Friday, February 1, 2013

Geomorphology, Part II

Giant Worms, Gold in Your Hair, and Another Way Volcanoes Can Kill

The fun and utility of geomorphology comes from what land-forms can tell us that we didn’t already know. An example of this is the Palouse region of western Washington State. Here we find hills composed of excellent sandy soil, covered with wheat farms that seem to have no “normal” (that is dendritic, or branching) water drainage pattern. When you drive through them on highway 91 it just looks “wrong” to a geologist. The area hosts the giant Palouse earthworm or Washington giant earthworm (Driloleirus americanus). There are other odd bits of unusual evidence lying around: boulder “erratics” – huge chunks of granite located in unexpected places. My wife reports seeing a huge granite boulder on the south side of a 1,100-ft-high (340 meter) ridge above the Columbia River, many kilometers from the source location of identical rock where they must have come from. How did these huge, Volkswagen-bus-sized erratics get moved such a long distance and then lifted over that ridge? 

Only in the 20th Century did geologists gain access to aircraft and air photos, and noticed that the Palouse “hills” looked like giant versions of ripple marks that you can see in stream beds. Further mapping and thinking led to the then-astounding conclusion that the Palouse represented the remnants of a gigantic series of floods in relatively recent history. The scale of the first flood was beyond anything that anyone could even begin to comprehend. As recently as 12,000 years ago, an ice dam formed at Glacial Lake Missoula (Idaho-Montana). Because it was an ice dam, when it failed it had burst catastrophically. How catastrophically? The modern Willamette Valley where you find Portland, Oregon, was once a canyon terrain, but is now filled in almost flat with sediment. How much sediment? Calculations suggest that the first of what may have been up to 72 sequential Missoula Floods carried with it 5,000 cubic kilometers of rock and debris down the Columbia River gorge! This was moved all the way from northeastern Washington State, down the Columbia River to Astoria, Oregon, and roared on out into the Pacific Ocean where it can now be mapped using sonar bathymetry systems.

As one practical example of applied geomorphology, there is a gold deposit in southern Venezuela called Chiricayen. The gold was first discovered when a Pemon Indian women went to a waterfall to bathe. When she returned, her family noticed bright flecks of gold in her black hair. The crucial issue was where did this gold come FROM?  If it all just collected at Chiricayen, then this was just a minor gold deposit. If it came from a particular sedimentary unit, then it implied a far larger and concentrated gold resource. The waterfall location was in a shelf along a cliff of a 1.7 billion-year-old sedimentary unit called the Roraima Formation. A quick glance at the site from a helicopter suggested that the shelf was a down-dropped block, fault-controlled. However, a geomorphologist gave careful examination to stereo-pair photos that I took from a helicopter passing in front of this apparent shelf. He pointed out that the sedimentary layer pattern (thick units and thin units in a particular order) was the same on the front of the shelf as in the neighboring walls. In other words, this was not a down-dropped catchment, Instead, one single layer above it probably hosted paleo-rivers, ancient meandering streams, that hosted gold from a distant (now weathered-away) source. This single observation provided a new understanding of how these gold deposits form – and redirected the gold exploration strategy of our host agency in Venezuela.

Geomorphology can have life-saving consequences, too. The rate of sediment movement off of a stratocone volcano like Mount Rainier has a huge impact on downstream communities. Recognizing lahar (water-and-volcanic debris flow) deposits from time past can indicate what can be expected in time future. In 1906 a “Pineapple Express” (a so-called “sky river” from the tropical western Pacific) dumped 20 inches or more of rainfall on the volcano, precipitating a huge flood. This led to the mistaken conclusion that making the Nisqually River straighter would make things safer. However, vast amounts of dredging and river-straightening made no one safer, but instead meant less salmon and more sediment movement to southern Puget Sound. 

It’s worse than that. Shortly after the 1980 eruption of Mount St Helens, a farmer asked Rocky Crandall, a USGS volcanologist, about boulders in his soil that made plowing difficult. Crandall recognized that this was part of a lahar – and traced the source to the BACK side of Mount Rainier. A lahar called the Osceola roared down here 5,600 years ago – and continued on to Puget sound, and then continued another 30 kilometers further under the water. Another lahar called the Electron did the same thing just 500 years ago. Lahar is an Indonesian word, and refers to a fast-moving wall of wet volcanic debris, not unlike wet concrete in consistency, traveling up to 60 kilometers per hour with car-sized boulders entrained in it. Nothing can stop these things, and one lahar killed 23,000 people in Colombia in 1982. It ripped the cathedral from its foundations and killed most of the occupants of Armero in just a few minutes. Ultimately, the recognition dawned on everyone that a crumbling volcano will unavoidably succumb to gravity. Geophysical studies have shown that the near (west) side of Mount Rainier is hydrothermally altered – old-time miners call rocks like this “punky” - and it is just waiting to collapse. The Nisqually Valley is literally filled flat with layer after layer of lahar deposits – up to 22 of them. Sadly, it is now also covered with human development - up to 500,000 people are exposed to the utter inevitability of a lahar. 

A careful geomorphology study has thus shown that the altered and friable west side of Mount Rainier will someday inevitably fall off and flow all the way to Puget Sound – sweeping absolutely everything before it. To try to protect the human population, the USGS has installed Acoustic Flow Monitors in the drainages leading down from the volcano. An artificial intelligence system monitors multiple sets of paired geophones 24 hours a day, and the entire system is coupled to a siren warning system. Calculations suggest that the inhabitants will have 45 to 60 minutes warning of an oncoming lahar from Mount Rainier, once a moderate-sized earthquake (or another eruption) sets free what is now called the Sunset Amphitheater.