Eerie afternoon in the Girdwood ghost forest
by J. Besl |
This is the second in a three-week series covering Anchorage Field Studies, a UAA geology class taught by Dr. Kristine Crossen. The course spans the first five weekends of each fall semester, starting with a brief classroom component before students head off to explore geologic sites across the Anchorage Bowl.
Dr. Kristine Crossen is one of the few UAA professors who has to consult a tide calendar before submitting her course schedule. That's because Anchorage Field Studies-her annual course highlighting the area's geologic hotspots-wouldn't be complete without a trip through the Girdwood ghost forest.
Anyone who's driven down the Seward Highway has seen the patches of dead trees that speckle the mudflats around Girdwood's old townsite. The trees-stripped to their trunks, perched at teetering angles-are a direct reminder of the 1964 earthquake. Some, though, indicate massive earthquakes that occurred even earlier.
For those not enrolled in Geology or Geography 101, here's a brief lesson. Anchorage is located on the Northern edge of the Ring of Fire, a geologically active area spanning the Pacific Ocean. Earth's tectonic plates are always shifting imperceptibly and, in Alaska's corner of the ocean, the denser Pacific plate is subducting-or sinking-under the less dense North American plate. As that massive slab of the earth gets buried-roughly along the line of the Aleutian Islands-the crust melts and fires back to the surface through active volcanoes. Recent evidence of this subduction zone occurred in 2009, when nearby Mount Redoubt spewed ash over the Anchorage area.
But these grinding, colliding tectonic plates also result in all-too-familiar earthquakes. In fact, thousands of quakes rattles through Alaska every week. Few, though, can rival the Good Friday earthquake of March 27, 1964-a shattering 9.2 event north of Prince William Sound that shocked the Anchorage Bowl for four and a half shaky minutes. The ghost forest serves as an eerie reminder of that powerful event.
XTRATUFFs in action
Massive quakes force bedrock to bend and buckle, but what happens in Anchorage where, as we learned last week, bedrock is hard to come by (the city is actually built above 800 feet of unconsolidated glacial till)? The students were about to find out.
Kris's ghost forest lecture started in a rather unconventional classroom-a coffee shop behind a gas station off the Seward Highway. While the rain splashed the windows, Kris instructed her students to grab a coffee and unpack their clipboards. There was too much to learn to let a little rain get in the way.
As cappuccinos and hot chocolates made the rounds, Kris explained the science behind the ghost forest. During the 1964 earthquake, the bedrock actually dropped nine feet, causing the mud, trees and everything else to drop as well. As the tides swept in, trees that once perched over the coastline were inundated with saltwater, soaking their roots and creating today's dead and decaying forest.
Students set out to observe the evidence firsthand, traipsing into the marshy reeds and putting their XTRATUFFs to use. Though the trees had dropped several feet, Kris pointed out that the surface had crept up their trunks. Decades of deposition have created a new surface and refilled the area with seven feet of silt. The surface at the waterline is composed of a hodgepodge of area rocks-sandstone from the Portage Valley, volcanic rock from Mount Spurr, granite from Mount Susitna-where all the sediments have been churned together, indicating oceanic origins. "It's all kinds of stuff brought in from the tidal action," said geology student Phil De Land.
In just 10 years after the quake, the Arm's gritty tides deposited seven feet of sediment over the tree roots. "Those trees you see are actually seven feet further into the ground," explained Phil. "They've just got a pile of sediment standing on top of them. You're looking at the middle of the tree."
Grimy, gritty layer cake
But some trees are buried even deeper.
The 1964 earthquake was a doozy-the second mightiest in recorded history-but the earth's plates have been shifting long before geologists kept track. When the students traveled into the ghost forest, they discovered that other trees were buried deeper in the muck, evidencing other massive quakes-and bedrock subsidence-from before 1964.
Bad news for the trees is good news for geologists, though. By digging into the mudflats around the ghost forest, a geologist can find periodic layers of organic material, or peat, made of plants growing at the surface. Each time the land subsided during a major earthquake, Turnagain Arm would deposit fresh silt and bury that layer of organic material. Its turned the whole area into a grimy layer cake, where thin levels of peat separate thick intervals of sediment. By dating the organic material, scientists can determine when the peat was buried and determine the frequency of major quakes in the area.
"That was the coolest part of the whole thing because it was truly tectonics in motion," said Ian Fox Minnock, a geology major from Maui, Hawai'i.
"It's a cycle-that mud goes down 700 to 900 feet before we hit bedrock, and that bedrock has been lowering," he continued. "It's crazy to think that earthquakes happen so often that that's how much the earth can subside. They are frequent-on a geologic time scale-and they are of high magnitude. That's permanent deformation to that area just through earthquakes alone."
We're #4! We're #4!
For Ian, Anchorage Field Studies has provided a valuable outlet to explore the area. He moved here specifically for the geology-"There's one rock in Hawaii and its basalt. In Alaska, there are so many rocks its ridiculous," he joked-and the class has certainly shown him the geology he specifically sought out by enrolling at UAA.
As a full-time student who lives on campus, the class provides a valuable outlet for Ian, too. "I don't drive because everything I do is on campus. I pretty much don't leave the same mile radius," he said. "It's great to get out and see geology not in a lab, but in the field, from Matanuska and the Chickloon formation down to Portage."
Although his research interests are in planetary geology-he's presenting on asteroids at a conference in Baltimore later this year-he'd recommend the class to anyone in the department. "You can't really get the idea of geology in Alaska without doing this field study," he noted. "Anybody who wants to do individual research and apply their skills really should do this field study. It's professional. It's advanced as well. We're not talking about geology 111 topics, we're talking about advanced integration of historical geology, petroleum, stratigraphy and everything. It's great."
Personal research aside, the class is a major benefit for any UAA graduate looking to stay in Alaska. "This town is #4 town in the country for geologists," Kris explained. "We have a huge geological industry. People who have a bachelor's degree-as long as they're well trained in the things we train them for-are going to get employed."
The awareness of Anchorage's geologic history is excellent preparation for a career in the field. "You live and work in Anchorage but, as soon as you get in an airplane, you're in Alaska," Kris smiled.
As the rain settled in again, the students headed back to the vans, cutting a path through a stand of live trees. When the next big earthquake hits, those trees will be the next in the long line of ghost forests that have dotted the coastline on Turnagain Arm.
For more on Anchorage Field Studies, read about last week's trip to Bird Point.
Written by J. Besl, UAA Office of University Advancement