1. Volcanic Obsidian In China

Home » Rocks » Igneous Rocks

Dark, reflective, obsidian sitting atop gray rocks near the Panum Crater in Mono County, California. Daniel Mayer Volcanic glass is unstable and tends to change spontaneously (devitrify) from the glassy to the crystalline state in periods of time that are relatively short by geologic standards; the material takes on a stony appearance due to. Obsidian is an 'extrusive” rock, which means it is made from magma that erupted out of a volcano. If it was an igneous rock that formed from magma underground and did not erupt, it would have been called an 'intrusive' rock. Obsidian: Can we get started already? I don't have all day!

Andesite is a fine-grained, extrusive igneous rock composed mainly of plagioclase with other minerals such as hornblende, pyroxene, and biotite. The specimen shown is about two inches (five centimeters) across.

What are Igneous Rocks?

Igneous rocks are formed from the solidification of molten rock material. Some form below Earth's surface. Some form on or above Earth's surface. We describe these two basic types:

Intrusive igneous rocks crystallize below Earth's surface, and the slow cooling that occurs there allows large crystals to form. Examples of intrusive igneous rocks are: diabase, diorite, gabbro, granite, pegmatite, and peridotite.

Extrusive igneous rocks erupt onto the surface, where they cool quickly to form small crystals. Some cool so quickly that they form an amorphous glass. These rocks include: andesite, basalt, dacite, obsidian, pumice, rhyolite, scoria, and tuff.

Pictures and brief descriptions of some common igneous rock types are shown on this page.

Dacite is a fine-grained, extrusive igneous rock that is usually light in color. It has a composition that is intermediate between rhyolite and andesite. The specimen shown is about four inches (ten centimeters) across.

Basalt is a fine-grained, dark-colored extrusive igneous rock composed mainly of plagioclase and pyroxene. The specimen shown is about two inches (five centimeters) across.

Diabase is an intrusive igneous rock composed mostly of plagioclase feldspar and pyroxene minerals. The grains in diabase are larger than those in basalt but smaller than those in gabbro. Diabase is used in the construction industry as trap rock or dimension stone. When the diabase contains colorful labradorite crystals, it makes an especially nice architectural stone.

Gabbro is a coarse-grained, dark-colored, intrusive igneous rock that contains feldspar, pyroxene, and sometimes olivine. The specimen shown above is about two inches (five centimeters) across.

Diorite is a coarse-grained, intrusive igneous rock that contains a mixture of feldspar, pyroxene, hornblende, and sometimes quartz. The specimen shown above is about two inches (five centimeters) across.

Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form. The specimen shown above is about two inches (five centimeters) across.

Granite is a coarse-grained, light-colored, intrusive igneous rock that contains mainly quartz, feldspar, and mica minerals. The specimen above is about two inches (five centimeters) across.

Peridotite is a coarse-grained intrusive igneous rock that is composed almost entirely of olivine. It may contain small amounts of amphibole, feldspar, quartz, or pyroxene. The specimen shown above is about two inches (five centimeters) across.

Pegmatite is a light-colored, extremely coarse-grained intrusive igneous rock. It forms near the margins of a magma chamber during the final phases of magma chamber crystallization. It often contains rare minerals that are not found in other parts of the magma chamber. The specimen shown above is about two inches (five centimeters) across.

Rhyolite is a light-colored, fine-grained, extrusive igneous rock that typically contains quartz and feldspar minerals. The specimen shown above is about two inches (five centimeters) across.

Pumice is a light-colored vesicular igneous rock. It forms through very rapid solidification of a melt. The vesicular texture is a result of gas trapped in the melt at the time of solidification. The specimen shown above is about two inches (five centimeters) across.

Scoria is a dark-colored, vesicular, extrusive igneous rock. The vesicles are a result of trapped gas within the melt at the time of solidification. It often forms as a frothy crust on the top of a lava flow or as material ejected from a volcanic vent and solidifying while airborne. The specimen shown above is about two inches (five centimeters) across.

Fire Opal is sometimes found filling cavities in rhyolite. Long after the rhyolite has cooled, silica-rich ground water moves through the rock, sometimes depositing gems like opal, red beryl, topaz, jasper, or agate in the cavities of the rock. This is one of many excellent geological photographs generously shared through a Creative Commons License by Didier Descouens.

Unakite is a colorful rock composed of green epidote and pink orthoclase. It is formed when granite, an igneous rock, is metamorphosed by hydrothermal activity. Attractive pieces of unakite are often used to make cabochons, tumbled stones, small sculptures, and other lapidary items. It is named after the Unaka Mountains of eastern Tennessee.

'Trap Rock' is a layman's term for any dark-colored igneous rock that is used to make crushed stone. This crushed stone can be used as road base material, or as an aggregate in concrete or asphalt. The most common types of trap rock are basalt, diabase, gabbro, and peridotite. Image copyright iStockphoto / Brilt.

The best way to learn about rocks is to have a collection of specimens to examine while you study. Seeing and handling the rocks will help you understand their composition and texture much better than reading about them on a website or in a book. The Geology.com store offers inexpensive rock collections that can be mailed anywhere in the United States or U.S. Territories. Mineral collections and instructive books are also available.

Welded Tuff is a rock that is composed of materials that were ejected from a volcano, fell to Earth, and then lithified into a rock. It is usually composed mainly of volcanic ash and sometimes contains larger size particles such as cinders. The specimen shown above is about two inches (five centimeters) across.

Obsidian is a massive volcanic glass. 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. It lacks layering, cleavage, foliation, phenocrysts, etc. It is just a piece of volcanic glass without further conditions. In the majority of cases obsidian solidified subaerially (on land). Volcanic glass that formed underwater has alternative names like tachylite and hyaloclastite.

Typical obsidian is either black or slightly reddish and often demonstrates beautiful conchoidal fracture. Width of sample 11 cm.

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 not always obsidian.

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. The first one 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.

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 framework structure of silica. 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.

Vesicular basaltic volcanic glass from Punalu’u Beach, Hawaii. Width of view 20 mm.
Obsidian with gray layers of rhyolitic pumice in Long Valley Caldera in California. Pumice and rhyolite have very similar composition but they differ in texture — pumice is highly vesicular while obsidian is massive.

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 Hawai’i that are composed of fragments of volcanic glass with basaltic composition.

How to identify small obsidian fragments like sand grains? In most cases it is not 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 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 usually dark and its surface is shiny.

Obsidian is not stable in the weathering environment but it does not mean that it can not last millions of years. Obsidian on the Moon may be billions of years old because the Moon is dry. The same applies here on the 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 unknown.

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

Obsidian from the Yellowstone National Park. Width of view 35 mm.

Volcanic Obsidian In China

Obsidian from NE California (The Cascades volcanic range). Width of view 22 mm.