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Diatomite Lake Invaded by Molten Lava - Frenchman Hills, WA

Pillow-palagonite complexes form when molten lava invades a water body and quenches. Pillows are the quenched blobs of lava. Palagonite or hyaloclastite is the loose, rusty-looking mess that often surrounds pillows. Palagonite is composed of basaltic glass, shards of basalt rock, and maybe even some cooked sediment. Its the alteration product of 1100 degC lava coming into contact with 25 degC water and flashing to steam. The stuff is loose and ravelly when formed and tends to remain so. Most of the time, you can just paw the stuff away with your hands. Pillow-palagonite complexes in the Columbia River Basalt are typically found at the base of flows and can be quite thick.

A spectacular late Miocene pillow-palagonite complex - one with a particularly interesting geologic history - is located along Road 9 SW on the south flank of the Frenchman Hills anticline. The south flank is known as Royal Slope, home to wine grape vineyards, fruit tree orchards, and a few remaining hay fields.

The roadcut along Rd 9 SW is located just below the Frenchman Hills crest between Dodson Road (~5 miles east) and Adams Rd (~5 miles west), Road 9 SW is the highest public road traversing Royal Slope. The newly-planted Stillwater Creek Vineyard is near by. Parking isn't great.

The pillow zone is about 4m thick. In this portion, the round pillows don't touch each other.

Pillows here all touch and nested together, their shapes established prior to cooling.

Concentric pattern and textbook glassy rind is due to rapid cooling.

Yellow mineralized zones associated with pillows contain common opal (hydrated amorphous silica). Most is not gem quality. Opal is associated with diatomite beds in surface mines west of here (IMERYS, Inc.) and with petrified wood in nearby exposures of the Ellensburg Fm. Some vein opal nearby appears to have formed in fractures after the basalt cooled, the result of mineral precipitation from circulating groundwater. Some opal chunks are found at Rd 9 SW and other nearby outcrops (i.e., intersection of Beverly-Burke Rd and Rd Q SW).

Diatomite is a friable, bright white sediment consisting of the fossilized, silica-rich tests of diatoms (hard-shelled microalgae). Marine and non-marine varieties exist. One cubic inch can contain the remains of 50,000,000 organisms. Diatomite, also called 'diatomaceous earth', 'kieselguhr', and 'moler', is used as a filter medium, as an abrasive, an additive, and for absorption. Though widely used since the early 1800s, it was recognized as an important industrial mineral in Washington in the 1920s-30s (Skinner et al., 1944).

Economic quantities of diatomite are mined from the Quincy and Squaw Creek deposits. Figure by Ebinghaus et al. (2019).

Diatomite beds at Quincy (Q) and Squaw Creek (SC) are slightly different age span the axis of the Hog Ranch-Naneum anticline. The axis parallels the Cacade crest and is approximated by the red dashed line. Shallow basin(s) in which the diatomite formed appear to be long-lived and to pre-date arching of both the N-S trending HRNA and the E-W trending Frenchman Hills anticline (Mackin, 1961, p. 25). My interpretation (and perhaps others' I've failed to recall) differs with that of Grolier and Bingham (1978, p. 11), who believed diatomite deposition "occurred at two different [Columbia River-affinity] quartzite-bearing gravels have been found within either deposit". Map modified from Bingham and Grolier (1966, Fig. 4). Other type section locations on map: V = Vantage Sandstone and Gingko flow, SG = Sand Hollow and Sentinel Gap flows, B = Beverly and Saddle Mts Members, PR = Priest Rapids Member, R = Roza Member.

Diatomite deposits accumulate on the floors of shallow lakes. Flood basalts followed valley bottoms. Thick calcretes in Eastern Washington are closely associated with alluvium and low landscape positions (i.e., floodplains). All three units occur together at George, WA, indicating this part of the landscape has remained a low spot for millions of years. At Frenchman Hills, Mackin (1961, p. 18) noted evidence of basalt flowing over both a dry surface (no lake) and wet sediment (lake present), "...this flow fills shallow channels cut in the [diatomite]. It is evident that the flow spread in the dry after disappearance of the lake..." At the same exposure, he observed clusters of "blocks and pillows...injected into the diatomite..the Priest Rapids flow clearly advanced into the lake in which the diatomite was formed". At the nearby Squaw Creek diatomite mine, "the diatomite was deposited in a lake impounded by the Sentinel Gap flow...the lake plain trenched by rivers before the spreading of the Roza flow" (p. 21). Photo: Washington Geologic Survey photo archives No. 3529.

Miocene basalt flowed into a body of water and over wet sediments (Ellensburg Fm). A baked "soil" marks the contact. Its not really a soil, because it was submerged when the lava showed up. The pillows basalt tells us that. What do you call sediment that lies on the bottom of a lake? Muck? Soil-muck? Baked soil-muck?

Here the same basalt flowed over the same wet sediment, but not into water. Baked muck-soil, but no pillows. I get the feeling this muddy stuff attenuated heat pretty quickly. The intensely baked zone is thin. Quality of the roadside exposure varies with the year.

Probably the coolest thing about the roadcut is these bright white dikes. They are not tectonic features or flood-related clastic dikes like those in the Touchet Beds, rather they are fractures filled with remobilized diatomite. Lake bottom mud was rapidly heated by the overriding lava and became mobile - a phreatic encounter. Note how the dike crosscuts the large pillow in the center of the photo. The fracture is younger than the pillow, if only by a few hours. But the fractures are not the whole story.

