Sedimentary Rocks Lecture Notes

Introduction

• Focus of the lecture: Sedimentary rocks, the second variety of rocks in the rocks chapter.
• Classification of rocks based on their formation:
  • Igneous rocks: Form from cooling and solidification of magma/lava.
  • Sedimentary rocks: Form from compaction and cementation of sediments.

Formation of Sedimentary Rocks

• Compaction: Squeezing together of sediments.
• Cementation: Gluing or sticking together of sediments.
• Sediments: Fragments of rock broken down by weathering (e.g., wind, ice, running water).
• Example: A boulder weathering over 10,000 years into smaller pieces.

Types of Sediment Sizes

• Six categories of sediment sizes:
  1. Clay
  2. Silt
  3. Sand
  4. Pebbles
  5. Cobbles
  6. Boulders
• Sand size: 0.006 cm - 0.2 cm.
• Boulder size: larger than 25.6 cm.
• Reference: Earth science reference tables (page 6).

Clastic Sedimentary Rocks

• Definition: Formed from compacted sediments (clasts).
• Classified by size and composed of inorganic materials.
• Key Types:
  • Conglomerate: Composed of mixed sizes of rounded sediments (e.g., cobbles, pebbles, sand).
  • Breccia: Composed of mixed sizes of angular sediments (not rounded).
  • Sandstone: Made of compacted sand grains.
  • Siltstone: Made of compacted silt.
  • Shale: Made of compacted clay particles.

Types of Sedimentary Rocks: Organic and Crystalline

• Crystalline Sedimentary Rocks: Form from chemical processes, can be evaporites or precipitates.

  • Examples:
    • Rock Salt: Forms from evaporated seawater (halite).
    • Rock Gypsum: Formed similarly to rock salt.
    • Dolostone: Formed through similar processes.
  • Precipitate: Forms when dissolved minerals crystallize out of solution (not evaporation).

• Organic (Bioclastic) Sedimentary Rocks: Form from compacted organic material.

  • Examples:
    • Coal: From compacted plant remains.
    • Coquina: A variety of limestone made of compacted shells.

Fossils and Sedimentary Rocks

• Only type of rock that may contain fossils.
• Fossils preserved due to lower temperatures and pressures in formation compared to igneous and metamorphic rocks, which destroy fossils.

Conclusion

• Importance of understanding sedimentary rocks and their formation processes.
• Reference to further studies in school.