Cascade Andesite

Main Ideas:

• Andesite is a rock type of intermediate composition
• Andesite volcanoes are large, steep-sides conical structures- "Stratovolcanoes"
• Cascade andesite is a result of subduction

On US 26 heading east from Portland toward Mount Hood there are a couple of dramatic road cuts exposing thick andesite flows (near mile marker 49). Take time to pull off to the side and examine a sample or two.
Andesite can be distinguished from basalt by its much lighter color.  It is typically a shade of gray.  This is a result of increased silica content (and consequently lower iron and magnesium).  If you look closely you will see small needle-like grains of white plagiolcase and black squarish pyroxene grains scattered in a very fine-grained gray matrix.  Because andesite contains more silica the lava is thicker and "stickier".  To understand why, imagine an ion of silica (Si) bonded to two oxygen ions (O2).  These ions bond because of their opposite charges- silica has a charge of 4+, and each oxygen ion has a charge of 2-.  In molten lava that has not yet soldified, the SiO2 forms a loose network, similar to adding oatmeal to boiling water.  The more oatmeal you add, the thicker it becomes (yeah, not the greatest analogy I admit!):

Because andesite is thicker (i.e. more viscous) than basalt it forms impressive, steep-sided cones that are clearly visible on the landscape (image below is from Google.maps.com).

 Background: Mt Hood began erupting about 700,000 years ago, a youngster in Oregon's history considering the oldest rocks in the state are 400 million years old.  At that time the western coast of the North American continent was the western edge of Idaho.  The area that Oregon now occupies was ocean (more on Oregon's earlier history in posts to come).   

Basically, the Cascades are a product of subduction- the collision between the N. American continent and the ocean seafloor to west.  The sinking slab of thin, dense seafloor began to partially melt about 50 million years after the slab began to sink.  The rising magma traveled upward toward the surface, probably incorporating some of the relatively silicic continental crust, making the resulting magma intermediate in composition.  The resulting magma had a subduction signature that is enriched in soluable elements like potassium (K) and Barium (Ba) relative to insoluable elements.

Lava Butte

OK, I'm not a professional photographer so this pic of Lava Butte doesn't quite do it justice (sigh).   But hopefully it gives you a feel for the area.  Lava Butte is one of several cinder cones associated with the Newberry Volcanic Complex (actually there are about 400!).   It's ~ 500' high and consists of basaltic to basaltic-andesite lava flows and cinders and bombs ejected explosively from the vent.  One of the striking things about Lava Butte is that as you're following the trails around the cone base you notice that there's not much of any vegetation growing on the lava.  It's "fresh" geologically-speaking.  I believe one of the rangers told us the most recent flows from Lava Butte have been carbon dated at 6,000 years ago.  You might have noticed that there are quite a few trees on the actual cinder cone itself.  The lava flows flowed from the sides and base of the cone, not the top.

Newberry Volcano

Getting to see Newberry Volcano was fantastic! The drive was about 3 hrs from Portland but definitely worth the trip. Newberry Volcano lies about 40 miles east of the Cascade Range just south of Bend, Oregon. Unlike the steep, impressive cone-shaped stratovolcanoes of the Cascades, Newberry Volcano forms a broad, low "shield" shape (hence the name shield volcano) approximately 40 miles long and 20 miles wide. This cross section from Alt and Hyndman's Roadside Geology of Oregon (1978) does a good job of showing how inconspicuous Newberry is on the landscape:

The Newberry crater is now collapsed, leaving a caldera 8 miles wide with two nice little lakes on either side of an obsidian flow. I was expecting to see almost exclusively basalt at a shield volcano, but the Newberry volcanic complex actually has a wide range of extrusive rock types.
MacLeod et al. (2006) does a good job of summarizing the petrology of the volcano, though it is much more information than the casual reader would be interested in. The younger lava flows are roughly 6,000 years old and tend to have slightly higher silica contents (basaltic andesites). The older lava flows tend to be more mafic, but they are not as well exposed as the younger flows, so most chemical studies show a greater proportion of basaltic andesites than is probably representative.

After the crater collapsed (as the magma chamber was emptied) episodes of pumice, ash, obsidian and rhyolite took place.

The Big Obsidian Flow, inside the caldera, is really something to see. In total, they are about 100 feet thick. If you want to collect some obsidian samples you can check out the rockhound quarries at Glass Buttes. Obsidian, or volcanic glass as it is commonly called, has the chemical composition of a high-silica rhyolite. The Roadside Geology authors pose an interesting question: if obsidian really forms as glass because it cooled too rapidly for crystals to form, then how do we explain the Big Obsidian Flow? Some of the individual flows are several feet thick in places. How would this cool so quickly as to prevent rhyolite from forming? Their explanation is that obsidian lavas contain very little water; too low for crystals to form. Seems plausible...however, some ingrained ideas are hard to part with.
NOTE: I wouldn't recommend sandels or open-toed shoes here.

A larger question in my mind is this: it appears that the Newberry Volcano gradually produced more silicic lavas throughout its lifespan. It's last eruption was a thick, viscous rhyolite magma. The volcano appears to be dormant though that's not certain. Is this evolution typical of other shield volcanoes in the area? I suspect it is. Can the compositional variations we see solely be the result of differentation in the magma chamber? Ah, something to sleep on.



References:

Alt D. and Hyndman D., 1978, Roadside Geology of Oregon, Mountain Press Publishing Co., p. 219-223

MacLeod N., Sherrod D., Chitwood L. and McKee E., 2006, Newberry Volcano, Oregon, USGS Geological Society Circular 838, http://www.nps.gov/history/history/online_booke/geology/publications/

Portland skyline

Coming soon: Newberry Volcano