Some Volcanic Rocks
Drop in to see these in person

Volcanic bomb (a small one).  Although smaller than a softball, this raindrop-shaped stone could fracture your skull if you were caught in a shower pyroclastic rain near a volcanic vent.  The dark color is due to mafic (magnesium-iron) minerals - this bomb is probably andesitic, perhaps basaltic.

Why is this volcanic bomb red in color?  This is from a very old (tens of thousands of years old) cinder cone, and the magnetite has oxidized, turning from magnetite (Fe₃O₄) to hematite (Fe₂O₃).  The original streamlined shape is still apparent.  Volcanic bombs leave the volcanic vent molten and solidify during their brief flight, developing this streamlined shape from air friction.

Volcanic bomb just a little bit too big to collect, in a cinder (cone) quarry near Clear Lake, California.  The toddler was almost 1 year old.

Large cinder.  Most cinders are filled with bubbles frozen into place as the melted rock cooled in the atmosphere.

Part of a lava 'icicle' shows flow structure.  Even flowing lava froze so quickly many bubbles were trapped.  Was this a bit of melted rock that leaked from the molten center of a bomb that broke when it hit the surface?  Watch a video of a cinder cone eruption at night and look for flashes of red and yellow from the dark, cold debris when a big pyroclastic hit the surface and either breaks or breaks the cool shell of rocks it crushes.

Solidified volcanic ash is called tuff.  No, tuff is not 'tough' compared to most other rocks, in fact it makes and excellent building stone because it is easily quarried, is stronger than adobe and relative low density.  This sample is rhyolitic.  Some small quartz crystals can be seen when I look at this rock under low magnification, using a geologist's tool called a hand lens.

The Bishop Tuff, a thick deposit near the Long Valley caldera, which erupted about 760,000 years ago.  The central section of this tuff is welded (glass-like) because it was so hot.

This andesitic tuff contains hornblende, needle-shaped black amphibole minerals.

Basalt is dense and black.  Andesite is often as black as basalt so sometimes it takes a thin section (thin polished wafer of rock) and microscope to discriminate between basalt and andesite.  This specimen has green olivine crystals, a diagnostic characteristic of basalt.

A different perspective of this basalt sample.

Obsidian can be black but this volcanic glass may well be rhyolitic (rich in silica).  Lava cooled so quickly that true mineral crystals did not get a chance to form from the entire melt.  The circular concoidal fracture is a diagnostic characteristic of glass.

Volcanic breccia.  Solid rock was shattered and the pieces (light gray) cemented together by hot ash (pinkish material).  Pinnacles National Monument contains mega-breccia: volcanic breccia with clasts (shattered fragments) as big as a house, these clasts themselves composed of breccia.

Porphyry is a volcanic rock characterized by a bi-modal crystal size.  Apparently crystals were leisurely forming in an underground magma chamber when an eruption occurred, cooling the mix of molten rock and crystals so quickly that the molten part crystallized without having time to organize into big crystals. 

Pumice - volcanic glass foam, the rock that floats. Bubble-filled melted rock foam cooled and formed a solid bubble-filled stone.  This sample was collected from the Gulf of Fonseca in 1971.  It floats.  Pumice is fragile, often crushed if buried by ash and broken down by abrasion if washed up and down a beach.  Before the invention of the inexpensive disposable razor, female students at the teachers' college were I was working used pumice to shave their legs.  Pumice is crushed for abrasive in soap (Lava Soap) and small pieces are used for cosmetic purposes (exfoliation, reducing calluses).  Some pumice is too dense to float.

Pumice floating in a pan of water.  Check out the video.

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