Download Spatial Scale of the Cascade Volcanic Arc as a result of Magma

Spatial Scale of the Cascade Volcanic Arc as a result of Magma Composition
By Timothy Randol
For Volcanic Landscapes, Geomorphology
December 9, 2015
Hi, I’m Tim and I am going to talking about Scale in the Cascade Volcanic Arc.
I originally wanted to research some intense article or study where the concept of
scale is applied to some break through model or idea within the Cascadian volcanic
arc. However, that proved to be a bit of a challenge in that almost every research
paper includes some figure with the phrase “scaled at” in its caption, thankfully
Leonard and Graeme recommended keeping it simple.
So, I decided that one of the most obvious curiousities regarding scale within
a volcanic landscape, in my opinion, is “What is with the size of the volcanoes?”
especially since the Cascades are within the same tectonic setting and they seem to
have varying elevations and volumes. What controls how high these peaks become?
My first thought was a time-scale thing where age was the main contributor;
a longer time period allows for more accumulation of lava flows. When I tried to plot
the main time of construction with elevations, I also tried to plot the elevations with
the latitudes hoping to see a clear trend that I could try talk about but there was no
such pattern you my undergrad eyes.
My second thought dealt with a little more geomorphology and brought to
the table the erosion rate as the number one contributor. Then I thought jeez, this is
bringing back some lost short-term memories of Deep Fluids and The Funk, which
are great memories! However, this contributing factor is not the main, large-scale
one.
When I found and dusted off my copy of Bierman & Montgomery’s Key
Concepts in Geomorphology, I flipped to the geomorphology of volcanic landscapes
and read the first sentence, which simply stated, “The intensity and spatial extent of
volcanically induced geomorphic change is directly related to the composition of the
magma.”
So, how does magmatic composition influence spatial scale? And the answer
is in the viscosity of the lava, which is controlled by the chemical make-up of the
molten rock that is ejected. Viscosity is the resistance to flow, so basically how sticky
the lava is. Composition affects the viscosity in a few ways; the one I want to talk
about is how the amount of silica, SiO2, within a melt helps resist flow. OxygenOxygen bonds, which are more abundant in silica-rich (felsic) lavas, are stronger
than oxygen-iron, -magnesium, or –calcium bonds, common in silica-poor lavas such
as basalt.
The stickiness of lava controls the large-scale aspect of the size and shape of
volcanoes by allowing the lava to build up in a vertical sense rather than spread out
horizontally. Higher viscosity allows for steep landforms within a limited region
surrounding the center, such as Mount Rainier, Mount St. Helens, and Mount Shasta.
Mt. Shasta in northeastern California is a large andesitic stratovolcano with
steep sides and a high elevation of 4,322 meters above sea level. It has a map area of
about 420 km2. What is interesting to keep track of is the volume of material making
up the landform, in Mt. Shasta’s case, its 350 km3.
Medicine Lake volcano is about 50 km northeast of Mt. Shasta, adjacent to the
axis of the Cascade Range. Its topography is an example of how mafic-rich lavas with
low-viscosity shape the landscape. This is mainly a shield volcano that consists of
small, cinder cones and low-sloping lava basaltic to basaltic-andesite flows that
expand over a map area of about 2000 km2. The total thickness of the shield volcano
is about 1,000 meters thick, much lower topography than Mount Shasta. However,
its total volume is almost double at 600 km3.
Tectonic setting is really what sets the stage for magma composition. So, why
do we get various ‘size-controlling’ magma types within this ocean-continental
volcanic arc setting?
Within most volcanic arcs we expect intermediate, andesitic magma as the
subducting plate causes partial melting of the felsic-rich continental plate. This
explains the string of stratovolcanoes running roughly N-S. The mafic-rich magma of
Medicine Lake are fed by sources east of the Cascade Range but still related to the
volcanic arc. Back-arc spreading occurs due to extension of the North American
plate as it overrides the Gorda, Juan de Fuca, and Explorer plate. This crustal
thinning allows mafic-rich molten rock to extrude directly from the mantle to the
surface creating bimodal volcanism in the eastern Cascade Range.