IS SEDIMENTARY ROCKS

Types of Rocks

Sedimentary Rocks

  • Presenter: Robinson S. Luzon
  • Course: IS 1
  • Institution: PSHS-ZRC

OBJECTIVE

  • To describe the different categories of sedimentary rocks.

Introduction

  • Scenario:
    • Imagine a rock on a hill.
    • Wind causes the rock to roll down, breaking it into two large pieces.
    • Rain washes these pieces away, leading to further deterioration as they tumble and bump into larger rocks.
    • Eventually, these pieces are deposited in a pit with other materials, eventually sticking together when dried by sunlight.
  • Definition of Sedimentary Rocks:
    • Formed from pre-existing rock and other materials (minerals, remains of living things).
    • Bits of these materials are called sediments.
    • The term sedimentary comes from the Latin word sedimentum, meaning "settling".
    • Sediments settle, compact, and cement to form rocks.
    • Similar to igneous rocks, sedimentary rocks are classified based on the origin of the sediments.

Detrital Sedimentary Rocks

  • Definition:
    • Formed from solid particles or dissolved materials.
    • Specifically formed from fragments of weathered rocks called detritus (from Latin deterere, meaning "wear away").
    • Composed mainly of clay minerals and quartz.
Particle Size Classification of Detrital Sedimentary Rocks
  • Importance:
    • Geologists differentiate among detrital sedimentary rocks based on particle size.
  • Classification Table:
    • Coarse (over 2 mm):
    • Sediment Name: Gravel
      • Rock Name: Varieties of gravel, rounded or angular particles indicate particle origin.
    • Medium (1/16 to 2 mm): Sand
    • Description of the texture and common formations.
    • Fine (1/16 to 1/256 mm): Silt
    • Very Fine (less than 1/256 mm): Clay
  • Gravel Characteristics:
    • Angular particles indicate proximity to the source of sediment.
Composition of Detrital Sedimentary Rocks
  • Naming Conventions:
    • Rock names can also reflect mineral composition alongside particle size.
    • Example: Quartz Sandstone (rich in quartz).
    • Mixed particle size rocks classified depending on the dominant size, e.g., Sandy Siltstone or Silty Sandstone.

Chemical and Biochemical Sedimentary Rocks

  • Formation Process:
    • Formed from materials transported to water bodies (lakes, seas).
    • Dissolved materials precipitate and settle out forming chemical sediments.
  • Example of Precipitation:
    • Leaving a salt solution to dry results in salt crystallization.
  • Biochemical Sediments:
    • Created from dissolved minerals extracted by water-dwelling organisms to form shells or hard parts.
    • Upon death, their remains gather on the ocean or lake floors.
  • Example of Rock Type:
    • Limestone: Composed of approximately 90% biochemical sediments, rest being chemical sediments from seawater.
Examples of Chemical and Biochemical Sedimentary Rocks
  • Limestone Variants:
    • Composition: Calcite, Calcium Carbonate (CaCO3)
    • Texture Types:
    • Non-clastic:
      • Fine to coarse crystalline.
      • Microcrystalline calcite.
    • Clastic:
      • Visible shells/shell fragments loosely cemented or various sizes shells cemented with calcite.
  • Additional Types of Rocks:
    • Quartz (SiO2): Very fine crystals, non-clastic.
    • Gypsum (CaSO4 · 2H2O): Non-clastic, fine to coarse crystalline.
    • Halite (NaCl): Non-clastic, fine to coarse crystalline.

Organic Sedimentary Rocks

  • Definition of Coal:
    • Organic sedimentary rock composed mostly of organic matter.
    • Different from charcoal used in grilling.
    • Forms from the compaction of a large mass of plants over extended periods.
Formation of Coal
  • Process of Formation:
    • Giant plants died in swamps before dinosaurs and were buried by water and dirt for millions of years.
    • Illustration depicts:
      • 100 million years ago: Plant decomposition under layers of dirt and water leads to coal formation.
    • Stages of Coal Formation:
    • Peat: Not yet coal, partially decayed plant matter in swamps/bogs; low heat content.
    • Lignite: Brown coal, low sulfur, limited supplies.
    • Bituminous: High heat content, commonly used, often high in sulfur.
    • Anthracite: Hard coal, highly desirable for its high heat and low sulfur content; limited supplies in most areas.

Features of Sedimentary Rocks

  • Importance in Earth History:
    • Sedimentary rocks illustrate Earth's historical environment through their layered structures known as strata or beds.
    • Strata Thickness: Ranges from extremely thin layers to very thick formations.
  • Fossils:
    • Remains/traces of prehistoric organisms found in sedimentary rocks.
    • Help geologists deduce the environmental conditions during the rock's formation:
    • Identifying whether strata were once terrestrial or aquatic.
    • Understanding the historical climate conditions.

Summary

  • General Definition:
    • Sedimentary rocks originate from sediments, which may include solid fragments from weathered rocks (detrital), precipitates from dissolved materials, or remains of organic matter.
  • Rock Types Distinction:
    • Detrital sedimentary rocks can be identified by particle size.
    • Chemical and biochemical sedimentary rocks are characterized both by particle size and composition.
  • Coal's Unique Origin:
    • Formed from extensive plant material buried and compacted over long periods.