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Welcome back and
we are in week two,
video two. Where we are going to continue on with the marine geology of the ocean
floor.
And in particular the actual materials
that are present. And what we are going to be talking about in this video
are the actual sediments
that are present. So first of all what are sediments? Well they are the loose,
chunks of rocks or minerals or dissolved ions that are present
either in
the water, as dissolved ions. Or
being deposited or transported along coastlines or on the ocean floor.
So the different types of sediments that we can see
and that we
collect and find on the ocean floor and on coastlines
you can see here. So we have ones that are called siliceous. Which
that term, siliceous, means there's quartz present. Which has a chemical formula of
SiO2. Silicon dioxide.
And we get siliceous sediments
through shells of various organisms building up
after
uh... those critters die and their shells sink the ocean floor.
And also if the water is actually very rich in
dissolved silica,
we can precipitate or form this
ooze. This kind of a gooey material that's all just made of silica
uh... on the ocean floor.
Which eventually will solidify into a sedimentary rock
if conditions are
right.
So we can also get very similar stuff
but made out of calcite. And calcite is what will react to
acid.
It has calcium carbonate in it or
calcite.
And these also come from organisms and if there's a lot of dissolved calcite in
the water column we can also
get this
calcareous ooze building up on the ocean floor as well.
When we get closer to continents we have terrestrial
or land based
sediments that are maybe traveling to the coast through rivers
and streams.
So we can see sands,
gravels,
and clays
depositing close to those
coastlines.
Also out in the deeper ocean we can see some fine-grained muds, some clays
that finally have a chance to settle out out of water
these clays
take a little while to actually settle.
Similar to like watching a leaf
or a feather or piece of paper
slowly settle
down onto the ground. Clays have a very similar structure
and it takes very calm water for them to actually settle out and be deposited on the ocean floor.
We can also get
detritus. Which is organic debris, dead organisms that
settled to the bottom of the ocean floor.
And if we're close to glaciers so close to the poles near Antarctica or
Greenland
we can get
glacial
debris being deposited. And these are very similar to the terrestrial deposits; sands,
gravels
that are going to be
transported by
glaciers.
So just to give you some pictures to kinda see what
beach sediments can be composed of
well a lot of the actual beach stand or gravels that you see is a direct
result or directly related to erosion from land and biologic activity
offshore. So
this idea of erosion from land. Well what kind of rocks are present
close to that beach. Where's that sediment coming from. And if you pick up a hand
full of sand on a beach
you might see a variety of different things.
I have
several different examples here. We've got this green one which is from
a green sand beach in Hawaii.
And the reason why it's so green is because the rocks that are
directly around this beach have lots of
this green mineral called olivine in them.
So the beach
tends to be very rich in olivine because it's
actively eroding out of the cliffs around that beach.
Same thing with this darker colored rock.
Our sediments, sorry, that you can see here
that
is eroded directly from
darker igneous rocks.
So we see
mayfic igneous rocks, like basalt's, starting to breakdown and form those
black sand beaches.
We can also see maybe bits of rock salt. So
halite
crystals in some areas maybe we have
a very shallow
portion of our coastline maybe a lagoon or
a tidal flat
where
when the tides go out
the water drains away.
We might have a little bit of a a puddle of water that
tends to evaporate and leave those salts behind. So we can build up crystals
that way.
Tide comes back in
and it would bring more of that salt water back into that basin
to replenish that
salt water supply.
Tide goes back out again,
water evaporates,
so on and so forth.
In places where maybe we don't
have a specific
igneous rock weathering away maybe we have a variety of different rocks
maybe we have mud stone, shale's, sandstones
maybe we have some igneous rocks breaking down.
We might get a variety of different
rock fragments. And those tend to be a kind of a darker, grayish brown black color in
your actual sediments.
And this bottom
picture here where it has this lighter color to it,
this light color is actual shell debris. So
in this case we have
much more biologic activity going on off the coastline. So we have lots of
ocean critters,
things that have shells
living and dying offshore soon as they die they're not
maybe
fixed to their original position on the ocean floor anymore. The waves can
wash them around. They break up into smaller particles
and then they get washed up on the beach.
So if we see lots of shell debris in your beach sand that's
a sign that there's lots of
biologic activity going on lots of things
living and dying off shore.
If we see more
of the rock
particles, rock fragments
and um...
mineral grains we might have less
biologic activity and more of just weathering of
terrestrial
rocks.
So that you can look closely
at the sediment and
some example here
this is a chunk
uh... of material that came from
an Ireland beach and
this is what you guys are going to be doing in lab this week. You're going to look at a
few different
sand samples of sediment samples and you're going to describe them you're going to look at
the grain size.
Do we have muds, sands, gravels present?
And in this one
i see
a mixture
of sands and gravels in this picture here.
And we are also going to look at the actual shape
of those grains. Are they rounded, are they angular, or are they kind of in-between.
So I would say these ones are
in the
middle category there. Maybe sub-rounded or sub-angular.
And then we're going to look at what it's made of.
