Geological Processes and Rock Classification

Fundamental Geological Processes

Glyptogenesis and Diagenesis

Glyptogenesis is the process by which weathering, erosion, transport, and sedimentation act upon rocks to give rise to sediments. Diagenesis refers to the processes of sediment compaction and cementation that cause the formation of sedimentary rocks.

Metamorphism and Magmatism

Metamorphism is the process where any type of rock is subjected to high pressure and/or temperature without changing its solid state. This modifies its structure into metamorphic rocks. Magmatism is the process by which magma originates and cools to form magmatic rocks.

Classification of Sedimentary Rocks

Sedimentary rocks are formed from accumulated sediments through the process of diagenesis.

Non-Detrital Sedimentary Rocks

• Carbonates: Formed from the precipitation of CaCO3. These are the most abundant limestones and are usually monomineralic (containing Calcite or Aragonite).
• Evaporites: Formed from the precipitation of gypsum, halite, and sylvite.
• Coals: Originate from organic matter. Coals of vegetable origin include:
  • Peat: 55% Carbon (approx. 2 million years old).
  • Lignite: 70% Carbon (approx. 10 million years old).
  • Coal: 85% Carbon (approx. 100 million years old).
  • Anthracite: 90% Carbon (approx. 200 million years old).
  • Graphite: 100% Carbon.
• Oil: Originates from plankton at the bottom of the sea.

Detrital Sedimentary Rocks

• Rudites: Formed by clasts larger than 1cm. Between the clasts, there is a matrix. When clasts are rounded, they are known as puddingstones or pebbles. When they are angular, they are known as breccias.
• Arenites (Sandstones): Consist of clasts ranging from 1/16mm to 2mm, held together by matrix and cement.
• Shale: Fine-grained rocks (less than 1/256mm). They contain matrix and cement. They can be identified with an HCl reaction; if there is no reaction, it is typically clay.

Magmatic Rock Categories

Magmatic rocks are formed in the Earth's interior through the cooling of magma.

• Intrusive (Plutonic): These have a granular texture and originate deep within the crust. They occupy mountain nuclei and are typically acidic rocks.
• Filonian (Subvolcanic): Formed at average depths with fast or slow cooling. They exhibit porphyritic, aplitic, or pegmatitic textures and range from acidic to intermediate.
• Extrusive (Volcanic): Formed at shallow depths through abrupt and rapid cooling. They have a vitreous or porphyritic texture as magma rises through crustal fissures.

Metamorphic Rock Types and Textures

Metamorphic rocks are formed in the Earth's interior through solid-state transformation.

Non-Foliated Rocks

• Marble: A calcium-rich, carbonatic rock that is whitish, glossy, and heavy.
• Quartzite: Formed from detrital sandstones (arenites). It has the same composition as quartz and may appear gray due to oxide.

Foliated Rocks

Foliation is the process by which minerals are oriented according to the direction of least resistance.

• Slate and Chert: Very brilliant, appearing as scales. This represents a mid-process of metamorphism.
• Phyllite: Black and very soft, showing a marked metamorphic process.
• Gneiss: Has the same components as granite but is arranged in distinct layers or bands.