Hey, there! Question for you: If you stumbled
upon this landscape, where would you think you were? The answer is probably obvious given
what kind of stories I tell, but humor me here. The pillowy rocks, the cracked veins,
the endless water, the hills — plants aside, this feels like the kind of landscape you
should see in, I don’t know, Hawai’i. Or Iceland. Definitely not the Upper Peninsula
of Michigan. But that’s exactly where we are. This is Presque Isle Point, aka Blackrocks. It’s
in the city of Marquette, and it’s a place that has fascinated me from the moment I first stepped
foot here. The problem? I couldn’t find good information about where this place came from,
or how it formed. So, I called for some help. Hey, Dr. Cannon, it's Alexis Dahl. This is the story of Blackrocks, but it’s also a
story about humans, and how we’re able to string together what the Earth was like a billion years
ago. It’s also a story about all the ways this corner of the planet has changed since then… and
even more surprisingly, all the ways it hasn’t. It's story time. No one knows exactly how old Blackrocks
is, because no one has made a definitive measurement on the rocks. But by comparing
this area to rocks that have been dated, some researchers have made an educated
guess is that the dark rock you see here is about 1.1 billion years old. So,
we’ll begin our story around there. At this time, this part of the world was likely
flat and relatively barren. The ground was made of rocks that were already at least a billion years
old themselves, and while there may have been some fungi decorating those rocks, land plants
hadn’t evolved yet. So, things were pretty quiet. Then, a great blob of magma likely rose
up from the mantle, and intruded into those ancient rocks. The magma probably didn’t
break the surface, so if you were standing here, you might not realize anything was happening.
But over time, that magma cooled, solidified, and formed a mass of material likely
about the size of Presque Isle. That rock is called peridotite. And
as the millennia progressed, the crust above this peridotite gradually eroded,
until the almost-island we see today was exposed. This is the basic story for how Presque Isle
likely formed, but it’s anything but the whole story. In fact, I don't even think this is the
most interesting version, because there are so many weird-looking features at Blackrocks in
particular that this explanation doesn’t cover. That’s the stuff I wanted to understand. And this
is where I started developing a sense of deja vu. To learn more about this place, I reached out
to Dr. Bill Cannon. He’s a Scientist Emeritus with the US Geological Survey, and although
his career has taken him all kinds of places, he’s been studying Michigan’s Upper Peninsula
on and off since the late 1960s. He’s one of the most knowledgeable people I know,
I had the privilege of learning from him in 2021 when I made a video about the
Sudbury Impact. So, I figured he might be able to help me understand Blackrocks, or
at least, would know somebody who could. Well, fast forward a couple of months, and Dr. Cannon was once again opening my
eyes to just how amazing this region is. I met Dr. Cannon at Sunset Point on Presque Isle, a place formerly known to me as “the spot where
I park my car.” In other words, I had given exactly no real thought to it. Which I quickly
realized had been a big mistake on my part. [Dr. Cannon] Okay, so this is this is the
Jacobsville Sandstone, which is roughly a billion years old, maybe a little bit younger.
We don't really know. This is the top of the peridotite, this stuff down here. And which we're
guessing is about 1.1 billion. So it might be 1.1 [billion]... The sandstones are 1. So, that's
about a hundred million years of a gap in here. This sort of gap in the geologic
record is called an unconformity. And they’re interesting, but not all that
rare. An unconformity can happen if, say, a bunch of rock up at the surface is
eroded and effectively disappears before the next rock layer is put down. In fact,
that’s exactly what happened here. A lot of the peridotite broke apart and weathered
away before a river swept in and deposited all the sand that would become Jacobsville
sandstone. But anyway, more from Dr. Cannon. [Dr. Cannon] Here you can see the... Why
don't we walk over? So here's another spot to see the unconformity. And so here, the
sandstone is almost directly on the peridotite. [Alexis] So this would be sandstone.
And is this the peridotite? [Dr. Cannon] This is probably, well... I think that's the weathered... the
very top of the peridotite right there. And it's very, very broken down
chemically, from being at the surface. [Dr. Cannon] So in addition to that, this
stuff I'm standing on right here is... what, in geologic terms, is a regolith, which means
that's kind of weathered, broken material, sort of trying to become a soil, that was
developed on top of this. The prototype is quite unstable near the Earth's surface, so it breaks
down. So this was like a loose gravelly surface. And some of this very basal part of
the Jacobsville Sandstone filtered down and around them. So interesting thing, if you stood on this surface a billion years
ago, you'd be at the Earth's surface, then. [Alexis] Oh, cool. Neat! [Dr. Cannon] And then, you know, shortly
after that, there was a river system that deposited the sandstone, gradually began to
inundate this area. And then it built up. And another interesting thing is
that — not from studies right here, but from studies around the region — we know
that if we were here a billion years ago, we would have been near the
equator and in a desert. [Alexis] Cool. So my understanding,
just... The definition of a desert is just area, an area
without a lot of rainfall? [Dr. Cannon] It's just an arid area, yeah. [Alexis] How can we tell this area was arid then? [Dr. Cannon] Can't tell it from right here. But
there's other features in both the sedimentary and the volcanic rocks that usually only
form in very arid environments. And we know it was near the equator because
of paleomagnetic studies. That is, when a lava flow, for instance
— when it solidifies and cools, it passes a certain temperature, and it locks
in the Earth's magnetic field at that time. This method is actually really cool. So, when
lava cools past a certain temperature, called the Curie Point, the magnetite in that lava locks in
a record of Earth’s magnetic field. Essentially, it "knows" (quote unquote) which way is north.
