VOLCANIC CURIOSITIES

Volcanic Bombs

These are fragments of liquid and/or plastic state lava, ejected into the air by means of a volcanic explosion (s). As the fragments move through the air, they twist and turn and can form spindle-shaped masses with surface markings. They range in size from a few millimeters to more than a meter in diameter. 

Two volcanic “bombs” from a relatively young volcanic eruption in central Arizona, USA are shown below. This first “bomb,” 6 cm width, is shown in two views, front and back. 

The second "bomb" is 9 cm wide.

Lava Stalactites

These are associated with lava flows. The lava was originally molten liquid matter  (magma). The stalactites shown below formed when the red-hot lava infiltrated cracks in the ceiling of a hollow lava tube and dripped down into the empty space. 

Two stalactites from a young lava-flow tube in central New Mexico, USA are shown.

The first stalactite (front and top views) is 7 cm high and 5 cm wide. The holes visible in the top view of the interior were caused by trapped air bubbles.

  

The other lava stalactite (front view) shown below is 9.5 high and 7 cm wide. If you look carefully at its side, you can see some green surface staining resulting from deposition of the copper mineral malachite.

Xenolith

The word “xenolith” is Greek, meaning a foreign (stranger) or guest rock. It is an inclusion of an older rock in a younger volcanic rock. The older rock is referred to as “country rock” or “wall rock” by geologists.

The following example of a xenolith is from the Mojave Desert in eastern central California. The xenolith is a small piece of granite, which is a plutonic igneous rock that formed inside the Earth. This xenolith has been incorporated as an inclusion into black volcanic rock (basalt lava, which is a volcanic igneous rock that formed from a lava flow or volcanic neck near the surface of the Earth). The age of the xenolith can be very old and indicative or an entirely different geologic temperature and pressure than the surrounding basalt. The whitish xenolith is 4.5 cm high and 2.5 cm wide. 

There is no indication that the temperature of the basalt lava affected the granite xenolith (inclusion) because there is no alteration of the granite where it made contact with the basalt. I have also included below a close-up image of a portion of this contact.