Early activity The Mazama vicinity formed gradually, its earliest deposits consisting of dacite from up to 1.28 million years ago. Between 725,000 and 500,000 years ago, rhyodacite was erupted, eventually forming a lava dome field with a volume of and dimensions of . This featured up to 40 rhyodacitic domes and lava flows, produced between 470,000 and 410,000 years ago before stratocone formation began. Mazama formed as a group of overlapping volcanic edifices, which included shield volcanoes and small composite cones. Cone-building eruptions took place within short periods of time. Shield volcanoes fed Mazama's expansion with basaltic andesite lava flows that covered large expanses on the slopes of the mountain. Forming lava fountains similar to those observed in
Hawaiian eruptions, the shield volcanoes erupted incandescent lava bombs and were deposited on the southern, western, and eastern flanks of the complex. They have average thicknesses of . Roughly 215,000 years ago, another flank vent erupted dacitic lava that coursed to the west for at least , with volcanic
dikes that trend toward the vent still present in the Mazama caldera wall. The complex remained inactive for about 40,000 years before it resumed activity, erupting andesitic lava from another flank vent, building a large shield volcano from 170,000 to 120,000 years ago. Other andesitic lava deposits can be observed in the caldera wall's northern side. The Mazama complex was inactive between 100,000 and 75,000 years ago. About 75,000 years ago, the volcano erupted effusive lava flows of andesite that formed thick deposits below Hillman Peak that extended at least from the caldera rim. Many of the major cone-creating eruptions at Mazama were effusive rather than violently explosive, but explosive eruptions about 70,000 years ago yielded silicic lava that created thick pyroclastic deposits. These deposits include Pumice Castle, an orange edifice at the eastern wall of the caldera, which formed by the fusion of glassy pumice fragments. Similar activity on the northern side of Cloudcap and the eastern segment of Llao Rock that ejected dacitic tephra and pyroclastic rock also formed fused edifices. About 50,000 years ago, a vent at Mazama erupted the Watchman flow, which filled a canyon at the southwestern wall of the complex. From 50,000 to 40,000 years ago, Mazama vents continued to erupt andesite lava flows onto the northern and southwestern slopes and create dacite lava domes on the southern flanks. These domes often collapsed and produced
pyroclastic flows that coursed down the southern slope of the volcano, forming deposits up to Devil's Backbone, a craggy volcanic dike that was exhumed when the volcano collapsed. Throughout its eruptive history, Mazama has produced basaltic andesite, andesite, and dacite lava. About 40,000 years ago, it underwent a dramatic shift to solely rhyodacite lava, which was highly viscous and had a silica content at about 70 percent. Between 30,000 and 25,000 years ago, rhyodacitic eruptions occurred at the complex, yielding pumiceous
tephra and lava flows from Grouse Hill, Steel Bay, and Redcloud Cliff. At Redcloud Cliff, a lava flow formed with glassy columns that touched glaciers, creating a large, inverted stone triangle on the eastern rim of Mazama. These same eruptions formed a nearly vertical-walled crater, producing pumice and creating a dome over the Redcloud vent. Grouse Hill's lava flow deposit and lava dome formed at about the same time, about 27,000 years ago. At the end of this eruptive sequence, rhyodacite lava domes formed on the northeastern slopes of the volcano. Between 100 and 200 years before the climactic eruption, Llao Rock, a thick block of dark lava with a volume of , was produced from a rhyodacite flow erupted from Mazama. It has a round center and extensions on the sides, resembling a
bird of prey. The formation of Llao Rock was preceded by explosive eruptions of rhyodacite, creating pumice and ash that were ejected high into the atmosphere and carried for several hundred miles into northern and eastern Washington state, eastern Oregon, and western Nevada. A large crater formed but was filled by subsequent lava flows. Shortly before the major, caldera-forming eruption at Mazama, the Cleetwood flow was produced. Also composed of rhyodacite lava, it originated from a vent on the northern side of the complex eastward of Llao Rock. It likely only occurred weeks or months before the major eruption, as Mazama's collapse formed a backward flow of lava when it cut through the Cleetwood lava, suggesting that the Cleetwood lava was still hot enough to flow and move. Tephra from the last major eruption that was emplaced atop Cleetwood deposits has been altered by heat and gas from the Cleetwood lava. Both these eruptive periods took place on the northern flank of the Mazama complex, not far from the magma chamber that produced the climactic eruption shortly after.
