Glass is commonly associated with human manufacturing, but nature also forms this transparent or translucent substance. Intense geological events produce natural glass, offering insights into Earth’s powerful forces and rapid transformations.
Glass from Volcanic Activity
Volcanic activity creates glass, notably obsidian. This natural glass forms when silica-rich felsic lava cools very rapidly, preventing crystalline structure. Swift cooling occurs when lava encounters cooler air or water, solidifying quickly. High viscosity further inhibits crystal growth, contributing to its glassy texture.
Obsidian is often jet-black, though impurities like hematite can produce red or brown varieties, and trapped gas bubbles can create a golden sheen. It has a distinctive glassy luster and is slightly harder than window glass. Obsidian is common in volcanic regions, forming along the margins of rhyolitic lava flows. Well-known occurrences include Mount Hekla in Iceland and Yellowstone National Park.
Humans utilized obsidian for its unique properties. Its ability to fracture with sharp edges, known as a conchoidal fracture, made it an ideal material for tools and weapons. Ancient civilizations, including Native Americans, Aztecs, and Greeks, fashioned obsidian into arrowheads, knives, and even mirrors. The sharpness of obsidian blades can surpass modern surgical steel.
Glass from Lightning Strikes
Lightning strikes forge natural glass, creating fulgurites, often called “fossilized lightning.” When a lightning bolt strikes sand or rock, the intense heat melts the material. This molten material then cools and solidifies quickly, preserving the lightning’s path through the ground.
Fulgurites form tubular or branching structures, resembling twisted roots or coral. Their shape reflects the lightning’s path as it disperses into the earth. These formations are primarily composed of silica glass, but their chemical composition varies based on the material struck. Sand fulgurites are often hollow tubes with a glassy interior, while rock fulgurites appear as thin glassy crusts on rock surfaces.
Fulgurites range in size from millimeters to several meters, with some found extending 15 meters deep. Although found worldwide, fulgurites are relatively rare compared to volcanic glass. Their formation requires specific conditions, including high silica content in the sand or rock, allowing for rapid melting and vitrification.
Glass from Asteroid Impacts
Another natural phenomenon creating glass is the impact of asteroids or meteorites on Earth’s surface. The heat and pressure generated by these impacts can melt terrestrial rock, forming various types of impact glass such as tektites and impactites. This process is known as impact metamorphism.
Tektites are gravel-sized bodies of natural glass formed from terrestrial debris ejected during impacts. When a meteorite strikes, the energy melts the target rock, and this molten material is ejected into the atmosphere. As these molten droplets travel through the air, they cool rapidly and solidify before falling back to Earth. Tektites are characterized by their homogeneous glass composition, low water content, and often contain microscopic bubbles.
Impactites, a broader category, include tektites but also encompass other rocks modified by meteorite impacts. They are terrestrial rocks created or altered by the heat, pressure, and shock of a meteorite collision. Impactites can be found directly within impact craters and include impact melt rocks where the target material has been melted and cooled. Unlike tektites, other forms of impact glass may solidify in place or travel only short distances.
Shared Properties of Natural Glass
Despite diverse origins, all natural glass shares fundamental characteristics. A defining property is their amorphous, or non-crystalline, structure. Unlike minerals, which have an organized internal atomic arrangement, natural glasses lack this regular structure because they cool too quickly for crystals to form. This disordered atomic arrangement results from the rapid cooling of molten material, whether from lava, lightning-struck sand, or impact-melted rock.
Natural glasses also exhibit a conchoidal fracture, meaning they break with smooth, curved, shell-like surfaces. This fracture occurs due to the absence of mineral crystals within the glass, allowing stresses to propagate evenly. Most natural glasses, including obsidian, fulgurites, and tektites, are rich in silica, an important component in their formation. This high silica content contributes to their hardness and durability, making them distinct from other rock types.