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.