Basalt. It’s the fine grained igneous rock that makes up the sea floor and sometimes spews forth from volcanoes on the continents.  End of story right?   Wrong!   That black rock sample I remember sitting in my classroom draw was interesting but it was about to become a whole lot more interesting when I first visited Hawaii back in the 1980’s.

Basalt is quite a common rock on Earth.  It makes up all the ocean floors as well as some huge areas of some of the continents.  It is a dark fine grained rock made up of the minerals olivine, pyroxene and plagioclase feldspars (notice…no quartz).  Almost always these minerals are too fine grained to be seen with the naked eye.  However sometimes Olivine can be seen as small sand-sized grains of olive-green glass in the specimen.

Olivine basalt

 

At other times larger crystals of plagioclase feldspar can be seen.

Plag in basalt

 

But on an active basaltic volcano, basalt can look very different from what most of us have seen in our classroom samples.  Basalt can be glassy black with amazing irredescence.  It can be bright red.  It can also be rough chocolate brown.  Basalt can be solid or full of gass bubbles.  It can be little tear shaped pieces (called Pele’s Tears) or thin drawn out glass fibers (called Pele’s Hair).  It can look like rope or even intestines!

basalt pahoehoe

Ropey pahoehoe basalt flow

 

IMG_5526

Dark grey pahoehoe flow over chocolate brown aa flow – all basalt of approximately the same age.

 

Basalt cinder

Basalt cinder from a fire fountain

 

red basalt

Red colors of basalt from close to vent

 

Reticulite

A delicate form of basalt froth – Reticulite

 


 

All this variation is due to the way the basalt has been erupted onto the Earth’s surface.  So while it is all the same rock type chemically, it’s appearance can be so varied.

 

Want to discover all this and more about basalt?  Why not join us on our Discover Hawaii Adventure.

 

The strange thing about Basalt.

5 thoughts on “The strange thing about Basalt.

  • November 7, 2015 at 11:11 am
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    Nice overview. However, I would not include amphibole among the typical minerals in basalts. That would be more typical of andesites. And, aside from mineralogy, much of this applies to the other major types of volcanic rock. 🙂

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    • November 7, 2015 at 5:28 pm
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      Thanks Bruce. I will recheck my text and adjust. I really would like it to be accurate for teachers.

      Reply
  • November 7, 2015 at 12:59 pm
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    The second photo looks more like rhyolite instead of basalt. Rhyolite would be more likely with those nice plagioclase phenocrysts. Where are those rocks from?

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    • November 7, 2015 at 1:02 pm
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      I agree they don’t look like a typical basalt, but they all are. They are all from the Big Island, Hawaii.

      Reply
  • April 25, 2017 at 11:51 am
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    Hi I live in forest falls Ca. I have found 8 peaces of basalt. All exhibit magnetic property with my compass. Under the microscope I have found beatful brown glass beads and crystal formations. My question is Where did it come from as I can’t find any evidence of flow and there are no near by valcanos. Any ideas or has anyone near me found any samples

    Reply

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