The Union Peak Volcano

     The Main Shield

Before glaciers modified its form, the volcano must have had a shape approximately like that of an inverted shield or saucer, as do many Icelandic volcanoes. It was not, however, a simple symmetrical mass, for in the later stages of its growth the products of eruption from the summit crater were augmented by thick and extensive flows from fissures and parasitic cones on. the lower flanks. For example, the massive flows of Bald Top undoubtedly issued from lateral fissures. Not far from the base of Bald Top, the pale-gray lavas are cut by irregular intrusions of dark, olivine-rich diabase. Similar intrusions elsewhere in the High Cascades are invariably related to volcanic vents.

There must also have been parasitic vents or fissures at the northeast base of the volcano from which large volumes of lava were extruded. One of these lay on or close to Arant Point. At the base of the bluff are dark-gray, dense lavas characterized by closely set, steep joints. Higher up, the lavas are much paler and the joints more widely spaced. On the summit the flow planes, which are gently inclined elsewhere, stand vertically or at high angles. These features suggest that Arant Point is the eroded remnant of a domical protrusion associated with a short and viscous flow from a near-by vent.

West of Bear Bluff, and at a few other places on Union Peak, there are narrow rock ridges where the flow planes run lengthwise and stand at high angles, as if the ridges represented the fissure feeders of flows. Very likely the thick, flat-topped pile of lava that forms Whitehorse Bluff escaped from such fissures.

But that most of the lavas of the Union Peak volcano were erupted from the central, summit vent is apparent from their quaquaversal dips. It is clear also that most of the flows were quite fluid, for thin sheets may be traced for long distances. Just to the west of the summit pinnacle, as many as twenty-five superposed flows may be seen in a small cliff section, their average thickness being no more than about 3 feet. Elsewhere flows 10 feet thick are not uncommon. Apparently the thicker flows are restricted to the lower slopes of the volcano, and probably most of them escaped from subsidiary vents.

A brief acquaintance suffices to distinguish the lavas of the Union Peak volcano from the andesites of Mount Mazama. The outstanding differences are briefly these: (I) Few of the Union Peak lavas carry large crystals of feldspar such as typify the Mazama flows; (2) they are rarely free from granules of olivine, a mineral notably rare in the Mazama andesites; (3) they are much poorer in basic inclusions; and (4) they are generally much more strongly jointed.

The dominant lava of Union Peak is a pale-gray, extremely dense rock relieved only by specks of olivine and pyroxene and tiny crystals of plagioclase. Normally the lavas weather to pale-gray and whitish crusts, though the joint planes are commonly tinted pink, owing to oxidation of iron by residual gases. In some flows, the tops and bottoms are extremely vesicular. Such scoriaceous flows are invariably darker than the poorly vesiculated flows, and are also distinguished by a higher content of glass. This relation between vesicularity on the one hand, and dark color and abundance of glass on the other, has been observed on many other Cascade volcanoes. Apparently the development of vesicularity leads to quick chilling of the magnetite-rich residual liquor; hence the dark, glassy groundmass. With the lava types just mentioned, and scattered haphazard among them, are deeply reddened flows which must have been exceptionally rich in gas.

Small amounts of cristobalite, tridymite, and opal are ubiquitous in the products of Union Peak. The opal normally occupies amygdules, but the other two minerals usually occur on joint planes or in seams cutting the lavas at random.

Explosive eruptions contributed little to the growth of the Union Peak volcano. The activity was almost wholly of the quiet, effusive type, so that lavas make up at least 95 per cent of the volume. Thin interbeds of red scoria indicate mild Strombolian eruptions at long intervals.

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