Hoodoo Mountain - Eruptive History

Eruptive History

Hoodoo Mountain is a well-exposed example of peralkaline, phonolitic ice-contact and subglacial volcanism. Studies have shown that a cycle occurs about every 24,000 years, after which activity shifts from subglacial to post-glacial eruptions and then back to subglacial again. This cyclical activity is unique to Hoodoo Mountain among the Northern Cordilleran volcanoes. Three cycles have been clearly identified, including one subglacial eruption and two post-glacial eruptions. Records show that from 85,000 to 80,000 years ago, activity was more intense than it is today. More than 90% of Hoodoo Mountain, which dates back to at least 100,000 years, formed during its early eruptive history and has grown steadily since then. Regional ice thickness during some of Hoodoo Mountain's subglacial eruptions might have been more than 2 km (1 mi) in thickness.

The main volcanics at Hoodoo Mountain are phonolite and comendic trachyte lavas and hyaloclastites, although some pyroclastic rocks also exist. The trachyte and comendic magmas erupted from Hoodoo Mountain and other Northern Cordilleran volcanoes are thought to have been created by fractionation of primary alkali basaltic magma in crustal reservoirs. The estimated volume of erupted material from Hoodoo Mountain is 17 km3 (4 cu mi), and at least six phases of eruptive activity are recognized at the volcano. The first phase of eruptive activity occurred 85,000 years ago in the Late Pleistocene stage of the Pleistocene epoch, producing lava flows, lava domes, and breccia deposits. These volcanics were erupted under more than 100 m (328 ft) of glacial ice to produce subglacial eruptions.

During the second eruptive phase 80,000 years ago, eruptions were no longer subglacial, but the volcano was surrounded by ice at least 800 m (2,625 ft) thick. Lava flows during this eruptive phase melted some of the thick glacial ice and ponded around the perimeters of Hoodoo Mountain, which quickly solidified to form a barrier behind which more lava piled up. Such ice-marginal lava flows are anomalously thick and are normally bounded by steep cliffs with fine-scale cooling joints and lots of volcanic glass. These ice-marginal lava flows form the massive lava cliffs near the base of Hoodoo Mountain and are among Hoodoo's most prominent features.

The third eruptive phase between 80 and 54 thousand years ago was followed by explosive activity that produced pyroclastic flows down the northern and western flanks of the mountain to create welded and unwelded ignimbrite deposits up to 100 m (328 ft) thick. This explosive activity is one of the few explosive eruptions throughout Hoodoo Mountain's eruptive history, and could have been caused by magma breaching thinner ice.

During the fourth phase of eruptive activity 54,000 years ago, eruptions were again no longer subglacial. Lava flows from this phase of activity overlie pyroclastic deposits from the third phase of activity and ice-dammed lavas from the seconed phase of activity.

Between 54 and 30 thousand years ago, the fifth phase of eruptive activity occurred subglacially and produced two distinct types of lava-breccia associations. The first subglacial eruption between 54 and 40 thousand years ago produced lava domes and monomict breccia, while the other subglacial eruption between 40 and 30 thousand years ago took place under thinner glacial ice.

The final phase of eruptive activity was effusive in nature with no explosive activity (VEI-0). This effusive volcanism produced lava flows with well-preserved lava channels on the northwest and southwest flanks of Hoodoo Mountain in the early Holocene epoch and are largely unglaciated, suggesting the final eruptive phase occurred in an ice-free environment. These lava flows originated from the mountain's flat-topped summit and volcanic vents on its flanks. Geologists do not always agree on the dates of these more recent eruptions, some dating them to nine thousand years ago, others to as recently as seven thousand years ago.