August 29, 2003

 Rock Identification: Igneous and Clastic Sedimentary

 

 

1. Types of rocks

 

  a) Igneous – formed from cooling magma or lava

   

    1. Random Crystalline texture

 

  b) Sedimentary – formed from accumulation and consolidation of material from existing rocks

     

    1. Clastic texture

 

   c) Metamorphic – formed from thermal or mineralogical alteration of existing rocks

    

     1. Foliated crystalline texture

 

2.  Igneous Rocks

 

a)      Magma vs. Lava

1.      Magma is formed by partial melting of the upper mantle at convergent and divergent plate boundaries

 

b)      Classification of Igneous Rocks

 

 1. Where Formed

 

            a) Intrusive (Plutonic)

                         Granite

                          Diorite

                          Gabbro

                          Peridotite

                          Dunite

Intrusive bodies Figure 5.1 p. 77

a) Batholith

b) Stock

c) Dike

d) Sill

 

b) Extrusive (Volcanic)

    Rhyolite

    Andesite
                Basalt
               Obsidian

    Pumice

    Scoria

 

     2. Mineral Composition

 

a) Felsic – generally light colored

b) Mafic – generally dark colored

c) Intermediate

 

Crystallization of Magmas

 

a)      Bowen’s Reaction Series

b)      Discontinuous vs. Continuous

 

    3. Texture – Determined by cooling rate

 

       Phaneritic – can be observed w/o aid of microscope

      

       Aphanitic – less than 0.062mm

 

       Porphyritic

       

        a) Phenocrysts – larger crystals

        b) Matrix or groundmass – smaller crystalline background

 

 

            Phaneritic                   Porphyritic                  Aphanitic

         

            Granite                         Granite

            Diorite                          Diorite 

            Gabbro                        Gabbro (rarely)

            Dunite                          Rhyolite (can)               Rhyolite

                                                Andesite (can)              Andesite

                                                Basalt (common)          Basalt       

 

4. Special Igneous rock Types

          

            a) Vesicular – scoria, pumice

b) Glassy – obsidian

c) Pyroclastic

 

 

 

Xenoliths

 

 

    

Sedimentary rocks (Clastic)

 

Three Types p. 98

 a) Detrital Sediment – Non Organic

 b) Biochemical Sediment - Organic

 c) Chemical Sediment - Precipitation

 

Depositional environments Fig. 6.12 p. 110

 

 

Textural Features p. 95

 a) Grain size – tells something about the transport history of the sediments

 

                                    Gravel size        >2mm (boulder, cobble, pebble-sized)

                                    Sand Size         2mm – 1/16 mm

                                    Silt Size            1/16 mm – 1/256 mm

                                    Clay Size          < 1/256 mm

 

 b) Grain Rounding

            Very Angular to Well Rounded

 

 c) Sorting

            Poorly to Well Sorted

 d) Composition

              Can deduce the source of the sediments

e) Color     

  1. Red – Iron rich oxidizing environment

              2. Gray to Black – reducing environments

f) Cement

1.      Chemically precipitated

2.      Commonly silica, calcite, dolomite, or iron oxide