Models of Composite Cones

To find and develop a hydrothermal system associated with a composite cone, it is necessary to understand the cone's structural framework, intrusive "plumbing," and thermal state. This information must be inferred from clues at the surface such as surface manifestations of hydrothermal activity, the age and composition of the volcano, and its fractures, faults, and hydrology (Sibbett, 1988). The ability to relate surface features to interior structure and thermal state depends on models that have been developed from geophysical models or through the examination of eroded composite cones. A variety of these models are discussed here; each is based on a different approach or tectonic setting.

Models Based on Mapping and Mining of Porphyry Copper Deposits in Deeply Eroded Composite Cones

Branch (1976) studied many composite cones in Papua New Guinea to determine if they contained volcanogenic ore bodies. Branch published a model of composite cones (shown in Fig. 7.15) that was based on his field observations of basaltic to andesitic volcanoes in various evolutionary stages. The composite cones developed over an island arc subduction zone in a region with a relatively thin crust. They were composed of interbedded lava flows, laharic breccias, and other pyroclastic deposits. Eruptive cycles were as short as 1 year or as long as 10,000 years; during the latest growth stages, the cones were permeated with shallow magma chambers, sills, and dikes. Hydrothermal systems developed during later stages extend into the core of the cone and through underlying basement

Fig. 7.14
North-south cross sections of Mount St. Helens in Washington illustrate changes resulting from
the sector collapse and eruption of May 18, 1980, and subsequent dome growth. (a) A pre-eruption
cross section shows: (1) the older volcanic center, consisting of nested dacite domes, pyroclastic 
flows, and mudflow breccias, which is cut by dikes; (2) andesitic and basaltic lava flows interbedded 
with volcanic breccias and scoria; and (3) several dacitic domes: the summit dome, erupted 370 years 
BP and the north slope dome erupted between 180 and 138 years ago). (b) A posteruption cross
section through the crater caused by sector collapse and the avalanche that preceded the most 
explosive phase of the eruption. Present-day development of the growing dacite dome within the 
crater is indicated. (Adapted from Voight et al ., 1981.)

rocks to form 1- to 2-km-diameter aureoles of hydro-thermal alteration around intrusions. Surface indicators of the hydrothermal system include andesitic, dacitic, or rhyolitic vents; fumaroles and sulfur deposits at the summit; collapse craters; hydrothermal explosion breccias; and a long cone history (>100,000 years).

Sillitoe's (1973) general model for composite cones and associated intrusive rocks was based on his observations of copper-porphyry deposits in the Andes of Chile and northern Argentina—a region where volcanoes overlie thick crust. The hydrothermal systems responsible for deposition of the porphyry copper deposits were established late in the history of the composite cones; subsequently, only dacitic or rhyolitic magmas were intruded and erupted. Observations of these systems at various levels exposed by erosion (or in mine shafts) indicate that large granodiorite plutons are present at shallow depths (~4 km below the summits) during late-stage activity (Fig. 7.16). The chief distinction between these volcanoes and those in island arcs is the difference in crustal thicknesses beneath them; processes within the thick crust of the Andean altiplano ultimately produce large silicic magma bodies that are emplaced at shallow depths.

Giggenbach (1989) developed a similar model for an Andean composite cone, the young, active volcano of Nevado del Ruiz in Colombia. His model was based upon the analysis of gases and waters from Ruiz' fumaroles and hot springs. According to Giggenbach's interpretation, there is a broad mass of crystallizing, degassing magma and rock from ~3 km to >15 km below the summit (the volcano's elevation is 5389 m). Intrusions below Ruiz may be surrounded by aureoles of vapor, a mixture of brine and vapor, and brine. The vapor and vapor-plus-brine aureoles are of the same shape and

Fig. 7.15
Simplified cross section through a "nearly extinct" mature composite cone. This model was developed
by Branch (1976), who studied many composite cones in Papua New Guinea to determine their
potential for volcanogenic ore bodies. The model is based upon studies of
volcanoes in various evolutionary stages.

areal extent as the zone of hydrothermal alteration within a mature andean composite cone, which was proposed by Sillitoe (1973). The model for Nevado del Ruiz has not yet been tested by drilling.

