Read the excerpt from "Geysers and Volcanoes."

Geysers erupt more often than volcanoes. Some geysers, such as Old Faithful, can erupt as often as every hour. Volcanoes are much less active, however. Most are quiet for years. Kilauea volcano in Hawaii is unusual. This volcano has been erupting without stopping since 1983. It may be the most active volcano in the world.

Which signal word in the passage is used to contrast geysers and volcanoes?

erupt
however
unusual
without