Lab 3 – Igneous Rocks
How the Earth Works Spring 2019
http://www.swisseduc.ch/stromboli/perm/iceland/eyafallajokull_20100414-en.html?id=1 How do we classify igneous rocks?
• Composition – Constituent minerals • Texture – Size, shape, and arrangement of minerals
• Give clues as to the geological setting and cooling history of the magma
Compositions of magmas and igneous rocks
Felsic/Silicic = contains 66-76% silica
(SiO2) = Rich in “Fel”dspar and “Sil”ica Intermediate = between felsic and mafic (52-66% silica)
Mafic = silica deficient (42-52% silica) =Rich in “Ma”gnesium and Iron “Ferric” Ultramafic = contains only 38-48% silica Textures of igneous rocks
Fine-grained: cooled fast Extrusive, or Volcanic = Aphanitic
Coarse-grained: cooled slowly Intrusive, or Plutonic = Phaneritic Pyroclastic rocks • Greek: pyro- = fire & -clastic = broken • Volcanic rocks that are transported by wind or water • Examples of these are tuff (consolidated volcanic ash) • VIDEO
http://mail.colonial.net/~hkaiter/volcanoes.html Pyroclastic rocks • Extremely “friable”- crumbly texture Textures of igneous rocks: Porphyritic Two-stage cooling: Cooled slowly initially - formed large crystals (phenocrysts) Then cooled rapidly – formed a finer matrix (groundmass)
Phenocrysts
Groundmass Textures of igneous rocks: Volcanic glass
Cooled too rapidly for a crystalline network to develop
Obsidian – glass, felsic volcanic rock Can identify by conchoidal fracture Textures of igneous rocks: Vesicular
A “vesicle” is fluid- or air-filled cavity or sac
The image shows a “vesicular basalt” Textures of igneous rocks: Pumice
Glassy, felsic volcanic rock that contains abundant vesicles Textures of igneous rocks: Scoria
Glassy, mafic volcanic rock that contains abundant vesicles Textures of igneous rocks: Pillow basalts
Youtube videos of pillow formation: http://www.youtube.com/watch?v=5G8W9sZtBTM http://www.youtube.com/watch?v=DdIUuUY0L9c&feature=related
http://academic.cuesta.edu/physci/Geology/pillow_lava.htm http://mail.colonial.net/~hkaiter/volcanoes.html Textures of igneous rocks: Columnar jointing Other textures of igneous rocks
• Pāhoehoe—ropey • A’a—rough blocky texture • Bombs—blobs of magma ejected from a volcano