Well-Mapped Examples of Eroded Composite Cones

Composite cones formed during Tertiary time usually have been deeply eroded and their interior structures are exposed. However, these cones are still sufficiently preserved to allow interpretation of the relationship between rock types and the structural framework. The cones we discuss here were chosen because their carefully executed three-dimensional maps with cross sections provide excellent examples of older composite cones.

Broken Top Volcano

At Broken Top volcano, in the southern Cascade Range of Oregon in the U.S., dissection by glaciation has provided a clear view of its interior workings (Crowe and Nolf, 1977). The cone consists of interbedded lava flows, laharic breccias, and other pyroclastic deposits illustrated in Fig. 7.17. The tall cone could not have remained standing without its complex internal framework, which is composed of dikes, sills, and small plutons. Early phases of cone construction were followed by collapse and the formation of a small summit caldera. After further cone construction, the edifice was intruded by plugs, radial and concentric dikes, and sills. Intrusions at Broken Top volcano constitute 5 to 20% of the cone volume.

Tieton Volcano

The Miocene-age Tieton Volcano of Washington has been deeply eroded, exposing radial dike swarms and plugs (Swanson, 1966). Originally, Tieton volcano had a basal diameter of ~11 km and a height of 2.4 km. The composite cone, made up of interbedded breccias, pyroclastic deposits, and block lava flows, overlies a shield composed of andesitic lavas. Within the southern, exposed half of the volcano, 200 dikes form a radial swarm; individual dikes are 2 to 6 m thick, are steeply dipping (70 to 90°), and are mostly andesitic. These dikes would have provided a substantial heat source if they were intruded over a fairly short period of time, but not if they were intruded piecemeal over tens of thousands of years. Such dikes

Fig. 7.16
This idealized cross section of a composite
cone and associated intrusive rocks is based
on observations of copper-porphyry
deposits in the Andes of Chile and northern
Argentina. Hydrothermal systems
responsible for the porphyry copper
deposits were established late in the history
of the composite cone; the intrusion and
eruption of rhyolitic magmas followed. In
this model, which is based on a region
with thick crust, the vertical and
horizontal scales are the same.
(After Sillitoe, 1973.)

Fig. 7.17
A cross section of Broken Top volcano in the southern Cascade Range of Oregon,
in the U.S. The cone consists of interbedded lava flows, laharic breccias, and other pyroclastic
deposits. The cone could not have supported itself without the complex internal framework of
dikes, sills, and small plutons—all of which give tall composite cones their stability.
(Adapted from Crowe and Nolf, 1977.)

Fig. 7.18
Diagram showing typical dike patterns radial
to the central conduit of a composite cone
and the parasitic cones on the volcano
flanks. The dike swarms follow the trend
of maximum horizontal compression.
(Adapted from Nakamura et al., 1977.)

also act as barriers to groundwater flow and thus can "compartmentalize" aquifers or parts of a hydrothermal system.

Volcanoes of the Aleutian Arc and Alaskan Peninsula

In their study of arc volcanoes of the Aleutian Islands and Alaskan Peninsula, Nakamura et al . (1977) mapped radial dike patterns and parasitic cones on volcano flanks. They concluded that dikes and flank vents on composite cones form elongate swarms in regions under compression and that the swarms follow the trend of the maximum horizontal compression (Fig. 7.18). Inference of the location and dimensions of such dike swarms must be made when siting exploration wells because although they provide the heat source, they also may act as barriers to groundwater flow (see Chapter 6).

