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Please note that not everyone in your class is likely to be at exactly the same level . With existing technology it is likely that dangerous volcanoes will continue to go unmonitored . This limitation is because the baseline , which is at its maximum of 2 km at the equator , decreases to the minimum usable ( about 700 m ) at those latitudes ( Vincent , 1993 ) . It is worth noting that deformation is likely to precede measurable seismic activity in many cases - - e.g ., a rock must be stressed before it fails - - suggesting that deformation changes may be the first indicator of an impending eruption . Red overcolor is not as important as the background color , which is the best visual indicator of ripeness . Later , the tips of infested iris turn brown and appear to senesce ( age prematurely ) , although the entire plant seldom dies . Currently , Aniakchak is not being monitored . Aluminum sulfate is not recommended as high rates of this compound can be toxic to roots . Moreover , SAR has a generic advantage over optical sensors because atmospheric correction is not needed . Control is not possible if root rot is extensive and infected plants should be discarded . In the future , it is likely that orbital SAR systems will be specifically designed to facilitate acquisition of interferometric data , and thus will include GPS and other tracking techniques to provide accurate information on the position and orientation of the platform . The SIR-C instrument addresses some of these concerns , although the low orbit inclination ( < 57deg . While many volcanoes experience periods of unrest and deformation that do not culminate in major eruptions , it is likely that many ( perhaps most ) of the world 's volcanoes that actually do erupt experience significant preeruption surface deformation ( Newhall and Dzurisin , 1988 ; Thatcher , 1990 ; Shimada et al ., 1990 ) .