Boring Volcanic Field, Oregon
On the day before the GSA meeting began, I participated in a field trip to the Boring Volcanic Field, a zone of anomalously-located volcanic vents around Portland, Oregon. The field is named for the Boring Hills, adjacent to the town of Boring, Oregon, which is named for a dude named "Boring." Kim Kastens noted this funny name on the Earth and Mind blog recently. The USGS maintains an information page on the field here.
Today, some photos...
Atop Rocky Butte, field trip leaders Rick Conrey (WSU) and Russ Evarts (USGS Menlo Park) orient the group with a map highlighting the various units comprising the Boring Volcanic Field:
Mount Hood hides its peak in the clouds:
At our first outcrop stop, the field trip participants get out and look at the Boring rocks:
Here, a Boring lava flow overlies Troutdale Formation fluvial gravels:
Annotated version for the untrained eye:
In places, a "baked" zone of contact metamorphism can be seen in the Troutdale as it got scorched by the lava that flowed on top of it (bright red), but the characteristic red color was missing underneath one spot, the central overhang in this photo:
Weird, huh? Maybe the metamorphosed sediments need a certain amount of rain-mediated chemical weathering before they "blush"?
Well-rounded clast from the Troutdale: vesicular basalt from the Columbia River Plateau:
Another nice Columbia River flood basalt boulder, this one with phenocrysts of plagioclase, and a concentric zonation of texture (massive in the center, vesicular towards the edges):
Plus, you can find cobbles derived from further afield: gneiss (from Idaho?), quartzite (Belt rock?), etc:
Between cobbles of the Troutdale, you can see hyaloclastic sand (immature sand with lots of hydrated basaltic glass fragments, apparently produced by interactions of magma and water in the source area, upstream):
More hyaloclastic sand:
Oooh! A "crack panel" on the side of some cooling columns at another stop! These horizontal slats are produced in individual fracture-propagation events, and each one concludes with a little ridge called an arrest line.
Mafic pyroclastics that underlie the lava flows at this second stop:
More mafic pyroclastics, on a cinder cone in Mount Tabor Park.
This is a pretty neat outcrop: you can see normal faults cutting these angle-of-repose inclined volcanic strata, presumably forming in slumping events.
Annotated version of this same photo, highlighting a marker layer and its offset along the fault:
The weather was pretty grim for this trip, so that was a bummer. But it's Portland, right? What did we really expect? Anyhow, I enjoyed being introduced to this suite of rocks -- boring out of context, but interesting given their location well west of the main axis of Cascade volcanism. Unfortunately, the field trip didn't really address why the Boring rocks are there. I was expecting some sort of detailed discussion of the possibilities: an evaluation of different models for their generation and passage to the surface... but that really didn't happen in any substantive way. So it wasn't the most amazing field trip I've ever gone on, but it was a nice day of checking out a cool suite of rocks.
Today, some photos...
Atop Rocky Butte, field trip leaders Rick Conrey (WSU) and Russ Evarts (USGS Menlo Park) orient the group with a map highlighting the various units comprising the Boring Volcanic Field:
Mount Hood hides its peak in the clouds:
At our first outcrop stop, the field trip participants get out and look at the Boring rocks:
Here, a Boring lava flow overlies Troutdale Formation fluvial gravels:
Annotated version for the untrained eye:
In places, a "baked" zone of contact metamorphism can be seen in the Troutdale as it got scorched by the lava that flowed on top of it (bright red), but the characteristic red color was missing underneath one spot, the central overhang in this photo:
Weird, huh? Maybe the metamorphosed sediments need a certain amount of rain-mediated chemical weathering before they "blush"?
Well-rounded clast from the Troutdale: vesicular basalt from the Columbia River Plateau:
Another nice Columbia River flood basalt boulder, this one with phenocrysts of plagioclase, and a concentric zonation of texture (massive in the center, vesicular towards the edges):
Plus, you can find cobbles derived from further afield: gneiss (from Idaho?), quartzite (Belt rock?), etc:
Between cobbles of the Troutdale, you can see hyaloclastic sand (immature sand with lots of hydrated basaltic glass fragments, apparently produced by interactions of magma and water in the source area, upstream):
More hyaloclastic sand:
Oooh! A "crack panel" on the side of some cooling columns at another stop! These horizontal slats are produced in individual fracture-propagation events, and each one concludes with a little ridge called an arrest line.
Mafic pyroclastics that underlie the lava flows at this second stop:
More mafic pyroclastics, on a cinder cone in Mount Tabor Park.
This is a pretty neat outcrop: you can see normal faults cutting these angle-of-repose inclined volcanic strata, presumably forming in slumping events.
Annotated version of this same photo, highlighting a marker layer and its offset along the fault:
The weather was pretty grim for this trip, so that was a bummer. But it's Portland, right? What did we really expect? Anyhow, I enjoyed being introduced to this suite of rocks -- boring out of context, but interesting given their location well west of the main axis of Cascade volcanism. Unfortunately, the field trip didn't really address why the Boring rocks are there. I was expecting some sort of detailed discussion of the possibilities: an evaluation of different models for their generation and passage to the surface... but that really didn't happen in any substantive way. So it wasn't the most amazing field trip I've ever gone on, but it was a nice day of checking out a cool suite of rocks.
Labels: meetings, oregon, pleistocene, pliocene, sediment, volcano
7 Comments:
Rich Conrey was my TA for at least one term of petrology (ig pet), and I think the second term as well (meta and sed pet). He's also the one that taught me how to make thin sections- so I've been able to look at rocks I've collected myself rather than just departmental specimens.
I've always been curious about the genesis of the Boring Volcanics... too bad that wasn't addressed. Still, nice photos.
Starting about 3 million to 2.4 million years ago, small eruptions
of olivine-rich basalt and basaltic andesite began to occur
throughout the Portland Basin, forming the Boring volcanic
field. Most of the eruptions produced small cinder cones and
a few lava flows, although some produced small shield volcanoes,
plugs, or flows that covered significant areas. The ages are
distributed fairly evenly from the onset to the most recent, at
about 120 ka.
Citation:Ian P. Madin - 2009
Oregon Department of Geology and Mineral Industries
Pg. 4 Portland, Oregon, geology by tram, train, and foot — Preprint, OREGON GEOLOGY, VOLUME 69, NUMBER 1, FALL 2009
Dennis, thanks for the info. The problem is how was magma created (or vented) so far from the arc axis? This area, in the tectonic regime, is forearc basin, not arc.
The the contact metamorphism zone needs to interact with rain water to turn red (which seems quite possible from your picture), then if you dig a few inches into it the red should disappear.
It looks like, from the CVO Boring Volcanic Field map, that the flows at Multnomah Falls - upper and lower - are not part of the field, possibly older? Did you look at basalt right along the old highway parallel to I-84?
I'm surprised that those Troutdale beds have been interpreted as "fluvial gravels", considering the absolutely horrible sorting--huge boulders, sand, mud(?) all mixed together willy-nilly. Or by "fluvial" do they mean "lahar"?
Cheers,
--Howard
@Howard- at some sites, the gravels and boulders are quite well sorted. There may be some lahar-style beds, but on-site, it's pretty clearly fluvial.
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