Volcanic glass

Volcanic glass is a unique state of matter formed from magma. It is described as an amorphous solid. This means the material lacks a highly ordered crystal structure. In a crystal, atoms are arranged in a tight, organized array. In a liquid, the atoms are highly disordered. Volcanic glass exists as an intermediate state between these two. It can also refer to the matrix found in fine-grained volcanic rocks.

The formation of volcanic glass requires a specific cooling process. It begins when magma cools very rapidly. If the cooling is fast enough, the magma becomes a supercooled liquid. Continued rapid cooling then transforms this liquid into an amorphous solid. This specific change occurs at the glass transition temperature. This temperature is not fixed. It depends on the cooling rate and the amount of dissolved water in the magma.

Chemical composition plays a major role in how glass forms. Magma that is rich in silica and low in water forms glass most easily. Rhyolite magmas have high silica levels. They can produce tephra made entirely of volcanic glass. They can also form glassy lava flows. In contrast, basalt magma has low silica levels. It is much harder for basalt to form glass. Consequently, basalt tephra usually contains some crystalline material known as quench crystals.

There are different types of basaltic glass based on their appearance. One type is called tachylite. Tachylite is opaque to transmitted light. This is because it contains many tiny oxide mineral crystals. Another type is sideromelane. This form is partially transparent because it has far fewer crystals. Sideromelane is mostly found in phreatomagmatic eruptions. These are eruptions where basalt magma is cooled very rapidly by contact with water.

Scientists categorize many different forms of volcanic glass. Pumice is considered a glass because it lacks a crystal structure. Scoria is the mafic or intermediate version of pumice. Scoria is typically denser and has larger vesicles, which are small holes. Other examples include obsidian and nodular obsidian called apache tears. There is also pitchstone, which is an acidic glass similar to obsidian but with higher water content. Some glass forms even look like hair or tears. Pele's hair consists of thin basaltic fibers. Pele's tears are basaltic drops. You might also see Limu o Pele, which are brownish-green flakes.

The method of cooling determines how much glass is created. Quenching by water is the most effective cooling mechanism. The next most effective method is cooling by air within an eruption column. The least effective way for glass to form is cooling at the bottom of a lava flow. In that case, the magma is in contact with the ground. This slower cooling makes it much harder to reach the glass transition temperature.

Volcanic glass is chemically unstable over long periods. Because it lacks a crystal structure, water reacts with it easily. This reaction removes certain elements from the glass. As these elements change, new minerals begin to form. This process can turn volcanic ash into solid rock relatively quickly. This alteration can also create important mineral deposits. Examples include zeolite and bentonite. These minerals are of great economic importance to humans.