Well-preserved basaltic pillow lava is preserved in the Elbow Lake Formation (Brown, 1987) in the Cascade foothills west of Mount Baker. You can visit them in an easy trip by a short drive up the Middle Fork Nooksack Road (BONUS– dunite mined from the Twin Sisters is nearby. Read to the end to find out about it). The outcrops are right along the road (but you should, really, get out of the car!). This is a worthwhile stop on your way up the MFN to do something else. Allow 30 minutes or so to walk the length of the outcrop. The 100-foot-high outcrops are, at first glance (and possibly your second and third) rather unprepossessing. But look closely and you’ll find good examples of pillows, some in cross-section, some longitudinal.
There are plenty of unbroken pillows, with the original curved outer surface perfectly preserved, exposed in three dimensions. The original basalt is metamorphosed to greenstone laced with occasional quartz veins left by hydrothermal alteration, possibly during subduction.These pillows were erupted onto the seafloor of the proto-Pacific Panthalassa Ocean, but the timing is poorly constrained. Ages obtained from various rock units in the Elbow Lake range from Pennsylvanian (ca. 300 Ma) to Permian (as young as 252 Ma) to Jurassic (ending 145 Ma) (Brown, 1987). The formation includes metasedimentary rocks, ribbon cherts, and basalt. These rocks were subducted beneath North America, and then accreted to the continental margin.The Elbow Lake is disrupted and outcrops are scattered all over the northwestern Cascades (Brown and others, 1987). The unit is everywhere faulted and generally a mess. These rocks were assigned to the Elbow Lake Formation by WWU geology professor Ned Brown on the 1987 1:100,000 geologic map compiled from the field work of over a dozen of this graduate students (Brown and others, 1987).
A more recent map (Lapin, 2000) maps this place in the Darrington phyllite but phyllite is not evident in these outcrops. If this relatively undeformed pillowed basalt is included with the phyllite, then they are a localized basalt flow and the Jurassic age would necessarily be the correct one. I don’t believe the Lapin mapping is accurate in this particular location.
If you are unfamiliar with the formation of pillow lavas, I have written about them elsewhere on this website, where you’ll also find links to other pillow localities in Washington.
From Mount Baker Highway, turn south on the Mosquito Lake Road a few miles east of the junction with Highway 9 South. This is east of Deming. To reach the Middle Fork Road from the Mount Baker Highway (State Route 542), turn right onto Mosquito Lake Road 17 miles east of I-5. This is 4 miles east Deming, and upvalley from the intersection of 542 with Highway 9 South. 4.8 miles (7.7 km) from Baker Highway, veer left onto the gravel Porter Creek Road, which becomes the Middle Fork Road. In 2.7 miles (4.3 km) a high rock cliff comes into view on the left, just after the side road on the right down to the Middle Fork Nooksack water diversion dam. Park in the graveled area in front of the rock face. You are there. I told you it was easy! You’ll be there in 35-40 minutes from I-5. The outcrops line the road for the next 500 yards. It is worth walking all the way to the end where the road curves left. Elevation is 1100 feet.
Start your visit at the western-most outcrop, the first you come to. Scan the lower 15 feet or so of the rock face, which is pretty clear of vegetation. If you are a confirmed pillow-phile, you may spot them right away-they are subtle. I hope the photos are helpful to you if you are not. Once you’ve picked out a few pillows here, you’ll begin to recognize more. In some places, the pillows shattered to fragments when the rind quenched on the seafloor to formed pillow breccia.
Walk on up the road. You’ll come to a high cliff with a large smooth apron of rock. The rock on either side and above is rougher. This is, I believe, a sheer plane resulting from faulting. Fault movement ground the rock smoothly. The rock above this apron, and to the west, is the remaining overlying rock; it is broken and rough on the faces adjacent to the fault surface.
In another rock face, parallel faults angle downward to the left. These are parallel with crude bedding preserved in these stacked pillows (lava flowed over slightly earlier flows). Look carefully and maybe you’ll see slickensides on the fault surfaces.
