PILLOW LAVA-HEART O’ THE HILLS ROAD, OLYMPIC NATIONAL PARK

Trip 2 PILLOWS IN WASHINGTON. Excerpted from the draft of Geology Underfoot in Western Washington by Dave Tucker (This place may be snow-covered in the winter. ONP road conditions website.)

Please note- all figures and text accomanying this article are copyrighted by the author, Dave Tucker.

Figure 1 Pillow lava outcrop along Heart o’ the Hills Road, Olympic National Park. (C) Dave Tucker. Click to enlarge any image.

Directions: This stop is along the road to Hurricane Ridge in Olympic National Park. It is reached by way of the winding Heart O’ the Hills Road.  From US 101 in Port Angeles, follow prominent signs leading south on Race Street. Olympic National Park Visitor Center is at the south edge of town and is a good place to get oriented. Immediately south of the Visitor Center, veer right on the Heart O’ the Hills Road at the junction with Mount Angeles Road. You now begin to climb higher and higher into the National Park. Stop at the entrance station 5.3 miles beyond the junction to pay your entrance fee. It is important to set your odometer to 0.0 at the pay station in order to find the pillow outcrop at 5.7 miles (9.1 km, elevation 3060’, 930 m). Watch the mileage carefully- the pullout is just around a curve to the right, a little off the road at a small gully with trees. If you miss it, there are other pullouts very close by, but on the other side of the curvy road. The preferred pullout is at

47° 59.787’, 123°23.210’

To learn about the formation of pillow structure in lava flows, please see the introduction to this series of field trips. The other trips visit

–Crescent basalt at Cape Disappointment (fabulous exposures!)
–Lopez, San Juan, and Cypress Islands in the San Juan Islands.

Figure 2 A classic cross section through a lava pillow along the Heart o’ the Hills Road, Olympic National Park. Radial cooling fractures, or ‘joints’, migrated inward from the solidified crust. (C) Dave Tucker

Pillow basalts of the Crescent Formation erupted onto the floor of the Pacific Ocean around 55 million years ago. Some geologists maintain that the Crescent basalts are an “accreted terrain”, a chunk of the Pacific Ocean floor that broke off the subducting oceanic plate and became attached to the leading edge of North America as it was subducted (e.g. Tabor and Cady, 1978). Some geologists from Western Washington University (Babcock and others, 1992) interpret this extremely thick section of submarine lava to be the remnants of a huge outpouring of lava on the sea floor of the North American side of the subduction zone, and therefore not accreted. The WWU geologists calculated the volume of Crescent basalt is upwards of 100,000 km³ (24,000 mi³). They measured a section of basalt in the Dosewallips valley on the east side of the Olympic peninsula, and found that the basalt was 10 miles (16 km) thick, with no structural or geochemical evidence for fault stacking. Consequently, the Crescent basalt may be the thickest section of basalt lava in the world! Some of the Crescent basalt is subaerial; it appears that most of it spread across the sea floor and formed seamounts, but some of the seamounts broke the surface to become sizable offshore island. Such a huge outpouring at the margin of the continent could result from the development of a “slab window”. I won’t go into that in detail here, but I will in the book.  A slab window forms when a spreading center between two oceanic plates is subducted at a triple junction. Large volumes of magma from the mantle can pass through the resulting gap in the subducted ocean crust, and pass through the thin continental margin to erupt on the continental shelf. The accompanying figure from the book might give you some idea if you aren’t familiar with the concept. Regardless of the tectonic environment when they formed, the pillows here are very cool.

Figure 3 A ‘slab window’ opens when a spreading center is subducted. (C) Dave Tucker

The bulbous shapes in the road cut are the lava pillows. Each of the pillows is a separate, short-lived lobe of a much larger lava eruption, and each of the pillows extends back into the outcrop. The pillowed flow in the road cut accumulated over a period of a few days to months or even years. During eruption, separate lobes flowed alongside or on top of earlier pillows. The road cut here is a vertical cross section through these successive lobes, exposed by erosion many millions of years after the fact.

The rock has a greenish cast to it, which is principally due to the green mineral chlorite. Vesicles, or gas bubbles left behind in the lava, are mostly filled with white secondary minerals- most of these are in the zeolite mineral family. White calcite forms patches and fills cracks in the lava. All these secondary minerals formed during low-grade hydrothermal metamorphism as hot water circulated through the lava as it thickened.

Figure 4 The downward-pointing ‘tail’ on this pillow indicates that the stack of lava along the Heart o’ the Hills Road has not been overturned. (C) Dave Tucker

Carefully walk along the road (it can be busy with tourists, rubber-necking at the road cuts just as you are) and observe the pillows up close. Ideal cross sections will reveal radiating fractures inside the pillow. These are cooling fractures that developed once the lava lobe halted and cooling solidified the rock from the outside in. Contraction from cooling is taken up by the fractures, the same way that columns form in subaerial lava. In a few places, you can see that the pillows are indeed elongated rounded ‘tubes’, sometimes changing direction as they advanced. A few of the pillows can be used to show which way was down when they erupted. When a pillow lobe advances over the top of adjacent earlier pillows, the bottom of the advancing lobe fills the gap between the rounded crests to make a V-shaped lower surface. Finding one of these is among the holy grails of pillow enthusiasts, as this feature reveals the original orientation of the stack. The pile of lava pillows here is not upright. The whole stack dips to the north, away from the road, about 50 degrees.

Figure 5 A steeply-dipping lava flow in the Crescent basalt overlies pillow breccia. Heart o’ the Hills Road, Olympic National park. (C) Dave Tucker

Two-tenths of a mile back down the road to the east is a sharp, dipping contact of pillow lava over shattered lava breccia (Figure 5). The lava initially shattered to form a layer of pillow breccia (Figure 6), and was then overridden by pillows later in the eruption. It is easier and safer to see this place on the way back down the road, as the large pullout is along the downhill lane.

References

Figure 6 Sometimes subaqueous lava lobes shatter as they quench, forming thick piles of ‘pillow breccia’. This example underlies the lava shown in the previous photo. Most of the white interstitial mineral is calcite. (C) Dave Tucker

Babcock, R.S., Burmester, R.F., Engebretson, D.C., Warnock, A. and Clark, K.P. 1992, A rifted margin origin for the Crescent Basalts and related rocks in the northern Coast Range volcanic province, Washington and British Columbia: Journal of Geophysical Research, v. 92, p. 6799-6821.

Tabor, R.W. and Cady, W.M. 1978. The structure of the Olympic Mountains, Washington–analysis of a subduction zone: U.S. Geological Survey Professional Paper 1033, 133 p.

Tabor, R.W. Geology of Olympic National Park (website, US Geological Survey, Geology in the National Parks series, http://geomaps.wr.usgs.gov/parks/olym/onpreface.html. Accessed November 2011.