Hello and welcome to your online lecture
of weathering and erosion so we're gonna
learn a little bit about the differences
between weathering and erosion and how
those words really begin the carving and
structure of our landscape so the photo
that I have picked here was one that I
took in my cell phone it's one of my
favorite photos it's called the Mobius
arch it's an iconic feature there in the
Eastern Sierra the rock and the
foreground is a most identified as a
Monzonite granite which means that it's
pea-sized grain crystalline structure
which means it cool slowly but not too
slowly the rock in the background that's
the Sierra Nevada a very different type
of granite that's the Yosemite granted
some of their crystalline size is over a
foot in diameter and so you know that
being said which one cooled the slowest
and I'd be the Sierra granite looking
here that's Mount Whitney that's our
tallest peak in the lower 48 states and
over fourteen thousand five hundred and
eight feet in elevation so what's
interesting about the idea of weathering
and erosion is that this is the process
that allows us to begin to break down
material and create a new landscape so
what's very interesting about this very
similar to that of Joshua Tree is this
very bulbous erosion that we're seeing
that's being caused by that weathering
and we'll talk a little bit about that
because it's actually quite unique so
let's begin well what is weathering
weathering is the physical or chemical
alteration of primary minerals that make
a parent material well what is a parent
material well that granite we saw in the
earlier photo that is parent material it
is the solid rock material in which all
things drive from in that sense of
looking at soils right you need to have
some solid rock first that then erodes
and turns into you know sediments so
that can then be lithified and turn into
a new style of rock so this is the
actual breaking of those materials now I
use the phrase no motion meaning that
when we're looking at weathering it's
just a process when we bring in the
value of motion that's been dealing more
with
erosion is that that motion now there
are two families of weathering there's
physical or mechanical and the other one
is chemical so physical or mechanical
means there's no change in the lithology
or the actual construction of that
material so a great example of physical
weathering would be if you throw a candy
bar in your mouth and you start chomping
down on the candy bar your teeth are
breaking the candy bar into smaller
pieces so if you were to then
regurgitate what you just shoot up a
little bit with just your teeth people
would know is still a Snickers bar it
would be gross but they know it's a
Snickers bar because you haven't changed
any of the composition of the elements
or the minerals that were within that
mass now chemical on the other side
would be if you swallowed the candy bar
and the acid in your stomach began
breaking it down and turning it back
into its smallest elements and then you
regurgitated it no one would know what
it was because it'd just be a disgusting
mess well that's because the chemicals
the acids in your stomach dissolve that
candy bar broke it down even further so
let's go back to physical or mechanical
so there's no change you know ethology
no change in the chemical composition
types that we see would be like
exfoliation frost wedging thermal
expansion the process of material
becoming wet and dry and wet and dry
will speak more to some of these in a
moment but these are the examples of
physical or mechanical so think of just
literally when the environment has the
ability to break things apart then
moving into chemical that's when you
actually change the composition so
there's a couple different types when
water is used such as hydrolysis that's
when you're using a solution to break
down material and change it oxygen can
be used in the process of oxidation or
the use of carbon dioxide or carbonation
or lastly just acids when you actually
pour an acid onto something and watch it
dissolve it changes its composition so a
great example of chemical weathering
would be when you're looking at photos
of the gargoyles or the headstones and
you in Italy and have been by today's
standards they all look like they've
melted like the dissolve
a little bit well that's exactly what's
happened is because of acid rain they've
been able to dissolve the composition of
that marble marble is usually some form
of carbonate made out of like limestone
that's been metamorphosed so which means
that it does have the ability to
dissolve using very weak acid now down
here there's an asterisk this
bioturbation I throw that in there just
cuz some people as long as it comes up
in conversation you know about plants
and animals do they have the ability to
break rocks apart and break material
apart and the answer is well they do
they do it's a little bit different than
this natural process that there is no
alteration or change what we do it's
kind of the process but plants and
animals do have that ability and we'll
see an example of that later so let's
move forward so let's look at an example
of exfoliation so perhaps you've seen a
photo like this before on the left and
this is Half Dome in fact these little
itty-bitty dots right here these are
people that are walking up to the top of
Half Dome what's interesting about Half
Dome is it may at first look like an
onion you can see these very distinct
sheets or slabs of rock that looked just
