Posted by: magmatist | August 17, 2010

Field work in Sherman Crater; summer 2010

By Dave Tucker  August 17, 2010

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Tony Reese's Hughes 500D below Sherman Crater, August 2010. click to enlarge images

There have been two rounds of fumarole gas sampling in Sherman Crater this summer, one on July 6th, the other on August 12. Melissa Park (WWU Geology) continued her thesis project, preparing an ice-radar profile of Sherman Crater’s ice thickness. She made her last set of profiles with a cast of many volunteer helpers in July. This required camping near the crater. On August 12, 2010, the first hand-held magnetometer traverse, and some new gravity readings were made in Sherman Crater by Sarah Polster (USGS). One more trip is planned, to retrieve the recording seismometer that has been buried at the south crater rim since last summer.

The July gas sampling was done by Dave Tucker, with the help of the team of scientists

The July Sherman Crater team, with the full range of MBVRC fundraiser shirts over the years. Left to right: K. Kemplin, D. McKeever, M. Park, D. Graber, D. Tucker, S. Linneman, C. Linneman.

and climbers  in the picture. Gas chemistry from that trip will soon be posted at the main MBVRC page, www.mbvrc.wwu.edu; dta from sampling in past years is available there now. Watch for a future update that will discuss the changes in Sherman Crater fumarole chemistry over the years, and for a separate update about Melissa Park’s radar work.

Sarah Polster measures gravitational force at Gravity Rock on the south rim of Sherman Crater

For the August crater trip, a helicopter flight was arranged as part of a USGS geophysics project run by Carol Finn (Denver). The party consisted of Sarah Polster, a recent WWU Geology graduate who works with Carol, John Scurlock, Flight Commander of the MBVRC air wing, and Dave Tucker, WWU. Tony Reese of Hi-Line Helicopters in Darrington, and a veteran of North Cascades flights since 1978, was our skilled and masterful pilot, flying his powerful Hughes 500D.

We landed on the glacier just south of Pooch Peak, the tottering pile of altered breccia and lava at the southwest corner of Sherman Crater, then climbed the short distance over smooth snow to the south rim. We hoped to dig out Jackie Caplan-Auerbach’s buried seismometer, but it is still under 4 feet of snow and frozen tephra. When it was buried last summer, in July, this place was completely melted out. Sarah measured gravitational force at Gravity Rock, then we slipped into the crater by downclimbing eight feet on delicately loose scree above a yawning bergschrund. It was only possible because a slender remnant of ice plastered on the slope allowed us to traverse to the safety of the crater snowfield. A belay made the crater entrance a lot safer.

Sarah Polster and the magnetometer shoulder rig on the floor of Sherman Crater.

Sarah loaded herself up with the awkward magnetometer, which includes a hefty battery beltpack, and a long, over-the shoulder pole with the magnetometers on the front end, a heavy counter weight on the back, all connected with a spaghetti of cables. That, plus she was carrying the gravimeter with its battery in her pack. We descended the easy snow for 400 m to the bottom of the crater, right up against the northern wall, then made an east-to-west magnetometer traverse. We went as far toward the east breach as we could, in the hopes of investigating and maybe even sampling the ‘Main Fumarole’, a.k.a. ‘Sulfur Cone’. However, it currently sits at the bottom of a steep ice wall, so we couldn’t get to it.

The Smiley fumarole in the west crater rim fumarole field- gas is streaming out of the Ti sampling tube before the Tygon hose is attached.

At the base of the west crater rim, John and I sampled gas from three fumaroles in the talus while Sarah wandered around getting more gravity data. The idea behind her projects is to better determine how much hydrothermally-altered rock underlies the crater area; this is the stuff that is altered to clay by the acidic fumarole gas permeating the area. The altered rock and clay has little coherence and is prone to gravitational sliding into valley bottoms to generate far-traveled lahars.

The fumaroles were hot, loud and blasting, as usual. The ground is too hot to touch around the dozens of vents on the west crater wall. Lots of sulphur crystals and encrustations grow among the rocks of the talus slope, either at active fumaroles or near ones that have become inactive. The fumaroles generally vent from between the rocks of the slope, and they generate a lot of gas, mostly H2O but also traces of CO2, and H2S; the latter lends its characteristic odor of rotten eggs.  Some people get nauseated by the stink, but to others it is the sweet smell that tells us that Baker is a live volcano.

Gas sampling. ~90 C Fumarole gas from the packed-up fumarole flows through the tubing and is condensed in the glass vacuum flask in the snow-filled plastic bag.

Gas is sampled by sticking a titanium tube into a fumarole vent. If a good connection is made, steamy gas will vent out the end of the tube. Mud is packed around the vent to reduce the amount of gas that can escape through the rubble around the vent; this might take some engineering, but is important. Once a good flow is established, a perforated rubber cork with a tygon tube is stuffed into the end of the tube– the tube immediately gets very hot, so big, heavy heat resistant gloves are needed to handle the tube. The other end of the tygon hose is plugged in to a glass vacuum flask, then the flask’s valve is opened and gas flows in. The flask sits in a plastic bag filled with snow to condense the gas into liquid. After the gas has run into the flask for around 5 minutes, the valve is closed, and the flask is then carefully packed in a foam container for the pack out.

The west wall of Sherman Crater rises above the Smiley fumaroles to the west rim.

It is possible to climb out via the rocky slope up to the west rim, but it may appear intimidating, especially with a big pack filled with glass flasks. There is no margin of error on that route- a slip could be very nasty and long. We retraced out steps to the south rim and down to the helicopter rendezvous- and out to Annie’s Pizza in Concrete. Yummy, and they just happen to have beer, too!

A word of caution, should you visit the crater- the rocks and clay of the crater are acidic, so it is a good idea to rinse everything when you get home- backpack, gloves, pants… I also always wash my rope and harness after.


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