Ring of Fire

There is no evidence that a catastrophic eruption at Yellowstone is imminent, and such events are unlikely to occur in the next few centuries. Scientists have also found no indication of an imminent smaller eruption of lava. 
Metropolitan Portland, Oregon, like Auckland, New Zealand, includes most of a Plio-Pleistocene volcanic field.
Ground deformation (swelling, subsidence, or cracking) is measured with a variety of techniques, including Electronic Distance Meters (EDM), the Global Positioning System (GPS), precise leveling surveys, strainmeters, and tiltmeters.
Lahars are commonly initiated by:
Instruments to measure sulfur dioxide and carbon dioxide can be mounted in aircraft to determine the quantity of gas being emitted on a daily basis. Such instruments can also be used in a ground-based mode.
Lahars can be of any size. They may be as small as several centimeters wide and deep, flowing less than one meter per second.
The Hawaiian shield volcanoes are the largest mountains on Earth.
Most volcanoes provide various types of warnings before eruptions begin. Although an explosive eruption could occur without warning, some premonitory events more likely will precede the next eruption.
Pyroclastic flows can be extremely destructive and deadly because of their high temperature and mobility.