Starting today’s post with a quick review, or introduction if you’re just joining in, the majority of rocks that make up Washington (and the greater Pacific Northwest) are exotic- meaning the rocks formed somewhere else on the planet and were transported here and added to North America through plate tectonic processes. The exotic rocks in Washington are grouped together into different terranes, delineated by similar rock types, ages, and where the rocks initially formed. Different types of terranes that were added to North America during the formation of the Pacific Northwest between ~250- 50 million years ago include volcanic island arcs (think the modern-day islands of Japan), small micro-continents (modern-day Madagascar or the island of New Guinea), or just large sections of the ocean floor.
The final major terrane to be added to the Pacific Northwest is a large group of volcanic rocks, an undersea volcanic plateau if you will, that were accreted around 50 million years ago to essentially form our modern coastline. It’s important to keep in mind here- this age of 50 million years is when the rocks were added to North America, their actual formation was around 7 million years earlier. These volcanic rocks are quite extensive, outcrops of them are present from Vancouver Island in the north to the Oregon Coast Range near the town of Roseburg in the south.
This is the terrane of Siletzia, which formed from voluminous eruptions of basaltic lava on the ocean floor. The exact mechanism is still debated, but likely either seamounts like the Hawaiian Islands, a region of intense volcanic activity similar to Iceland, or maybe it formed a little closer to the continental margin! Either way, the lavas that form a lot of Siletzia were erupted underwater.
Underwater? How do we know?
And here’s where the photo comes in! While it not look as impressive as a giant granitic mountain wonderland like the Enchantments, the rocks here still tell a great story! This is a photo of a small part of Siletzia a short hike from the Hurricane Ridge Visitor Center in Olympic National Park. For scale, the photo is about 3 meters/ 9ish feet across.
The exact rocks here are pillow basalts- the same general flavor as the ubiquitous Columbia River Flood Basalts of eastern Washington, but quite a bit older and also exotic!
Magma/ lava with a basaltic composition has a very low viscosity, it’s about as runny or fluid as molten rock can get. When these really “runny” lavas erupt on the ocean floor, the outside of the flow will cool/ crystallize/ harden into rock almost instantaneously when in contact with the cold ocean water. We call this process “quenching”, and it also insulates the interior of the lava flow! The lava keeps erupting, and breaks out into smaller round-ish lobes. As more and more lava erupts, more and more of these lobes form and pile up on top of each other.
If this pile of lobes is then uplifted into a mountain range, the Olympics for example, erosion will expose these lobes… and will end up looking like a stack of round pillows!
Each one of the round-ish “blobs” in this photo is an individual pillow, and this distinct morphology of the lava lets us know that it formed underwater, despite being at over 5000 ft elevation today! Fun features that are relatively straightforward to identify when on the trail or from you car window J
Think we’ll continue with “Exotic Washington” next week, but it will be a much deeper post…