🌻 Liz Madsen 🌻🌾🍂🍁🎃🇺🇸🇺🇦

Who doesn’t fall in love with a volcanic bomb? This is one from a basaltic fissure eruption in the Pliocene-Pleistocene Potrillo volcanic field on the flanks of the southern Rio Grande Rift just west of El Paso, Texas but in southern New Mexico. These are formed in Strombolian type eruptions in which gas laden basaltic magmas rise upwards and saturate, forming bubbles. These bubbles over pressurize and cause the lava to fragment, essentially the bubbles burst. But because the viscosity of basaltic liquids is low, these bubbles can migrate and some can float out of the lava or coalescece with other bubbles to form larger bubbles. These larger bubbles can range from cm scale to even meters. When these bubbles burst, they can launch a large molten slug of lava high into the sky. As the molten slug flies through the sky it chills from the outside in, freezing in whatever vesicularity the lava started with. In large slugs, however, there is not enough time for the interior to quench so when they hit the ground, they have a hard rind but a liquid core. Bubble nucleation and growth are controlled by kinetics, so somewhat delayed. Thus bubbles continue to nucleate and grow in the core even after the slug has come to rest on the ground. This is why the interior of the slug is characterized by larger bubbles and a more frothy texture. Because the core is still molten when it hits the ground, the impact causes the slug to flatten like a pancake as seen in this example. The quench rind also preserves the flow lines imparted by friction between the molten slug and the air as it flew through the sky. Now you know how volcanic bombs form and what controls their morphologies and textures.