We have no fecking idea WHY the things erupted in the first place, why they started when they did, and why they (at least apparently) stopped.
Now the Pacific Northwest is volcano Disneyland. No argument there. The only thing you really need to know about plate tectonics is that there are three basic types of ways that the ginormous shell-bits that clad the outside of the Earth interact.
There are places where the plates move past each other - these are transform boundaries and you probably know of one right off the top of your head. The San Andreas Fault Zone is a monster transform margin, with Southern Californa (which is really part of the Pacific) moving northwest relative to North America.
But here in the Pacific Northwest we live next to the third kind of plate margin - the one where the plates are moving together. This is called a convergent margin and comes in two flavors. Where continent meets continent you get the world's biggest head-on, and the rocks pile up into a big ol' heap; think the Alps, or the Himalayas.
But then they move.
And it's real wrath of God stuff; buildings fall over, the dead rising from their graves, dogs and cats living together.
Ginormous earthquakes, possibly the largest on Earth.
So. Okay, my family lives in Natural Disaster Central.
But getting beyond that; the thing is that we pretty much understand how we get the big stratocones. The ocean crust gets shoved down into the hot mantle, melts, mixes with and melts granite continent crust as it rises and erupts as a mixture of ocean basalt and continental granite - we call the stuff andesite, and there's tons of the stuff all over the Cascades.
And I've told you about the Columbia River Basalt, the incredible thickness of flood basalt that forms much of the bedrock everywhere around the Portland area.
But the Boring Lavas are very different.
"as compared to the Yakima, (Boring Lava) is gray rather than dark gray to black, and the jointing is generally massive or blocky rather than columnar or brickbat. Still more characteristic of the Boring Lava, as seen in thin section, is the meshwork of minute plagioclase laths (polotaxitic texture) commonly with open spaces between the laths (diktytaxitic texture). The Boring Lava contains olivine, rare in Yakima Basalt, and there is a very distinct geochemical difference between the two types of lavas" (Allen, 1975, Volcanoes of the Portland Area, Oregon: State of Oregon, Department of Geology and Mineral Industries, The ORE-BIN, v.37, no.9, September 1975)Boring vents tend to be either "shield volcanoes" or cinder cones rather than flow basalt like the CRB or stratified cones like Cascade volcanoes.
But - again - the big mystery is - why here?
Cascade volcanism is directly related to the tectonics of convergent margins. We think the CRBs are the product of a superheated geyser of magma from deep in the mantle - a "hotspot" or mantle plume.
But the Borings?
Their location and rock type suggest they're related to convergent tectonics, but that's like saying the Mitt Romney is a cluelessly entitled gasbag because he comes for a family of rich Mormons - it's a condition, not an explanation. Don Swanson admitted he was pretty baffled:
"The setting of the basalt field is puzzling and not understood. The vents lie well west of the crest of the Cascades, and those such as Kelly Butte, Mount Tabor, and the vents in the Portland Hills are in and even west of the Portland basin." (Swanson and others, 1989, IGC Field Trip T106: Cenozoic Volcanism in the Cascade Range and Columbia Plateau, Southern Washington and Northernmost Oregon: American Geophysical Union Field Trip Guidebook T106)They do seem to be related to fault lines or fractures in the Portland Basin, where magma can leak through. They might be a sort of "forearc" volcanism, similar in nature to the small spreading basins that sometimes form between the subduction trench and the volcanic arc it generates.
We're not really sure.
We're also not sure of why they started erupting - about 2.7 million years ago - and why they stopped - in some cases, less than 300,000 years ago.
The big Cascades peaks; Hood, St. Helens, Rainier, Shasta - are all about a million years old. The older rocks, the so-called "Western Cascades" that underlie the big stratos, predate the subduction of the Pacific rise at the western edge of the old Farallon plate at about 40 million years ago and shut off about 5 million years ago, near the end of the Pliocene.
So what happened 2.7 million years ago to kick-start the Boring Lavas?
We have no idea.
But we do know that the Boring Lavas provide us with some of our most beautiful scenery and our most attractive places, from Mount Tabor to Elk Point high atop Sylvan Hill, from Larch Mountain in the east to Bob's Mountain up north.
One of the best places to see the Lava is, not surprisingly, down deep in the earth; in the Oregon Zoo/Washington Park station of the west side MAX light rail line.
Here the Tri-Met designers placed a portion of one of the rock cores that Portland geotechs L.R. Squier drilled back in the Nineties for the tunnel that runs through the West Hills; it does a wonderful job of showing you not just the Boring Lavas themselves but their relationship to the older, and younger, rocks and sediments that make up the bones of our Portland earth.
here's a great one, by the delightful Dana Hunter over at En Tequila Es Verdad, complete with a full-contact geologic discussion of the B-5 core on display. Dana utterly rocks the rocks, too - she's a manic for geology, and her blog has a ton of other great geo stuff for your delectation.
Here's another, from the blog Subway Nut; Jeremiah Cox, the blogger, is more into the actual train station than the geology, but his pictures of the core there are incredible...
here's the U.S. Geological Survey's cross-section through the West Hills; it's amazing, and - like many of the wonders in life - the pink parts are the best.