like the layers of an onion well let's
talk let's talk about why this occurs so
this is a very simplistic time-lapse
when you know when it's it's orogeny
then it began to be exposed and then
through full exfoliation so when the
granitic pluton was then cooling and
becoming someone from a molten material
to iraq as its solidified it stored all
this pressure from all the material
around it and something that's not
viewed here the atmosphere above it so
it's stored all of that stress and
tension and pressure well now we can see
that over you know a series of erosion
we've eroded this upper part of material
away now exposing some of this pluton
well what happens well we see that the
pressure is able to release so since
that rock was so compressed and tight
kind of like imagine like a dog pile
your that phrase were somebody jumps
down everyone jumps on top of them being
that person on the bottom you're storing
all that stress and all that tension
well once everyone starts getting off
you're able to stretch
and actually expand and in that process
it fractures along these weaknesses
within the rock creating these joints so
then what we do is we create this very
large bulbous or rounded feature well
what about a Half Dome it's only half a
dome it's a good point glaciation took
part of that away so that's a different
process the glaciers not been involved
in the Yosemite Valley it would not look
the same way as it does a great example
of again are these bulbous things
perhaps you've been to Joshua Tree it's
that same process and if you go look at
that type of granite you can actually
see the layers so maybe you've also
heard of Yosemite is early large Rock
Falls they get all the time
well those Rock Falls a core because
these large slabs of rock they joined
and they pop off because that pressure
is able to release in large sheets of
granite fall off how big several
football stadium sizes and just sheer
distance and width so very large amounts
of rock moving forward we can look at
frost wedging this is another example of
that I use my watch for scale this at
one point was one rock water gets in
there then the water freezes and as it
is water freezes it expands resulting in
actually wedging or tearing the rock
apart so this now what was once one Rock
is now broken into four smaller rocks to
make things a little complicated it's
kind of hard to see but you can see some
discoloration down here we can also see
there's probably some chemical
weathering in there as well but it makes
a process that we see here is actually
frost wedging so here's an example of
chemical here's oxidation as a photo I
took of Red Rock Canyon in cantle
California so here we have beautiful
stratigraphy that's been exposed and
very rich layers of the gray as the
whites and the reds so oxidation is the
process of atmospheric oxygen reacting
with with the rock to produce an oxide
known as rusting so there was there was
already a metal that was deposited
within that material this red material
is actually different mixtures of basalt
from volcanic flows so then as what ends
up happening in this result is that
because of oxygen and water they begins
to rust what's also interesting about
this picture
particular is that you see there's no
oxidation occurring in these whiter
areas that's actually volcanic ash so we
can see there's multiple layers of ash
and then we can see that there's basalt
and ashen basalt so and so forth what's
interesting about that is one that they
are able to take samples of this ash to
see where the volcanic scum from most of
the volcanic sir are local in California
but there are deposits from Yellowstone
National Park all the way in California
that is observed in this landscape here
another thing that we can look at is you
may be observing that well there's some
layers that stick out farther than
others well the rocks that stick out
further are more resistant to weathering
the ones that are you know being pulled
in in a sense or more susceptible to
erosion meaning that the material
through weathering is able to transfer
away so if you think about it know the
differences between hard rock and
powdered volcanic ash the ash is able to
wash away a lot easier than that harder
volcanic rock moving forward here's just
an example of biological here is a tree
stuck between a hard place there's a
joke a joke and then we have this here
this gal here is standing next to this
huge pile of debris that's actually soil
that's been relocated by ants so that is
an actual anthill but the ant colony is
not just above but is also below so they
took all that material and built this
out between a mixture of their saliva
and the grains themselves it feels like
it's concrete it's a very solid mass but
these are very prominent in Africa so
let's try a couple examples here's a
photo is this mechanical or chemical or
both so it
you guess that part of it was chemical
because of the red oxides you'd be
correct if you're seeing it this large
piece of rock looks like it must have
fallen from somewhere that must have
been some process of mechanical then
you're also correct here's another one
mechanical or chemical well in this
process we can see that this was a rock
signed here essentially this material
was once here the ocean crashes against
that making it weak underneath so this
would be part of a mechanical or
physical process and here's one more
mechanical or chemical or both well this
one here is definitely looking more at
