#DEFINE OBSIDIAN ROCK HOW TO#

In nature, erupting a glass flow requires an unusually viscous magma, one that has both a high silicon dioxide (silica) content and a very low water content.

Glass, be it volcanic or manmade, cools quickly from a molten liquid without forming crystals, the building blocks of the minerals that make up most rocks. So what is obsidian? Obsidian is dense volcanic glass, usually rhyolitic (the fine-grained equivalent of granite) in composition and black in color. Width of view 22 mm.No matter how many lava flows you've seen in Hawaiʻi, we guarantee you'll be astounded. Obsidian from NE California (The Cascades volcanic range). Obsidian from the Yellowstone National Park. Here are two pictures of obsidian grains picked from coarse-grained sand samples. However, obsidian formations older than the Cenozoic (it began 65 million years ago) are unknown. In dry areas obsidian can last pretty long. The same applies here on the Earth as well. Obsidian on the Moon may be billions of years old because the Moon is dry. Obsidian is not stable in the weathering environment but it does not mean that it can not last millions of years. Obsidian is usually dark and its surface is shiny. Red color is caused by the same stuff that gives red color to weathered basalt, desert sand and K-feldspar. This color is caused by minute inclusions and tiny crystals in the glass. This means that fracture surface is smoothly curving (like a seashell). Obsidian has a strong luster and conchoidal fracture. Obsidian is usually black although reddish varieties are pretty common also.

#DEFINE OBSIDIAN ROCK HOW TO#

How to identify small obsidian fragments like sand grains? In most cases it is not too hard to do by optical examination only. There are some nice black sand beaches in Hawai’i that are composed of fragments of volcanic glass with basaltic composition. This is the case if basaltic lava is flowing into the water. Is some volcanic glass still basaltic in composition? Yes, but in this case the cooling has been really rapid. Pumice and rhyolite have very similar composition but they differ in texture - pumice is highly vesicular while obsidian is massive. Obsidian with gray layers of rhyolitic pumice in Long Valley Caldera in California. Vesicular basaltic volcanic glass from Punalu’u Beach, Hawaii. I think this point is important and worth paying attention to because lots of people seem to think that rhyolitic magma is more viscous than basaltic magma only because its temperature is usually lower. If the magma contains lots of metals (cations) then it is less viscous because these cations break the framework structure of silica. There are countless bridges (chemical bonds) between oxygen anions of silica (SiO 2) which is the reason why this magma is so hard to move. Why is that important? Because silica causes magma to polymerize. This lava is the thickest because it has the highest silica content. Most obsidians are rhyolitic in composition.

So obsidian forms from viscous magma only? Usually yes, but not always. The result is that everything solidifies just randomly as glass. 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. I already gave a subtle hint what the second problem might be. There is very little of it when viscous magma is pushed out of a volcano and cools rapidly. Large crystals need a lot of time to grow. Most of it is not crystallized because the crystals had two difficult problems which restricted their growth. Volcanic glass is an igneous rock that is composed of largely uncrystallized magmatic material. It is definitely not wrong to use ‘volcanic glass’ instead of ‘obsidian’ but you should be careful the other way around - volcanic glass is not always obsidian. So the volcanic glass and obsidian are not synonyms although in many cases you can freely use both terms. Typical obsidian is either black or slightly reddish and often demonstrates beautiful conchoidal fracture. Volcanic glass that formed underwater has alternative names like tachylite and hyaloclastite. In the majority of cases obsidian solidified subaerially (on land). It is just a piece of volcanic glass without further conditions. It lacks layering, cleavage, foliation, phenocrysts, etc. The term ‘massive’ has several (although related) meanings in geology but here it means that the rock (obsidian is a rock type, not a mineral) is homogenous.

Define obsidian rock