Obsidian occurs as a flow, not as an explosive eruption in contrast to a vesiculated rhoylite pumice or dacite. This difference is due to the difference in composition, specifically volatile, gas, content. If magma cools slowly, deep within the crust, the resulting rock is called intrusive or plutonic. The slow cooling process allows crystals to grow large, giving the intrusive igneous rock a coarse-grained or phaneritic texture. The individual crystals in phaneritic texture are readily visible to the unaided eye.

Are obsidian explosive or slow flow and what slow cooling produces igneous rocks with large crystals and a texture?

All materials are destroyed explosively when their chemical bonds are collectively destroyed. Any material under stress begins by accumulating energy, transforming it with the progressive breaking of bonds before the final stage of explosion. This sequence depends on time, temperature, the material's polarization properties and the strain rate . When the material is cooled below its glass transition temperature, its energy storage capacity is increased, and the transformation will take place with crystallization, but probably with an increase in porosity. In the presence of a fluid, the zeta potential will either accelerate or retard the transformation. A proper analysis of the rock should enable us to predict all its behavior under stress, including seismic behavior.

Regards.

Dr Claude Le Gressus thank you for your contribution to the discussion

Slow cooling means the individual mineral grains have a very long time to grow, so they grow to a relatively large size. Intrusive rocks have a coarse grained texture. If magma cools slowly, deep within the crust, the resulting rock is called intrusive or plutonic. The slow cooling process allows crystals to grow large, giving the intrusive igneous rock a coarse-grained or phaneritic texture. Igneous rocks contain randomly-arranged crystals. If the magma cools quickly, the crystals will be large. If the magma cools slowly, the crystals will be small. Igneous rocks contain randomly-arranged crystals. If the magma cools quickly, the crystals will be large (coarse grained). If the magma cools slowly, the crystals will be small. Obsidian occurs as a flow, not as an explosive eruption in contrast to a vesiculated rhoylite pumice or dacite. This difference is due to the difference in composition, specifically volatile, gas, content.