Download Calc alk volcanism

Calc-alkaline Volcanism - Case Histories
Cascades
The eruption of Mt. St. Helens in 1980 served as a wake-up call for people living in the
Pacific Northwest. The blast, the largest in two centuries, devastated 200 square miles,
killed 57 people and buried towns up to 250 miles away in several centimeters of ash. As
large as the eruption was, it pales in comparison to the eruption of Mt. Mazma 6000 years
ago.
Both Mt. Mazma and Mt. St. Helens are a part of the Cascade Range which stretches
from southern British Columbia to northern California (See below). Technically, both Mt.
Shasta and Lassen Peak in California are a part of the range. Most geologists would place
the southern terminus of the Cascades at Sutter's Butte, an extinct volcano just northeast
of Sacramento. Interestingly, if one steps east and south into the Sierra Nevada a distance
of about 120 miles you are at Mammoth and the Long Valley caldera. Is the volcanic
activity at Mammoth related to that of the Cascades?
The volcanism of the Cascades is thought to be due to the subduction of the Juan de Fuca,
Gorda and Explorer plates beneath the Pacific Northwest. The trench, located 50 miles
off the coastline is termed the Cascadia Subduction Zone. The above three plates are
actually the remaining small slivers of the much larger Farallon plate, the same plate
responsible for Miocene subduction in southern California. When the East Pacific Rise
collided with continental North America (about 30 Ma) the San Andreas Fault formed
and subsequently lengthened itself to the north and south with time. Eventually the last
remnants of the subduction zone will disappear as North America overrides the remainder
of the plate and the spreading center.
Recent studies have shown that no fewer than 12 Cascade volcanoes have been active
within the last 2500 years (See Below). Of these St. Helens and Mt. Shasta appear to be
the most active, but this may be somewhat deceptive. Due to years of diligent work by
the U.S. Geological Survey the volcanics around Mt. St. Helens have been more
extensively studied and age dated than anywhere else in the Cascades. Similarly, Mt
Shasta has been the center of research efforts by many universities as well as the USGS.
Thus, it is possible the better mapped and sampled a volcano is the more extensive it's
eruptive history.
Cascades volcanoes are generally "classic" composite volcanoes characterized violent
pyroclastic eruptions (See below). The St. Helens ash cloud reached 60,000 feet into the
atmosphere and may well have gone higher had not a significant volume of ash been
discharged as a laterally directed blast. Mudflows devastated the Toutle River and
blocked the ship channel in the Columbia River. To the north at Mt. Rainer potential
mudflows pose a significant hazard to the Seattle-Tacoma suburbs. Lava flows are often
restricted to rhyolite/andesite domes that rarely leave the local vicinity of the volcano, but
in the southern Cascades basaltic lava flows have been extensive.
The 3-D graphic shows the subduction of the Juan de Fuca plate beneath the North
American plate. It also portrays the consistent pattern of earthquakes associated with the
subduction and faulting of the upper plate. Shallow focus crustal earthquakes are
probably triggered by rising magma that generates arc extension. The more severe, deep
focus, earthquakes probably result from decoupling of the down going Juan de Fuca
plate. Plate locking may also trigger large magnitude earthquakes along the slab interface.
Evidence for these quakes is somewhat tenuous since most epicenters would be well
offshore and no "subduction" quake of significant magnitude has stuck the Pacific
Northwest in recorded history.
In class we talked about the diapiric spacing of rising magma plumes along a subduction
zone. The view northward from the Three Sisters in central Oregon provides excellent
support for that hypothesis. In the foreground are the Three Sisters Peaks. Mt Jefferson
lies about 40 miles to the north and 40 miles north of that is Mt. Hood. In the far distance
are Mt. Adams and Mt. Rainer.
The central and northern Cascades are dominantly andesitic in composition. No basalts
are present. In contrast, the volcanics of the Southern Cascades range in composition
from basalt to rhyolite. This compositional difference can be related to geology of the
underlying crustal rocks. In the northern and central Cascades no pre-Tertiary continental
rocks are present beneath the lava flows. Thus the volcanic piles are effectively setting on
oceanic crust and partial melting results in the typical andesite of an island arc. However,
in the southern Cascades the volcanic piles lie on pre-Tertiary continental rocks resulting
in contamination of rising magmas and a much more varied suite of volcanic rocks.
Tonga-Kermadec-New Zealand Arc
The Tonga-Kermadec arc lies in the south Pacific and stretches from the Fiji Islands in
the north southward to New Zealand. The Tonga-Kermadec portion of the arc system lies
on basaltic oceanic crust. In contrast, the New Zealand volcanics lie on Australian
continental crust. Several trends can be seen:
1. New Zealand magmas are very diverse, but dominantly silicic.


87Sr/86Sr ratios are >.706.
Distinct enrichment in incompatible elements occurs (Rb, Cs, Ba, U)
2. Tonga-Kermadec magamas are tholeutic basalt to andesite.


87Sr/86Sr ratios <.704.
The trace element suite is compatible with a mantle derived partial melt and is
depleted in typical incompatible elements.
Apparently there are two distinctly different calc-alkaline magma suites:

basalt-andesite associated with oceanic crust and characterized by
1. low 87Sr/86Sr ratio
2. depleted incompatible element suite
3. derived by partial melting of basaltic oceanic crust in descending
lithospheric plate

highly diversified, but distinctly felsic suite of rocks
1. 87Sr/86Sr ratios >.706
2. enriched in incompatible elements
3. derived through contamination by silicic crust during ascent of magmas.
Andes Mountains
The Andes are often cited as the type example of a calc-alkaline volcanic chain. There are
three active segments of the arc-trench system (See diagram)> The northern segment is
characterized by dominantly basaltic lavas, the central segment by andesitic lavas and the
southern segment by a diverse suite of rocks. What factors do you think are responsible
for the differences in rock types? In general we can conclude:


volcanism only seems to be associated with the more steeply dipping
portions of the volcanic chain (25°)
thickness of the overlying wedge of continental crust is less a factor than
the ratio of continental crust to oceanic crust
Island Arcs
The figure below summarizes the important features of an arc system. It should be
stressed that this is a highly generalized model and that most arc systems do not have all
the illustrated features. In fact the outer arc island comprised of sediments scraped off the
down going oceanic slab is rarely present. Some suggest Santa Catalina Island may
represent the remnants of the outer arc island system here in southern California. Back
arc basins are present in some arc systems but absent in many. Back arc basins are areas
of active spreading characterized by thick sequences of basaltic lava. Although no back
arc basin has ever been described in southern California are the basaltic lavas of the
Mojave related to back arc spreading? Or are they related to the East Pacific rise?
The final figure attempts to show "possible" source areas for island arc magmas. As you
can see a number of sources have been suggested. Is there only one source or can there be
a variety of different sources?