JAC - 07-3 HW - Geological Time v6 (PDF PRINT FOR CLARITY)
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University of North Dakota *
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101
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Geology
Date
Dec 6, 2023
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12
Uploaded by CommodoreFalconPerson959
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Geological Time Exercise
To help you with this exercise, I attached a copy of the Geologic Time Scale on the last page. Part I. Geological Time Scale
1. The time circle below can be used to depict Earth history. It is like a big clock. The Earth began about 4.6 billion years ago. (Bya stand for billion of years ago; mya stands for millions of years ago.) Going clockwise around the diagram, the Earth gets younger until we get to the present day (at “12" O’clock, again.) The largest divisions are eras. Era began ended Precambrian era 4.6 bya 540 mya Paleozoic era 542 mya 251 mya Mesozoic era 251 mya 65 mya Cenozoic era 65 mya today On the time circle below, clearly show the duration of each of the Eras. (Draw lines and show pie-shaped wedges for each.)
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2. Key events in Earth History include: age of earth 4.6 bya oldest life 3.8 bya first multicelled organisms 600 mya dinosaurs 250 mya primates 50 mya hominids 3.8 mya (most of) your birthdays 1990s sometime On the time circle below: use arrows and clear labels to show where each of the above events plots in Earth History.
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Part II. Four Corners Region, United States
The map below map shows the crude physiography of the Four Corners area of the United States (Colorado, New Mexico, Arizona, Utah). The highest elevations are in the Uinta Mtns, the Rocky Mtns and the Sangre de Cristo Mtns (really just an extension of the Rockys). High elevations are also found in an area labeled the Mogollon Mtns, although this is really not one range, but a number of different uplifted areas. In the central part of the area, the Colorado Plateau is an area of moderate elevation. Around the edges of the four corners region, elevations are lower.
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The map below shows the ages of the bedrock in the Four Corners area. Darker colors are older rock. The black regions have Precambrian bedrock, dark gray have Paleozoic bedrock, light gray have Mesozoic bedrock, and white areas are Cenozoic bedrock.
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3. Look at the map on the previous page. The rock ages in the Uintas form a bull’s-eye pattern, with oldest rocks in the center. The Rocky’s display a similar pattern, although it is strung out in the North-
South direction. These patterns are typical of mountain ranges around the world. Explain the patterns! Why are ancient rocks often exposed in the centers of mountain ranges, with rocks getting younger toward the outside? This pattern is representative of an anticline, where older rock is pushed up from the center of a formation due to tectonic activity, and younger rock is pushed out and away from the center of the formation. 4. Very old rocks are also exposed at the base of the Grand Canyon; note the sickle shaped blob on the map (previous page) in northern Arizona. Yet, the rocks all around the Grand Canyon are significantly younger. Why? The older rock exposed at the base of the Grand Canyon were uncovered by millions of years of erosion by a river cutting through the rock. The rocks all around the Grand Canyon on the other hand are significantly younger because all we can visibly see are the topmost layers of sedimentary rock. Since these rocks have not undergone the same level of erosion as the rocks in the Grand Canyon, the younger rock layers remain and have not yet been weathered away to expose the older rock layers underneath.
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Most of the formations exposed in the Grand Canyon have ages ranging over 100's of millions of years. The bottom-most formations are billions of years old. Geologists use stratigraphic columns to describe the geology and rocks of different ages in a given area. I’ve given you a copy of an “ideal” stratigraphic column for the Grand Canyon here. In this figure, the column on the left contains general ages and formation names. The cartoon on the right shows the relationships between the different formations, and also which ones weather and erode Qto form cliffs or slopes. 5. In the Canyon bottom, near the Colorado River, formations of the Grand Canyon Supergroup (marked with a two-headed arrow) are sloping with respect to the overlying formations. Why? Explain how they came to be this way. The Unkar group is tilted due to a tectonic event. The sloping layers were first deposited horizontally, and then, some time after being deposited, there was some sort of tectonic event that tilted them at an angle, and then they were eroded until they were flat on top, to allow for the sediments that make up the overlaying sedimentary rock to be deposited.
