The lava tubes people see most, and occasionally tour, are created by flowing lava (http://tmblr.co/Zyv2Js1YY_xmG ). The lava cools most rapidly at the top, causing a crust to form that stands up even if the lava flow ends. This is a different type of lava tube.
This lava tube is found alongside the Coso Volcanic Field in California and it was created by erosion. Water flowing over rapids and waterfalls formed eddies that stayed in the same spot and cut downwards. This entire cave is a vertical tube with an almost rifle-style pattern left on the walls. No scale is given but it’s probably less than a meter across.
“During thunderstorms, approximately ten Coulombs of charge — some 10^20 electrons — are exchanged with every bolt, representing the release of an incredible build-up of energy.
During a volcanic eruption, however, the incredible heats cause neutral atoms to become ions, either positively or negatively charged, which then separate due to differences in masses, temperatures and physical cross-sections. The aerodynamics separates the particles even farther, and when the threshold of breakdown voltage is crossed, a lightning strike occurs.”