Upon consideration, you might think that with a name like Harlequin Lake there would at least be some harlequin ducks floating around. Au contraire, my friends! Harlequin Lake, situated inside Yellowstone National Park between Madison and West Yellowstone, is not known for its ducks. It is, however, known for two other notable features:
1) a lava flow
2) beavers
This particular lava flow is not any ordinary lava flow. It has a remarkable reverse paleomagnetic polarity. Approximately every 200,000 to 300,000 years the magnetic polarity of the Earth flips. These links to web sites of PBS Nova and the British Geologic Survey each offer a fine explanation of this phenomenon. And beavers are truly among the most astounding of Earth’s engineers, whether two-footed or four. I simply had to go there!
IMG_8924 HarlequinLake
Harlequin Lake
Without getting deep into something I know next to nothing about, I will say that as molten lava flows and cools and solidifies, any iron-bearing minerals in it (such as hematite) adjust themselves according to the Earth’s magnetic North-South orientation of the day. What is so interesting about the Harlequin Lake flow, which occurred around 840,000 years ago, is that, at the time the flow occurred, North was apparently South and South was apparently North. If you had stood there with a compass, the needle would have pointed South instead of North.
According to Christiansen (2001), the Mount Jackson Rhyolite is a sequence of eight lava flows at the beginning of the third and most recent volcanic cycle of the Yellowstone hot spot. The Harlequin Lake flow occurred during this sequence.
So the other day I dragged Friday–hiking–buddy Sacha out with me, to see what we could see. We each had a compass, the better to investigate this remnant magnetism. We wanted to find out for ourselves if the needle would indeed deflect South. A perusal of any beaver activity in the area was also on the agenda.
IMG_8938ShouldBeThe HarlequinLakeFlow
Alleged Harlequin Lake flow
IMG_8919Sacha_ Compass_AndThe Hill
Looking for North
At the trail head parking lot I looked at the geologic map for the zillionth time. I stared intently at the hill across the highway. This had to be the right place. The hillside, however, was dense and impenetrable with a 23–year old lodgepole pine forest, sprouted after the 1988 fires. I knew that lodgepole cones could drop 10,000 seeds per acre after a fire and this hill looked to me like every single one of them had taken firm root. We decided to check out the beaver situation first and then attack the hill on the way out.
It would have been really nice to spot a beaver or two in action, slapping mud onto newly–arranged lodge logs with their flat fat tails, but since they are crepuscular little critters, and it being around noon, we were not really surprised that we didn’t see them. What we did see, though, were half a dozen lodges scattered about at intervals on the far side of the lake. We also noticed that a considerable portion of the forest edging the lake had succumbed to the beaver’s incessant appetite.
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Beaver lodge on Harlequin Lake
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A marvel of engineering
IMG_8932Beavers BeenHere
Beavers been here!
I’m not sure what fascinates me so about beavers, but I do a short ranger program on them once a week and have learned some amazing beaver facts. They mate for life, are driven by the sound of running water to build dams and lodges that are marvels of engineering, can swim underwater for ½ mile and remain underwater for up to 15 minutes, can gnaw a 5–inch diameter tree in three minutes, and have two sets of lips! Yes, it’s true. Not one person I have spoken with knows this fact about beaver lips. If you think about it, two sets of lips make perfect sense. Beavers often gnaw trees beneath the water of lakes and ponds – the set of lips behind their teeth remain closed while they chomp on the tree. When they are ready to swallow that tasty bit of willow or aspen, the lips in front of their teeth close and the inside lips open, allowing the beaver to swallow.
After our foray into all things beaver we decided we could not let a bunch of trees hinder us from discovering all things paleomagnetic. We proceeded to thrash our way loudly up the hill to a large outcrop, placing our compasses directly on various rocks along the way.
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On the rocks
IMG_8936 MountHaynes
Mt. Haynes
I was positive we were right on the Harlequin Lake flow and that we would soon see the compass needle quiver and zip around to point towards Mt. Haynes towards the South. I’ve seen this phenomenon in southern Utah. In the Condor Canyon Formation there is a layer of ash called the Swett Tuff member. If you happen to find yourself standing, say, 20 yards or more away from this Swett Tuff while holding a good compass, the needle will point north as expected. But move ever so slowly closer and closer to the Swett Tuff and your compass needle will start to quiver. Keep moving closer, and closer, and closer until you are within inches of the tuff at which point your compass needle will have flipped. Your compass needle will now be pointing to the south.
Dang! We poked around for 45 minutes or so, placing the compasses here and there directly on the rhyolite. But much to our dismay our compass needles never did deflect to the south. Why? I have no idea. According to my geologic map we were in the right place.
The next time I go searching for reverse paleomagnetic polarity, I will definitely walk every square inch of that hill. Perhaps I can first hire a beaver to gnaw down all the trees, too.
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Reference:
Christiansen, R.L., 2001, The Quaternary and Pliocene Yellowstone Plateau Volcanic Field of Wyoming, Idaho, and Montana, USGS Professional Paper 729-G.
IMG_8967 HarlequinLakeArea
