Although they are often grouped together as one unit, the three mountains have their own individual geology and eruptive history. Neither North Sister nor Middle Sister has erupted in the last 14,000 years, and it is considered unlikely that either will ever erupt again. South Sister last erupted about 2,000 years ago and could erupt in the future, threatening life within the region. After satellite imagery detected tectonic uplift near South Sister in 2000, the United States Geological Survey improved monitoring in the immediate area.