Pyroclastic deposit. This rock is hardened tephra, ejected during a volcanic eruption. This type of deposit is called a bread-crust bomb, so- called because of its distinctive outer crust. During its formation, an outer crust of solidified material formed on the bomb, while the interior remained plastic. Gas inside the core of the bomb expanded, causing the outer skin of rock to stretch and crack. The outer surface cracked open when it landed, revealing the core. This bomb was ejected by the now extinct Hohentweil volcano, in Baden-Wurttemberg, Germany.