How was the Salish Sea formed?

Ready to talk geology?  In order to understand the biology of a place, you have to understand a little bit about the underlying geology of a place.  All of what follows is a gross oversimplification because that is what you get when you squeeze 200+ million years of geologic history into fewer than 850 words.  So, bear with me…here we go…

If you were asked what geological forces formed the Salish Sea, I suspect you would say glaciers! or volcanoes!  You would be correct, but we also need to think about uplift and erosion…over a very long period of time…

Breaking away

As you likely recall from high school, our modern continents used to be part of one giant supercontinent called Pangea.¹  The western edge of the ancient landmass that became the North American continent used to be just west of Spokane and Pullman, WA.²  When plate tectonics started to pull Pangea apart about 200 million years ago, the North American plate started to move west, taking the North American continent with it.³  Thus began an ongoing collision with oceanic plates that would transform the western edge of the North American continent.

Adding on

Over the last 200 million years, the transformation of the northwest edge of the North American continent has been dramatic.

As oceanic crust moved under the North American plate, collisions with sizable volcanic island chains fused new land to the continent that would become major parts of the Coast Mountain Range and Vancouver Island.⁴  These collisions also caused crushing, folding, slipping and uplifting of sea floor that would become the Cascade and Olympic mountain ranges.⁵

The ongoing subduction of oceanic plate material also caused the formation of multiple volcanic arcs that erupted through the new land to fuel the growth of the Coast and Cascade Mountain Ranges.  The massive volcanoes we see today – Mt. Garibaldi, Mt. Baker, Glacier Peak, Mt. Rainier, Mt. Adams, Mt. St. Helens, Mt. Hood, and peaks as far south as Mt. Lassen – are only the most recent to stand along the spine of the Cascade Range.  Many more ancient volcanoes along these arcs have been eroded away.⁶

Carving out

The transformation of our corner of the North American continent continued through the Ice Ages.  Glaciers descended from the north.  Not once, not twice, but at least four times over the course of nearly 2 million years.⁷  Squeezing between the Cascade and Olympic mountain ranges, they scoured the surface of the land, deepening the massive river valleys and fjords that characterize our modern landscape.  They also deposited clay, sand, gravel, and rocks throughout the region.  The last ice sheet in this region reached Tenino, just south of Olympia, WA before retreating about 12,000 years ago.⁸

Imagine Seattle under 3,300 feet of ice! Or Vancouver under closer to 8,000 feet! The Washington State Department of Natural Resources produced this map showing the maximum extent of Puget Lobe of the Cordilleran ice sheet.⁹  There was also a lobe that stretched west out to the Pacific Ocean called, not surprisingly, the Juan de Fuca Lobe.

When the last ice sheet retreated, it left behind the deep fjords, channels, and a scattering of islands throughout the inland sea.  Those deep channels allow cold, dense, nutrient-rich saltwater from the Pacific to enter the inland sea.  The glaciers and ongoing erosion from rainfall also established vast river systems that drain freshwater filled with nutrients, and more recently pollutants, from the watershed into the sea.  The upwelling of cold water from the Pacific mixes with the fresh water from the rivers to create a highly productive estuary. 

Attracting life

The word “productive” basically means lots of food for the marine food web.  The microscopic phytoplankton and zooplankton thrive on the nutrients in the water.  Larger animals consume the plankton and so on up through the invertebrates, fish, marine mammals, birds, and humans.  We all depend on this nutrient-rich water which is a direct result of the geology here.

This is a photo taken from a BC Ferry of freshwater filled with sediment from the Fraser river meeting saltwater in the Strait of Georgia.  Eventually, these bodies of water mix and distribute nutrients throughout the Salish Sea.

So, you see, knowing even a little bit about the geology of the Salish Sea can provide a helpful perspective on how the ecosystem we have today operates.  The adding-on and uplifting of land, volcanoes, glaciers, and ongoing erosion have all enabled the food web that sustains the more than 3,000 marine animal species¹⁰ that call the Salish Sea home.

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Go Deeper!

Go Deeper to explore the ocean and the Salish Sea through a curated list of articles, books, films, and other resources.

To learn more about the topic in this article, check out the following resources:

• The Burke Museum.  Northwest Origins
• Washington State Department of Natural Resources. Puget Lowland
• Washington State Department of Natural Resources. Washington’s Glacial Geology
• Hruby, Kate. SeaDoc Society. Salish Sea Geology Rocks
• Benedict, Audrey DeLella and Gaydos, Joseph K. The Salish Sea, Jewel of the Pacific Northwest

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End Notes

^1-7 Townsend, Catherine L, and John T Figge. “Northwest Origins.” The Burke Museum, 2002, www.burkemuseum.org/geo_history_wa/.

⁸ Puget Lowland. Washington State Department of Natural Resources, www.dnr.wa.gov/programs-and-services/geology/explore-popular-geology/geologic-provinces-washington/puget-lowland#glacial-features.1.

⁹ Washington’s Glacial Geology. Washington State Department of Natural Resources, www.dnr.wa.gov/programs-and-services/geology/glaciers#the-cordilleran-ice-sheet.2.

¹⁰ “About the Salish Sea.” SeaDoc Society, www.seadocsociety.org/about-the-salish-sea.