Vesicular and Amygdaloidal Textures

Vesicles are fossil bubbles. So, a rock that looks like Swiss cheese has a vesicular texture. Vesicles range in shape from spherical to elongated and in size from ~1mm to more than 1 cm. Basalts are most commonly vesicular.

The drop in pressure that a magma experiences as it flows from underground to the Earth's surface allows water and gases in the lava to form bubbles. If the bubbles do not get large enough to pop, they are frozen in the lava as vesicles. Amygdaloids are simply vesicles that have been filled in with a secondary mineral long after the flow cooled. Such secondary minerals are commonly white: quartz, calcite, or zeolite. (A secondary mineral is one that formed after the rock originally formed.)

This is a fine example of a vesicular basalt. If you click on the picture to enlarge it, you will see tiny white 'laths' of plagioclase feldspar. This sample is thus vesicular, fine-grained, and porphyritic.
This basalt sample is so full of vesicles that it is more hole than basalt. It is amazing to me how much dissolved water and gas a molten rock can hold.
These are small vesicles, only a millimeter or so across.

The greenish brown crystals are olivine. They should appear olivine green, but it is difficult to get the color of the olivine to photograph properly.

This hand sample of vesicular basalt has olivine phenocrysts. Like the sample at the top, this one is vesicular, fine-grained, and porphyritic. A close-up appears below.
Here some vesicles and olivine phenocrysts may be seen. The white spots are where the sample was scratched. When you collect some special rock to display on your mantle piece, pack it carefully so that it doesn't rub against other rocks or hard objects!

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