section_path: Erupted material/Lava flows
section_name: Lava flows
paragraph_text: Lava flows
  
  The form and style of an eruption of a volcano is largely determined by the composition of the lava it erupts. The viscosity (how fluid the lava is) and the amount of dissolved gas are the most important characteristics of magma, and both are largely determined by the amount of silica in the magma. Magma rich in silica is much more viscous than silica-poor magma, and silica-rich magma also tends to contain more dissolved gases.
  
  Lava can be broadly classified into four different compositions:
  
  Mafic lava flows show two varieties of surface texture: ʻ A ʻ a (pronounced) and pāhoehoe (), both Hawaiian words. ʻ A ʻ a is characterized by a rough, clinkery surface and is the typical texture of cooler basalt lava flows. Pāhoehoe is characterized by its smooth and often ropey or wrinkly surface and is generally formed from more fluid lava flows. Pāhoehoe flows are sometimes observed to transition to ʻ a ʻ a flows as they move away from the vent, but never the reverse.
  
  More silicic lava flows take the form of block lava, where the flow is covered with angular, vesicle-poor blocks. Rhyolitic flows typically consist largely of obsidian.