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Holes in Rocks

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Openings of all kinds are found in all kinds of rocks. Here are the most important types of holes in geology (natural ones, not the holes that geologists make). Sometimes a hole qualifies for more than one name, so be careful with your observations.

1. Druse

Druses are small cavities that are lined with crystals of the same minerals that are found in the host rock. "Druse" may also refer to a surface carpeted with crystals, one with a drusy texture. The word is from German.

2. Geode

Geodes are small to medium-sized cavities, typically found in limestone or shale beds. They are usually lined with at least a thin layer of chalcedony, and they often have a drusy lining of quartz or calcite crystals. More rarely, the drusy lining is other carbonate or sulfate minerals. Geodes are capable of weathering out of the rock as discrete concretions or nodules.

3. Lithophysa

Lithophysae are found in high-silica lavas like rhyolite and obsidian: they are round hollows lined or filled with feldspar or quartz in concentric layers. It's not always clear whether to consider them bubbles or droplets (spherulites), but if they empty out they are clearly holes. The name is Latin, meaning "rock bubble"; lithophysa is the singular and lithophysae is the plural.

4. Miarolitic cavity

This is a special type of small cavity found in coarse-grained igneous rocks like granite, especially in late-stage settings such as pegmatites. Miarolitic cavities feature crystals of the same minerals as the rest of the rock (the groundmass) protruding into them. The name comes from the Italian miarolo, the local dialect name of the granite near Lago Maggiore whose crystal-lined pockets were once famous among mineral collectors.

5. Mold

Molds are the openings left behind when minerals dissolve or when dead organisms decay. The material that subsequently fills a mold is a cast. Fossils are the most common kind of cast, and casts of easily dissolved minerals like halite are also known. Molds are temporary things, geologically speaking.

6. Pholad boring

Pholads are small bivalves that bore holes into shore rocks a few centimeters across, living their lives inside that shelter and sticking their siphuncles out to filter the seawater. If you're at a rocky shore, or if you suspect that a rock has once been there, then look for these biological holes, a type of organic weathering. Other marine creatures make marks in rocks, too, but the real holes generally belong to pholads.

7. Pit

Pit is the general name for a hole in sedimentary rock that is produced by weathering. Small pits are typical of alveolar or honeycomb weathering, and large pits are called tafoni.

8. Pocket

Pocket is a term used by rockhounds or miners for any hole with crystals in it. Geologists don't use the word.

9. Pore

The tiny spaces between the individual grains of rocks and soil are called pores. The pores in a rock collectively make up its porosity, which is an important property to know in groundwater and geotechnical studies.

10. Vesicle

Vesicles are gas bubbles in lava that has solidified. Lava that is full of bubbles is said to have a vesicular texture. The word comes from the Latin for "little bladder." Vesicles that fill with minerals are called amygdules; that is, if a vesicle is like a mold, an amygdule is like a cast.

11. Vug

Vugs are small cavities lined with crystals, like druses, but unlike druses the mineral crystals lining vugs are different minerals from those of the host rock. The word comes from Cornish.

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