Hi folks, Dr. Chapman here. Today we're going to visit Joshua Tree National Park and explore
shear zones, the tectonic fate of California,
and the connection between granite and French fries.
Joshua Tree National Park is chiefly composed of intrusive igneous rocks that are part of
the coastal Cordilleran batholiths.
These batholiths are the remnants of the Mesozoic
continental arc and include the Sierra Nevada
to the north and the Peninsular Ranges to
the south.
The batholiths were likely once connected,
but they were sliced in two and disrupted
by the San Andreas Fault that cuts between
them.
Joshua Tree National Park is located right
where the San Andreas fault cuts through – in
a region called the Eastern Transverse Ranges.
They are called the Transverse Ranges because
they trend east-west, perpendicular to most
of the other main mountain ranges in California,
like the Sierra Nevada that runs north-south.
These mountains were not always oriented east-west
though.
Paleomagnetic data suggests that the Transverse
ranges were rotated by the San Andreas and
other faults in the area.
The San Andreas is nominally the boundary
between the North American Plate and the Pacific plate.
However, this plate boundary is not a single
fault, but more of a wide zone of distributed
deformation made up of many faults and small
crustal blocks between the faults – called
a shear zone.
Joshua Tree is in the southern part of a feature
called the Eastern California Shear Zone.
If geologists starting calling it the Eastern
California Shear Zone Mega Awesome – they
could use the acronym ECSZMA.
just a thought….
The Eastern California Shear zone is about
125 kilometers wide and has a overall right-lateral
sense of motion – meaning if you are standing
on one side of the fault looking across to
the other side, it is moving to your right.
As the edges of the shear zone move, faults
within the shear zone accommodate the motion
and the blocks within the shear zone are rotated
and tilted.
Almost all of the faults in the Eastern California
Shear zone are currently active
and produce small earthquakes every day.
The last really big earthquake in the National
Park was in 1992 with the magnitude 6.1 Joshua
Tree earthquake followed a couple months later
by the magnitude 7.3 Landers earthquake, whose
epicenter was located just north of the park.
One of the coolest things about the Eastern
California Shear zone is that it might represent
the birth of a new plate boundary.
Some geoscientists suspect that in the future,
a new plate boundary may form inland of the
San Andreas fault, on the eastern side of
the Sierra Nevada Mountains.
GPS data shows this region is moving about
1 cm/yr - that’s a third as fast as the
main San Andreas fault.
It’s not too hard to imagine that the existing
faults in eastern California may start to
link up with one another and form one major through-going fault system.
If you’ve ever heard the myth that California will one day fall into the sea, or fall into the ocean,
this scenario, with a new plate boundary  forming in Eastern California might be the closest to reality.
But you'll need to wait around 15 million years.
The new fault system appears to splay off
the main San Andreas fault right in the vicinity
of Joshua Tree National Park.
The jostling of the crustal blocks in the
park has caused the west side of the park,
the side next to the San Andreas Fault, to
be uplifted and tilted.
The igneous rocks in the west side of the
park were once located around 20 kilometers deep
inside the Earth, while the igneous rocks on the east
side of the park were located around 5 kilometers.
Geologists call this type of exposure a crustal
section, because it’s like taking a slice
or cross-section into the earth.
Tilted crustal sections like Joshua Tree provide
invaluable information on what continental
arcs look like inside of the Earth.
Most visitors to Joshua Tree do not come to
see faults scarps or crustal sections – they
come to see the fantastical rock shapes - features like Skull Rock
The rock shapes are a product of mechanical and
chemical weathering and erosion.
Erosion and weathering are often conflated,
but weathering breaks down rocks in place
with little to no movement, while erosion involves
the movement and transport of rocks and sediment.
Mechanical weathering forcibly breaks apart rocks and grinds them down into smaller sizes.
This can be achieved through roots growing into cracks, the thermal expansion and contraction
with temperature changes, and from the release of pressure as rocks are exhumed towards
the surface.