Nakamura's observations can be applied to many composite cones that exhibit sector collapse. This type of collapse may occur parallel to the dilational stress within the volcano when the volcano's flanks are forced outward (Siebert, 1984). However, it is more likely that the collapse process is related to the shape of the dike-sill complex within the cone. Composite cones devel-

oped in regions with homogeneous stress are supported by a radial framework of dikes and sills, whereas cones developed in regions strongly influenced by the regional stress regime have dikes that are located mostly along a line parallel to the maximum horizontal compression. Parallel dike systems support only the part of the volcano below a line of parasitic vents; unsupported flanks, with only rare dikes or sills, are subject to sector collapse.

Summer Coon Volcano

The mid-Tertiary-age Summer Coon composite cone in Colorado, mapped by Lipman (1968), is a deeply dissected cone in which the nearly circular complex of stocks is exposed, as are the radial dikes shown in Fig. 7.19. These silicic dikes are as much as 4.8 km long and 60 m wide. Remnants of the cone consist of interbedded tuff-breccias and lava flows of mafic to intermediate

Fig. 7.19
Geologic map and cross section of the mid-Tertiary-age Summer Coon
composite cone in Colorado. This is a deeply dissected cone in which both the
nearly circular complex of stocks and the radial dikes are exposed.
(Adapted from Lipman, 1968.)

compositions. The symmetry of the dike complex reflects fracturing and dike emplacement controlled by stress around the central pluton or plutons and little superimposed tectonic control. The volcano's location on thick continental crust makes the Summer Coon volcano model more similar to volcanoes of the Andean altiplano than to the island arc volcanoes.

Mount St. Helens

Major structural changes to Mount St. Helens in Washington were caused by the sector collapse and eruption on May 18, 1980, in addition to subsequent dome growth. Before the eruption, Mt. Saint Helens consisted of (1) an older volcanic center with nested dacite domes, pyroclastic flows, and mudflow breccias, which was cut by dikes (Fig. 7.13); (2) andesitic and basaltic lava flows interbedded with volcanic breccias and scoria; and (3) dacitic domes (the 370-year BP summit dome and the 180- to 138-year BP domes of the north slope).

After the sector collapse and an avalanche that preceded the most explosive phase of the eruption at Mount St. Helens, an ampitheater-shaped crater remained. The dacite dome within the crater continues to grow (Voight et al ., 1981).

A Facies Model

Ruapehu

Ruapehu volcano in New Zealand, elevation 2797 m, was constructed by four major cone-building phases over the last 250,000 years. The 110-km³ composite cone is characterized by many abrupt lateral and vertical facies changes (Hackett and Houghton, 1989).

The central and flank vents, aligned along a north-northeast-trending lineament, consist of plug- and dome-like intrusions, thin lava flows, welded pyroclastic fall deposits, and vent breccias. Most of the vent areas have been hydrothermally altered. Proximal (near-vent) facies are made up of mostly block lava flows and lesser ashfall and laharic breccia deposits. The near-vent cones were intruded by numerous thin (0.5- to 5-m wide) dikes that are compositionally identical to adjacent lavas.

Hackett and Houghton concluded that the distal (ring plain) facies consists of interbedded epiclastic volcanic deposits that are interbedded with ashfall deposits and fluvially reworked avalanche or volcanic mudflows. This plain forms a 6- to 15-km-wide girdle around the composite cone, which is formed by overlapping alluvial fans. Hummocky mounds on the plain are part of a sector collapse avalanche deposit.

Hackett and Houghton also determined that the cones at Ruapehu are mostly constructed of lavas and domes and that pyroclastic and epiclastic materials are found mostly on the ring plain.

A Model Based on Heat Flow Measurements

Hakone Volcano

Iriyama and Oki (1978) measured temperature profiles in Hakone Volcano, Japan. However, the model was for a composite cone that grew within a small caldera. Hydrothermal activity is most intense in the eastern half of the caldera—probably because recharged groundwater from the lake in the western half of the caldera flows toward sea level in the east (Fig. 7.20). The thermal regime is most likely related to the caldera and not to the overlying composite cone.