And then in this case here i see
chunks of what looks like, actually
a light-colored igneous rock. I see some mineral grains, I see some quartz. So
I'd say we have a mixture of rock fragments and quarts. I don't see any
calcite. Any
of those shell debris. So
this is probably an area where we have lots of weathering from
the land. Very little biologic activity offshore.
Another example of
an environment where maybe there's a little bit more biologic activity,
less weathering
from the local rocks in the area
because I see some darker colors in there. Those are probably rock fragments.
And then I see bits and pieces of shells
that have been broken down
and slightly rounded. So
we can also test these with hydrochloric acid
to see if they have any calcite
present.
So these mostly sand sized grains.
Pretty well sorted. We don't see a mixture.
And then I would say these are pretty
uh... in that sub-rounded category as well.
And then of course
little bit of rock fragments, but mostly shell material. So this would be
maybe an area where there is
lots of tide pools or reefs off shore.
So just to give you an idea of where some of these things are going to be forming, well
coarser sediments
are going to be closer to land. So, on our continental shelf,
beaches,
that's where we'll see the gravels, and the sands and maybe the coarser
uh... shell material.
As we get further and further out onto that continental shelf, we're still kinda in that
maybe muds, sands
and broken up shell material. Depending on
if you have a reef nearby.
And then when we get further out into the deeper ocean this is where
we start to have the clays starting to settle out. This is where we can get the
silica ooze and the calcite ooze
forming on our ocean floor.
Now the silica tends to be in deeper, cooler water
because calcite
tends to be unstable and it dissolves in the ocean water
when it's cold
and deeper.
If we have
warmer ocean water
the silica is not stable and it gets dissolved and the calcite is what crystallizes.
So on these
continental shelves, in warm environments, we're going to see lots of this calcareous ooze building up.
Also in the
upper portion of the water column what you see here is the sunlight zone.
That area is where we have some of these microscopic critters
floating in the water column that might be made of, their shells might be made of calcite or
silica.
Once they die, they settle to the ocean floor
and build up
overtime. And then of course we have like a whale or a fish that dies out in the open
ocean,
it's going to sink to the bottom
and we'll get some of that detritus building up as well.
So
to look at the actual thickness of sediment
we take a look at maybe the United States first of all,
Which we can see right over here.
And we
go on the east coast and follow and go out toward the center of the Atlantic ocean
we notice we have lots of reds close to the coast and as we get further and further out we get
into the blues.
So what's happening here is we have
thick deposits of sediments close to the coastline. And we have
thinner deposits as you get further and further out into the center of the ocean.
There's a few reasons for this.
One reason is because we have lots of weathering on the coastline
bringing and transporting a lot of those courser materials. Building up right
along the edge of that continent.
The other reason
could be the age of the ocean floor.
So at the very center of the Atlantic ocean there's a divergent plate boundary
pulling those continents apart.
The younger rocks are going to be towards the center
of that ocean. The older rocks towards the outside.
The longer those rocks had been sitting there, the older they are, the more material can build up on that
ocean floor.
We'll talk about this a little bit more detail when we get to plate
tectonics next week. So
where do we have different types of sediment on the ocean floor?
This is a screen capture shot
of
the media that you'll have available to you on the course website.
Courtesy of Frank Granshaw.
And what this media shows you are the various types of
deposits. So we have
terrigenous, so that's going to be
derived by
terrestrial environments. So on the actual
continents.
Biogenous
deposits are going to have some kind of biologic activity
associated with them. And then this last one
that's where we have some type of hydro-thermal,
or water
and hot
fluids mixing together to form those sediments. So
when you use this media
you can actually scroll over each of these different locations and highlight
them.
So I scroll over say the clay deposits will highlight
the actual locations were clays are being deposited actively on the ocean floor.
And then when you get a look at this media on your own
you can actually see some pictures of what that might look like
on the right-hand side of the screen.
I'll let you guys explore that on your own so you can see some pictures and
explore a little bit.
So if I look at calcareous here we can see the calcareous
debris building up in
some of the warmer portions of the oceans.
And then if we look at silicious
we see silicious sediments
most in the cooler temperature environments.
So pretty interesting if we look at clays
those are the glacial sediments. Those are building
right at the edges of those
glacial environments continental margin sediments
right around the edges of the continents closest to the
source of those sediments.
So when we take a look at what's
made of the whole entire ocean floor, what would we typically see?
If we were to take an
ocean voyage and take a core sample of
the sediments and the rocks that are below those sediments what we would see
so we would see gabbro on the bottom,and then
we would see basalt on top of that and
and then we would see layers of sediments on top of the basalt's. Depending on where
you are on the globe those sediments might be calcareous, might be
land-based sediments. Just
kinda depends on what latitude you are, how close are you to the continents.
And then layers of sedimentary rocks if we're right next to the actual continent you might see
instead of basalt and gabbro you might see
sedimentary rocks with sediments on top of those.
So to kind of wrap
things up we just went through and talked about the different sediments that are present on
the ocean floor. How we
identify them and where they're located.
In the next video we're going to come back and talk about uplift, gradation, surface processes
and how we collect these materials.