Pyroclastic Rocks
Pyroclastic rocks are composed of fragmented volcanic products ejected from explosive volcanic eruptions of viscous silica-rich magmas. Individual eruptive fragments are called pyroclasts ("fire or hot fragments"). Once pyroclasts have fallen to the ground they remain as unconsolidated tephra unless hot enough to fuse together or lithify through compaction and cemetation into a pyroclastic rock, a tuff.
Volcanic Ash - Tephra particles less than 2 mm in diameter
Montana, USA
Size: 10 cm in diameter
Pele's Hair - Molten fragments drawn by wind into threads of volcanic glass
Size: 3 cm
Lapilli - Tephra between 2 and 64 mm in diameter
Mt. Helka, Iceland
Size: 1-2 cm fragments
Lava Bombs - Tephra particles greater than 64 mm in diameter
Size: 10-22 cm
Volcanic Bomb
Volcanic bombs are large ejected pyroclasts which acquire rounded aerodynamic shapes during their travel through the air. These spindleform or fusiform bombs form from high to moderately fluid basaltic magma. Spinning during flight leaves these bombs looking elongated or almond shaped.
Size: (Upper) 35 cm / (Middle) 25 cm / (Lower) 26 cm
Crystal Tuff
Mineral fragments originating from a partially crystallized magma dominate.
Size: 17 cm
Accretionary Lapilli Tuff
Balls of volcanic ash that form around a wet nucleus falling through a volcanic ash cloud.
Size: 15 cm
Welded Tuff
Here glass shards were hot enough to deform and fuse together after depostion, resulting in a welded or eutaxitic texture.
Size: 13 cm
Pumice Tuff - Lithified ash with lapilli sized pumice fragments
Atacama Desert, Chile
Size: 10 cm