Geology Lecture 3: The Rock Cycle

Lecture 3: The Rock Cycle

• The rock cycle is a continuous process where rocks are transformed from one type to another through various geological processes.
• Rocks are formed from minerals, which are naturally occurring, inorganic, crystalline solids with a specific chemical composition.
• Rocks are aggregates of one or more minerals.
• Rocks are coherent, in contrast to a handful of sand.

Mineral Properties

• Minerals are naturally occurring, inorganic, crystalline solids with a specific chemical composition.
• Minerals have a repeating crystalline structure.

Rock Types

• Igneous rocks: Formed from the cooling and solidification of magma or lava.
  • Extrusive igneous rocks (volcanic): Cool quickly on the Earth's surface, forming fine-grained textures.
    • Examples: Rhyolite, Andesite, Basalt.
  • Intrusive igneous rocks (plutonic): Cool slowly beneath the Earth's surface, forming coarse-grained textures.
    • Examples: Granite, Diorite, Gabbro.
  • Classification is based on the minerals present and the texture.
• Sedimentary rocks: Formed from the accumulation and cementation of sediments. Sediments are weathered and eroded from pre-existing rocks or precipitates. They undergo lithification.
  • Types: Detrital (fragments from pre-existing rocks) and chemical/biochemical (minerals extracted from solutions or produced by organisms).
    • Examples: Sandstone, conglomerate, limestone.
• Metamorphic rocks: Formed from the transformation of existing rocks by heat, pressure, or chemical reactions. They do not melt completely.
  • Types: Foliated (due to pressure creating layers) and nonfoliated (doesn't have layers).
    • Examples: Slate, schist, gneiss, marble, quartzite.
  • Classification based on the composition and texture (foliated/nonfoliated).

Sedimentary Rock Formation: Lithification

• Sediments accumulate
• Sediments are compacted by weight of overlying material.
• Sediments are cemented by minerals.
• Sediment particle sizes: Gravel, sand, silt, and clay

Rock Cycle Processes

• Weathering and erosion: Breaks down rocks into smaller pieces.
• Transportation: Moves sediments.
• Deposition: Accumulates sediments.
• Lithification: Compacts and cements sediments to form sedimentary rocks.
• Metamorphism: Alters rocks by heat, pressure, or chemical reactions.
• Melting: Creates magma, which can cool and solidify to form igneous rocks.
• Uplift and exposure: Brings rocks to the surface.

Igneous Rock Classification

• Aphanitic: Rapid cooling, small crystals.
• Phaneritic: Slow cooling, large crystals.

Metamorphic Rock Formation

• Igneous or sedimentary rocks are changed by heat and pressure
• Types of changes:
  • Compositional (new minerals form).
  • Textural (minerals align) - Foliated textures.
• Depending on the degree of metamorphism, or the original rock type, different types of metamorphic rocks result.

Foliated vs. Non-foliated Textures

• Foliated: layers in the rock result from pressure.
  • Examples: Slate, schist, gneiss
• Non-foliated: Rocks that cooled/were formed without layers
  • Examples: Marble, quartzite