And even as the continents move and that lava rock travels to a new place on the globe, it’s still
pointing to wherever “north” was when it cooled. So, by looking at a lava rock and comparing
that to what they know about the Earth’s magnetic field, geologists can get a
sense of where a lava originated. And when they look at lava rocks from this
area from about a billion years ago, that’s a big part of what tells them that
modern-day Michigan used to be near the equator. I always find that kind of fascinating as a
geologist to think that I could've been here a billion years ago, and I could have been
standing right here on the surface. And if I looked around the area, all these hills
— pretty much, these hills would be more or less the same as they are now. They've
been sculpted a little bit by glaciers, but this topography was here a billion years ago. [Alexis] Really? And is that just
because it's all volcanic or...? [Dr. Cannon] Well, it was — it was
eroded into these hills. But then it was all covered up by the Jacobsville
Sandstone. So all that topography was underneath this thick pile of
sandstone until pretty recently, probably during the ice ages. And the ice gouged
it back out and re-exposed that topography. [Alexis] Moments like this are gold
for me. It’s where I really feel myself starting to understand a place, and start to
appreciate it in a new way. It’s also just fascinating to learn how geologists can study
something that happened so far in the past. Now, most of the rock at Presque Isle Point has
changed significantly in the last billion-plus years. It used to be made almost entirely of
the mineral olivine, which you might recognize from its gem form peridot. But today, most of
that has been chemically altered and turned into the mineral serpentine, which has the
same chemical composition but is more stable. If you’re curious, the transformation
of olivine into serpentine is called serpentinization, which I think is just lovely. After this, Dr. Cannon and I walked
over to Blackrocks proper. The rocks here used to be farther underground
than the peridotite at Sunset Point, and they haven’t been up at the surface as
long. So, this peridotite is more fresh looking, and doesn’t have the same
veining as rock elsewhere. Going into this video, I really thought that
this was going to be the most interesting part of the afternoon. The rocks here are
just so weird. But then, this happened. [Dr. Cannon] Geologically, it's not
as interesting out here. You know, it's all... pretty much all one thing. [Alexis laughing] [Alexis] I also find that somewhat fascinating because you're
like... Or I guess my perspective over here, I'm like, "Wow, this is this is super
cool. This is super interesting." [Dr. Cannon] Lots of rocks,
but they're all the same. [Alexis] You're like, "Well, you know!" [Dr. Cannon] It's sort of like reading the same
page of a history book over and over again. It's interesting history, but it's one thing. I mean,
I think the surfaces that we're walking on... the top of these is probably
almost a glaciated surface, that's all kind of smoothed down. So I think
that's where the glacier was passing over it. [Alexis] After this, we walked over to
an area I’ve been curious about for ages: This section of rocky beach has really
weathered peridotite on one side, and much fresher-looking rock on the other.
Dr. Cannon pointed out that this region on the left has been a lot more chemically
altered. And here’s one way he could tell. [Dr. Cannon] So, here's one of our geology
tools. It's a stud finder, a magnet. And some... it's probably altered too much
here to be very magnetic. But I think if we tried that over there on those other rocks,
it would be... We can try that if you want. [Alexis] Oh, yeah! Now you've got me curious. [Dr. Cannon] Yeah, I can see these
are... these are attracting the magnet. [Alexis] Oh, okay. So the magnet's on the bottom? [Dr. Cannon] The magnet's on the bottom, yep. [Alexis] Oh, cool. [Dr. Cannon] So one way you can map these, then, is by their magnetic pattern. You can either
do it on the ground carrying a magnetometer, going back and forth, or [it's] mostly done
now from aircraft. One of the ways that you get magnetite in a rock like this, is as the
original olivine breaks down — olivine is a magnesium iron silicate. And the serpentine
it forms from usually has more magnesium. So, it frees up some iron, and the iron ends up
mostly as the mineral magnetite. So if you look at a section of this under a microscope, you will
probably see that there's very little, very tiny, dusty grains of magnetite scattered all through
it, and that's what gets that magnetic attraction. Something I’ve been trying to practice lately is
not being afraid to ask follow-up questions. Like, I could have learned the basic story
of Blackrocks and Presque Isle from the beginning of this video and then just
called it a day — but by not allowing myself to continue to being curious,
I would’ve missed out on an adventure, and a better understanding of one
of my favorite places in Marquette. Overall, Blackrocks is a bit like a puzzle that
geologists still don’t have all the pieces to, but that’s a big part of what fascinates me about
it. I mean, there are still opportunities for scientists to come out here and learn new pieces
of this story. I don’t know. Something about the potential of the undiscovered really just
gets to me. And you know what I think is maybe the coolest part of all: Wherever you are in the
world, I bet there's a place like that near you. Thanks for joining me for my first story
of 2023, and many thanks to Vlogbrothers for sponsoring this video. If you'd like to help
me tell more stories like this in the new year, there are two major things you can do
to help. One is to share this video if you liked it to help more folks find
my work. And the other is you can click the link on-screen or in the description
to learn more about Patreon and Buy Me a Coffee. The latter doesn't literally
buy me coffees, just for the record. One way or another, I’m glad you’re here.
I hope you learned something that makes you think about the world just a little
differently, and I’ll see you next time.
