The Quaternary edifice of Mount St. Helens volcano was built upon a deeply eroded terrane of gently folded and altered volcanic and plutonic rocks that represent the core of the Tertiary Cascade magmatic arc. These rocks constitute an east dipping homoclinal sequence, several kilometers thick, of subaerially erupted mafic to silicic flows and volcaniclastic strata; K-Ar ages from this section range from about 28 to 23 Ma (late Oligocene and earliest Miocene), which corresponds to an apparent lull in Cascade volcanism to the north of Mount Rainier. Volcanism was essentially continuous during this period of time, and neither a well-defined base nor top of the section is exposed within the mapped area. Basalt and basaltic andesite dominate the lower part of the mid-Tertiary section, whereas andesitic and dacitic rocks comprise most of the upper part. This section was intruded by numerous mafic to silicic dikes, sills, and irregular plutonic bodies, most no more than a few million years younger than their host rocks, and subjected to pervasive burial metamorphism and widespread hydrothermal alteration. Large areas of hornfelsed rock surrounding even relatively small intrusions indicate that the proportion of plutonic rock becomes significantly greater at shallow depth beneath the existing erosion surface.

A large granitic pluton intruded the mid-Tertiary section north of Spirit Lake at about 21 Ma. The Earl porohyry copper deposit occurs within the pluton but appears too young (17 Ma) to be genetically related to it. In contrast to the rather continuous and voluminous Oligocene to early Miocene activity, volcanism since then in the Mount St. Helens area has been localized and volumetrically minor. Products of three younger eruptive periods have been recognized: a sequence of 15 m.y. old pyroxene andesite flows resting unconformably on mid-Tertiary strata south of Mount St. Helens, widespread shallow dikes and sills of pyroxene andesite between 10 and 8 m.y. old, and compositionally diverse rocks erupted during the past 3 m.y. The Quaternary lavas are more potassic than the Tertiary lavas and typically contain phenocrysts of hornblende and biotite, which are absent from the older rocks. A number of Tertiary structures define a broad NNE trending zone that may reflect a deep-seated lithospheric flaw that has controlled the locus of Cascade magmatism in southern Washington for the past 25 m.y. Mount St. Helens lies within this zone at the intersection of the NNW striking St. Helens seismic zone (SHZ) and an ENE trending alignment of Pleistocene silicic plug-domes. No surface breakage has been detected along the SHZ, which is apparently very young.

The linear zone of silicic vents is probably controlled by a fault that has been interpreted from seismic records to occur directly beneath the volcano. This zone parallels the directions of regional maximum horizontal compressive stress and North America/Juan de Fuca plate convergence. Mount St. Helens is an example of a low-volume tectonically controlled magmatic system in an early stage of development. ¿American Geophysical Union 1987