Transcript The Uplifting Experience of Utah`s Henry Mountains: A Sill Lee Way

The Uplifting Experience of Utah’s Henry
Mountains: A Silly Way to Intrude on the Origins
of Igneous Rocks
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Our journey to the Henry Mountains takes us out to the southeastern part of Utah
and in a region of the Western United States known as the Colorado Plateau. The
Colorado Plateau is an elevated, mildly folded and faulted land area (all features
created were due to plate tectonics), that was formed as a single geological unit.
Geologists call land areas that have similar geological features because how they
formed provinces. Geologic provinces can also be classified by age, mineral
resources it contains, and so on; but since we are looking at the geological history,
we will use the first stated definition.
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As the crust stretched, regions south and west of the Colorado Plateau uplift broke
up into a multitude of down-dropped valleys and elongated mountains. This
formed another geologic province known as the Basin and Range Province. Yet
for some reason not fully understood, the neighboring Colorado Plateau was able
to preserve its structure and remained a single tectonic block. Eventually, the great
block of Colorado Plateau crust rose a kilometer higher than the Basin and Range.
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The Henry Mountains are located in the southeastern lower section of
Utah on the Colorado Plateau. The Colorado plateau lifted as a single
tectonic block, but how did the Henry Mountains lift on top? Notice it
produces a watershed in which rivers flow around it.
Henry Mountains
location marked
by red dot.
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The most direct peak behind me is called Mount Hillers (elevation 10,723
feet), which is part of the Henry Mountain range. The sedimentary rock
layers around the mountain are relatively flat and those on the mountain
are nearly vertical.
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These are two pictures from the side of Mount Hillers at an elevation of 7035 feet looking down slope.
The picture on the left shows the near vertical orientation or positioning of some sandstone layers. Since
this orientation of rock layers is so steep it sets the stage for an erosional process known as mass wasting.
Weathering of rock layers into angular fragments formed and loosened by the wedging effect of the
freezing and thawing of water on the bedrock are constantly assisted by gravity moving down slope
forming what is known as talus. Notice once again the relatively flat rock background at the base of the
mountain peak in the foreground of both photos, which is the foreground in the slide before.
Malice in the talus. Besides the difficulty of
climbing a talus slope, two steps forward for
every one step up; you have to be careful
about who is down slope from you so they are
not harmed by rock slides that can occur as
you climb the loose or unconsolidated slope.
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Geologist talk about orientation of rock layers as strike and dip. Dip refers to the
angle of the rock layer is inclined form a horizontal layer (like in this case, the
water) Strike is the compass direction of the line formed by a “dipping” rock
layer meeting a horizontal surface (the water). Strike is always perpendicular to
dip of the rock layer, thus they always form right angles to one another. When
know the strike and dip of the rock layer, you know the rocks attitude.
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Geologist use a special surveying compass to measure strike and dip. The compass also consists of a
device to measure the incline or dip of rock layers called a clinometer ( think of it as an “incline-meter”).
It has a bulls eye level to keep the compass horizontal to measure strike like in photo A and a clinometer
level which is adjusted when reading the dip of the rock from horizontal by rotating the clinometers till
the air bubble is between the lines in the clinometer’s level to measure dip like in photo B.
Photo A
Photo B
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We are going to watch a video clip about how to use a geology surveying
or Brunton compass to measure strike and dip to find the rock layer’s
attitude, so you can see it actually in action. At this point you need to end
the slide show and go to slide 9 and click on the icon below because an
Internet connection is needed to view the movie. Click out the slide show
at the end to return to the PowerPoint. You may want to watch it twice to
answer your worksheet completely.
http://naturebytesvideo.com/bytes_G-J/geomap-strikedip.html
Click here for Dip and Strike video
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In class we learned about geological time unit like eras and periods. We also learned these units of time
were based on originally the fossil record found in rock layers and compared to each other to provide a
relative measure to a rocks age. More recently in the last century, an absolute age has been determined
with radiometric dating based off the decay rate of radioisotopes.
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• Since the Cenozoic is the
most recent era, there
tends to be more evidence
and hence the periods can
be broken done into
smaller units of time
called epochs.
