Volcanic glass

Hot melted rock can turn into glass.

This happens when the rock cools very fast. Water can make it cool even faster.

Some glass looks like tiny threads. It can also look like small tears.

Some glass is even like seaweed.

One kind of glass is called pumice. It has no crystal shapes inside.

This glass can change over time. Water can touch it and change it. This can turn ash into solid rock.

It is a very special way for rocks to form.

Volcanic glass is a special kind of rock. It forms when magma cools very fast. Magma is hot, melted rock.

Usually, cooling rock forms crystals. Crystals are neat, ordered shapes. But volcanic glass has no crystal structure. It is an amorphous solid. This means its parts are messy and disordered.

Magma with lots of silica makes glass easily. Silica is a mineral found in the rock. Water can also help. Cooling by water is the best way to make glass. This is called quenching.

There are many types of this glass. Some look like tiny threads called Pele's hair. Others look like drops called Pele's tears. Pumice is also a type of glass.

Volcanic glass can change over time. It is chemically unstable. This means it does not stay the same. Water can touch the glass. This makes it break down. It can turn volcanic ash into solid rock. It can even make new minerals. Some of these minerals are useful to people.

Volcanic glass is a very special kind of matter. It is an amorphous solid. This means it has no neat crystal structure. Most rocks form crystals as they cool down. Crystals have a very ordered and tight pattern. Glass is different because it is messy and disordered. It sits between a liquid and a crystal. This state makes it very interesting to scientists.

How does this glass form? It all starts with hot magma. The magma must cool down very quickly. If it cools fast, it becomes a supercooled liquid. Further rapid cooling turns it into an amorphous solid. This happens at the glass transition temperature. This temperature changes based on how fast things cool. It also depends on how much water is in the magma. Magma with high silica and low water makes glass easily.

Scientists study many different types of this glass. Some types are named after famous places or people. For example, there is a type called Pele's hair. These look like thin threads or fibers. There are also Pele's tears which look like drops. Some glass is called obsidian or pitchstone. Another type is called pumice, which has no crystal structure. You might also see scoria, which is denser than pumice.

There are specific names for basaltic glass too. One kind is called tachylite. It is opaque because it has many tiny crystals. Another kind is called sideromelane. It is partially transparent because it has fewer crystals. Sideromelane often forms during phreatomagmatic eruptions. These are eruptions where basalt magma hits water. Water is the best way to cool magma quickly. This process is called quenching.

Volcanic glass does not stay the same forever. It is chemically unstable. This means it breaks down over time. Water can react with the glass quite easily. This reaction removes some parts of the glass. Then, new minerals begin to form. This can turn volcanic ash into solid rock. This change can even create useful minerals. Some of these are called zeolite and bentonite. These minerals have economic importance to people.

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.