Hi folks, Dr. Chapman here. In this video
we're going to explore Grand Teton
National Park
and learn about cratons, the Teton fault,
and the formation of Jackson Hole.
Grand Teton National Park in western
Wyoming is famous for its alpine scenery,
but it also exposes some of the oldest
rocks in North America -
metamorphic rocks that formed 2.8
billion years ago during the Archean Eon.
These ancient rocks are part of a
geologic province called the Wyoming
Craton.
Cratons are very old and stable parts of
the continental lithosphere
and often form the nucleus or core of
the continents and of tectonic plates.
Where the crystalline basement rocks
that form cratons are exposed at the
surface they're called
shields, like the Canadian shield.
In most places these basement rocks are
covered by layers of much younger
sedimentary rocks
and are called continental or cratonic
platforms. Almost all of the midwest
region in the United States is a
cratonic platform.
Until relatively recently in geologic
history the Tetons were also part of
this platform.
During the Proterozoic Eon, the eon after
the Archean, the Wyoming Craton was
sutured together with many other
lithospheric blocks to form an
amalgamation of
geologic provinces collectively called
the North American craton or
Laurentia. Laurentia is like an onion -
Well, it has layers that you keep peeling
away to get to the center.
The pithy core of the onion is the
really ancient Archean terranes like
the Wyoming and Superior cratons.
The next layers are slightly younger
terranes like the Yavapai,
Mazatzal and Grenville provinces that
were added to North America between one
and two billion years ago.  The youngest
layers, the skin of the onion,
are the
mountain ranges surrounding Laurentia.
These include the Appalachians to the
east, the Ouachita Mountains to the south,
and the Cordillera to the west. Geologists
call the tectonic events that created
these mountains orogenies and refer to
the collection
of mountains and basins related to these
events as orogens or orogenic systems.
For example, the Appalachian
Mountains were chiefly created during
the Appalachian orogeny with the
assembly of Pangaea.
Not only were the edges of the North
American continent deformed during these orogenies but in many cases additional
terranes or small crustal blocks
were added to the continent. That's how
continents grow, 
like snapping on legos.
The metamorphic rocks in Grand Teton
National Park suggest
the same processes were occurring during the Archean Eon
and the region experienced continental
collision,
similar to the orogeny that created the
Appalachians or is still creating the
Himalaya. 
Geoscientists know the interior of the
Earth was much hotter during the
Archean
and there are many unanswered questions about what plate tectonics
looked like in the past or even when it
started.
There's a growing theory in the geologic
community that life may not have
developed
or even evolved on Earth without plate
tectonics. 
The Tetons contain some of the oldest
evidence for plate tectonics on Earth
and it may also help us understand what
plate tectonics looks like on other
planets with different
thermal conditions.  The rocks in the
Tetons are old but there are
older rocks on Earth.  Every year or so a
new study comes out claiming to have
discovered the absolute oldest rock, but right now the oldest
rocks are about 4 billion years old and
are from Canada and Greenland,
part of the Canadian Shield. There are
even older minerals that have been
discovered in
sedimentary rocks.  Zircon crystals from
the Jack Hills Quartzite in Australia
have been dated to be about 4.4 billion
years old.
These zircon crystals originally formed
in an igneous rock,
but there's no traces of the parent rock
left.  The zircons were eroded
out of the parent rock, presumably went
through the rock cycle for
1.4 billion years, and then were
deposited in a sandstone about 3 billion
years ago.
The sandstone was metamorphosed into a
quartzite but the zircon grains were
more or less unaffected by the
metamorphism.
We call these crystals detrital grains or
detrital zircon as opposed to primary or
igneous zircon
because they were part of the detritus
that make up sedimentary rocks.
The only thing on Earth older than the
Jack Hills zircons are meteorites that
formed during the birth of the solar
system and the formation
of Earth itself about 4.5 billion years
ago.
The reason why very old rocks are so
rare is because the Earth recycles
itself.
through plate tectonics and the
rock cycle.