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6. Most of the contacts between formations in the above cartoon are shown as squiggly lines, not straight lines. I marked some of them with black dots (but there are others). For example, look at the line that separates the Surprise Canyon Formation from the Redwall Limestone, the line that separates the Redwall Limestone from the Temple Butte Formation, the line that separates the Unclassified Dolomite from the Muav Limestone, or the line at the bottom of the Tapeats Sandstone. These squiggly lines are unconformities
. What is an unconformity and how does one form? An unconformity is a gap in the geologic record that could indicate periods of crust deformation, erosion, and sea level variations. Unconformities usually occur in stratified rocks, and are usually found in sedimentary rock. Unconformities form when there is a substantial break in the surfaces between two rock formations, and indicate a gap in the geologic record. These unconformities can represent periods of time where sediments were not deposited, or when erosion occurred that completely removed whole layers of rock before depositing new sediment. 7. (You may wish to look at the complete geological time scale at the end of this handout.) Check out the table below, and consider the periods of the Paleozoic Era. In the Grand Canyon, some of the rock units (formations) of some periods are much thicker than formations of other periods? In fact, there are no rocks associated with two (of the Paleozoic) periods. Which ones? Why are there no rocks of those ages? Speculate but be logical. In the Grand Canyon there are no rock formations that are associated with the Silurian, and Ordovician Paleozoic periods. There are likely no rocks of these ages to be found due to periods of Geological Time where little to no sediment was deposited, and times when much erosion occurred. This is mostly due to sea level. During these periods of time, the sea level was probably very low and the rocks from these time periods were exposed, which did not give much sediment the chance to be deposited to form new rock layers, and the exposed rock was likely eroded completely. Formations of the Grand Canyon
millions of years (mya) age lithology formation Permian limestone shale sandstone shale Kaibab Toroweap Coconino Hermit Shale Pennsylvanian sandstone & shale Supai Group 323-354 mya Mississippian sandstone & conglomerate limestone Surprise Canyon Redwall Devonian limestone Temple Butte Unclassified dolomite Cambrian limestone shale sandstone Muav Bright Angel Tapeats Late and middle Precambrian sandstone, quartzite, shale, limestone Grand Canyon Supergroup 1600-1800 mya = 1.6 - 1.8 bya Early Precambrian granite schist Zoroaster Vishnu
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9. On the previous page, put the correct number of millions of years in the left hand column of the table. For example, the Vishnu Schist and Zoroaster granite formed about 1600-1800 mya = 1.6-1.8 bya. (I also filled in the Mississippian for you.) From the oldest rocks to the youngest rocks, how many millions of years of earth history are exposed in the Grand Canyon? According to the table, there are roughly 1,550 million years, or 1.5 billion years, of earth history exposed in the Grand Canyon. 10. The Grand Canyon is about 5,000 feet deep. That is equal to 5,000x12" = 60,000 inches. Geologists think of rocks piling up slowly over the many long years of geologic time. Although it is not correct in detail, let’s simplify and assume for the moment that the rocks exposed in the Grand Canyon formed continuously at a steady rate over time. From question #9, you know how many years it took. So, how many inches of rock formed per year? (You may get a very small answer.) If the Grand Canyon formed at a steady rate over time, it would have formed 0.00004 inches per year. 11. If you look at the lithologies in the table on the previous page, you will see that the rocks of the Grand Canyon are highly variable. Many different rock types. What does this mean to a geologist? Why all the variations in lithology from one time period to the next? Highly variable rocks to a geologist means that there are rocks with highly variable compositions and characteristics that make up the Grand Canyon. There are variations in the lithology from one period to the next due to differing environmental conditions as the rocks formed. As the environment changed over time, different substances and minerals became either more or less readily available, which led to variations in the rock that formed at each period. 12. The Red Wall Limestone exists in the Grand Canyon area. It is Mississippian in age. Although the Red Wall Formation itself does not exist everywhere in the western States, other limestones of Mississippian age can be found just about everywhere. Why do you suppose this is the case? If we look at these rocks of the same age (Mississippian age rocks), why are they the same across most of the western U.S.? (Note that rocks of other ages are not as uniform.) Mississippian age limestone can be found just about everywhere because it formed during the time that all the continents were connected in the supercontinent Pangaea. Due to this, all or at least most of the limestone formed during the Mississippian period would have formed in the same place, and then, as the continental plates that made up Pangaea broke apart and drifted away from each other, it would have broken up the Mississippian age rock as well and carried a portion of it on each continental plate as they drifted to form new continents. This rock originating from the same land mass would also play a role in all the rock being the same, due to the fact that all the rock would have formed from the same sedimentary deposits, and therefore would have similar, or identical chemical compositions regardless of where the rock was eventually found.
