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

Compositions of and igneous rocks

Felsic/Silicic = contains 66-76% silica

(SiO2) = Rich in “Fel”dspar and “Sil”ica Intermediate = between felsic and (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 (consolidated ) • 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 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 ” Textures of igneous rocks:

Glassy, felsic that contains abundant vesicles Textures of igneous rocks: Scoria

Glassy, mafic volcanic rock that contains abundant vesicles Textures of igneous rocks: Pillow

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