Holes in a rock: vesicles or weathering?

Describe the cavities before naming the rock

“Holes in the rock” is a useful start. Make it more specific without jumping to a name: are the openings round, stretched, irregular, smooth-edged, sharp-edged, isolated, clustered, aligned, or concentrated on one surface? Are they enclosed in the rock or open from weathered pits at the outside?

Vesicles are cavities left when gases escape and expand in cooling lava. They can be small, rounded, lens-shaped, tubular, or irregular. That makes them an important volcanic-rock clue, but a cavity visible in one photo is not automatically a vesicle.

Photograph the pattern, not only one hole

1. Take a whole-specimen view. Show whether the cavities occur throughout the rock, only near an outer surface, or in a particular band.
2. Take a sharp close view. Include several openings and the material between them. A single dark shadow is not enough to describe the pattern.
3. Add a scale beside the specimen. A scale helps show relative size without obscuring the surface. See our rock scale photography guide.
4. Record the surface condition. Note whether the face is dry, wet, weathered, coated, cut, or an existing fresh-looking edge. Do not damage the specimen to create a new surface.

When holes may be vesicles

In volcanic rocks, vesicles commonly formed as gas bubbles grew in lava before it solidified. National Park Service guidance describes vesicular texture as abundant cavities created by gas expansion in fluid lava. Pumice and scoria are highly vesicular volcanic materials, but their colour, density, texture, and the spacing and form of cavities can differ.

A useful photo description might be: “many rounded cavities dispersed through a dark, fine-looking rock.” That is stronger than calling the specimen basalt, scoria, or pumice before other evidence has been checked.

When holes may be weathering instead

Some cavities are made after a rock forms. The National Park Service describes rounded holes in certain sandstone surfaces where softer mineral pockets eroded faster than the surrounding rock. Honeycomb weathering can also create clustered openings in exposed rock faces. These processes can make a weathered surface look sponge-like even when the rock did not form from gas bubbles.

Ask whether the openings are mainly at the exposed outside, whether they follow a layer or line, and whether the interior looks different from the weathered face. These questions do not prove an answer, but they separate a surface-weathering possibility from a rock-wide volcanic texture.

Keep the limits of the photo clear

A picture cannot measure density, determine mineral composition, or show every internal cavity. It also cannot tell whether a similar-looking hole was once filled by another mineral. Use the rock identification tool for a tentative visual description, preserve the discovery setting in a location note, and use the AI rock identification checklist if the suggested name conflicts with what you observe.

If the evidence remains mixed, write “cavities visible; origin undetermined from this photo.” That is more useful than claiming a volcanic rock type from holes alone.

Sources for this guide

• U.S. Geological Survey: vesicles in volcanic rock
• National Park Service: vesicular texture in igneous rocks
• National Park Service: gas vesicles in basalt
• National Park Service: weathered cavities in sandstone
• National Park Service: honeycomb weathering