The Geologic History of Washington State & Kittitas County Jana Jones Mabry

Geologic History of Washington State Overview of the processes responsible for the underlying geology Overview of different geologic provinces of the state Overview of the geology specific to the area of Snoqualmie Pass and Lake Easton State Park

Weathering and Erosion The Geologic Processes responsible for the landscape features seen in Washington Plate Tectonics Volcanism lava flows Glaciation Alpine Glaciers Massive Flood Events Weathering and Erosion

Plate Tectonics The theory and study of the earth that describes the processes of continental formation and movements. It proposes explanations for how, why, and when continents are built and how they move. It is the process responsible for the physical conveyance of materials that form the bedrock geology of Washington State.

Volcanism Three types of volcanic processes are associated with Washington Geology Batholiths and plutons Magma trapped and cooled under the surface. Volcanoes Magma that escapes through a vent in an explosive event or the repeated rise of magma to the surface. Plateau Basalt Provinces

Glaciation The accumulation and movement of flowing ice. In Washington the most significant glaciers occurred between 1.6 million years ago to about 10,000 years ago. The glaciers carved sharp features in the high peaks created glacial troughs, lakes, and ice dams that led to massive flood events deposited glacial debris called till and loess along their paths

Overview of the Geologic History of Washington State

The oldest rocks in Washington State are part of the 1 billion year old Belt Supergroup The North American Continental Coastline was located approximately 30 miles west of the the present day border of Idaho and Washington

The continued addition of material to the North American Continent was through the process of Plate Tectonics Plate tectonics conveyed small volcanic islands and fragments of other continents called terranes onto the North American Continent and accreted them in the area of the existing coastline.

The first terrane to arrive was the Okanogan Micro-continent at 190 to 160 million years ago This micro-continent crushed the coastal plain that had been building for 600 million years (800 to 200 myBP) between the North American Continent and the Okanogan creating an area of displaced marine sedimentary materials called the Kootenay Arc

The next major addition occurred 90 to 100 million years ago with the arrival of the North Cascade Terranes The North Cascade Terranes are believed to be made up of a group of 6 different islands. Each individual terrane can be distinguished by its unique rock type. These terranes were covered later by volcanic materials and complicated their interpretation.

The Insular Terrane arrived behind the North Cascade Terranes at about 100 million years ago This terrane is believed to have been two large pieces that make up the basement rocks under the San Juan Islands and the Blue Mountains

The last event to add to the Washington coastline was approximately 25 million years ago is called the Crescent Terrane located in the areas of the Puget Lowlands, Olympic Peninsula and the Willapa Hills The Crescent Terrane is believed to be a portion of ocean crust that was stranded between an extinct trench and the present day trench. Some how more buoyant rocks were carried underneath the continental crust, then escaped pushing the overlying rocks as much as 2 miles above the ocean crust

More Changes occurred to the landscape with the eruptions of the Columbia River Basalt Group between 17.5 to 6 million years ago These basalt lava flows originated from dike swarms and cracks in the earth called fissures in the area where Washington, Oregon and Idaho come together Lava flowed out onto the surface in 120 individual flows covering 100s of square miles of this region

Pleistocene ice sheets and glaciers that occurred from 2 million to 500 years ago The area we are in was carved by the glaciers and the sharp features on the high peaks are called aretes, horns, cirques and hanging valleys. Many of the lakes were created by the glaciers and glacial outwash called till and loess blanket the region.

The last significant volcanic events to shape the Washington landscape were the birth of the Cascade Volcanoes between 1 million and 75,000 years ago Creation of the Cascade Volcanoes was a direct result of plate tectonics and the subduction of the Fallaron Plate under the western coast of North America. The volcanoes were created in a progressively northern direction beginning in Northern California and now extending into Canada.

The Cascade Volcanoes of Washington were the icing on the cake that make up the geology of the state Plate tectonics and the resulting volcanism remain the most active and significant geologic processes that affect geology of Washington. The Juan de Fuca Plate, is still subducting under the northern-most portion of the North American Continent. It is responsible for the earthquakes we experience, the changes to the topography, and controls the energy flow and climatic conditions of the region.

The Ice Sheet and Glaciers that have occurred over the past 2million years have had a significant impact on the topography of this area. The Yakima Glacier flowed from the Snoqualmie Summit to Thorp The three depressions now holding Keechelus, Kachess and Cle Elum Lakes held glaciers that fed into the main stream Yakima Glacier.It created the U-shaped valley in which the present day Yakima River flows and covered the bedrock geology of the whole area with glacial debris and alluvial (water deposited) materials.

The Glaciers in this region carved the paths and natural depressions now used as reservoirs for the south-central region

Lake Easton is a man made lake created where the Yakima and Kachess Rivers come together. This reservoir was built 1928 through excavation and removal of the glacial till. Its primary function is to supply water for the agriculture of south-central Washington and a variety of recreational activities to the central region. The view to the east of the park provides a glimpse into the rich geologic history of the region

The Rocks of Central Washington and Lake Easton Volcanic (Igneous) Intrusive (cooled underground as batholiths) Granite Mounts Stewart and Si Metamorphic (cooked) Ingalls Tectonic Complex Easton Schist Sedimentary Glacial Debris River Deposits