Geology Notes: Rock Types, Metamorphism, and Wilson Cycle

Rock Classification and Grain Size

• Rocks are formed from minerals, which are naturally occurring crystalline substances; rocks are formed by mixing minerals together.
• Three major classes of rocks: igneous, sedimentary, and metamorphic.
• Within sedimentary particle sizes, a smaller class is called silt, and the final particles are clay.
• Particle size thresholds mentioned:
  • Particles smaller than $0.002\text{ mm}$ are considered clay-size particles.
  • A common cutoff used is $0.001\text{ mm}$ for even finer distinctions.
• Sand-sized grains are visible in sandstone, illustrating the presence of coarser particles within sedimentary rocks.
• Sorting by grain size occurs when sediments are moved by running water or other processes; materials are separated into groups of similar size. Note: this use of "sorting" here is different from general everyday usage.
• The idea that a mixture of different grain sizes can become lithified into solid rock is described (mixture becomes rock through lithification, i.e., compaction and cementation).

Sedimentary Processes and Precipitation

• Precipitation process described with limestone and calcium carbonate:
  • Water trickling down through gaps in limestone may dissolve calcium carbonate (CaCO₃).
  • As water reaches a surface (e.g., roof of a cave) and evaporates, pure water enters the gas phase, leaving behind CaCO₃ grains on that spot, effectively cementing or depositing CaCO₃ at that location.
  • Repeated rain events over a long time (storm after storm for a century or so) can build up the deposit into a noticeable CaCO₃ precipitate layer (often described as a “milk” of calcium carbonate).
• In swampy environments, plants contribute to a distinctive sedimentary product:
  • Very acidic soils inhibit gas exchange and bacterial activity.
  • When vegetation dies, slow decomposition leads to accumulation of organic matter, forming a thick, soft, organic-rich layer called peat.
  • Peat accumulates in layers over time (soft layer buildup) and represents an organic-rich component of sedimentary sequences.
• Practical note: peat is an important transitional material in the sedimentary/coal sequence (peat formation is part of the broader sedimentary and geological record), and its presence signals particular environmental conditions (swampy, acidic soils, low decomposition).

Igneous Rocks: Cooling and Textures

• Magma cooling inside the Earth (intrusive) vs cooling near or at the surface (extrusive) determines crystal size and texture:
  • Intrusive (plutonic) cooling occurs underground, where surrounding rock acts as an insulating blanket.
  • The insulating surroundings slow cooling, allowing crystals to grow larger, producing a coarse-grained texture with visible crystals of different minerals.
  • The longer magma spends in a given temperature range, the larger the crystals can become; slower cooling yields coarser textures.
  • In contrast, when magma reaches the surface (extrusive), rapid cooling tends to produce smaller crystals or a glassy texture.
• Textural examples related to cooling and emplacement:
  • If cooling occurs underground (intrusive), you get large, discernible crystals with a mix of minerals.
  • If magma flows to the surface and spreads out (extrusive), it can form sheet-like deposits (large, flat sheets) and finer-grained rocks due to rapid cooling.
• Magma pathways and intrusion types:
  • Dikes: vertical intrusions that cut through surrounding rock via cracks; magma moves through vertical fractures.
  • Sills: horizontal intrusions that spread between preexisting rock layers along weak zones.
  • Laccoliths: dome-shaped intrusions where magma accumulates between layers and creates a dome.
• The basic distinction between intrusive and extrusive textures explains the appearance and crystallinity of many igneous rocks.

Metamorphic Rocks: Pressure, Temperature, and Texture

• Metamorphic rocks are rocks that have been altered at the molecular level due to exposure to pressure and/or temperature.
• The extent of metamorphism is described in terms of grade:
  • Low-grade metamorphism involves relatively modest changes.
  • High-grade metamorphism involves more dramatic mineralogical and structural changes.
• Metamorphism can occur via different settings:
  • Contact metamorphism (low-intensity): changes to rocks surrounding an intrusive magma body due to heat transfer from the magma.
  • Regional metamorphism (high-intensity): widespread metamorphism over large areas, typically associated with deep burial under thick sequence of overlying rocks, with substantial pressures.
• General characteristic: metamorphism tends to produce new textures and structures (e.g., foliation or banding) due to alignment of minerals under directed pressure.
• The textual description emphasizes that regional metamorphism occurs under very high pressures (described as tens to hundreds of thousands of feet of overburden), producing harder rocks than the original.
• The narrative also notes a banding effect in metamorphic rocks caused by the stretching and alignment of mineral crystals in response to directional pressure.

The Rock Cycle, Weathering, and Erosion: Dynamic Earth

• The rock cycle connects igneous, sedimentary, and metamorphic processes through time, driven by weathering, erosion, transport, deposition, burial, heating, and deformation.
• Isostatic adjustment and glacial processes influence landscape evolution:
  • Isostatic adjustment: crustal responses to loading/unloading (e.g., mountain-building and erosion can alter vertical crustal position).
  • Ice sheets grow and decay (glaciation) influencing deposition and erosion patterns.
• Erosion and deposition feed sedimentary cycles, while tectonic processes rework rocks into metamorphic and igneous forms, closing the cycle.

Plate Tectonics and the Wilson Cycle

• Plate movements create and destroy ocean basins through three main boundary types:
  • Divergent boundaries: plates move apart; mantle upwelling creates new oceanic crust and seafloor spreading.
  • Convergent boundaries: plates move toward each other; oceanic crust can subduct beneath other plates.
  • Transform boundaries: plates slide past one another laterally.
• Wilson cycle: a long-term cycle describing the formation and destruction of oceans and continents over extremely long timescales (roughly on the order of hundreds of millions to around a billion years).
  • Stage 1: Divergence and creation of a young ocean basin, with new seafloor forming and subsidence allowing basin development.
  • Stage 2: Continued spreading leads to a mature ocean basin; sea-level accommodation supports broad shallow seas and accumulation of oceanic crust.
  • Stage 3: Subduction begins along margins of the ocean basin, consuming oceanic crust as it is pushed back into the mantle.
  • Stage 4: Ocean basin becomes smaller and eventually is destroyed; continents collide and mountains are formed, leading to continental crust assembly and the creation of a supercontinent.
  • Stage 5: Subduction-related processes and mantle convection eventually drive rifting in the supercontinent, creating new divergent boundaries and initiating the formation of a new ocean basin.
• The cycle illustrates how oceans form, expand, close, and vanish, driven by plate tectonics and mantle dynamics.
• A key example discussed is the most recent supercontinent, Pangaea:
  • The prefix "Pan-" means all; hence Pangaea means all Earth’s lands joined together.

Connections and Implications

• The material connects geochemical processes (precipitation of CaCO₃, peat formation) with large-scale tectonics (Wilson cycle) and long-term planetary evolution (formation and breakup of supercontinents).
• Implications span environmental science (sedimentation and soil acidity), resource geology (visibility of ore-forming processes in igneous and metamorphic settings), and planetary evolution (cycle of continents and oceans).
• Ethical and practical considerations arise in the interpretation of rock records for natural resources, environmental change, and hazards associated with tectonic activity (e.g., volcanic activity linked to magma movement and formation of dikes/sills).