Poking Around Pillow Basalt of the Oregon Coast

Published 08/31/2011

(Oregon Coast) – The shorelines of Oregon are filled with a huge variety of beach environments, from sandy and fluffy to rocky and craggy – and much in between. Wandering the Oregon coast is often a heady journey of discovery in the world of geology, as just a few miles can abruptly change in scenery and landscape, with each one telling a wild geologic tale (above: Fishing Rock State Recreations Site near Depoe Bay is a good example of this kind of rock).

One of those is the more rounded rocky areas you see around Depoe Bay – and in some spots around Yachats, among others. They’re less pokey versions of the more craggy, sharp edges you find in most of the Yachats area, Oceanside, Cannon Beach or other spots.

Bubbly basalt on a really large scale at the hidden cliffs at Depoe Bay

These are called pillow basalts. Not because they’re suitable for laying your head on – indeed they’re as hard as any of that black rock that typifies the Oregon coast. It’s for their more rounded structure, a kind of bubble shape in a way.

Pillow basalt happens when hot lava hits the water by erupting on the ocean floor – something which happened often around these coastal parts some 70 to 40 million years ago.

As they ooze out into the cold ocean, they quickly form in the shape of some kind of lobe and the outer surface chills immediately. But lava may still be erupting underneath and pushing its way upwards, which then caused the newly-formed object to crack, and more lava would push out from that opening, forming yet another lobe bud.

More pillow basalt dominates downtown Depoe Bay

Sometimes, these would break off and cool into giant, singular masses. Others would cool in vast formations of them – like a bed of pillow basalt structures. These had an interesting way of “settling” into the gaps between the other lobes and formations, causing accumulations of them.

These can sometimes be hundreds of meters thick.

Epochs of lying around beneath the ocean, rising and falling in height over those eons, and being worn away by water and elements caused these to round out even further.

You can find plenty around Depoe Bay’s downtown area, but they’re more common and more dramatic at the secretive north end (accessed by Sunset Ave.) and at Fishing Rock State Park, between Depoe Bay and Lincoln City.

Curious pillow basalt at Yachats - though not all of the area is typical of this

Yachats

Depoe Bay

Fishing Rock State Park

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