Learning from the past

There was a recent paper in Science Advances that’s gotten a lot of press lately. It discusses the possible collapse of the Atlantic meridional overturning circulation (AMOC) due to warming from climate change. This is not a new idea, here’s a July 2023 paper in Nature Communications saying the same thing (and forecasting a timeline between 2025 and the end of the century).

Basically, colder and more saline ocean water is more dense than warmer and less saline ocean water and these density differences lead to the development of areas of sinking water which drive deep currents (called thermohaline currents). These thermohaline currents are effectively mixing oceanic waters around the globe and hugely important in global climate.

In the AMOC, the Gulf Stream and North Atlantic Current bring warmer waters from the equatorial regions northward. At higher latitudes, this now colder water sinks down to drive deep thermohaline currents circulating water back south. If global temperatures continue to warm, the melting Greenland ice cap will dump enough freshwater into the North Atlantic to potentially disrupt the AMOC. This will have profound climatic implications for all of us.

Geologists, who have a longer view of things, know that the AMOC hasn’t always existed and there have been times during the last ice age when it has repeatedly collapsed and restarted. One of those times it collapsed may be tied to events right here in the Hudson Valley.

Some papers have suggested that a time known as the Intra-Allerød cold period that began around 13,360 years before present (B.P.) was due to temporary collapse of the AMOC. As the mighty continental glaciers melted back into Canada around 13,400 B.P., large amounts of meltwater were flowing south down through the Hudson Valley. Glacial moraines (ridges of glacial sediment called till) formed dams down by the Hudson Highlands formed a large freshwater lake called Lake Albany. Lake Albany was over 150 miles long and over 200 feet deep in places.

Image from Woods Hole Oceanographic Institute

 

Also dammed up was what’s now Lake Ontario. It was much larger at this time and called Lake Iroquois (misspelled in the diagram below). Eventually Lake Iroquois broke through the dam and massive amounts of meltwater raced down the Hudson Valley and into the Atlantic Ocean (the whole story is a bit more complicated but you get the picture). There is evidence for this event both in the sediments of the Hudson Valley as well as features on the continental shelf seafloor out from the mouth of the Hudson River.

 The hypothesis is that this massive influx of freshwater disrupted the AMOC and led to a cooling period known in paleoclimatology as the Intra-Allerød cold period. It’s not fully accepted due to difficulties in getting exact dates and correlations for events occurring thousands of year ago, but certainly an intriguing hypothesis and perhaps indicative of events going on in the present day with the Greenland ice cap.

 It also shows how the study of geology can help us understand events occurring in the modern day.

New Plates

 Just a short post today...

The Hudson Valley Geologist's wife bought me new customized license plates for Valentine's Day. I love them!

Obsidian

If you frequent certain areas of social media, or visit new age mineral shops, you may have seen stuff touted as obsidian that is nothing but colored glass.

Real obsidian is volcanic glass. It's formed from volcanic eruptions when lava (molten rock) cools so quickly minerals don't have time to nucleate and grow. Most obsidian is black and glassy with distinctive conchoidal fracture.

Conchoidal fracture can be seen in the sample above and is the curved, shell-like way obsidian (and other glass) breaks leaving sharp edges. Native people around the world took advantage of this fracture pattern to use obsidian to make knives and spear tips.

Some obsidian has inclusions of a white mineral called cristobalite (a form of quartz) leading it to be called snowflake obsidian which is very pretty when tumbled or carved.

Mahogany obsidian has a brown coloration from some trapped iron oxide in the rock.

There are also some rare varieties of obsidian (sheen, rainbow, or fire obsidian) that show a metallic sheen or colorful iridescence. This is usually from microscopic gas bubbles or different mineral inclusions that reflect various wavelengths of light.

When visiting metaphysical shops that sell crystals, as I'm wont to do out of curiosity (I usually don't buy from them because I can go to rock and mineral shows and buy genuine samples at a fraction of the price), I have started seeing a lot of colored glass being sold as "obsidian". Here are some examples from Etsy.

While marketed as obsidian, and sometimes priced at hundreds of dollars, these are really just hunks of colored glass. They're usually formed as a by-product of steel manufacturing and found in industrial dumps. One variety that I know of is artificially manufactured from Mt St Helens ash (Helenite) and sold as natural. It about as natural as your kitchen window.

These are often marketed as Andara crystal, blue, green, or red obsidian, Gaia obsidian, Aqua Lemuria, and other made-up names with sometimes outrageous magical claims associated. Break a green beer bottle if you want some green glass - it will have about the same healing energy as these "crystals" and you can drink the beer beforehand.

Wallace Creek and the San Andreas Fault

 This post is about an area a little far afield from the Hudson Valley but about a geologically significant area - it's featured in the lab manual I use for my Physical Geology course at SUNY Ulster.  It's about a place I visited on vacation a couple of years ago.

Wallace Creek is on the Carizzo Plain halfway between Los Angeles and San Francisco, east of the Coast Range, and in a dry, arid desert environment. I drove 7 miles or so down a dirt road to get there. (why I own an all-wheel drive vehicle with high clearance!).

What makes Wallace Creek so interesting is that it crosses the San Andreas Fault - the boundary between the North American tectonic plate and the Pacific tectonic plate. The Pacific Plate is trying to move northwards with respect to the North American Plate and, after stresses build up for a while, it eventually slips along one segment or another generating an earthquake (sometimes a large one).

On January 9, 1857, this segment of the fault actually moved 6 meters (20 feet or so) during the 7.9 magnitude Fort Tejon earthquake. As the fault moves over the years (centuries), the creek has become offset where it crosses the fault clearly showing the direction and magnitude of movement.

Here's a nice aerial view of the offset.

This view is looking south. On the left is the North American Plate and on the right is the Pacific Plate. You can clearly see the offset of Wallace Creek in the center of the photo with the Pacific Plate moving north (toward the bottom of the photo). The fault scarp of the San Andreas is clearly visible.

What's it look like on the ground? Below is the interpretive sign at the parking area.

Here's a look back to the parking area (two black dots) from the creek. My wife and stepson stayed in the car with the AC running because it was around 100 degrees F (the Hudson Valley Geologist is made of sterner stuff and will suffer for geology). The white in the distance is a soda (alkaline) lake bed. The path to the left follows the fault scarp of the San Andreas Fault.

The feature is a bit difficult to see from a ground picture, but the creek is running away from me here (it's dry but marked by the greenery in the ditch). In the distance it bends left (onto the Pacific Plate) and in the foreground where I'm standing it turns right onto the North American Plate (off the picture). Looking straight ahead (north) is looking along the axis of the San Andreas Plate.

After visiting Wallace Creek, we stopped at the Parkfield-Coalinga Bridge across Little Cholame Creek for a photo op. The bridge literally crosses the San Andreas Fault.

 I was so excited I couldn't resist a little pole dancing here (I'll keep by day job).