Several bulbous pillows have eroded right out of the cliffs and are lying in the talus or along the road. I picked out a nice foot-long pillow and left it sitting beside the road, but forgot to pick it up on the way back to bring home. Finders keepers, I guess.
The easternmost exposure may be the best. The rock was drier when I was there, and steeply-dipping pillows are very obvious. The largest are 4-5 feet long. The stack of bulbous basalt lava pillows are clear to see, and the section shows them 3-D.
When I visited, a 2-foot rounded dunite erratic was lying beside the road. The surface is weathered a bright brownish orange. It must have rolled down the hill from the till overlying the bedrock.
Which reminds me of that dunite BONUS I promised you. Drive back toward Mosquito Lake Road. Before you get there, notice a pile of greenish crushed rock on the left. You can drive down a short road on one side to reach the Mosquito Lake Road. The Olivine Corporation quarries dunite high on the north end of the Twin Sisters, across the Middle Fork and several miles upstream. This famous rock body formed in the Earth’s mantle. Dunite is principally the mineral olivine, a magnesium-ron silicate. The olivine in the Twin Sisters dunite is principally the forsterite variety containing 90% Mg and only 10% Fe ((Mg90,Fe10)2SiO4) in the mineral lattice (Ragan, 1963). The high proportion of magnesium gives this rock an extremely high melting point: about 1900 C (over 3200 F). The other variety of olivine, fayalite , is rich in iron and poor in magnesium, and melts at a mere 1200 C (2190 F). For the chemically-inclined, the formula for pure fayalite is (Fe)2SiO4- note the total absence of Mg. So, this particular body of olivine is great for making refactory bricks to line high-temperature kilns and ovens. Truckloads of crushed dunite are hauled down the Middle Fork Road and dumped in a pile near the junction with the Mosquito Lake Road. Ordinary dunite boulders weather with an orangish crust, and the rock is very hard and difficult to break open with a hammer. Because the dunite in this pile is already crushed to fist-sized chunks, this is a great place to examine rocks from the mantle; once again, you barely need to get out of the car. Fresh surfaces (most of them in this pile) are the typical greenish gray of dunite. Look closely to see 1-mm black flecks. This is the mineral chromite, a source of chromium. These disseminated crystals make up less than 1% of the dunite.
Dunite is receiving a new type of attention (Goldberg and others). Crushed forsterite-rich olivine reacts readily with CO2 in the atmosphere. One liter of crushed olivine can sequester the CO2 given off from burning 1 liter of petroleum. One ton of olivine can dispose of approximately two-thirds of a ton of CO2, depending on the proportion of Mg in the olivine. The simple reaction is:
(Mg,Fe)2SiO4 + CO2 = SiO2 + MgCO3 +FeO + H2O
One of these days I’ll get around to writing up a field trip to the dunite quarry, and a visit to the Twin Sisters proper.
Brown, E.H., 1987, Structural geology and accretionary history of the Northwest Cascades system, Washington and British Columbia: Geological Society of America Bulletin, v. 99, P. 201-14.
Brown, E.H., and a host of others, 1987, Geologic map of the Northwest Cascades: Geological Society of America Map and Chart Series MC-61, scale 1:100,000, 10 p. Not available on-line.
Goldberg and others, 1998, CO2 mineral sequestration studied in US: DOE National Energy Technology Laboratory. http://www.netl.doe.gov/publications/proceedings/01/carbon_seq/6c1.pdf accessed 12/29/12.
Lapin, T. J., 2000, Geologic Map of the Bellingham 1:100,000 Quadrangle, Washington: Washington Division of Geology and Earth Resources Open File Report 2000-5. Download maps at this DGER website.
Ragan, D. M, 1963, Emplacement of the Twin Sister Dunite, Washington: American Journal of Science, v. 261, p. 549-65. http://www.dnr.wa.gov/publications/ger_reprint8_twin_sisters_dunite.pdf accessed 12/29/12.