Igneous Rocks

Igneous Rocks

1. Types

   • igneous rocks are crystalline rocks formed by the cooling and solidification of magma, they normally compose of interlocking crystals in random orientation
   • intrusive
     • the magma cools and solidifies within the eath amd ususally form large crystals due to slow cooling
   • extrusive
     • rock forms due to rapid cooling at the earth's surface normally a more fine grained texture

2. Composition

   • mafic
     • generally dark in colour, rick in magnesium and iron but poor in silica, they contain large amounts of olivine and pyroxene
     • examples
       • Gabbro
         • coarse grained, formed intrusively, grey with very little quartz
       • Basalt
         • this is a highly abundant igneous rock, it is fine grained which proves that it is an extrusive rock and is black in colour
   • felsic
     • normally light in colour , high in potassium and sodium, meaning that it is rich in silica. There are large amounts of quartz micas and feldspar
     • examples
       • granite
         • coarse grained intrusive, quartz and feldspar
       • rhyolite
         • extrusive igneous so fine grained
   • ultramafic- high temperatures compared to felsic. MAfic more suseptible to weathering

3. Structure

   • jointing is the greatest structural weakness, sometimes joints are filled in by mineral deposits these are called veins

4. Texture

   • Glassy: smooth and crystals
   • Aphanitic: fine grained
   • Phaneritic: coarse grained
   • Porphyritic: large crystals with matrix of smaller crystals
   • Pyroclastic: fragmental appearence