.
Questions and phone orders: 10 AM - 10 PM Pacific (760) 920-6465
Sold Out

basalt - hand/display specimen of typical non-vesicular basalt

$ 9.50

Back in stock soon. Inquire.

This basalt is from a Tertiary flow east of the summit of Montgomery Pass in Mineral County, Nevada. It lives up to its description as being aphanitic, meaning the crystals are not visible to the naked eye. The roots are a, Greek for not, and phaneros, Greek for visible.

Basalt is rich in magnesium and iron, poor in silica, allowing flows to move and cool rapidly. Silica makes lava sticky, so unlike basalt flows, an andesite magma, with over 50% silica, is reluctant to flow and plugs up volcanic vents, leading to explosive eruptions, in contrast to the basaltic lavas of Hawaii, where tourists can approach a vent with considerably less risk.

The rapid cooling of this basalt makes the crystals of what are most likely plagioclase, pyroxine and olivine, too small to see. Aphanitic, somewhat dense, and black or dark gray are the characters a student would use to make the identification. 

Many basalt flows are vesicular, with large gas bubbles that were trapped when the lava cooled. We occasionally have examples in stock.

As you can see from the QAPF diagram, the compositions of basalt and andesite can overlap and both are variable within the yellow polygon in the diagram, though andesite would be closer to the quartz side of the polygon. Their similarity can cause a student to identify andesite as basalt, acceptable, since accurate identifications are best made with thing sections and a petrographic microscope, allowing the percentage of quartz to be measured.

 

 

 

Select a specimen: When more than one specimen is shown, you can select a particular specimen by telling us what is in the photo with it, a black and silver pen, a black mechanical pencil, or one of those plus some number of coins, or you can let us make the selection.

Making multiple purchases? Click on the "combine shipping" button in the shopping cart. We'll send an invoice with combined shipping. A link in that invoice will bring you back to checkout, no hassle.

Shipping: By Priority Mail or USPS Ground Advantage, whichever is cheaper, unless we find a better rate. Click > here < for shipping rates. Use back button to return to this page.