A geologist who has studied Southeast Alaska’s rocks and minerals for decades is creating a new geologic map of the region. That map could have a wide range of applications — from educating tourists, to helping prospectors find gold, to helping communities prepare for landslides.
Arizona-based geologist George Gehrels has studied Southeast Alaska’s rocks and minerals for over 40 years. He stopped through Petersburg this summer to do field research and tell the public a little bit about the geologic history of the earth beneath their feet. He wrapped up his summer research trip to rainy Southeast still tanned from the Tucson sun, where he’s a geology professor at the University of Arizona.
He’s been doing field research around the Coast Mountains since 1981 — going out on boats, carving up rock samples, and inspecting them with a tiny magnifying glass. But Gehrels said every summer he spends here is a little different.
“Sometimes we have big groups of students with us, sometimes I’m out with just a colleague and another professor or two,” he said. “Sometimes, I’ve been by myself with my dog — and then she’s the geo-assistant for me!”
So, what makes the region’s rocks so special? He said it starts with a process called “metamorphosis” — that’s when huge amounts of heat and pressure change the formation of a rock. Gehrels describes Central Southeast stones in terms that almost sound tasty: crusty bits, flaky bits and what he calls the “Crunch Zone.”
“Petersburg is kind of in the big ‘Crunch Zone’ of Southeast Alaska,” Gehrels said. “The plates are moving — North America, we know, has been moving westward for about 200 million years. As North America moves westward, it’s kind of like a bulldozer — or kind of like a snowplow — moving over the Pacific Ocean off to the west.”
Gehrels said Petersburg was right smack in the path of that snowplow (or, bulldozer) about 100 million years ago, getting crammed up against another piece of the continent at a speed of about five centimeters per year — or, about as fast as your fingernails and hair can grow. That means Petersburg is part of the critical boundary where land was deposited onto the west coast of North America. Gehrels said all that movement and metamorphosis produced some pretty cool rocks.
“Many Petersburg people know about Garnet Ledge down by Wrangell,” Gehrels said. “Those garnets there are a result of this collision. When the rocks get metamorphosed, minerals grow like garnet. So, those are world-class.”
That’s all to say that Wrangell’s garnets are world-class. Gehrels said the same metamorphic event formed the blood red crystals that can be picked out of the hillsides in neighboring Petersburg. But those garnets didn’t get hot enough in the formation process to be as good as Wrangell’s — they have more impurities.
Gehrels said the continental “Crunch Zone” produced some of the other rocks that characterize Central Southeast — there’s lots of sparkly schist and stripey gneiss and coarse quartzite.
But there’s more to his research than just cataloging cool rocks. Gehrels is gearing up to produce a geologic map of the area with some scientists with the Alaska State Survey. He said that map could be useful for people who might want to extract certain minerals — like gold — from the area. Or, it could help tour guides explain the geological significance of the landscape to visitors.
“Tourism is a huge part of the economy in Southeast Alaska right now,” Gehrels said. “The people on the tour ships, maybe they don’t realize it, but one of the main reasons they come here is because of geology. It’s fabulous, it’s what controls our landscape. Tracy Arm, you know, one of the most beautiful places on the planet — it owes its beauty in large part to the geology.”
But, even more importantly, the information they’re gathering for the map could also help people evaluate the stability of the slopes around them, to protect their communities from disaster — like when Wrangell experienced a deadly landslide last fall.
“That’s a key topic here in the Wrangell-Petersburg area right now, after the big landslide,” Gehrels said. “And all of that kind of depends on knowing a little bit about the bedrock geology in Southeast Alaska. So that’s what we’re trying to do.”
He said his team is developing an interactive digital map, where users can zoom in and out, so they can see the level of detail they want — kind of like Google Maps. But he said he’ll still print out a few old-fashioned paper versions — just because he likes them. He expects it’ll probably take his team a year to finish the map, after which, it will be available to the public.
