Volcanoes can be either tall and thin (see above picture), or wide and squat. They are randomly generated, so you will never see the same volcano twice. They are made out of a new block called Basalt, which has no use other than aesthetics and microblocks. It also has 1-3 lava source blocks on or near the top, so you can comprehensively never go underground to get enough obsidian to make a nether portal. However, volcanoes are somewhat rare, so finding enough of them to make a nether portal is near impossible. Also, since the lava generates in a random location on the volcano, forest fires are a real danger, so be on the lookout when exploring.