The most common form of mechanical weathering
is the abrasion of rock by wind, water, or
ice laden that is with sand and other smaller rocks.
The most common types of chemical weathering
are
Dissolution: like dissolving limestone in
slightly acidic rainwater
Oxidation, which gives iron-rich rocks their rusty
color
And hydrolysis, which is particularly important
at places like Joshua Tree, were the dominant
rock type is granitic
Actually, Joshua Tree National Park is a good
proxy for the average composition of the continental
crust globally, which is also granitic.
Continental crust and the rocks at Joshua
Tree are chiefly composed of feldspar –
mainly the plagioclase variety.
So let’s considering the chemical weathering
of plagioclase by hydrolysis
The mineral formula for plagioclase is Na
Al Si3 O8
For the hydrolysis reaction to balance, we’ll
need two plagioclase formulas and double our
number of ions.
To this we’ll add water, which of course
is H2O
Most rainwater and groundwater is slightly
acidic, so we’ll also add a couple extra
hydrogen ions to lower the PH level.
Ok, this is where the magic happens
when the acidic water is added, the plagioclase
crystal is hydrated and hydroxyl or OH groups
form. This is where the hydrolysis process
gets its name.
This helps break apart the mineral into more
basic, stable components like quartz, which
is SiO2
An easy way to think about this is that basic
quartz sand is a feldspar weathering byproduct
Hydrolysis also frees up the sodium ions,
which are dissolved into the groundwater of
surface waters.
What you have left over is a new mineral with
the formula, Al2 Si2 O5 OH4
This is the mineral kaolinite and it is a
very common type of clay
So starting with feldspar and water - we ended
up with clay, quartz sand, and dissolved sodium ions.
Water is able to seep into tiny pore spaces
in the outer surface of rocks
And because the hydrated clay mineral kaolinite
expands and takes up more space than the original
feldspar crystal, hydrolysis can actually
break apart rocks and reveal more fresh
mineral surface area to weather.
The process ends up rounding off sharp edges
and corners and the end result is the rounded
forms you see in Joshua Tree National Park.
What is even more awesome is that this entire
process likely took place underground.
The modern, dry arid climate of Joshua Tree
National Park was established about a million
years ago during the Pleistocene epoch.
Prior to that time, the region was much wetter
and cooler and the landscape consisted of
rolling green hills with a thick soil mantle.
The weathering and rounding of the granite
took place in this soil.
As the climate changed and the vegetation
thinned out, the area was more prone to erosion
and the granite shapes buried beneath the
ground were unearthed.
For this reason, geologists sometimes call Joshua Tree National Park a fossil landscape.
I mentioned that hydrolysis of feldspar ends
up releasing sodium ions into water.
These positively charged cations will be looking
for something to bond with in order to have
a neutral charge.
Sodium’s favorite bonding target is chlorine.
The atmosphere and all surface waters on Earth have trace amounts of chlorine – mainly produced
in volcanic eruptions.
Sodium bonding with chlorine produces. NaCl
– which is salt.
This salt makes its way from the granitic
outcrop into streams and then rivers and eventually
makes its way into the ocean.
The reason the oceans are salty is because
of the weathering of continental crust, which
is predominantly made of feldspar.
Around 1900, geologist John Joly estimated
the age of the Earth to be about 100 million
years old by using the amount of salt in all the major rivers on Earth and calculating how long
it would take for enough salt to build up
in the oceans to match its current level of saltiness.
Joly was only off by about 4.5 billion years.
Mainly because he underestimated how deep the oceans
are, he didn’t accurately account for the
evaporation of seawater and the formation of rock salt,
and he didn’t know about plate tectonics,
which recycles rocks.
Next time you reach for the salt shaker, remember
that some random feldspar crystal in a nameless
granitic rock in a far away mountain range
somewhere gave up its sodium ions for you
and some volcano somewhere else on Earth spouted
chlorine into the atmosphere where it swirled
around until it found its sodium soulmate
and they bonded, and evaporated, and ended
up on your French fries.
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