Pavlof: A volcano without typical signs of unrest?

The ash plume produced during the August 2007 Pavlof eruption in Alaska was approximately 17,000 to 18,000 feet high. (Photo by Chris Waythomas, 2007, Alaska Volcano Observatory/U.S. Geological Survey).

As magma migrates to shallow depths, it causes the volcano to grow and expand, resulting in upward and outward ground displacements that can be seen on local ground deformation instruments. The increased pressure below the surface — due to moving magma — can cause the surrounding rocks to break, resulting in small earthquakes. But do volcanoes always show such clear indicators that they may erupt?