National Geographic : 1992 Dec
..- Approximate er . Calg limit of ashfall B.C. ALTA. SASK. cwr CAN Bi g blasts of the past U. S. volcanoes have often erupted N.DA with enormous force, sending ash *Bism far afield. Mount Mazama ex ploded around 4900 B.C ., forming e S. DAK. Crater Lake. An eruption at N Sioux Falls. Yellowstone 600,000 years ago was among the largest. Floods of basalt inundated Omaha IOW the Northwest about 15 million years ago. Lesser BTAH Ove 'ae flows and eruptions are shown in light red. Swichita MO The U. S. Geological Survey lists 44 sites iahomt with the potential querque OKLAto erupt, perhaps n Diego .7oe ** ARK. 5 explosively. Dallas Jacks Paso A Active volcano O o Potential volcanic activity O 400 e Recent volcanic rock (0-5 million years ago) MILES d1 Columbia River basalts (about is million years ago) NO CARTORAPHIC DIVISION (aou 15illo G ATCP S Measuring almost imperceptible changes in the landscape, lasers in the caldera at Long Valley, California, help gauge its potential for eruption. Earthquakes throughoutthe 1980s raised concerns at the nearby Mammoth Lakes resort area, background. Magma is on the move beneath the caldera, and Long Valley's violent past warrants the continued monitoring of the volcano.