Giant Worms, Gold in Your Hair, and Another Way Volcanoes 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.
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