Climactic eruption event, while the third drawing displays an image of steam eruptions. The final drawing depicts Mazama today, with Garfield Peak on the left, Wizard Island within Crater Lake, and Llao Rock to the right of the lake. Mazama's climactic eruption has been dated to about 6,845 ± 50 years ago via radiocarbon dating, or about 7,700 years ago via
dendrochronology. Other scientists have determined ages of 6,730 ± 40 years ago or roughly 7,470–7,620 calendar years ago, as well as 7,627 ± 150 calendar years ago. Fallout from the eruption continued for roughly three years, though the major eruption only occurred for a few days. The eruption is thought to have occurred during autumn, as inferred from pollen data. The eruptive activity that led to Mazama's collapse proceeded in two phases. During the first phase, shortly after the eruption of the
Cleetwood flow, a vent at a higher elevation on the northern side of the Mazama complex (but still below the summit) produced a high ash column into the
stratosphere and ejecting over half the eruption's total magma. The second stage erupted material from a ring of vents. the Mazama edifice had an estimated volume of , though it may have exceeded taking glacial erosion into account. After the climactic eruption, Mazama's peak was completely destroyed, replaced by a depression with a depth of surrounded by cliffs. The volume of magma erupted from Mazama during this eruption was 14 cubic miles (61 km3). The lava produced was zoned vertically according to arrangement within the source magma chamber, and was calc-alkaline in composition. It consisted of roughly 90 percent uniform rhyodacitic pumice, which contained about 10 percent
phenocrysts, the rest made up of crystalline andesite scoria and mafic crystals. The total eruptive volume adds up to 42 cubic miles (176 km3) which would make it over twice the size of the Mazama ash. Mazama's climactic eruption was 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 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 (VEI) of 7, as determined from eruption cloud height, eruptive volume, and qualitative observations.
Mazama Ash In the western United States and Canada, volcanic ash often forms distinct layers among recent geological deposits, which can be used for
stratigraphy among geologists and
archaeologists. Given the large distribution of the historic eruption of Mount Mazama,
Mazama Ash serves as a common geological marker. Mazama Ash had a minimum fallout area of , while ash from northwestern Washington's
Glacier Peak volcano, known as Glacier Peak Ash, encompassed an area of more than .
Mazama Ash is found at a higher layer than the
Glacier Peak ash, estimated to have deposited over 13,000 years ago. Mazama Ash is the most widely distributed
tephra layer from the late Quaternary in the United States and southwestern Canada, extending to eight states to the west and three Canadian provinces. It forms orange colored deposits. Like the Glacier Peak Ash deposits, Mazama Ash is well preserved in the Pacific Northwest. It is distinguishable from the lump pumice deposits ejected from the Glacier Peak volcano, which contain more
phenocrysts. Mazama Ash also has more
soda,
yttrium,
ytterbium, and
zirconium, and less silica and lime than eruptive products from Glacier Peak, and it forms finer deposits than Glacier Peak Ash. With an age between 6,000 and 7,000 years, Mazama Ash corresponds to the estimated time for Mazama's climactic eruption 6,600 years ago, identified by
radiocarbon dating of wood charred by ashflows. Some of the Glacier Peak ash has been found in strata beneath Mazama Ash in Washington state and in eastern
Idaho, to the southeast of
Glacier Peak. Studies of Glacier Peak Ash deposits suggest that they are older than Mazama Ash, at about 12,000 years old. Glacier Peak Ash has not been found in deposits less than 10,000 years old, and nearly all human artifacts have been found above its deposits, except for a site at Birch Creek where non-diagnostic artifacts were discovered beneath Glacier Peak Ash.
Recent activity and potential hazards 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 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 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 above the floor of Crater Lake; just two percent of Wizard Island sits above the water surface. 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 , 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. Landslides could flood adjacent shoreline regions, but they are not likely to induce failure of the caldera's walls, as they rise more than 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. 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. Though the population within of Mount Mazama is only about 50 people, more than 270,000 live within of the volcano. traveling at speeds from . Landslides could threaten visitors to the national park or researchers by creating tsunami-sized waves, also potentially damaging infrastructure surrounding the lake. == Geothermal energy ==