Tachylite facts for kids
Tachylite (pronounced TAK-ə-lyte) is a special type of natural glass. It forms when hot, melted rock, called basaltic lava, cools down super fast. Think of it like making glass in a factory, but nature does it with volcanoes! Tachylite is a dark, igneous rock. It's usually black or dark brown and has a shiny, almost greasy look, like resin. It's also very brittle, meaning it breaks easily. You can find it in thin sheets or veins within other rocks. The name "tachylite" comes from an ancient Greek word meaning "swift," which makes sense because it forms so quickly.
Tachylite often looks like black tar or pitch. Sometimes it has tiny bubbles or small round shapes inside. If you hit it with a hammer, it shatters easily. You might even see tiny crystals of minerals like feldspar or olivine if you look closely. Over time, tachylite can change color to dark brown or red as its iron content reacts with air. This rock forms in places where lava cools very rapidly. It's less common than other volcanic glasses, like obsidian, because basaltic lava usually prefers to form crystals rather than glass. You can tell tachylite apart from other volcanic glasses because small pieces of it will melt and stick together when heated.
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Tachylite: Nature's Fast-Cooled Glass
Where Tachylite is Found: Volcanic Ash and Cinders
One common place to find tachylite is in the fine ash and cinders that erupt from basaltic volcanoes. These "cinders" are often spongy pieces of tachylite. They have a few larger crystals mixed into the black glass. You can see these tachylite volcanic bombs and scoria in places like Iceland, Stromboli, and Mount Etna.
These bubbly pieces of tachylite are also found all over the bottom of the ocean. They can be scattered in deep-sea mud or form layers. Sometimes, they get coated with manganese, a metal from the seawater. Over time, these tachylite fragments change. They become a dark red color because their iron reacts with oxygen and water. This changed basic glass is called "palagonite". You might find layers of soft palagonite surrounding a core of fresh tachylite. In palagonite, the original minerals have also changed. However, fresh tachylite glass often contains tiny, sharp crystals of feldspar, augite, and olivine. Palagonite tuffs, which are rocks made of this altered ash, are found in places like Iceland, Sicily, and the Canary Islands. Some of these tuffs even contain fossils or are mixed with glacial deposits.
Where Tachylite is Found: Lava Flows and Caves
Tachylite can also form within lava flows. While most basaltic rocks have some glass, completely glassy types are less common. Tachylite often appears at the very edges of thin lava flows or sills. These edges cool so quickly that they become glass, sometimes only a millimeter thick. As you go further into the flow, the rock becomes crystalline basalt.
In the Hawaiian Islands, volcanoes have poured out huge amounts of black basalt lava. This lava is very runny when it comes out. The rapid cooling in the air stops it from forming crystals completely. Many of these lavas are bubbly or spongy. Their surfaces can be very rough or form smooth, wave-like shapes. Amazing caves are found where the outside of a lava flow hardened, but the liquid inside flowed away. Inside these caves, you can see "stalactites" and "stalagmites" made of black tachylite. These formations usually have a hollow center with walls of dark brown glass. They contain tiny crystal skeletons of augite, olivine, and feldspar.
At the Kīlauea volcano in Hawaii, thin clouds of steam constantly rise. As vapor bubbles escape the molten rock, they carry tiny fibers of basalt into the air. These fibers cool instantly and become thin threads of tachylite. People call these "Pele's Hair," named after the Hawaiian goddess of volcanoes. Under a microscope, they look like almost pure glass with tiny air bubbles. The Hawaiian Islands are one of the few places where glassy basaltic lavas are so common. A small area in Victoria, Australia, called Spring Hill, also has tachylite. Ancient Aboriginal people used this tachylite to make sharp stone tools.
Where Tachylite is Found: Inside Earth's Cracks
A third way tachylite forms is along the edges of dikes and sills. These are cracks in other rocks that get filled with molten lava. The tachylite forms a thin layer, sometimes just a tiny fraction of an inch thick, on the edge of the crystalline rock. It can look like a thin coating of tar. However, sometimes these glassy veins can be several inches thick.
In these locations, tachylite usually doesn't have many bubbles. Instead, it often shows clear flow patterns, like lines in the glass. You might also see round, dark brown spots called "spherulites." These spherulites have a radiating structure, like spokes on a wheel, and can have different colors. The glass itself is always brittle. Common tiny crystals found here include olivine, augite, feldspar, and specks of magnetite. At the very edges, the glass can be perfectly clear of crystals. But it quickly changes into the normal crystalline rock. These spherulites can be quite large, sometimes a quarter of an inch or more across. Over time, this tachylite can also change into a red palagonite substance. Because there are no steam bubbles, the tachylite found in dikes often stays fresh longer than the bubbly ash. You can find these types of tachylites in places like Scotland, northern England, and parts of Germany.
See also
- Pseudotachylite
- Vitrophyre