Rocks are transformed over time and
their previous lives are
lost to history. Relatively thin
and dense oceanic lithosphere is
particularly susceptible
to being destroyed because it is readily
subducted back into the mantle.
The oldest oceanic crust remaining on
Earth formed during the Jurassic period
around 200 million years ago, which
sounds old but is nowhere close to the
billion-year-old continental crust.
The reason we can even see the Archean
rocks in Grand Teton National Park is
because of the Teton fault and the
formation of the Tetons,
which is entirely unrelated to the
deformation and orogeny that produced
the ancient metamorphic rocks.  In fact,
the Teton Range is one of the youngest
mountain ranges in North America.
The Tetons have been uplifting for about
10 million years and rapidly uplifting
for the last 2 million years.
The Teton fault is a normal fault that
runs along the eastern side of the
Tetons.
The Teton Range has been uplifted about
two kilometers on the west side of the
fault
and the east side of the fault has
dropped down around eight kilometers
for a total of approximately 10
kilometers of fault offset -
more than the height of Mt. Everest.
The uplifted side
is being eroded to form the ragged
Tetons and the down-dropped side creates a
hole or basin that is being filled up
with sediment
to form Jackson Valley or Jackson Hole.
The Jackson Hole basin and the Teton
Range are the easternmost portion of the
Basin and Range province in the western
U.S.
It's called the Basin and Range because
it's full of normal faults that uplift
mountain ranges on one side
and drop down basins on the other. The
Teton fault is one of the most
active normal faults in the Basin and Range
province and the Tetons are being
uplifted around two millimeters per year.
This is just an average rate -  it doesn't
really mean the mountains are rising
each year.
Almost all of the uplift occurs during
major earthquakes when the fault
ruptures.
The last major earthquakes in the region
occurred around 4,600 , 
8,000 , and 10,000 years
ago  - suggesting a reoccurrence interval
of 2 to 6 thousand years.
Geoscientists use the timing and
magnitude of past earthquakes to
estimate the probability of future
earthquakes
and to help with natural hazard
assessment and preparedness.
Based on the previous episodes of
faulting, geologists have
warned that an earthquake as large as
magnitude 7.5
could strike the area. Nearby normal
faults in the Basin and Range province in
Montana and Idaho ruptured in 1959 and
1983, producing magnitude 7.3 earthquakes.
These quakes are among the top 20
strongest earthquakes in the United
States in the last 100 years.
In some ways, the entire face of the
Teton Range is one gigantic fault scarp,
but the Teton fault can be seen in many
places where it offsets young geomorphic
features including glacial moraines.
A moraine is an accumulation of
unconsolidated rock and debris
pushed along or carried by a glacier.
When a glacier melts or retreats it
leaves the moraine behind like a high
tide line.
Many of the lakes in Grand Teton
National Park including Jackson and
Jenny Lake
formed when moraines left behind by
glaciers dammed up streams and rivers.
Where these moraines cross the Teton
fault, they're offset
by up to 30 meters. The youngest glacial
episode in Grand Teton National Park
is locally called the Pinedale
glaciation and occurred from around 30,000 to 10,000 years ago. The Pinedale is
part of a global climatic event called
the Last Glacial Maximum or LGM.
The LGM is one of five major glacial
periods in the last half million years
and it's likely one of tens of thousands
of glaciations in earth history
that have advanced and retreated over the
landscape.
The oldest known glaciation on Earth is
the Archean Pongola glaciation,
or Pongola ice age, named after glacial
deposits in South Africa.
The glaciation is roughly the same age
as the metamorphic rocks in the Teton
Range. 
There's a poetic symmetry between the
modern landscape - dominated by
mountains carved up by glaciers and
valleys filled with their detritus.
And an ancient landscape, a 3 billion year
old landscape that we can only imagine,
but that may have had many of the same
features.
Hey, thanks for watching. Check out some
more videos and share them with friends
and family.
Take care.
