Volcanic glass facts for kids
Volcanic glass is a special kind of rock that forms when hot, melted rock (called magma) cools down super fast. Think of it like making regular glass for windows! When magma cools quickly, it doesn't have time to form tiny crystals. Instead, it becomes a solid that's smooth and doesn't have a regular, repeating pattern like most rocks. This makes it "amorphous," which means "without a clear shape."
You can find volcanic glass as part of other volcanic rocks, or as its own unique glassy rock.
Contents
How Volcanic Glass Forms
The Secret of Fast Cooling
Volcanic glass forms when magma cools down very, very quickly. When magma cools fast, it gets colder than the temperature where it would normally start forming crystals. It becomes a "supercooled" liquid. If it keeps cooling rapidly, it turns into a solid without any crystals – that's volcanic glass!
This change from a supercooled liquid to glass happens at a special temperature. How fast the magma cools and how much water is dissolved in it both play a role in this process.
Why Some Magma Makes Better Glass
Magma that has a lot of a mineral called silica (like the kind that forms rhyolite rock) and not much water can turn into glass very easily. This is why you might find whole lava flows made of glassy rhyolite. Even volcanic ash can be tiny pieces of volcanic glass.
But basalt magma, which has less silica, finds it harder to form glass. So, basalt ash usually has some tiny crystals mixed in. Basalt needs to cool below about 700 degrees Celsius to become glass.
We can see how cooling affects glass by looking at two types of basaltic glass: tachylite and sideromelane. Tachylite looks dark and you can't see through it because it has many tiny crystals inside. Sideromelane, however, is clearer because it has fewer crystals. Sideromelane often forms when basalt magma cools super fast by touching water. This happens in underwater eruptions or when lava flows into the ocean, creating pillow lavas.
Different Ways Magma Cools Down
The best way for magma to cool fast enough to make glass is by touching water. Imagine hot lava hitting the ocean! Cooling by air during a volcanic eruption is also effective. This is how tiny glass shards can form in eruption columns. The slowest way is when lava cools slowly at the bottom of a flow, touching the ground. This slower cooling usually allows crystals to form.
Types of Volcanic Glass
The most well-known type of volcanic glass is obsidian. It's a dark, shiny glass that forms from rhyolite magma, which is rich in silica. People have used obsidian for tools and decorations for thousands of years!
But there are many other cool kinds of volcanic glass too!
- Pumice: This is a very light, bubbly rock that can even float on water! It's considered a glass because it doesn't have a crystal structure.
- Scoria: Similar to pumice, but usually darker, heavier, and with bigger bubbles. It comes from magma with less silica.
- Apache tears: These are small, rounded pieces of obsidian, often found loose on the ground.
- Tachylite: A dark, opaque glass that forms from basalt magma (low silica).
- Sideromelane: A clearer, greenish-brown glass, also from basalt magma, but cooled even faster than tachylite.
- Palagonite: This is what happens when basaltic glass changes over time, often turning yellowish or brownish.
- Hyaloclastite: A rock made from broken pieces of sideromelane and palagonite, often formed when lava erupts underwater.
- Pele's hair: Imagine thin, golden-brown strands of glass, like hair! These form when lava droplets are stretched by the wind during an eruption.
- Pele's tears: Small, tear-shaped drops of volcanic glass, often found with Pele's hair.
- Limu o Pele (Pele's seaweed): These are thin, wavy sheets of volcanic glass, looking a bit like dried seaweed.
- Pitchstone: A duller, less shiny volcanic glass than obsidian, because it contains more water.
What Happens to Volcanic Glass Over Time?
Volcanic glass doesn't last forever in its original form. It's not very stable over long periods. Water molecules are very good at reacting with the glass. They slowly break it down and carry away some of its parts.
As this happens, new minerals can start to form in its place. This process is called "alteration." It's one of the fastest ways that volcanic ash can turn into solid rock at cooler temperatures.
This natural change in volcanic glass is important! For example, it has helped create valuable deposits of minerals like zeolite and bentonite, which are used in many industries.
