April 06, 2006
Thursday

Ketchikan, Alaska - Road builders blast them, children throw them, gardeners curse at them and collectors pick them up. What are they? Why rocks of course! And as most local know, Revillagigedo Island is one large mass protruding from the ocean which earned it the knick-name "The Rock."

This past Friday evening the Southeast Alaska Discovery Center sponsored their weekly Friday Night Insight Program to a large crowd gathered in their comfortable theatre. The guest speaker was Julie Rowe who gave a highly informative presentation on "History in Rocks - The Geology of Ketchikan."

Rowe works as a U. S. Forest Service Recreation Specialist but spoke as a private citizen using her past experience as an Interpretive Park Ranger and her education in Evolutionary Biology and Geology to educate those present on how and why Revillagigedo Island came to be.

The defining word in that last statement was Evolutionary. Rowe started her presentation at the very beginning, a time in Earth's history when Alaska did not exist. Those in attendance that possessed a basic understanding of geology found it easy to follow along with Rowe as she progressed from the Cenozoic Age through to present day.

With the assistance of a Power Point presentation and a geologic map and reports issued by the United States Geological Survey, Rowe was able to demonstrate the process of folding and faulting of the earth's crust known as tectonics.

The audience viewed slides and diagrams that demonstrated how large land masses called "terranes" would collide with, and in essence, become welded onto the North American continent. Different terranes would collide with our continent at different times in geologic history.

The enormous heat and pressure created by these collisions would melt the rock and change their components. Over a period of millions of years, one terrane after another would crash into the continent changing the composition of the rock involved and causing the land mass to rise up to the surface.

The process known as "subduction" occurs when two plates of earth's crust collide and the extreme energy involved forces one plate to dive underneath. Picture a large truck and a compact car involved in a head on collision whereby the truck would literally climb over the compact car. Due to this subduction, volcanic activity began in the area of Southeast Alaska.

Rowe went on to explain the process of "metamorphism" which is how rocks changed or were altered by the extreme heat and pressure mentioned earlier. Much like a baker gathering the ingredients to make bread, each ingredient has its own characteristics but once blended and baked, the heat involved has caused the ingredients to change or metamorphosis in an entirely new substance.

According to Rowe there are three terranes in the Ketchikan and Misty Fiords area. These terranes collided during the Mesozoic Age, between 66 and 245 million years ago. Separating each of these terranes are, (thankfully) inactive faults.

The Stikine Terrane is in the northeast area of Misty Fiords and extends to the Hyder area. The Tracy Arm Terrane covers the length of Southeast Alaska, most of the Portland Peninsula and Rudyerd Bay. Taku Terrane is the largest by far, encompassing all of Revillagigedo Island, the western edge of the Portland Peninsula and much of the Cleveland Peninsula.

Once the earth moved into the Tertiary period, based on Rowe's research, the terranes had finished colliding in our area but volcanic activity continued. The well-known New Eddystone Rock in the Behm Canal is a perfect example of what is known as a volcanic plug.

Rowe described a plug as the "throat" or "neck" of a volcano. Once the neck is filled with magma and the volcano becomes dormant, natural erosion from weather occurs and eventually all that remains is the erosion resistant plug.

Since Rowe had transported the audience through the geologic history of the area and explained how the terranes were altered by metamorphism she proceeded to discuss the results of this metamorphosis that we see around us every day.

Most residents would describe our local rock as granite and shale and they would be only partially correct. The dark grey, slightly shiny rock most common on our island is called phyllite. A wonderful display can be seen on North Tongass Highway along the stretch from Wolf Point to WalMart.

Phyllite began its life on the ocean floor as nothing more than sediment, it hardened over time and became shale. During tectonic collisions the shale is heated under tremendous pressure where it becomes slate. When slate is compressed even more it become phyllite. The flaky texture makes phyllite easy to recognize. Depending on how light reflects on the rock it tends to shine due to the tiny mica crystals that are beginning to form.

Another common rock in this area is basalt which is a certain type of lava that is dark brown and contains many holes that were formed by gas bubbles. Relatively new in geological terms, only 600,000 years old, New Eddystone Rock is basalt.

A prominent light and dark banded rock common to our area is known as gneiss which may have begun as a sedimentary rock (formed by fragments deposited by wind, water or ice) or as igneous rock (formed by the solidification of magma or lava).

What many refer to on our island as granite is actually granodiorite. This is a plutonic crust that has re-melted and re-cooled as it rose to the surface during a collision of terranes. On North Tongass Highway, across from the scenic outlook is an excellent example of granodiorite recognized by its grey to black specks on its white background.

On a stretch of South Tongass Highway, just past Herring Cove and slightly before the old cannery one can see rocks that are a part of the gabbro pluton. Gabbro has the same mineral composition as basalt but unlike basalt, it was formed by magma that cooled underground.

In conclusion, Rowe stated that although she was not thoroughly pleased with the book Roadside Geology of Alaska, she tentatively recommended it for its basic descriptions of the rocks in each terrane adding that there simply aren't any other books on the Geology of Alaska.

As the presentation ended the audience moved to the front of the theatre to peruse the rock samples and to study the USGS quadrangle of Ketchikan and the surrounding area that Rowe provided.

This week's Friday Night Insight program to be held on April 7th at 7:00 P.M. will be "Gardening in Ketchikan" presented by local horticulturist Jeannie Blackmore. Jeannie will share her knowledge of gardening using her personal homegrown experiences as examples.

On the Web:

United States Geological Survey
http://www.usgs.gov/

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Ketchikan, Alaska