RockOn #10

You start with 400 parent atoms of a particular radioactive type, which decays to give stable offspring. You wait just long enough for three half lives to pass. You should expect to have how many parent atoms remaining (on average): 400. 200. 100. 50. 25.

50. Feedback: After one half-life, you've gone from 400 parents to 200; after a second half-life you go from 200 parents to 100, and after a third half-life you go from 100 parents to 50. (Typical studies of radioactive decay use many more atoms, to avoid statistical fluctuations, but the question says "on average", so we asked you about 400 rather than 400,000,000,000,000 to make the math easier.)

Two yellow lines have been drawn on the picture by the instructional team. These lines follow an interesting surface, which separate flat-lying sedimentary rocks, on top, from slanting sedimentary rocks beneath. This surface is: A great fault, where pull-apart Death-Valley-type faulting dropped rocks so that they could be preserved from erosion and seen today. A great unconformity, with sedimentary rocks above resting on older sedimentary rocks below. A great unconformity, with sedimentary rocks above resting on igneous and metamorphic rocks below. A great unconformity, with sedimentary rocks above resting on younger sedimentary rocks below. A great fault, where push-together action shoved the upper rocks over the lower ones.

A great unconformity, with sedimentary rocks above resting on older sedimentary rocks below. Feedback: John Wesley Powell, of the United States Geological Survey, and the leader of the first boat trip through the Grand Canyon, called the feature marked by the yellow lines "The Great Unconformity". It separates horizontal Paleozoic sedimentary rocks, above, from inclined Precambrian sedimentary rocks, below.

Geological evidence based on several radiometric techniques has provided a scientifically well-accepted age for the Earth. Represent that age of the Earth as the 100-yard length of a football field, and any time interval can be represented as some distance on the field. (So something that lasted one-tenth of the age of the Earth would be ten yards, and something that lasted one-half of the age of the Earth would be fifty yards.) On this scale, the time from when dinosaur extinction made space for large mammals, until today, would be represented by how far on the football field? A little over 1 yard. Over 90 yards. 60 yards. Over 80 yards. 50 yards.

A little over 1 yard. Feedback: If the 4.6 billion years of Earth history are 100 yards, then the 65 million years since the dinosaur extinction are a little under 1.5 yards, hence a bit over 1 yard.

Which is the oldest sedimentary rock layer: B C E D F

C Feedback: The package of sediments C, D, E, and F is upside-down, as shown by the footprints and mud cracks, so C is the oldest one.

Which is younger: Unconformity K. Fault H. Unconformity L. Fault J. Fault I.

Fault H. Feedback: Unconformity L is cut by fault I, so is older than I. Fault I is cut by fault J, so is older than J. Fault J is cut by unconformity K so is older than K. Unconformity K is cut by intrusion G so is older than G, and intrusion G is cut by fault H so is older than H. