GLG 101 Lesson 4
1. Describe how temperature, pressure, chemically active fluids, and time control the mineralogical and
textural characteristics of metamorphic rocks. Provide examples using specific metamorphic rocks,
foliated textures, and metamorphic environments.
The composition of a elements that make a rock under low pressure but high heat will be a different
rock than if it was high heat and low pressure, for example, Phylite, a medium foliated texture, that is
under low heat but high pressure will form Blueschist, but under slightly lower pressure, it will become
greenschist. There are facies or groups of rock that form under the same pressure and temperature
conditions, so no matter where they are on the planet, the conditions that create blueschist help
geologist understand what is happening under the crust. Water can sometime affect and change the
rock by introducing a new temperature, maybe the water is cooler than the surrounding rock, and will
lower the temperature of the surrounding rock. The water may be a hydrothermal solution and
therefore introduce new elements into the surrounding country rock. Wherever subduction occurs, the
subducted rock is pushed to a greater depth and heated. How far it has been oushed changes what it
becomes.
2. For a metamorphic rock, is it always possible to determine the original parent rock prior to
metamorphism? Why or why not? List three parent rocks and give a possible metamorphic equivalent
for each one.
No, not always. If a rock has been metamorphized under higher pressure, and if hydrothermal solution
introduced any new elements, and if the crystallization has changed in the rock then there may be no
way of knowing what the parent rock was. Shale is most likely the parent rock of slate and mudstone
and siltstone, while limestone is the parent rock of Marble, and quartz sandstone is the parent of
quartzite.
3. From what you have learned about hydrothermal solutions with respect to metamorphic rocks, what
can you conclude about their role in the emplacement of metallic ore minerals?
Hydrothermal solutions are mainly responsible for veins of metallic ore mineral deposits. The solution
works its way through spaces in rocks and facilitates the exchange of electrons, and changing the
surrounding rock into metallic ore.
4. Describe the formation of two nonrenewable energy resources. Why are these not considered
renewable? (Include two factors.)
Hydroelectric and wind, plants. They are renewable because the resource to generate the energy is
easily replaceable, in a short amount . Energy resources such as oil cannot be renewed, once they are
mined and used, it takes millions of years to generate new deposits.
5. List economically important minerals that can be found as placer deposits. What physical or chemical
properties of these minerals make them candidates for this type of accumulation?
Platinum diamonds and tin are all placer deposits. Placer deposits are heavier than most mineral which
make then easy to sort in water currents. Placer mineral s are also very durable and are much more
difficult to break down by either mechanical or chemical weathering.
1.Where is the Great Unconformity best displayed, and in what ways is it significant to our
interpretation of Earth's history? (10 points)
GLG103 Lesson
Refer to the stratigraphic column:
The Great Unconformity is best displayed, and studied, at the base of the Grand Canyon,
where the Vishnu and the Grand Canyon Super Group meet the Tapeats Sandstone. It is
approximately 1 billion years of missing rock history. That rock while eroded didn’t just
disappear, there is evidence of it the sudden boom of carbon based life forms that bloomed in
the Precambrian Age. The mass erosion of the Billion year of rock layers was a catalyst in plant
and animal life on Earth. Life forms adjusted to the high mineral content of the waters during
those millennia. Evolving mechanisms to absorb and use the abundant mineral of calcium
phosphate, calcium carbonate, and silicon dioxide to build shells, bones and teeth, giving rise to
small sea creatures and eventually land animals. Fossils found in the Precambrian layers of rock
show the progressive, and impressively fast development of life on Earth due to the dissolved
and eroded layers of mineral rich rock evidenced in the Great Unconformity.
2.What types of unconformities are represented by the Great Unconformity? (Provide specific rock
layers) (10 points)
Where the Vishnu Schist meets the Tapeats in a nonconformity, a place where the Vishnu shows erosion
before being overlaid by the Tapeats Sandstone layer because the igneous intrusion has been exposed
and eroded. Along the top of the Grand Canyon Supergroup where it meets the Tapeats Sandstone is an
angular unconformity. Below the Tapeats sandstone represents the Great Unconformity.
3.Using the stratigraphic column of the Grand Canyon provided;
identify two other unconformities within the Precambrian (i.e. formations below the Great
Unconformity).
You must name the unconformities and support your answer with the amount of time missing, show
your math. (10 points)
The layers that make up the Grand Canyon Super group, the Unkar Group, Nankoweap
Formation, Chuar Group, and Sixtymile Formation, show 460 million years (1200-740 million
years) of rock layers that each show disconformities, or erosion before another layer is
deposited on top. This would indicate the ocean waters regressing, exposing the layer to erosion
and then the ocean covering it again and depositing another layer of sedimentation. Between
the Unkar Group layer and the Nankoweap Formation layer there is 200 million years of erosion
forming a disconformity. Nankoweap Formation layer only has 130 million years of erosion,
showing a more complete history of time in those layers. Finally, between the Chaur and the
Sitxtymile Formation there is even less rock history missing due to a disconformity, only a few
hundred million years. This whole group was then tilted by some event and these layers
altogether show an angular unconformity as they are wedged in sideways under the Tapeats
Sandstone layer.
The base rock, the Vishnu Schist, shows a nonconformity where it meets both the Grand
Canyon Super Group and the Tapeats Sandstone. This means the Vishnu Schist is the oldest layer
exposed in the Grand Canyon, so far, was exposed to erosion before the Unkar Group was
deposited. Between the Unkar Group layer and the Vishnu there is almost 500 million years of
missing rock record. The Vishnu Schist layer is more than 1680 million years old, but the Unkar
Group is only 1200 million years at its oldest, leaving 480 million years, that’s more than the
whole of the Grand Canyon Group, just missing. The Vishnu and the Grand Canyon Super Group
were then tilted and eroded, a nonconformity where Vishnu meets Tapeats, but an Angular
nonconformity where the Grand Canyon Super group meets Tapeats. The Tapeats Sandstone
layer is deposited, 525 million years ago, leaving a missing record of 215 million years. After that
the Paleozoic Rock layers above have much less rock history missing between each layer,
showing a rather rhythmic moving cycle of deposition due to ocean coverage and erosion due to
exposure to the elements.