Mysterious Lava Flows and Fountains

Looking at all these brown and grey cliffs of basalt, who would have thought that geologists could figure out that they had once been shot out of cracks in the ground as buoyant convection plumes of magma? Well leave it to Dr. Richard James Brown and others (2014) to decipher some of the processes that allowed them to model how these basaltic rocks formed. The rocks located off George Knott Road and Texas Draw (Figure 1) and the surrounding area are about 16 million years old and made up of the Roza Member of the Columbia River Basalt - a much larger igneous province that covers much of eastern Washington and Oregon and continued all the way to the Pacific Ocean through the Columbia River Gorge (Kasbohm and Schone, 2018).

Dr. Richard J. Brown and others (2014) found that the Roza Member erupted out from a linear vent system that is about 180 km long, from just north of Enterprise in NE Oregon to north of Rock Creek in SE Washington (Figure 2). This means they found out that it was divided into two segments the South and North snake rivers. The South of the Snake River is a vent system that was seen as magma intruding through the ground (dikes), as individual pieces of rocks with lots of air holes (or scoria, the basaltic equivalent of pumice), and in many places along this coulee these pieces of scoria had been welded together – giving a wonderful example of the intense heat during their deposition. Pyroclastic deposits and the proximal lavas are generally poorly exposed on the South of the Snake River that there was spatter, pyroclastic breccia, spongy pāhoehoe lobes, and thin to thick, dense lava. On the North of the Snake River they were looking at the rocks and saw that the proximal pyroclastic deposits and spongy and shelly pāhoehoe lavas that go around the vents and are usually along the fissure too (Thordarson and Self, 1998; Brown et al., 2014). So, based on mapping of the different thicknesses, sizes and amount of welding of scoria along Texas Draw Dr. Brown and others (2014) recreated the eruptive plume that was a localized area of intense eruptions along the fissure system (Figure 3).

From an eruptive plume of this size you might expect a release of a massive amount of climate-altering gases that can affect climate change and the extinction of animals and plants, but how did this happen? Gases (volatiles) were released from the magma during the fountaining of magma from the eruptions. They found out that it was injected into the atmosphere by ash-bearing eruption plumes or thermal convection (gas-laden plumes) above pyroclastic fountains (Stothers et al., 1986; Woods, 1993). If the ash and the smoke reaches a certain height it will affect our climate change systems. But it wasn’t just the eruption along Texas Draw, the Roza Member had pyroclastic deposits intermittently exposed along with a >180-km-long vent system (Swanson et al., 1975; Martin, 1989; Thordarson and Self, 1998). Geologist think these eruptions were typified by vigorous lava fountaining that went over 10 km into the air and erupted at multiple zones along the fissure system, must have been a rough place to survive in eastern WA, 16 million years ago!