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13. Why are there no rocks younger than Permian age exposed at the Grand Canyon? Why no rocks older than the Zoroaster and Vishnu Formations? There are no rocks younger than Permian age exposed because any rocks younger than that have already been eroded away completely, and it is also possible that the Grand Canyon has not been underwater since that time, which would leave no opportunity for sediments to be deposited that would eventually form new rock. There are no rocks older than the Zoroaster and Vishnu formations, possibly because the temperature of the Earth prior to these formations forming was too hot causing the rock to melt before it could fully
solidify, and the
Earth didn’t cool down enough for rock to fully form until just before the Vishnu Schist formed.
14. The Kaibab Limestone is a very important formation at the Grand Canyon. It forms the Canyon Rim. Yet, just a 150 miles northeast, in Canyonlands National Park, it is missing even though rocks younger and older than the Kaibab are present. Why? Speculate! The Kaibab Limestone is most likely missing from Canyonlands National Park for a few reasons. It’s possible that when the sediments that formed the Kaibab Limestone were deposited, the Grand Canyon was underwater in an inland sea, which allowed the sediments to be deposited there. It’s possible that at this same time, the area that makes up Canyonlands National Park was not underwater, and instead protruded from the surface of the water, which did not allow enough of the sediments that make up the Kaibab Limestone to be deposited there, while at the same time any sediment that did manage to be deposited was most likely eroded away due to the area being exposed to the elements. 15. The map above shows National Parks (and Monument Valley) in Utah and Arizona. The table below lists the general ages of rocks found in each place. Parkland
age of most rocks
Grand Canyon Precambrian => Permian Bryce Canyon Jurassic => Tertiary Zion Permian => Jurassic Canyonlands Pennsylvanian => Jurassic Arches Jurassic Capitol Reef Permian => Cretaceous Monument Valley Permian => Triassic In which Parkland are the oldest rocks exposed? (Ignore sediment on top.) In which are the youngest rocks exposed. Why? Why are older rocks exposed in some parks and younger rocks in others? The oldest rocks are exposed in the Grand Canyon, and the youngest rocks are found in Capitol Reef. The Grand Canyon has exposed rock from the Precambrian era, which is the oldest eon in Geological time, where as Capitol Reef has exposed rock from the cretaceous period, which is the youngest time period represented in this list. Some parks have older rocks exposed compared to others, because these parks have probably undergone large amounts of weathering and erosion to expose the older rock.
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16. Getting back to an earlier question: Precambrian rocks are not exposed in most National Parks of the Four Corners region. In fact, we only see them in Rocky Mountain National Park and in the Grand Canyon. Why just those parks? i.e., What are the characteristics or features of those parks that means we can see such old rocks? These parks are located in an area where major orogenies occurred during the Precambrian time period, pushing Precambrian rocks to the surface through tectonic activity. In Rocky Mountain National Park, as Precambrian rock was pushed upward, the younger rock on top moved away and was eroded, making the Precambrian rock visible. In the Grand Canyon, Precambrian rock became exposed due to a river running through the canyon for millions of years, eroding away the rock layer by layer until it eroded deep enough to expose the Precambrian rock layers deep in the crust. 17. The youngest Precambrian rocks are about 570,000,000 years old. That is, about 570 million years old. Or, 0.570 billion years old. What percent of Earth history does this represent? How many human generations could this represent (assuming we had been around since the beginning)? This represents about 12.4% of Earth’s history. If we assume that a human generation is about 30 years, this percentage of Earth’s history would represent 19,000,000, or 19 million, human generations.
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