental collision and the consolidation of a new supercontinent.

Diagenesis and Sedimentary Rock Formation

Diagenesis is the set of processes by which sediment becomes sedimentary rock. These processes include:

• Compaction: Reduction in the volume of a rock.
• Cementation: Due to the precipitation of minerals dissolved in water that passes through the grains of sediment.

Classification of Sedimentary Rocks

Detrital Sedimentary Rocks

Formed from rock fragments that have been carried as solid matter. They differ by grain size:

• Conglomerates: Coarse-grained rocks (>2mm) resulting from the diagenesis of gravel. There are two types:
  • Conglomerate: Rounded clasts from long transport.
  • Breccia: Angular clasts from short transport.
• Sandstone: Medium-grained (2-1/16mm) resulting from the diagenesis of sands.
• Siltstone (Limolita): Fine-grained (1/16-1/256mm) formed by the diagenesis of silts.
• Argillites: Very fine-grained, formed by the diagenesis of clays (arcillas).

Chemical and Organic Rocks

• Carbonate Rocks: Formed by substances dissolved in water that precipitate. These include Limestone (calcium carbonate; lumaquelas if they contain many shells) and Marl (clay and limestone).
• Evaporite Rocks: Formed by the precipitation of salts from minerals dissolved in water after intense evaporation.