GEO1111 Winter 2023 Lecture 4/5: Earth Materials II, III

Types of Rocks

• Igneous Rocks: Formed through the solidification of magma.
• Sedimentary Rocks: Comprised of particles that settle and accumulate in layers.
• Metamorphic Rocks: Result from the alteration of existing rocks due to temperature, pressure, and chemical processes without melting.

What is a Sedimentary Rock?

• Definition: Sedimentary rocks are formed from sediments which are rock fragments that settle and accumulate after being transported or precipitated.
• Etymology: The term is derived from the Latin "sedimentum" meaning "to settle".
• Origin: Result from mechanical and chemical breakdown of pre-existing rocks.
• Statistics: Comprise 5% by volume of the Earth's outer 15 km and 75% of exposed rocks.
• Significance: Contain evidence of past environments, including tectonics, climate, and life (fossils).
• Classification: Based on the source of the material:
  • Detrital/Clastic Rocks: Composed of transported sediment as solid particles.
  • Chemical Rocks: Formed when sediments precipitate from solutions or organic activity (like shells/skeletons).

Formation Process of Sedimentary Rocks

1. Weathering: Breakdown of rocks into smaller particles.
2. Erosion (Mass Wasting): Movement of these particles.
3. Transport: Movement of sediments to other locations.
4. Deposition: Settling of these sediments in layers.
5. Lithification: Process involving compaction and cementation of sediments into solid rock.
   • Physical Features: These features indicate how sediments were deposited and the environmental conditions during deposition.

Weathering

• Physical Weathering: Physical breakdown of rocks.
• Chemical Weathering: Alteration of minerals in rocks due to chemical reactions (e.g., oxidation).
• Factors Influencing Weathering:
  • Parent Rock: Composition (e.g., granite vs. salt).
  • Climate: Temperature and rainfall influence breakdown rates (e.g., deserts vs. rainforests).
  • Vegetation: Presence of soil and plant life impacts weathering rates.
  • Time: The longer the exposure, the more weathering may occur.

Types of Sedimentary Rocks

• Classes:
  • Chemical Rocks: Form from the crystallization of minerals from water (e.g., limestone, halite).
  • Organic Rocks: Composed of organic material (e.g., coal from plant decay).
  • Biochemical Rocks: Shells and skeletons of organisms cemented together.
  • Clastic Rocks: Comprised of broken pieces of other rocks and minerals, categorized by size:
  • Gravel: >2 mm (used in conglomerates).
  • Sand: 1/16-2 mm (sandstone).
  • Mud: <1/16 mm (shale).

Characteristics of Clastic Sedimentary Rocks

• Chief Constituents:
  • Common minerals: quartz, feldspar, micas, clay minerals.
• Texture: Reflects the process of deposition, influenced by:
  • Grain Size: Related to energy levels of transport medium; smaller grains often deposited last.
  • Grain Sorting: Refers to how different sizes are distributed in sediment; well-sorted vs. poorly sorted.
  • Grain Shape: Angular vs. rounded shapes depend on transport distance and energy.

Common Sedimentary Rock Types

• Mudstone: Composed of clay to silt-sized particles; deposited in calm water (e.g., lakes).
• Sandstone: Made of sand-sized particles, mostly quartz; forms in various environments (rivers, beaches).
• Conglomerate: Contains rounded gravel-sized particles in a finer matrix.
• Breccia: Similar to conglomerate but with angular fragments.

Economic Considerations of Sedimentary Rocks

• Resources:
  • Energy resources: coal and petroleum.
  • Mineral sources: iron, aluminum, manganese.
  • Salts and phosphates for fertilizers.
• Scientific Importance: Fossils and geological history reconstruction.

Introduction to Metamorphic Rocks

• Definition: Metamorphic rocks are formed from existing rocks (igneous, sedimentary, or other metamorphic rocks) that undergo changes in their texture, mineralogy, and chemical composition via heat, pressure, and fluids without melting.
• Metamorphism Process: Involves temperature and pressure that break atomic bonds and alter minerals.

Factors Influencing Metamorphism

1. Temperature: Increases in temperature lead to recrystallization and formation of stable minerals.
2. Pressure: Two types of pressure:
   • Lithostatic (Confining): Equal pressure from all directions, makes rocks denser.
   • Directed (Differential): Pressure in one direction, causes foliation (layering).
3. Fluid Activity: Presence of water (H2O) and gases (CO2) enhances metamorphic reactions and can lead to hydrothermal alteration.

Metamorphic Environments

• Types:
  • Regional Metamorphism: Occurs over large areas due to tectonic forces, producing foliated rocks.
  • Contact Metamorphism: Occurs adjacent to igneous intrusions due to heat from magma.
  • Hydrothermal Metamorphism: Caused by hot, ion-rich fluids circulating through rocks; typically occurs at mid-ocean ridges.

Classification of Metamorphic Rocks

• Texture-Based Classification:
  • Foliated: Contains aligned minerals, often seen in schist and gneiss.
  • Non-foliated: Lack of layering; composed of interlocking crystals (e.g., marble and quartzite).

Types of Foliated Rocks

• Slate: Very fine-grained, derived from shale.
• Phyllite: Slightly higher metamorphic grade than slate with a glossy sheen.
• Schist: Characterized by larger, visible minerals and enhanced schistosity due to increased metamorphism.
• Gneiss: A high-grade rock, characterized by alternating light and dark bands.
• Migmatite: Formed at very high temperatures where partial melting occurs.

Rock Cycle Summary

• Igneous Rocks: Formed by crystallization.
• Sedimentary Rocks: Formed through weathering, erosion, and lithification.
• Metamorphic Rocks: Created through metamorphism under heat and pressure.
• Cycle: Rocks can transition between these forms due to geological processes.