CH 1 2 MN Geological History Source

>[NICOLE KJERSTEN] So for us
to understand the landscape

of Minnesota as we know it today,

we need to understand where it came from.

And to understand that,
we have to look back

to what's happened in this space
since the beginning of Earth.

Earth was formed about 4.6 billion
(that's billion with a B) years ago,

and a lot has happened since then.

And so we will briefly go over
the geological history of Minnesota.

If you're interested in this topic in depth,

I would encourage you
to take a geology course.

All we need to know is some context

for how Minnesota got to be
what Minnesota is today

because we know the landscape
is so tightly connected

to the life that lives in Minnesota.

Okay, so the Earth, the Earth crust --

so that's the outer layer of Earth
that's made of rock and dirt and such --

it has many, many layers,

with the oldest layers being
at the bottom of the crust

and the youngest layers being at the top.

And the observation
that Earth has these layers

and that we find different types
of fossils within these layers,

and that there are different
elements within those fossils

or within those rocks
that change over time --

the isotopic ratios change over time --

from all of that information,
we can date the Earth.

And we've been able to date the Earth

such that we hypothesize that it was
formed about 4.6 billion years ago.

And by looking at these different layers
and dating these different layers,

we have divided the Earth
into four major time periods.

And what I want to do is,

I just want to walk through
each of those time periods

and provide a little bit more context
for what was happening

in the landscape that we know of
as Minnesota during those different times.

So we're going to start
in the Precambrian era.

So this is the oldest and longest
period of geological time.

It went from when the Earth
was formed 4.6 billion years ago

to about 600 million
(million, with an M) years ago.

And during this time across the globe

is when we saw the formation
of oceans and continents.

And I should say, at this point,
when we're talking about continents,

we're talking about
one giant conglomeration

of all the continents
that we know today

that was called Pangea,
just one giant landmass.

During this time, there was
lots of volcanic activity,

so that's putting a lot of different
elements into the atmosphere,

and volcanoes are responsible
for forming bedrock

and the different types
of rocks that we see on Earth.

Not all of them, but a good deal of them.

And we know that at this point in time,

Minnesota actually --or what
we know of today as Minnesota --

was covered by shallow seas.

So that is the reason we have a lot of
iron deposits in northeastern Minnesota

and why we mine for iron in that area.

This is a time period when we also
start to see the earliest life form,

even though if we look at
this figure here, the fossil record,

there's not many fossils
that were preserved from that time.

Okay. But there are rocks and there
are two types of rock formations

that I want you to know about
that are present here in Minnesota

that originated during
this Precambrian period.

The first is the Morton Gneiss and
then the second is the Canadian Shield.

Let's talk about the Morton Gneiss first.

This is the oldest type of rock
that is found in Minnesota

and it is located along
the Minnesota River Valley

kind of near Granite Falls,
if you know where that is.

So here's a map of Minnesota,

and this orange rusty color

is shading in the areas where you
can find Morton Gneiss type of rock.

[pronounced] "Neese"? "Ness"?
I think I'm pronouncing that correctly.

(One of those ways is correct, at least.)

The second type of rock
is the Canadian Shield,

and this is really just
a massive layer of rock

that is present not only in Minnesota,
but also a good portion of Canada.

And so here's a map of the
northern portion of North America,

and all of this area shaded in purple

represents the rock that makes up
the Canadian Shield.

And if you've gone into central northern
Minnesota or northeastern Minnesota,

you can see this exposed rock

in various areas such as
the Boundary Waters Canoe Area.

These exposed rocks are actually have been
around for millions to billions of years.

And you can also see it along
these cliffs in the North Shore.

Okay, so back to our timeline,

we're now going to move
to the Paleozoic era.

And this was a time period
600 to 225 million years ago.

And believe it or not,
at this point in time,

Minnesota is actually thought
to be located near the equator.

So remember, when all the continents
are mushed together in Pangea

before plate tectonic separated them
into the locations that they are today,

not everything is in
the place that it is today

[corrects self] Not everything
was in the place that it is today.

And so Minnesota is actually
located near the equator,

which means that there was a lot warmer
temperatures, maybe more moisture.

And that is one of the reasons
why in the fossil record,

we see evidence of tropical
and subtropical plants.

It's also important to note
that there were continental seas

that were present in Minnesota at this time,

which is why in the fossil record,

we can see evidence of marine
organisms being present in Minnesota,

despite the fact that we
don't have an ocean today.

Okay, so the next time period
is the Mesozoic period.

This was between 225
and 65 million years ago.

Again, we're still under water.

There's still continental seas
covering at least portions of the state.

It's also when we see
the first flowering plants,

the first birds, the first mammals,

even though this is sometimes referred to
as the period as the age of the dinosaurs.

So dinosaurs tend to be more
prevalent than mammals at this point.

And it's also what's happening with
where Minnesota is located on the globe is,

it is starting to get pulled
more northwards,

and so it's getting pulled towards the poles,

and it's starting to experience
a cooler and somewhat drier climate

than what it did when it was near the equator.

And here's just an example of some
of the fossils that have been found

in Minnesota during this time period

such as small, small vertebrate
animals and flowering plants

that have been preserved
in the different layers of rocks

that have been dated to the Mesozoic.

Okay, and then our final
time period is the Cenozoic.

That is from 65 million years ago to present.

And this is known as the age of mammals,

where we see the dinosaurs have died out
and the mammals are starting to rule.

It's also a period where
there's intense glaciation,

and so Minnesota has made it to where
it presently is located on the globe.

And not only did Minnesota
see a cooling period,

but there was just a large
cooling period across the globe,

which resulted in glacier formations.

And glaciers had a huge impact
on the present landscape,

which we'll learn more
about in the next lecture.

Which we will get to after I wrap this up
and say thank you for your attention.

And like I've already previewed,
we will talk about glaciers next.