Watching Washington Evolve

Our talented cartographers have put together a great animation showing the evolution of Washington geology. Based on the previous work of Jack Powell and John Figge, the cartoon shows the accretion of terranes through geologic time from the Neoproterozoic (~750 million years ago) to the present. It demonstrates how the breakup and reconstitution of ancient supercontinents, and subsequent volcanism and sedimentation resulted in the complex geology we see in Washington today. The final slide (present-day terrane map) also derived from work presented on the DGER Geologic map of Washington-Northwest quadrant (Dragovich and others, 2002) and the USGS Geologic map of the North Cascade Range, Washington map (Haugerud and Tabor, 2009), as well as more detailed data from a number of other DGER and USGS geologists.

Animated GIF showing the geologic evolution of Washington State.
Animated GIF showing the geologic evolution of Washington State (Courtesy of Ian Hubert, DNR). Click to see larger.

The general story of the Evolution of Washington:

The geology of Washington is the result of a complex history of tectonic events. It is an amalgam of various terranes, accreted through geologic time. A terrane is a discrete block of oceanic or continental material of analogous geologic history compared with the surrounding bodies of rock. The terranes of Washington resulted from continental evolution whereby pieces of ancient continents have broken off and reattached to form different continents.

The oldest rocks in Washington can be traced back to the supercontinent of Rodinia, which is thought to have formed 1.1 billion years ago. About 750 million years ago (Neoproterozoic Era), Rodinia broke apart and the area just west of Spokane became the shoreline for the continent of Laurentia. This new coast transitioned into an active subduction zone (a subduction zone is a plate boundary where one plate is sinking underneath the other, usually with an ocean plate being crushed beneath continental crust). The Laurentia subduction zone existed from about 350 to 170 million years ago (Paleozoic Era). The Kootenay terrane formed during that time period.

Eventually a series of large volcanic island arcs were transported towards the continent through the subduction process. As the oceanic plate they were riding on subducted beneath primordial Washington, the islands were accreted as they smashed against the coastline. This extended the coastline further westward. The first of these island terranes was the Intermontane superterrane that formed about 190 to 170 million years ago (during the Jurassic Period); the Intermontane terrane is located in the modern Okanogan Highlands.

The accretionary process was repeated several more times with a group of archipelagos that docked against coastline as their oceanic plate was overridden. This Insular superterrane extended from the present-day North Cascades up to Vancouver Island.

Between 55 and 20 million years ago (Eocene to Miocene Epochs), intrusives (magma bodies that have cooled and solidified underground) spread throughout the older terranes, and volcanics covered much of them with lava as magma centers migrated westward with the subduction zone. Contemporaneously, during the Eocene to Miocene Epochs, (from about 37 to 25 million years ago), the Siletz-Crescent superterrane accreted onto the west side of the state. The Siletz-Crescent is composed of sea floor basalts and sedimentary rocks. It currently ranges from the Olympic Peninsula region, southward though western Oregon.

About 17 to 6 million years ago (Miocene Epoch), a series of flood basalts, called the Columbia River Basalt Group, erupted out of fissures in southeast Washington and northeast Oregon. These flows covered the eastern half of Washington State and northern Oregon with lava hundreds to thousands of feet thick, deeply burying portions of the accreted terranes.

The Pleistocene (2.6 million to 12,000 years ago) ice age brought ice cover to the northern half of Washington State. Ice dams formed, blocking large waterways and creating reservoirs that eventually sourced huge floods when the dams burst. The massive volumes of floodwater scoured eastern Washington, forming dramatic scablands still visible today. Finally, modern volcanism continues to reshape the Cascades and the geology of Washington State today.

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