9 comments on “Basaltic…? Felsic…? Porphyritic…? Confusion!

  1. Great tutorial. Learn something every day. I had thought that obsidian was slowly cooling rather than quickly cooling rhyolitic magma. The gabbro looks a lot like various granites I have come across over the years. Way, way cool. Thank you for the continuing education. Cheers –


  2. I think a “simple” glossary would help! Once it gets beyond the basic classification of silica weight percentage (carbonatite, komatiite, basalt, basaltic andesite, andesite, dacite, rhyodacite, rhyolite) I’m hopeless! 😀


    • Hi MrMJF, with “carbonatite, komatiite, basalt…etc.” you know quite a lot already! Beyond those, i.e. further classification of a particular type, you’d either need to have professional geological knowledge with year-long field experience or to know your particular mountain like the back of your hand. Division could be by amount of main constituent minerals, by type of accessory minerals, by amount of accessory minerals etc. Just see this “enlightening” description (from the German Wiki page for Andesite):

      “Andesites consist of 0 to 20% quartz and plagioclase feldspar, pyroxene, amphibole and biotite. If quartz is absent, Andesites contain Foids and are called Foid-bearing Andesite. There are also garnet-bearing and cordierite-bearing andesites; They are called [just that]. If hornblende is present, they are called hornblende andesites. In the presence of olivine, andesite has no quartz (olivine forms only in a silica-undersaturated environment); If Andesites occur close to basalts, they usually have neither amphibole nor biotite, and the proportion of plagioclase is low…”

      I found this “Igneous Rock Identification Chart” and put it on our Basics pages, maybe this could be helpful for first ID-ing in the field:


  3. Wow, Камбальный – Kambalny!? KVERT: Eruption of the southernmost volcano of Kamchatka on March 25 was a complete surprise to volcanologists; 5-6 km high ash column stretched a few hundred km SW. No earthquakes or steam emissions had been observed beforehand. According to tephrachronology the last serious eruption was 600 years ago, a period of increased activity happened abt. 250 years ago. There is a very active hydrothermal area on the slopes of the volcano. Aviation code: Orange

    From: http://www.kronoki.ru/news/1187


  4. Also on 25 March: Vulcanian explosion from Sakurajima/Minamidake crater. The explosion seems to have come from the Minamidake summit crater and was relatively powerful; it appears to have produced small pyroclastic flows but only a relatively low ash plume (JMA indicates 500 m, but this is probably underestimated).
    The volcano continued to vent ash (strombolian activity ?) for several hours following the initial eruption, then quieted down, but had a second eruption at 22:28 local time, with an eruption cloud rising 1400 m.
    (from volcanodiscovery.com)


    • Hi Phil, thank you for your kind encouragement! Re your comment on the “Basidian” (or was it Obsalt? 😉 ) – do you have by any chance a picture of your work to share? I used to have a lapidary workshop (working mostly with rocks), so naturally, I am interested to see what others do.

      This is the original of my avatar, I made it abt. 10 yrs ago from a beach rock:


  5. Pingback: End-Permian Extinction Due to Contact Metamorphism? | Watts Up With That?

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