By Dave Tucker December 17, 2011
Everybody loves pillow lava. Come, on admit it. Who doesn’t take a second look at pillows? These rounded lava structures are the quintessence of geologic exotica: the result of the odd give-and-take dynamic of incandescent lava flowing underwater. Pillows may form in two general environments 1) in a subaqueous eruption of low viscosity lava, or 2) when the same sort of lava flows over land (‘subaerial lava’) and enters the sea or a lake as an entrant lava flow and becomes subaqueous. All the examples I’ve visited are basaltic, but andesitic pillows have been identified in a few places in the world.
There are many places to see lava pillows in Washington. This collection of field trips will direct you to a few widely scattered sites. Keep these locations in mind so you can visit them when you are in the relevant areas. References for the material I present are listed at the end of each trip. We’ll visit:
- Crescent basalt at Cape Disappointment (fabulous exposures!)
- Crescent basalt on the road to Hurricane Ridge.
- Several sites in the San Juan Islands.
- Columbia River basalt in a couple of places
The first two places will be featured in my upcoming book, Geology Underfoot in Western Washington. I’d really appreciate it if anyone who uses the directions and descriptions given in this online version lets me know via comment if you they were good, merely adequate or not useful. Any comments about the discussion below or the trips that follow are very welcome. Note that text, photos and drawings on these pages are copyrighted, as they will eventually appear in the book.
HOW PILLOWS FORM
Pillows form when the outer surface of flowing lava is quickly chilled by seawater. Rock is an excellent insulator, so the lava inside this very thin rind remains molten. Think of each of these lobes as a ‘finger’ a foot or two across of incandescent orange lava within a shiny dark skin, advancing in a herky-jerky fashion. The leading ‘snout’ of the lava lobe advances, something like a bead of thick caulking squeezed out of a caulking gun, or toothpaste from a tube. The surface of the incandescent lava is quickly quenched by seawater, but the internal pressure from the ongoing extrusion of lava at the vent very quickly causes the hard rind to split open. Interior lava stretches and then spurts out through the crack in another short advance, until its outer surface solidifies.
The process continues until there is insufficient lava pressure to break through the rind; this is either because the lava finds a more efficient finger to continue the advance, or the eruption wanes. Pillow lobes, then, are underwater lava delivery systems. Each pillow is really a cross section through a miniature (usually) filled lava tube. The famous subway tunnel-sized lava tube at Ape Cave at Mount Saint Helens (which will be visited and described in the book) is an example of one of these writ large, where the lava has partially drained out. Spaces between pillows are often filled with glassy ‘pillow breccia’, fragments of basalt shattered during rapidly quenching. If the lava is advancing down a steep underwater slope, or at a rapid pace, the lava may be mostly breccia. The internet has numerous videos of pillow lobes forming in Hawai’i, taken by scuba divers. A recommended site is http://www.pmel.noaa.gov/vents/nemo/explorer/concepts/pillows.html.
Go visit some pillow lavas in person. The active blue links take you to the relevant field guides as I publish them. If you are a subscriber to this blog, you will be notified as I publish them.
—Elbow Lake Formation, Middle Fork Nooksack, Whatcom County.
–Columbia River basalt:
1) Sand Hollow, east shore of Columbia River near Vantage
2) Manastash Ridge view point, I-82 south of Ellensburg
Please send information about other great locations you know about, and photos!