Bread-crust blocks are rounded or angular lumps with a smooth, glassy crust broken by deep cracks and fissures, exposing the frothy, vesicular core of the bomb, and reminiscent of a well-baked, crusty loaf of bread. Bread-crust blocks are formed when lumps of viscous, gas-rich lava are ejected from the vent: the outer crust chills quickly to form the glassy crust, but the interior remains hot, and continues to vesiculate, frothing up the interior. As in a decently baked loaf, the internal expansion causes the brittle outer crust to crack. This block was found in the Socompa debris avalanche deposit. It shows that hot magma was present within the volcano when the catastrophic flank failure took place, since the vesiculated brittle block would have broken up during transport. Similar blocks are often found as ejected bombs in vulcanian pyroclastic deposits. (Fig. 9.20).

Part 1       Part 2       Part 3       Part 4       Next Slide


 To VolcanoWorld