ACTIVITY 10.1 Geologic Structures Inquiry Name: Course/Section: Date: A. Recall
ID: 292405 • Letter: A
Question
ACTIVITY 10.1 Geologic Structures Inquiry Name: Course/Section: Date: A. Recall the Law of Original Horizontality, which states that layers of sedimentary rock and lava tend to accumulate as relatively horizontal layers. The layers may remain undisturbed or they may be deformed-have their original shape or form disturbed in some way. Analayze each image below. Beneath each image answer these two questions: Are the rocks deformed or not deformed? If deformed, then describe how and why you think the rocks were deformed. 1. Grand Canyon, Arizona rock layers. 2. Cliff face about 400 m tall, photograph by N.J. Silberling). laska (USGS 3. Quartzite, Maria Mts, Riverside County, California (USGS photograph by W.B. Hamilton). 4. Sandstone on a steep wall about 100 m tall, Little Colorado River Gorge, Navajo Nation, Arizona REFLECT&DISCUSS; Based on your analysis of deformed rocks above, classify rock deformation into two categories, and note what images B. above would be in each category. Be prepared to explain your to other geologists 273Explanation / Answer
A.
1. Grand Canyon, Arizona rock layers
Answer: The rock in this area is not deformed as the rock layers are horizontal and follow the law of original horizontality.
2. Cliff face about 400 m tall, south-central Alaska
Answer: The rock in the area is deformed. The deformation is in the form of tilting of beds and faulting. The rock is interpretted as deformed because the rocks are tilted and faulted and do not obey the the law of original horizontality.
3.Quartzite, Maria Mts, Riverside Country, California
Answer: The rock in the area is deformed. The deformation is in the form of folding of beds. The rock is interpretted as deformed as we can see folds developed in the rock. The rocks when deposited are usually deposited as horizontal layers according to the law of original horizontality but here we see folds which is due to the deformation taken place on the rock.
4. Sandstone on a steep wall about 100 m tall. Little Colorado River Gorge, Navajo Nation, Arizona:
Answer: The rock in the area is not deformed because the rocks are seen to be horizontally stacked over one another obeying the law of original horizontality.
B.
The above deformed rocks can be classfied into two categories:
1)Brittle deformation: Picture 2 (Cliff face about 400 m tall, south-central Alaska ) is an example of brittle deformation. Faulting has taken place in the area as shown in the picture.Rocks are brittle under normal temperature and pressure. And if deformation stresses such as tensile stresses act on the rock which can rupture the rock then faulting occurs.
2) Ductile deformation:pic 3(Quartzite, Maria Mts, Riverside Country, California) is an example of ductile deformation. Rocks are brittle under normal temperature and pressure. If the temperature, pressure and strain rate on rock is increased rocks can occur as ductile/ Plastic material and hence can flow/Bend. The rock in the given picture shows bending indicative of ductile/plastic material. This suggests that the rock in the past was in ductile conditions i.e temperature and pressure were so high that rocks could flow/bend. After which the upliftment processes led to cooling and "maintaining of the shape of the folded rocks".
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