22 The Union Peak Volcano – The Summit Plug

The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams

The Foundations of Mount Mazama


The Union Peak Volcano

     The Summit Plug

The conspicuous pinnacle at the top of Union Peak owes its existence to a vertical-sided intrusion into the summit cone. So does the great monolith of Thielsen. The Union Peak plug, like the enclosing cone, is approximately oval in outline and elongated in a north-south direction. It measures 1/2 by 1/3 mile across. The material of which it is composed does not differ essentially from the lavas of the surrounding shield, though much of it is slightly coarser. By far the dominant rock is a pale-gray, massive lava cut by strong joints of irregular trend, the spacing of which generally becomes closer toward the margins of the intrusion. Locally this pale lava is microvesicular, but large vesicles are rare. Flow banding is obscure. Along the northern base of the intrusion, the banding may lie flat or dip inward at low angles, but elsewhere it dips inward steeply or stands vertically. Associated with the pale lava is a darker and denser lava which seems to have been injected later.

After the plug had congealed in the central conduit, iron- and silica-bearing gases continued to rise through it, depositing plates of hematite and crystals of tridymite and cristobalite on joint faces and on the sides of irregular vesicles. Especially fine specimens may be obtained near the eastern margin of the plug on the trail to the summit.

Mention may be made in passing that on the topmost crags of Union Peak the lava has suffered considerable fusion from strokes of lightning. Particularly is this the case on the edges and corners of protruding rocks, though flat faces may also be coated with thin ribbons of olive-green, vesicular glass and punctured by tiny glass-lined pits. Diller has described similar fulgurites from the summit of Mount Thielsen.23 They are rarely absent from the summits of the higher volcanoes of the Cascade Range.

Finally, when the plug was intruded into the summit cone of Union Peak, it not only deflected the bedding of the enclosing tuffs, but enveloped part of the cone and carried it upward. This explains the occurrence of three detached masses of well bedded, gray and black, scoriaceous tuff breccia within the plug.

Whether the lava forming the Union Peak plug rose above the floor of the summit crater as a steep-sided Pelean dome or spine, or failed altogether to escape from the conduit, cannot be determined. The manner in which the included and enclosing tuffs were disturbed suggests that when the lava rose it was extremely viscous. Even after parts of the plug had solidified, there may have been continued differential uplift as new magma came from below. Such uplift might account for the polished and slickensided faces found on some of the blocks near the summit. The blocky carapaces of Pelean domes commonly show the effects of differential movement in this manner.