<P> The United States Geological Survey has referred to the 7,700 years ago Mazama eruption as the largest explosive eruption within the Cascades in the past million years, and one of the largest eruptions during the Holocene epoch . Considering Mazama's eruptive output within the past 420,000 years, it may have produced more than 43 cubic miles (180 km) of eruptive volume, making it either the third or fourth most productive Quaternary volcanic center in the Cascade Range . The eruption had a Volcanic Explosivity Index of 7, as determined from eruption cloud height, eruptive volume, and qualitative observations . </P> <P> Since the climactic eruption 7,700 years ago, all eruptive activity at Mazama has occurred within the caldera . After the caldera formed, the original crater was widened by avalanches from the walls . These gave a scalloped profile to the caldera, such as the coves at Llao Bay, Steel Bay, and Grotto Cove . The Chaski slide, for example, the most prominent landslide detectable on the caldera wall, occurred long after the formation of the crater . Located on the southern side, it contains lava blocks between 850 to 1,400 feet (260 to 430 m) in length, many of which now reside underwater on the floor of Crater Lake . As the crater filled with landslides, lava, and water, the caldera's appearance changed over time . New cones and lava fields have formed inside the caldera, all of which have been submerged except for Wizard Island . Mapping the caldera floor with high - resolution multibeam echo sounders, vehicle - motion sounders and navigators, and a dual - differential global positioning system (DGPS), scientists from the US government and universities detected volcanoes and landslide deposits under the water . Between 7,700 and 7,200 years ago, the central lava platform, the Merriam Cone, and Wizard Island were produced, as well as lava flows . These eruptions produced 1 cubic mile (4.2 km) of andesitic lava, half of that in the Wizard Island cone . Wizard Island lava interacted with water to form breccia piles, and as the water levels rose, only the top of the Wizard Island edifice stayed above the water . The edifice has a lavaberg shape, as it sits above a larger, ovular pedestal rising 1,200 feet (370 m) above the floor of Crater Lake; just two percent of Wizard Island sits above the water surface . At the summit of the above - water cone, a small lava flow from a lava fountain appears, located on the southwestern flank . There is also a volcanic plug poking out of the crater floor as a body of dark lava . Lava blocks with diameters of 6 to 7 feet (1.8 to 2.1 m) can be seen along the rim of the crater, possibly produced from later stages of cone - building eruptions that created Wizard Island . The central platform was created by a similar vent eastward of Wizard Island, made up of effusive lava that created lava fields northward and eastward of its vent . Merriam cone does not have a summit crater, but rises to 1,300 feet (400 m) above the northern lake floor . Merriam cone was produced underwater, and was probably formed at the same or similar time as Wizard Island and the central platform . </P> <P> Mazama was last active about 4,800 years ago, when an eruption took place near Wizard Island's eastern base . This eruption took place underwater, and it produced viscous lava that created a rhyodacite lava dome, about 2,400 years after the first period of postcaldera activity . Given that Mazama has had periods of sporadic eruptions for 420,000 years, the United States Geological Survey thinks that it is "virtually certain" that Mazama will erupt again in the future . Mazama is considered dormant, but it remains monitored by the United States Geological Survey Cascades Volcano Observatory . Future eruptions would likely take place near the western side of the complex and within the caldera rim; they could occur underwater . The ejection of lava rich in gas from shallow water could produce huge ash columns, but submarine eruptions at deeper depths may decrease the explosiveness of the event . Nonetheless, the rapid mixing of water and lava could produce dangerous pyroclastic surges, which are more gaseous and less solid than pyroclastic flows . Such flows could pass over topographic barriers, move rock fragments at rapid speeds of 800 feet per second (240 m / s), and travel several miles from their vent . Mazama is unlikely to produce mudflows far from the caldera, though a nearby vent outside the caldera could erupt and mix with snow . Eruptions are unlikely to produce waves that extend beyond Crater Lake, but powerful explosions could produce tall waves in the caldera . An eruption as explosive as the one 7,700 years ago is unlikely given that it would require larger volumes of magma than are known to be available within the Mazama vicinity . Moreover, landslides could flood adjacent shoreline regions, but they are not likely to induce failure of the caldera's walls, as they rise more than 500 feet (150 m) above the lake's surface . Earthquakes from the nearby West Klamath Lake fault zone could reach magnitudes up to 7.0 on the Richter scale, but these only occur every 3,000 to 10,000 years; they could generate tall waves by creating landslides . Though local earthquakes from volcanic activity would create motion in the lake, they would likely only reach maximum magnitudes of 5.0 on the Richter scale . However, Crater Lake is poorly monitored, and not highly active seismically among the Cascade volcanoes . The largest earthquake threat originates from the Cascadia subduction zone, which could produce earthquakes with a magnitude of 8 or 9 that could lead to huge waves in Crater Lake . </P> <P> Though the population within 6.2 miles (10 km) of Mount Mazama is only about 50 people, more than 270,000 live within 62 miles (100 km) of the volcano . Still, highly consequential eruptions at Mazama seem unlikely in the near future . Large, pyroclastic eruptions like the climactic eruption are unlikely for thousands of years . Similarly, the release of lethal gases like carbon dioxide from Crater Lake, similar to the eruption from Lake Nyos in Cameroon in 1986, is unlikely as a result of the natural mixing of deep and surface water within the lake . There is no known mechanism by which Crater Lake's water could be catastrophically drained or by which the caldera walls would fail suddenly . Even minor hydrothermal explosions could potentially be dangerous, as the ejection of ballistic blocks or tall waves could threaten people standing near the lake, traveling at speeds from 330 to 820 feet per second (100 to 250 m / s). Likewise, landslides could threaten visitors to the national park or researchers by creating tsunami - sized waves, also potentially damaging infrastructure surrounding the lake . </P>

When was the last time crater lake volcano erupted