• The Henry Mountains
formed in the Tertiary
period(more correctly
called the Neogene
Period) in the early
Miocene epoch. So about
how many years ago did
that occur?
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Remember, geologist can’t literally travel back in time so they use assumptions or principles to base their
interpretations of the evidence on. The Principle of Uniformitarianism, processes occur today like they
did in the past, leads to the basis of a puzzle concerning the Henry Mountains when they were first
studied. One, assumption principle is that all rock layers first form horizontally. This forms the basis of
the next concept, the Law of Superposition. This states when we find a sequence or strata of these
horizontally layered rocks, the one on the bottom must of formed first, so it is the oldest, and the one on
the top must of formed last, thereby, it must be the youngest. The dilemma with the Henry Mountains is
as we climb the mountain’s slope we find the older rock layers on the way up to the top and the youngest
on the bottom. What is happening or should I say, what happened?
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The major clue came from the igneous rock structure found at the top of the mountain.
Remember, intrusive igneous rocks cooled slowly deep underground and characterized by
large crystals and extrusive igneous rocks cool quickly near or on the surface of the earth..
Recall from past studies, that mafic igneous rocks are dark color, high in iron (Fe) and
magnesium (Mg), and more dense compared to felsic igneous rocks which are light in color,
high in aluminum (Al), sodium (Na) , and potassium (K), and are less dense. Then there is a
group that fall between these two types of subclasses called the intermediates
Intrusive forms:
-gabbro (mafic)
-granite (felsic)
Intermediate forms:
-andesite (mafic/felsic)
-diorite (mafic/felsic)
Extrusive forms:
-basalt (mafic)
-rhyolite (felsic)
-scoria (mafic)
-pumice (felsic)
-obsidian (felsic)
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The man credited for finding out the answer
to this puzzle was Karl Grove Gilbert. He
studied the geology of these mountains in
1875-1876. He coined the term to describe
the characteristic shape of some of the
“pools” intrusive igneous rock that cored the
mountain as laccoliths. Laccoliths (in the
lower right picture) are somewhat large
intrusions of magma injected at shallow
depths between sedimentary rock layers that
resulted in uplift (doming) and folding of the
preexisting rock strata above the intrusion.
He believed this was how the Henry
Mountains formed. After the mountains were
in place, erosion gradually removed the
surface rocks, revealing the volcanic core of
the range.
This type of a pluton
(a crystallized magma chamber exposed by
erosion named after Pluto, Roman God of the
underworld) also caused the younger rocks to
be uplifted and eroded away, which allowed
the older cracked or faulted rocks to be come
exposed at the surface. Furthermore, he saw
other intrusions of magma that flowed and
crystallized between rock layers forming sills
and cut across them and crystallized forming
dikes. These were too uplifted and exposed
at the surface.
Karl Grove Gilbert
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Sills and laccoliths are known as concordant intrusive igneous rock
structures because they form parallel manner between the surrounding
rock layers. The laccoliths in the Henry mountain range are are believed
to form from sills that merged and when they could not flow horizontally
anymore, and they vertically started to lift the layers that trapped them.
Laccolith might sound like a “silly” name, but that is how they started out.
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Dikes are shallow intrusions or plutons that cut across pre-existing rock
layers or bodies. Since they cut across pre-existing structure, dikes are
referred to as being discordant
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This photo from Unaweep Canyon , another uplifted area on
the Colorado Plateau located in southwestern Colorado,
shows pegmatite dikes cutting across a large exposed
Precambrian pluton of a type of granite called monzonite.
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This is a geological cross-section of Mount Hillers in the Henry mountains. It shows how the older rock
layers were lifted and exposed at higher strata levels due to the erosion than the young rock layers that
used to be “super positioned” above them. The lettered rock layers represent periods of time. The
youngest at the “K” is the Cretaceous, ‘J” is the Jurassic, “T” is the Triassic, and “P” represents the
Permian , the oldest rock layer. The “SZ” represents the shatter zone where the rock layers and sills are
dipped angled nearly vertical. This is the area on the south side of the mountain which were
photographed.
