Obsidian – glass from volcanoes

Black color, strong luster, and conchoidal fracture make the identification of obsidian relatively easy.

Obsidian is a massive volcanic glass.

All right, what does it mean? What is ‘massive’? This term has several (although related) meanings in geology but here it means that the rock (obsidian is a rock type, not a mineral) is homogenous. It lacks layering, cleavage, foliation, phenocrysts, etc. It is just a piece of volcanic glass without further conditions. In majority of cases obsidian solidified subaerially (on land). Volcanic glass that formed underwater has alternative names like tachylite and hyaloclastite.

So the volcanic glass and obsidian are not synonyms although in many cases you can freely use both terms. It is definitely not wrong to use ‘volcanic glass’ instead of ‘obsidian’ but you should be careful the other way around.

Volcanic glass is an igneous rock that is composed of largely uncrystallized magmatic material. Most of it is not crystallized because the crystals had two difficult problems which restricted their growth. First is time. Large crystals need a lot of time to grow. There is very little of it when viscous magma is pushed out of a volcano and cools rapidly. I already gave a subtle hint what the second problem might be. It is the viscosity of magma/lava. If the magmatic body is very thick or viscous, the crystals have a really hard time forming because they simply don’t have new material coming in if almost nothing is able to move inside the magma body. The result is that everything solidifies just randomly as glass.

Vesicular basaltic volcanic glass from Punalu'u Beach, Hawaii. The width of the view is 10 mm.

So obsidian forms from viscous magma only? Usually yes, but not always. Most obsidians are rhyolitic in composition. This lava is the thickest because it has the highest silica content. Why is that important? Because silica causes magma to polymerize. There are countless bridges (chemical bonds) between oxygen anions of silica (SiO₂) which is the reason why this magma is so hard to move. If the magma contains lots of metals (cations) then it is less viscous because these cations break the network structure of silica. I think this point is important and worth paying attention because many folks (I mean geologists) seem to think that rhyolitic magma is more viscous than basaltic magma only because its temperature is usually lower. That is only one and less important part of the answer.

Is some volcanic glass still basaltic in composition? Yes, but in this case the cooling has been really rapid. This is the case if basaltic lava is flowing into the water. There are some nice black sand beaches in Hawaii that are composed of fragments of volcanic glass with basaltic composition.

How to identify obsidian fragments in sand? In most cases it isn’t too hard to do by optical examination only. Obsidian is usually black although reddish varieties are pretty common also. Obsidian has a strong luster and conchoidal fracture. This means that fracture surface is smoothly curving (like a seashell).

Obsidian is usually black. This color is caused by the minute inclusions and tiny crystals in the glass. Red color is caused by the same stuff that gives red color to weathered basalt, desert sand and K-feldspar. It is mineral hematite (iron oxide).

Obsidian is not stable in the weathering environment but it doesn’t mean that it can’t last millions of years. Obsidian on the Moon may be billions of years old because the Moon is dry. Same applies here on Earth as well. In dry areas obsidian can last pretty long. However, obsidian formations older than the Cenozoic (it began 65 million years ago) are not known.

Here are two pictures of obsidian grains picked from coarse-grained sand samples.

Obsidian from the Yellowstone NP. The width of the view is 35 mm.

Obsidian from NE California (The Cascades volcanic range). The width of the view is 22 mm.