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.