the mechanical process because it has
been abrasion this has been sandblast
this is actually a future found in Death
Valley this archway is a little bit
unique this is over an archers National
Monument this is a mixture of mechanical
and chemical so we can see this is red
so there must been some oxides that have
rusted but this hole that's been carved
in between it's definitely part of a
process of mechanical erosion so what
does that mean well there are some great
videos that are online I always suggest
there's a free video is through Bill Nye
the Science Guy and it's called
weathering and erosion if you want to
see a little bit more about that I
highly highly suggest it I think it's a
great video to watch so let's talk about
just a few more things and then we'll
wrap up this is a really complex diagram
but I guess don't in here because I want
to talk about it real briefly so Bowen
was a scientist that put pretty much
common sense into a graph saying that
you know I get it there must be certain
types of minerals that are more
susceptible to weathering and some that
are not that are more resilient and able
to withstand so looking at these three
minerals down here which formed and
lower temperatures and/or the last two
whether these cooled in this particular
case perhaps the slowest we have
potassium feldspar muscovite mica and
quartz these are the last to crystallize
this is pretty much your recipe for most
household granite everything on this
side over here
sodium rich and calcium rich feldspars
these are more susceptible to weathering
so
I'm not asking you to memorize this
diagram at all I'm asking you to
understand that there isn't what we call
the Bowens reaction series they just
does put things in an order
understanding that they're some minerals
are more susceptible to weathering than
others and again that brings you back to
when we talked about this diagram here
these are obviously whatever makes up
this rock is much more susceptance than
this material down here moving forward
so what is erosion well again erosion is
the actual think motion here's a photo
of it's called sub risky point and death
valley this is called the badlands
meaning that this land is bad nothing
grows there but look at how rich these
little canyons are they're being carved
out from water this material is very
soft so we have obviously mechanical
processes involved but it's also the
process of water is removing it and
that's that idea of erosion now as this
material moves away it has to go
somewhere and after we bring in the
values of stratigraphy so names aren't
important to this sequence but these two
individuals Nicholas steno and also
looking at William Smith they're the
ones who went back and said you know
what there must be a law of
superposition meaning the order in which
things are deposited so we can
understand that when looking at
weathering and erosion is that this is a
screen shot of Avenue s there on the
Palmdale Lancaster area just before the
San Andreas Fault zone we can see that
these rocks are bent and folded and
moved around you can really see it right
here in fact this is what we did
acknowledge as a syncline well we know
that because of the laws of
superposition that this in this material
would have been deposited in a sequence
of lateral sequence as sort of a more
horizontal innocence so in this case we
can see there's been some form of
compression that forced these rocks to
create this shape
well then fast-forward almost a hundred
years William Smith was is what we
identify as the father of stratigraphy
being the first to actually make a
geologic map and so we did this activity
in our lab
perhaps if we looked at how do we put
this back in order how do we take this
rock diagram separate it and pull it
apart and make it all horizontal so we
understand the order in which things had
been laid so that brings us what to the
last slide which is the principles of
lateral  continuity
excuse me the principles of lateral
continuity just means that everything
and what this way was all conformed nice
and layered made sense undeformed great
but because of change these layers can
often be skewed or they can be offset or
they can be a little more complicated to
understand so the job of the structural
geologist is to take this vision and
find a way to turn it back into this so
we can understand the process so says at
the time of deposition sedimentary
layers are continuous for very long
distances if they terminate abruptly
then they have undergone deformation or
they've been eroded the problem with
deformation and erosion specifically
erosion is that actually information
gets lost look at this here see how
thick this yellow layer is see how
little it is down there all that history
has been removed so when looking at
perhaps the geologic timescale or a
geologic history we lose at information
the two most common causes of abrupt
been determination or faulting and what
we identify is unconformity x' we'll
learn a little bit later in the text
about unconformities, there are three in
particular so make sure that you're
familiar with those very briefly and
then lastly sedimentary layers can taper
laterally into other units meaning that
at some point this material will erode
and preps move on top of here and then
again even more complicate the situation
at hand well thank you for joining me
for this lecture again there's
additional videos and features that are
found within the modules of the canvas
shell and until then we'll talk soon
