A Geologic History of the Methow Valley By Dana Visalli www.methownaturalist.com/
[email protected] One could start a geologic history of the Methow Valley with the sto- ry of plate tectonics, because much of the drama written in the rocks is inscribed there due to the forces of moving plates. 200 million years ago (mya) most of what is now northern Washington existed as several large islands, each one a scrap of continent, lying somewhere out in the Pacific Ocean. One after the other they were pushed up against the existing continent and became a part of it. The earth has at its center a metallic core, thought to be 85% iron, al- though no one has been there to check, and 5% nickel. The core is about the size of the moon. Surrounding the core is the mantle, a thick shell of dark green and black rocks called peridotite that com- prise the largest part of our planet. Peridotite is an iron-manganese silicate, which means it is composed of iron, manganese, oxygen and This cartoonish map illustrates how the earth's crust is broken silicon. Any rock with iron will be heavier that those without iron, up into 12 major plates, which are in slow but constant motion. and will be drawn downward by gravity. The continents tend to be on different plates than the oceans. A thin outer skin called the crust covers the peridotite of the mantle nearly everywhere. Oceanic crust is composed mostly of the rock gabbro, which like peridotite is relatively dense.