Vesicles and Amygdules
Vesicles are commonly found in volcanic rocks– that is, in rocks that solidified from lava. Molten rock– magma and lava– often contains dissolved gases such as water and carbon dioxide. When lava erupts, these gases expand and often are able to escape to the atmosphere as the lava cools and solidifies to form a volcanic rock. Sometimes, particularly in very small, interior vesicles, the gases do not escape and geologists are able to study these gas (or sometimes fluid) inclusions to learn more about the gases and fluids that were in the lava when it erupted. When there are many vesicles in a volcanic rock, this indicates that the lava from which the rock formed was very rich in gases.
Since vesicles are open cavities, they sometimes become filled in with secondary alteration minerals such as calcite, quartz, or zeolite. When vesicles are filled with a secondary alteration mineral, they are called amygdules. Since calcite, quartz, and other secondary minerals are generally light in color compared to the brown or black volcanic rock, amygdules can make a volcanic rock look spotted
When you describe a volcanic rock, you can say that it is “vesciular” if it contains vesicles and that is it “amygdaloidal” if it contains amygdules.
The word vesicle generally applies only to cavities formed in volcanic rocks by gases expanding in lava. The word “vug”– another v word– is another term used to describe cavities in rocks. Vugs can be formed in a variety of ways– such as through faulting, folding, or collapse of a rock– and are often partially filled with secondary minerals.
Vugs (also spelled: vugh, or vugg)
A vug is a small to medium-sized cavity inside rock. It may be formed through a variety of processes. Most commonly, cracks and fissures opened by tectonic activity, which are partially filled by quartz, calcite, and other secondary minerals. Open spaces within ancient collapse breccias are another important source of vugs. Vugs may also form when mineral crystals or fossils inside a rock matrix are later removed through erosion or dissolution processes, leaving behind irregular voids. The inner surfaces of such vugs are often coated with a crystal druse (a coating of fine crystals on a rock fracture surface, vein or within a vug. Fine crystals are often found in vugs where the open space allows the free development of external crystal form. The term vug is not applied to veins and fissures that have become completely filled, but may be applied to any small cavities within such veins.
Geodes
Geodes are a common vug-formed rock, although that term is usually reserved for more rounded crystal-lined cavities in sedimentary rocks and ancient lavas.
Logging Requirements
To confirm your visit to GZ, please send the answers to the following questions to the CO at the top of this cache page.
1). The million dollar question! What type of cavities can be found in the rock at GZ? How do you know - explain?
2) What is the length and width of any one of the larger cavities found in the rocks at GZ.
3) Take a close look at the crystal growth in the cavities at GZ. Describe the colour, shape and texture of the crystals you see. Do the completely fill the cavities, or are they more of a lining.
4) After examining the rocks at GZ, use the cache description to hypothesize how these cavities may have been formed.
5) Last one, I promise. Approximately, how many cavities are found in 1 square foot of the rock at GZ.
6) Now one for fun! What dairy product do these rocks most remind you of?
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