Geology Lesson #4, with Capt. Paul and Dr. Nat (also known as Paul and Nathalie Brandes), this one is at an undisclosed location in the woods, here in the Upper Peninsula. Once again, I’m not going to pretend that I know anything about what the Capt. and the Dr. are talking about, so today’s notes have been written by Capt. Paul:

“For this Cam Notes, we’re going out of this world! Well, not really, but this site has extraterrestrial origins.

The photos are of the ejecta from the Sudbury Impact Event, an object (some say meteor, some say comet; it’s not totally known yet) 6-9 miles across struck the area 1.849 billion years ago. Debris from the impact is spread over an area 620,000 square miles. Rock ejecta, like what is shown in the photos, have been found as far away as 500 miles in Minnesota. Another fascinating thing about this is that it is believed that the impact 1.85 billion years ago may have had something to do with ending the formation of the Banded Iron Formations like Jasper Knob in Ishpeming.

There is still ongoing research in the area, hence the “undisclosed location”, but I can tell you it’s in the woods north of Ishpeming.”

Dr. Nathalie, your enthusiasm is infectious.

This is all so interesting! I wonder if what we have in our yard
is just “rocks” or some of this?? I can’t say we are very north
of Ishpeming but we are north of US41! So cool. I would love
for Capt. Paul and Dr. Nat to talk about Black Rocks in
Marquette. Or did I miss that? Thank you!

By Mary Drew at Pasty Central (Mdrew) on Monday, August 24, 2020 - 10:08 pm:
Again, we join Dr. Nat for a bit more information about these ejecta from well over a billion years ago. That pretty much blows my mind, as I can’t wrap my head around something from that long ago. I wonder how they date it that far back?

Great question, Mary!

There are several different ways to date a rock, but the most common for very old specimens are Potassium-Argon (K-Ar) dating and Argon-Argon (40Ar/39Ar) dating. In K-Ar dating, it is based on the time measurement of radioactive decay of an isotope of potassium into argon. Potassium is a common element found in very old rocks. When these radioactive isotopes decay, they release argon into the atmosphere while the rock is still molten; however, once the rock solidifies, argon is trapped in the crystal structure of the rock. That time since the rock crystalized can be measured by measuring the ratio between the parent potassium and the daughter argon. Since argon has a long half-life (1.248 billion years), it can be used to date these very old rocks.

To reduce the amount of contamination (and thus error) in time measurements, the 40Ar/39Ar dating method was developed. K-Ar required splitting samples into two for separate potassium and argon testing. 40Ar/39Ar testing requires only a single rock fragment or mineral crystal and uses only a single argon isotope measurement. In this method, the sample is crushed and irradiated to produce the 39Ar isotope in the sample. Then, the sample is degassed using a furnace to release the argon gas, which has radioactively decayed over time. The isotopes contained within this gas are then measured to produce an age date.

I know this seems a bit technical, but this is only the Cliffs Notes version. There are several good articles out there that go into this much deeper than I have here.

Laurie:
We did a video out on Presque Isle and while not directly on Black Rocks, it shows a neat feature that does tie into Black Rocks. Stay tuned...

Thank you, everyone!
I'm glad you are enjoying these photographs and short videos. I love telling the epic story of the Earth that is written in the rocks.

Fascinating and amazing!! To be able to determine that was the origin by looking at the rocks does blow your mind!

Dr. Nat you are amazing! You explanations are even understandable to an accountant, which is going some.

Laurie B. (Ratherberiding), i agree - looking at 'common rocks' now has me wondering if they aren't something special!

"Common rocks" are special. They all tell us something about our planet. I have often told my students that even a grain of sand has a story to tell.

"To see a World in a Grain of Sand..."
(William Blake)

Thank you Dr Nat & Capt. Paul. We got the lesson I was waiting for at the undisclosed location. Of course now I have to try to locate it. If I do I'll keep the secret.

Even rocks have clocks!! I'd heard of Zircon dating using Uranium and Lead and Carbon dating for organic material but Potassium and Argon dating was news. Wonderful.

I've been trying to come up with a modern day, around the household or backyard analogy to help explain the dating technology that doesn't require nuclear chemistry and physics. Maybe something that estimates when that chocolate coated ice cream bar was removed from the freezer since the vanilla ice cream melts faster than the harder chocolate coating does?

You've motivated me to do some more reading, specifically on the Paleoprotozoic era (during which the Sudbury impact happened). One amazing thing I learned was that the Earth then had about a 20 hour day and the year lasted about 450 days! I read it on the internet so it must be true? I figure the eventual slowing down of the Earth's rotation probably was caused because more mass moved closer to the surface. (Remember when you are spinning in a chair and bring your arms in closer and you start to spin faster?...it's called conservation of energy). I wonder what caused it to start spinning in the first place.

The original spin of Earth is related to how the planets originally formed. Early on in Earth's history, a Mars-sized object collided with the planet and sped up rotation, and created the Moon. Earth's rotation has been slowing since then. Every 100,000 years about two seconds are lost from the day due to tidal friction.

Smarter every day. Thanks.

Any thoughts on the contribution to slowing that my theory might have? Or maybe the opposite effects if Earth's diameter might be shrinking?

Still, it would seem that due to volcanics, the mantle is very slowly becoming less dense and the crust more massive, thus mass is being moved outward. I guess though that the crust is potentially subsiding downward (synclines?) in other areas (ocean floor and elsewhere) such that our little blue spheriod's rotational moment of inertia stays in equilibrium? Or maybe not?

Given the negative acceleration rate data, if nothing changes (fat chance I'm guessing) and it proceeds linearly, then I calculate it should stop rotating in a little over 4.3 billion years.

It might make for an interesting math simulation to try to predict the changes in tidal tables as the slowdown proceeds.

Assuming that 1.8 billion years ago the day was just 20 hours long, further mathematical analysis reveals that the average rate of rotational slowdown during this period has been 0.8 seconds per 100,000 years which is substantially less than today's rate of 2 seconds per 100,000 years.

All this means the rate of slowing down is getting greater. With just 2 data points it's somewhat unreliable to calculate the rate of change in the negative acceleration rate (deceleration) but the given data results in a value of +1.4 seconds per 100,000 years per billion years. (in electromechanical systems it's called the Jerk rate or the rate of change of acceleration, (negative acceleration, positive jerk in this case).

Volcanism and the evolution of Earth's crust is quite complex. In fact, due to volcanism, the crust will slowly become less dense. When mantle material is melted, it is partially melted. The part that melts and erupts is less dense than the parent rock because the lower density minerals also have lower melting temperatures.

How this could affect things over time, I have no idea. But that's the beauty of science... there's always more to ponder and discover.