The depth of the Miocene lake approximates the thickness of the pillow-palagonite complex. Pillows and the mobilized diatomite intermingle. The pillow in the photo is enclosed by white, which suggests the lava flow was invasive into the lake bed sediment and fluidized the white diatomaceous mud which escaped into permeable cracks and pore spaces in the partially-quenched palagonite. The white stuff is high in the outcrop. The dikes taper downward, not upward. The center of the lake and main body of the diatomite seems to be mostly east of the exposure.

White diatomite surrounds pillows.

Fractures filled with mobilized diatomite descend from a larger, coherent mass.

A photo by Ebinghaus et al. shows "Diatomite interbed of the Silica Road locality (Squaw Creek Member), invaded by basalt of the Roza Member. Spiracle on the left side indicates blow-out of water steam."

Sketch by I. C. Russell of a now-lost exposure at Barnard Street at 8th in Spokane (Russell, 1897, p. 53; Pardee and Bryan, 1925, p. 14). A basalt flow moved into a lake, overriding and deforming unconsolidated bottom sediments. The bed of "micaceous clay" is thin-bedded, sandy, and containing leaf fossils. The "conglomerate" is an uncemented gravel containing clasts up to boulder size. Its a baked and loaded contact. Russell continues, "The bottom of the basaltic sheet has rounded, smooth-surfaced protuberances...which the gravel and bowlders, together with detached [stratified rip-up] fragments of the micaceous clay, frequently 12 to 16 inches in diameter, extend upward for 8 or 10 feet [as is common when wet sediment is rapidly loaded]. Fragments of the micaceous clays in contact with the basalt show evidences, in their compactness and reddish color, of the effect of the heat of the lava which came in contact with them...The loose deposits were disturbed and forced along in front of the flowing lava until the resistance became sufficient to check its progress...Still liquid lava broke out from the face of the arrested flow and advanced beyond the previously cooled border, overflowed the accumulation of stones and fragments of the micaceous clay, and again forced up a ridge of the loose material. This process was repeated several times...". The geologic story at Bernard Street is not identical to that at Frenchman Hills, but its close: Unconsolidated sediment responding to inundation by a piping-hot lava flow. Albert Mann describes diatomite collected from the Latah Fm near Mica near Spokane, WA (Pardee & Bryan 1926, p. 51).

According to Bingham and Grolier (1966),

...zones of pillows and palagonite at the base of many flows and the silicified logs or stumps between pillows or within the palagonite [that] all show that the lava flows advanced across lakes, streams, and forests. The lakes in which sedimentation took place owed their existence to disruption of drainage by advancing lava flows, and possibly to subsidence of the lava field as well.

And observed this same diatomite bed at other locations,

...the diatomite bed ends against the front of a Priest Rapids flow...the lake was impounded by this flow...relationships are believed to be similar to those between the Squaw Creek and the Sentinel Gap flow...the flow advanced into the lake rather than over the dry lake bed...the Quincy lake was impounded by one...flow and destroyed by another...

An active diatomite mine is visible from Frenchman Coulee near The Feathers. Two diatomite beds are distinguished above and below the Roza flow (Schminke, 1967; Brown, 1968, Fig. 2; Tolan et al., 2009),

On the horizon across the coulee to the north are bright white piles of sediment removed from the interbed between the Roza Member and Priest Rapids Member. The Roza flow at this locality invaded the diatomite and is an invasive flow. The diatomite is not in its proper stratigraphic position, and actually belongs to the Squaw Creek Member and not the Quincy Member of the Ellensburg Formation.

Stratigraphic position of two diatomite beds in Brown (1968, Fig. 2).

The fracture fills contain basalt fragments (chunks of wall rock) and faint indications of fluidized flow.

Open pore space in the palagonite appears to have been filled by the white stuff.

See how the stringers and pockets of white are found above and below pillows and in small fractures throughout the complex? To me, that speaks to a diatomite bed several meter thick invaded and displaced by invading lava. Ignore the light gray calcrete capping the outcrop. The calcrete, a paleosol, is Pleistocene in age - millions of years younger than the basalt. See how the uppermost pillows are truncated by it? The younger calcrete unconformably overlies the Miocene pillow-palagonite complex. Now go back to ignoring it.

A network of filled fractures crosscuts a pillow near the top of the exposure, giving it a shattered look.

Map of diatomite occurrences in the Western U.S. (USGS/Wallace et al., 2006). A number of articles describe the Quincy diatomite deposit near George, WA (Silica Rd), mines in the western Frenchman Hills (Adams Rd), and the smaller Squaw Creek diatomite (T15N, R20E) west of Vantage (Godby, 1914; Lowell, 1930; Swift, 1940; Skinner et al., 1944; Makin, 1949, 1961; Bingham and Grolier, 1966; Livingston, 1966; Burlington Northern, 1971; Campbell, 1975, Ralston, 1984; Brunstad, 1987; Houseman, 2006; Menicucci et al., 2008; Menicucci et al., 2016). The mines near George closed decades ago, but mining continues in the Frenchman Hills today. Two busy surface operations can be seen from the Beverly-Burke Road near where it crosses the crest of the anticline (IMERYS,


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