The Okanogan Dome is visible along State Route 20, mostly between Tonasket and Wauconda, in eastern Washington. However, it spans the mountains eastward to Republic, Washington (Alt and Hyndman 1994:47-50).
The Okanogan Dome consists mostly of a huge mass of granite, 20 miles in diameter. A 5 to 10 mile wide shroud of gneiss mantles it. The gneiss, however, looks a lot like the granite, only streakier (Alt and Hyndman 1994: 49).
Okanogan Petroglyphs by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License. Pictographs (both old and modern) are drawn on the outcrops hither and tither throughout the Okanogan Dome. Note the streaks underneath the drawings on the rock. This may be gneiss. To be honest, this outcrop contained both gneiss/granite and meta-sedimentary rocks, and the patina is thick. Cool rock art though, eh?
The gneiss is apparently older than the granite. As the Okanogan micro-continent’s rocks heated, they melted and formed the granite and gneiss. The rising, molten granite rose up, dragging the gneiss formed from the older rocks up along with it. This formed the Okanogan Dome (Alt and Hyndman 1994:49).
All this happened between 50 to 70 million years ago, which is 30 to 50 million years AFTER the Okanogan micro-continent docked on the then North America west coast. This means that the Okanogan Trench still sucked ocean floor into itself 50 million years ago. The docking of a micro-continent usually moves the trench, since it essentially plugs it. The lighter materials (like granite) can’t sink, so the trench moves to the other side of the continent. In this case, the Okanogan Trench moved west to the Tonasket area after docking, but not after 30 to 50 million years (so says the Okanogan Dome [Alt and Hyndman 1994:50]).
Republic West Rocks by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License. As State Route 20 winds east into Republic, the east side of the Okanogan Dome is buried by 50 million-year-old volcanic rhyolite ash.
What’s all this about docking micro-continents? Here’s a brief sum-up.
Much of the Pacific Northwest, particularly Washington state, west of Idaho is formed of micro-continents. They are islands formed of much lighter material than the sea floor. The islands move east from Asia towards the oceanic trench on the northern United States. One hundred million years ago, the trench was located on Idaho’s western boundary. The Okanogan micro-continent was the first of these islands to dock. Japan is another micro-continent, which will eventually dock on the west coast.
Alt, David D., and Donald W. Hyndman
1994 Roadside Geology of Washington. Mountain Press Publishing Company, Missoula, Montana.
Okanogan Dome: Formed 50 to 70 MYA: After Micro-Continent Docked by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License.