III. Lifetimes of Composite Volcanoes
It is important at the outset to distinguish the active life of a volcano from its longevity as a topographic entity. A volcano can exist as a topographic entity for a considerable time after its active lifedefined as the time span over which volcanic eruptions occuris over. The time span over which it remains recognizable is largely a function of degradation rate (the "background erosion" of Section IIE), which is a function of climate. In arid regions such as the central Andes, 20-Ma-old volcanoes are well preserved and easily recognizable. In more temperate climates, this time span is considerably shorter. The discussion below focuses on the active lifetime of a composite volcano.
The general impression from the sparse and largely poorly constrained geochronological data on composite volcanoes is that these are long-lived. Observational evidence suggests that during this long lifetime periods of rapid aggradation punctuate the slow background of degradation. Unfortunately, these data do not address more detailed questions that are critical to really addressing the time scales over which volcanoes operate. For instance, does a composite volcano form in one continuous episode or multiple episodes with periods of repose? How long are the eruptive episodes and the repose periods? What are the rates of eruption that characterize periods of aggradation? To answer these questions, one must address issues such as eruption rates during eruptive episodes vs background eruptive rates, episodic cone growth rates vs average long-term cone growth rates, length of repose intervals, eruption recurrence time scales, time-volume-composition relationships, and even the meaning of "active" and "dormant."
The paucity of knowledge exists because addressing these questions requires detailed high-precision geochronology tied to detailed stratigraphy and few studies that have attempted this. The seminal work on the Mount Adams volcanic field in the USA distinguishes itself as a standard, which has recently been complemented by studies of Tatara-San Pedro in central Chile and Tongariro on the North Island of New Zealand. These detailed studies afford a general framework (see Table IV) within which many of the questions above can be addressed. Some of the main observations are
1. Composite volcanoes grow in spurts. The main cone of Mount Adams was built in three main episodes between ~520-490, 460-425, and 40-10 ka at eruption rates of 1.6-5 km3/ka. At Tatara-San Pedro the two youngest composite cones of Pellado and Tatara-San Pedro formed between 188-83 and 90-19 ka, respectively. So these two composite cones formed during ~100 ka of eruptive activity at a minimum rate of ~0.2-0.3 km3/ka. At Tongariro, seven large cones of 2-17 km3 were built in periods ranging from 2.5-270 ka (or 2.5-70 ky) at rates of 0.09-1 km3/ka. These data suggest that the length and rate of cone-building episodes are quite variable.
Less comprehensive data from other places confirm this variability. The 250-km³ Kluchevskoy volcano (Kamchatka) has been built in the past 7000 years at an extremely high rate of 8-35 km³/ka, whereas the similarly symmetric cone of Mount Fuji (Japan) appears to have been constructed in two phases from 80 to 10 ka and from 5 ka to the present. At Mount St. Helens the 79- km³ cone has been built in the past 40 ka at a rate of 2 km³/ka, whereas the 76 km³ cone of Parinacota (Chile/Bolivia) is thought to have formed in the past 250 ka at peak rates of around 0.6 km3/ka. Santa Maria and Fuego (Guatemala) are thought to have been constructed entirely in the past 100,000 years.
These reasonably short (generally <100 ka) cone-building episodes might be considered the typical timescales to reach "maturity"an approach to the steady-state equilibrium after which overall size is maintained roughly constant by the opposing effects of construction and erosion discussed earlier and the physical controls outlined in Fig. 5. Increases in the volume of the overall edifice beyond this are typically a function of subsequent vent migration.
2. Composite volcanoes stay active for periods of up to 500 ka. Mount Adams in the Cascades dates back to ~520 ka and appears to be typical of other large composite cones in the Cascade arc of North America. Although the data are sparse, Mounts Baker, Rainier, Hood, Jefferson, Mazama, and Shasta are thought to have lasted 300 ka. Once a steady-state edifice has been constructed, there appears to be no clear relationship between volume and longevity.
The observations from the Cascades are ratified from other regions. Tongariro (New Zealand) had been active since ~250-275 ka. In the southern volcanic zone of the Andes, composite volcanoes are thought to have been active for periods of up to 300 ka. Volcan Parinacota, in the central Andes, was initiated ~250 ka, after the vent shifted slightly s outhward from its older twin cone Pomerape (Fig. 6e).
3. Volcanic systems may remain active between widely spaced pulses of peak activity. At Mount Adams, the main volcanic field remained continuously active from 940 to 10 ka with an average rate of eruption of 0.05-1 km³/ka with breaks in activity up to 30 ka. At Tatara-San Pedro, the system was active from ~930 to 19 ka at an overall eruption rate of 0.09-0.12 km³/yr (adjusted for glaciation associated volume reduction. At Tongariro, the background rate of activity since inception ~250-275 ka is 0.17-0.2 km³/ka and the background rates of activity are considerably lower than the peak rates of activity discussed earlier (Table IV).
4. Recurrence time intervals between eruptions at historicallyactive composite volcanoes display a wide range. Volcanoes such as Sakurajima (Kyushu) and Sangay (Ecuador) have been erupting persistently, on an almost daily basis for decades. More typically, composite volcanoes erupt on the 0.1 to 10-year timescale, as exemplified by Fuego (Guatemala), Klyuchevskoy (Kamchatka), Lascar (Chile), Pavlof (Alaska), and Arenal (Costa Rica). Activity at these volcanoes consists of clusters of minor outbursts, commonly vulcanian, of varying frequency. The next characteristic timescale is the hundred to hundreds of years timescale that characterizes many of our best-known recently active volcanoes such as Mount Pinatubo (Luzon), Mount St. Helens (Cascades), and Mount Fuji (Honshu). Lastly, volcanoes such as Mounts Baker, Hood, Rainier, and Adams that have had few true eruptions in the Holocene may typify a class of composite volcanoes that operate on the timescale of millennia.
TABLE IV Volume-Time Relationships for Well-Studied Active Composite Volcanoes
a Volume of lavas preserved. Original volume assuming 50-95% of material erupted between 930 and 200 ka has been removed by erosion due to glaciation is much higher.
b Based on preserved volume. If no erosion is assumed, rate would be 0.2-0.3, which is similar to peak rates.
c Applies to main period of cone growth, 110-130 ka.
No doubt our understanding of composite volcano behavior and evolution is biased by the historical record (Table IV), which may be too truncated relative to the timescales of cyclic volcanic activity to be representative.
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