Volcanism and Extrusive Rocks

Welcome to Poorna Pal's 'Earth Revealed' telecourse (Geol 101: Physical Geology; section #7695) at the Glendale Community College

  • Volcanic Rocks

Home | Quizzes  |  Module 1 | Module 2 | Module 3 | Module 4 | Module 5           05/05/15


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How Igneous Rocks Form?






Click on the above image to access this online edition of Robert Tilling's USGS publication "Volcanoes"



Visit the URL:
for the animation of igneous rocks shown above and for more information on the igneous rocks
  • Large Igneous Provinces

or LIPs are voluminous emplacements of predominantly mafic extrusive and intrusive rock whose origins lie in processes other than 'normal' seafloor spreading. LIPs include continental flood basalts and associated intrusive rocks, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups, and ocean basin flood basalts. Click on this map of LIPs, shown below, to read about the research on LIPs.

  • Notable LIPs on Land

    The most notable of continental flood basalts and flood basalt provinces are:


    • Columbia River Basalts (~15 Ma),

    • Ethiopian Traps (~38 Na),

    • Deccan Traps (~65 Ma),

    • Parana/Serra Geral lavas (~135 Ma),

    • Karoo/Stromberg, Patagonian and Ferrar lavas (~180 Ma),

    • Siberian Traps (~250 Ma), and

    • Keweenawan Lavas (~1,250 Ma).


Explore Volcano World at the URL: http://volcano.und.nodak.edu/vw.html. Also available on-line,at http://adsbit.harvard.edu/books/bvtp/toc.html is the treatise: Basaltic Volcanism on the Terrestrial Planets. For extra-terrestrial volcanism, try the URL: http://pubs.usgs.gov/gip/volc/extraterrestrial.html

  • Other Kinds of Volcanism:

    • Spreading Submarine Ridges:

      Spreading submarine ridges and rises (e.g., Reykjanes/ Mid-Atlantic Ridge, East Pacific Rise, hydrothermal vents) and associated volcanic islands (e.g., Iceland).

    • Volcanism at the Subduction Zones

      Convergent plate margins  have volcanism towards the edge of the plate that is being subducted (e.g., Cascade Ranges and the Pacific Ring of Fire)..


    • 'Hot Spots' and Aseismic Ridges:


      Intra-plate or hot-spot volcanism (e.g., Hawaii-Emperor Seamounts, Yellowstone-Snake River volcanics)

The sketch alongside, taken from the URL: http://vulcan.wr.usgs.gov/Glossary/Plate
explains the entire cycle of volcanism from Juan de Fuca Ridge to Juan de Fuca subduction (i.e., the Filled Trench) and the Cascades.


  • Volcanic materials/products and rock classification:

    • Volcanic Rocks:

      Increasing order of silica (SiO2) content,
      from silica-rich rhyolite at one end and the more common basalt at the other, with the compositionally intermediate andesite denoting crustal contamination of the
      magma on its passage through the crust. 

Why andesites on Mars, then?

  • Other products :


    • Cinder cones/ Pyroclastics (Ash, Cinders, Blocks/Bombs, Lahars)

    • Gases and Gas Clouds: Nuees Ardentes, Toxics, Climate Change

    • Other materials: Pyroclasts, Volcanic Breccia


  • Textures of Volcanic Rocks:
    • Glassy (Obsidian),
    • Vesicular (Scoria, Pumice),
    • Aphanitic (Andesite, Basalt, Rhyolite),
    • Porphyritic (Andesite Porphyry, Basalt Porphyry, and Rhyolite Porphyry).

Visit the USGS volcanoe sites, starting with http://volcanoes.usgs.gov/ and its links

Also try the site: http://interactive2.usgs.gov/learningweb/explorer/topic_rocks_igneous.htm




  • Volcanic Precursors and/or Predictors?
    • Seismicity (Harmonic Tremors?)
    • Bulging or Uptilting?
    • Exotic Gas Emanations?

Volcanism and Climate

Try http://www.etl.noaa.gov/about/review/aq/post for a comparison of the climate effects of Mt. Pinatubo and El Chichon events.



American Geophysical Union has an interesting discussion of terrestrial degassing at the URL: http://www.agu.org/sci_soc/eosvarekamp.html

Also try the URL: http://unr.edu/homepage/fbiondi/BiondiFessenden1999E.pdf