Geologic – 04 Geologic History

Crater Lake National Park: Geologic Resources Management Issues Scoping Summary

Geologic History

The Cascades formed as a result of the collision of the San Juan de Fuca plate with the North American plate, a collision still in progress. The oceanic crust is being thrust under or subducted beneath the North American crust. This subduction began in the Early to Middle Tertiary as evidenced by the lava flows and pyroclastics of the Western Cascades. Some of these volcanics have been given formational names such as the Eocene Colestin Formation and the Little Butte Volcanics of Oligocene and early Miocene age (Baldwin, 1976). The Colestin Formation consists mainly of andesitic tuff, conglomerate, and tuffaceous sediments in the southern Oregon Cascades. The Little Butte Volcanics consist of a variety of flows and tuffs from 3,000 feet to 15,000 feet thick. Along the western margin of the Cascades in the northern part of Oregon, the middle Miocene flood basalts of the Columbia River overlie the Little Butte volcanics. Other volcanic formations of Middle to Upper Miocene include the Sardine Formation, Rhododendron Formation and the Boring Agglomerate (Baldwin, 1976). None of these formations are exposed in CRLA, but probably form the foundation upon which the volcanoes of the high Cascades are built.

By Late Miocene, as subduction continued, volcanic activity shifted slightly eastward forming the High Cascades. In the Pliocene, eruption of basalt and andesite flows built up lava plateaus upon which Mount Mazama began to form. A small island in Crater Lake known as the “Phantom Ship” is believed to be the remains of a small cone that formed in the Pleistocene as a precursor to Mazama. Mount Mazama is a composite volcano or stratovolcano; that is, it was built by alternating eruptions of lava flows (mostly andesitic) and pyroclastics, along with abundant intrusive dikes and sills. Mazama had a large number parasitic cones and flank eruptions represented today by Mount Scott (Harris and Tuttle, 1983).

During the Pleistocene, glaciation developed as Mazama began to grow. Much of Mazama was probably ice-covered during its eruptive history. The caldera walls of Crater Lake display glacial deposits interbedded with lava flows. The evidence indicates that there were several episodes of glacial advance and retreat, the latter likely due to eruptions of Mazama. Soil horizons with charred vegetation at the top of till deposits and overlain by lava flows indicate that there were quiescent periods between eruptions during which vegetation flourished.

Geomagnetic data indicate that most of the lavas of the High Cascades were of normal polarity. Since the last reversal (to normal polarity) took place about 700,000 years ago, most of the eruptive activity has taken place since that time (Cranson, 1982). At its maximum height, Mazama may have been over 12,000 feet and similar in size to Mt. Hood (Baldwin, 1976). Eruptions began to be more siliceous forming dacite domes, siliceous flows, and dacitic pyroclastics. These eruptions tended to be from vents north of the central vent: the vent of the Watchman andesite, the Hillman Peak cone, the vents of the Llao Rock, Cleetwood, and Redcloud dacite flows (Harris and Tuttle, 1983).

About 6,600 year ago Mount Mazama erupted a huge amount of ash that spread over the entire Pacific Northwest, from southern British Columbia and Alberta to northern California and Nevada, and east covering most of Idaho and the western one-third of Montana. It is estimated that 15 to 17 cubic miles of material was blown off the mountain. Mazama collapsed into a caldera 5 miles wide and 4,000 feet deep (Harris and Tuttle, 1983). It has been estimated that the amount of material blown out of the volcano plus the amount of lava that flowed out far exceeds the material of the cone that was lost. The difference must have come from the magma chamber which evacuated resulting in the collapse.

After this defining eruption, there was further activity that resulted in the formation of Wizard Island and two smaller cones on the caldera floor. Wizard Island rise about 2,600 feet above the caldera floor but is only 774 feet above water level. The larger of the two smaller cones, Merriam cone, lies near the southern edge of the caldera and, though submerged, rises about 1320 feet from the caldera floor (Harris, 1980). The accumulation of rain and snow resulted in the lake we see today.


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