Shiny black volcanic glass sample with gray patches, which resemble flowers or "snowflakes," hence the name of this variety of obsidian. An extrusive igneous rock, obsidian is formed when felsic lava extruded from a volcano colls rapidly with minimal crystal growth. The snowflakes in this sample are actually radiating inclusions of the mineral cristobalite and formed through devitrification of the original obsidian. Snowflake obsidian was used by Native American peoples as a valuable commodity that was traded extensively throughout North America.
- Catalog Number:
- 45995
- Object/Specimen Description:
- Specimen Count:
- 1
- Cabinet:
- 25
- Drawer/Shelf:
- 01
While underground, magma (molten rock) achieves temperatures of 700 degrees C to 1,300 degrees C (1,300 degrees F to 2,400 degrees F). At Earth's surface, the magma, now known as lava, spreads out and starts cooling. On land, highly viscous (thick, slow-moving) lava may flow downhill, burning and then covering everything in its path. Under the ocean, lava may cool into domed or tube-like shapes that look like puffy pillows. Sometimes the solidified rock piles so high that it reaches the surface and becomes an island, like the Hawaiian Islands. Because the lava undergoes a fast drop in temperature, it lacks enough time to form a well-defined crystalline structure. If it freezes quickly enough, lava that is rich in silica, also known as felsic lava, becomes a pure glass called obsidian, which breaks into pieces with sharp, curved edges.
You can tell a lot about the history of igneous rocks by looking at the size of crystals within them. Rocks that cool quickly contain small crystals, while slow-cooled rocks are filled with large crystals. When magma erupts at the Earth's surface, heat radiates out from the lava allowing it to cool rapidly and the atoms and molecules do not have time to grow into large crystals before the lava solidifies. The resulting rock has such small crystal grains that humans have a difficult time distinguishing them, even with a handheld lens. Geologists describe the texture of these fine-grained igneous rocks as "aphanitic," from the Greek word meaning "unseen." Deep inside the Earth's crust, the magma cools much more slowly because the surrounding rocks insulate the magma from rapid heat loss. This allows the crystals to grow into mineral grains that are easier for humans to see. Geologists describe the resulting coarse-grained rock texture as phaneritic, from the Greek word meaning "visible." Some igneous rocks contain crystals that are much larger than the crystals in the matrix surrounding them. Scientists call these specimens, which resemble a chocolate-chip cookie, porphyritic rocks, and the larger crystals are called phenocrysts. The phenocrysts had started to form within the magma before it later cooled rapidly, probably due to that magma erupting at a volcano.