Examples; A=laccolith B=dike C=sill
Photographed
area
A
C
B
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This is an example of is an uplifted sedimentary rock layer or outcrop consisting of
sandstone known as the Entrada formation from the Jurassic period.
A formation is a distinguishable (having specific characteristics) rock unit having a
recognizable similarity that it can be used for mapping, describing, or interpreting the
geology of an area . The dip measured here was about 78 degrees. Using this information,
do you know the attitude of the rock layer? Explain.
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This is an uplifted sill on the
southern side of Mount Hiller in
the shatter zone. The type of
igneous rock making up the
laccolith and all the surrounding
sills and dikes is diorite, an
intermediate light colored felsic in
the igneous rock classification.
How could you tell whether this
or any other diorite outcrop was a
sill or a dike intrusion?
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Here is a close up where the intrusive diorite sill made contact with a sedimentary
sandstone rock layer (part of the Entrada formation). The whitish zone between
the diorite and the sandstone is referred to as a bake zone. This is where the heat
from the sill chemical altered a part of the sandstone that was in direct contact
with it. This is an example of contact _______________ which we talked about in
class before.
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Here is another uplifted sill of diorite. If you look above the whitish tan bake
zone on the right hand side of the paper, you will notice fragments of the bake
zone are inside the dioritic sill. These foreign fragments that are not related to the
igneous rock they are in are called xenoliths. How do you think they got there?
What is older, the sill of diorite or the xenoliths? Explain.
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The crystal size of this Mount Hillers sill of diorite gives us another clue to its history.
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An igneous rock’s grain size is referred to as texture.
Phaneritic is the term used to describe textured rocks that are comprised of large
crystals that are clearly visible to the eye with or without a hand lens or binocular
microscope. However, the diorite found in the Henry Mountains, has two different
grain sizes in it so it is called a diorite porphyry. The small mineral crystals are called
ground mass and the large mineral crystals are known as phenocrysts. Since there are
two distinct crystal sizes, this igneous rock must have cooled at two different rates.
Think back to what you know about igneous rock formation. Which crystal form
cooled quickly? Which one cooled slowly? How does this possibly relate to the
relative depth where they formed underground?
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The picture on the left is diorite which the crystal size is relatively the
same size. On the right is a specimen of a diorite porphyry from the
Henry Mountains where there are distinct crystal size differences. Why
are pencils in picture? Is this because this is the ‘write” way of
photographing rocks? In the porphyry picture on the right, what color
represents the groundmass? Which color represents the phenocrysts?
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Why was the laccolith, sills and dikes of Mount Hillers made out of diorite? How
come it was not another type of igneous intrusive rock? What determines what
type of igneous rock forms from magma? Similar questions were asked by young
scientist in the 1920’s by the name of N. L. Bowen.
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Bowen through experimenting with magma mixtures he made in lab came up with a scheme
how mineral mixtures (remember rocks are mixtures of minerals), due to different
temperatures would crystallize out of the melt. This change in magma chemistry due to
precipitation of solid crystals due to changing temperature of the melt became known as the
Bowen Reaction Series. Crystallization refers to a what state change? Does that process
have a specific name? Be fluid with your thoughts, but give a solid answer.
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This picture diagram of the Bowen Reaction Series show the various minerals
found in magma. The minerals at the top crystallize out a high temperatures and
as you go down the “series”, those minerals crystallize out a lower temperatures.
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We are going to leave the Henry Mountains and Mount Hillers and proceed to an
activity using Bowen’s reaction series to better understand the chemical change
and evolution of the magma chamber that made this laccolith mountain.
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• People most all the time
think of magma in in these
types of situations.
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•
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However, the Henry Mountains
provides us a silly way to intrude
on the origins of igneous rocks and
provides a very uplifting
experience, that I am very fond of
for “sedimentary” reasons.
Hopefully, this PowerPoint has
provided you the right attitude to
strike out on an adventure of
looking at igneous rocks from a
different slant. I know this is a
dippy thing to say, but after
studying the Henry Mountains in
terms of igneous rocks, you can’t
take everything for granite.
Note: The sandstone tilted behind
me is a clastic example of uplift.
Plants grow weather they can and
slowly breakdown the rock.
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