Which is most clearly a topic sentence? Volcanoes may be scary, but they are the result of simple and very
natural processes. Volcanoes are holes in the Earth's crust where
substances from beneath the surface, such as hot gases and molten
rock, can escape. "You can think of a volcano as a paper cut or a
pimple on the Earth," says volcanologist Roberta Alvarez. They're
usually not a big deal on the scale of the whole planet, but they're a big
deal to us." Sometimes these eruptions can be small and ongoing,
such as the Mt. Kilauea eruption in Hawaii that has been occurring
since 1983. Other times, as was the case with Mt. St. Helens in
Oregon, the eruptions are abrupt and violent. When the lava and ash
emitted from a volcano cool and harden, they gradually form
mountains.