Hi folks. Dr. Chapman here. Today we'll
learn about the geology of Denali
National Park,
the concept of isostasy, and what makes
Denali so high.
Denali, at 6190 meters, is the highest
mountain in North America.
Globally, there are hundreds of peaks
that reach higher elevations,
chiefly in the Himalayas and Andes, but Denali may have the greatest topographic
relief in the world.
Topographic relief is how far above the
surrounding terrain the mountain rises.
The vertical distance from the base to the top of Denali is around 5600 meters.
For comparison, the topographic relief of
Mt. Everest is around 4 700 meters.
If we include mountains with their bases
below sea level, Mauna Kea on the Big
Island of Hawaii is the tallest mountain on Earth with
over 10,000 meters of relief from the
sea floor to the summit.
It's hard for mountains to reach such
high elevations because gravity is
working to pull them down.
Part of the reason Mauna Kea was able to
become so tall is that most the mountain
is surrounded by water instead of air and the pressure
from the water pushing in
on the sides of the mountain help keep
it from collapsing.
Likewise, scientists have suggested that
lower gravity on other planets and
asteroids may allow mountains to reach
even greater heights.
The two tallest in the solar system are
Olympus Mons on Mars and a peak on the asteroid Vesta,
both of which are about 26 kilometers high.
Back on earth geoscientists have
suggested that because of gravity it may be impossible
for mountains to ever grow to elevations
much higher than Mt. Everest - 
more than 9 km or so. For very
high mountain ranges there's a delicate
balance between the force of gravity
pulling the mountain down
and the tectonic forces pushing the
mountain up.
Near the summit of Mt. Everest, there
are actually normal faults or
extensional faults.
These are the type of faults formed when
Earth's crust is being pulled apart or
collapsing from gravity.
Extensional faults are also responsible
for creating the lowest elevation in
North America -
the Badwater Basin in Death Valley
National Park, which is 86 meters below
sea level.
Of course there are lower points in the
oceans, the deepest being the Mariana
Trench in the Pacific Ocean that is more
than 11 kilometers below sea level.
If we plot all of the elevations and
depths on Earth
we can see these have a bimodal
distribution meaning that the data
clusters into two peaks on the plot.
These peaks are the average elevation of
the oceans and continents.
The reason we have this bimodal
distribution is because the oceanic
crust
and continental crust are different
thicknesses and because they are made of
different types of rocks
with different densities. Density and
thickness are the two main controls
on isostasy. Isostasy describes how
buoyant the crust is with respect to the
mantle.
Another way of saying this is how high
the crust floats within the mantle.
You may heard that the majority of an iceberg is
actually submerged beneath the water.
This is because ice is only a little
less dense than water,
so only a little bit floats above the
surface.
If we put a block of wood in water it
floats higher than ice
because it's less dense than the ice and
the density difference between the wood
and the water is greater than the density difference between ice and water
Oceanic crust is made of more dense rock than continental crust
and it floats lower in the mantle.
Oceanic crust
also floats lower in the mantle because
it is much thinner than continental
crust.
If we want a really tall iceberg we need
a really thick column of ice and because only a
small percentage of the iceberg sticks
out above the water,
we need to add a lot of thickness to get
just a little bit more elevation.
Mountain ranges work much the same way.
To create a mountain range
2 km high  - like the average
elevation of the Alaska range where
Denali National Park is - 
we need to make the crust about 15
kilometers thicker than normal.
This extra thickness is largely hidden
beneath the Earth's surface,
like an iceberg, it's called a mountain
root or crustal root.
If you look at a map that geophysicists
created showing the
thickness of the crust in Alaska, you can
see the crust beneath the Alaska Range
and Denali is much thicker than
elsewhere in the state.
There's also an area of thick crust in
northern Alaska  - that's the
Brooks Range. Almost everywhere you go on Earth, where there are mountains there is
also thicker than average crust,
which is around 35 kilometers. Because of isostacy, it takes a lot of work  - a lot
of crustal thickening to make mountains.
You can't just make a pile of rocks six
kilometers high to make a six kilometer
high mountain like Denali.
The high elevation of Denali and the
Alaska Range was produced by
crustal thickening in a restraining bend
in the Denali strike slip fault.
Strike slip faults often have curves or
bends in them.
When a strike slip fault curves across
the direction of motion,
it's called a restraining bend and
results in compressional stresses and
contractional faulting like reverse
faults.
When a strike slip fault curves away
from the direction of motion,
it's called a releasing bend. These areas
develop extensional stresses and normal
faults as the crust is pulled apart
and a hole or basin is formed that can
fill with sediment.
The Denali fault curves across the
direction of motion and is a restraining
bend.
So it's an area of contraction and
thickening. The last major earthquake on
the Denali Fault,
a magnitude 7.9 earthquake in 2002,
started on a thrust fault in the
restraining bend before branching out to
the main strike slip part of the fault.
Movement on similar thrust faults helped
to thicken the crust and increase the
elevation of Denali.
Geologists estimate that Denali has
increased in elevation by about 3
kilometers in the last 6 million years.
Of course, Denali is eroding at the same
time it is rising and the total amount
of rock uplift in that 6 million year period is
around 10 kilometers.
It's a hard concept to wrap your head
around, but rock uplift is not the same
thing as surface uplift.
Erosion can be slower or faster than
rock uplift - 
meaning Earth's surface may be gaining
or losing elevation over
geologic time.  The movement of rocks
relative to Earth's surface is called
exhumation.
In the case of Denali, the rocks have
exhumed about 7kilometers in the
last 6 million years, 
which is the amount of rock uplift minus
the increase in elevation.  So what changed six million
years ago that caused Denali to start to
rapidly uplift?
The collision of the Yakutat microplate
with southern Alaska.
The Denali fault is a major strike-slip
fault, but is not a plate boundary like
the San Andreas fault.
The Denali fault occurs within the North
American plate. 
The boundary between the North American
plate and the Pacific Plate is located
almost 500 kilometers to the south of
Denali National Park.
The Pacific Plate subducts beneath
southwestern Alaska
and slides along the Queen Charlotte-
Fairweather strike-slip fault in
southeast Alaska.
The Yakutat microplate was carried along
with the Pacific Plate
and is now being jammed into the corner
of south central Alaska.
The Yakutat microplate is an oceanic
plateau that is about 30 kilometers
thick  - around 5 times thicker than
normal oceanic crust.
Because of isostasy, this extra thickness
makes the Yakutat microplate
much more buoyant and it floats much
higher in the mantle, more like
continental crust.
The extra buoyancy also means that it
resists subduction like continental
crust.
The Yakutat microplate is partly
subducting and partly accreting
and colliding with North America. The
part that's subducting subducts at a
very low angle
as it resists sinking because of its
buoyancy.
The low angle causes the Yakutat slab to
scrape against the base of the Alaskan
lithosphere,
which geoscientists often call increased
plate coupling.
All this scraping and colliding
is pushing southern Alaska along the
Denali fault
and other faults resulting in
counterclockwise rotation of the crust.
At the apex of this rotation and the
curve in the Denali fault
is the Alaska range and Denali National
Park.
So what makes Denali so high? It's isostasy.
The high elevation is isostatically
supported by thick crust
and the crust is thick because of a
restraining bend in the Denali fault.
And the Denali fault is especially
active and curved because the Yakutat
microplate is colliding with southern Alaska and
pushing the crust out of the way.
And the Yakutat microplate is colliding
because it is anomalously thick
and buoyant and doesn't want to subduct.
And the reason it's buoyant
is because of isostasy.
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