Level Mountain Range - Geology - Eruptive History - Shield Volcano Eruptive Period

Shield Volcano Eruptive Period

The first phase of activity resulted in the creation of the massive oval-shaped shield volcano 14.9 million years ago during the mid-Miocene period. The shield volcano forms a broad cliff-bounded lava plateau, 70 km (43 mi) long and 45 km (28 mi) wide, with an average thickness of 750 m (2,461 ft) and an elevation of 1,400 m (4,593 ft) on the eroded Nahlin Plateau marking extensive Neogene regional uplift. The shield is built of four distinctive stratigraphic units dominated by 1 to 8 m thick basaltic lava flows separated by thin discontinuous breccias, sporadic tuff horizons, and local lenses of fluvial, lacustrine, and glacial sediment. Ropy pahoehoe lava flows predominate over blocky aa lava flows, breccias, and tuffs, attesting to a fluid and effusive character for the volcanism. Radial flow directions and quaquaversal dips suggest that most lava flows originated from central vents rather than flank vents. Long masses of magmatic rock exist throughout the shield but are rare, thin, and radially oriented. These long masses of magmatic rock, known as dikes, are intermittent features related to high-level volcanotectonic processess within the shield.

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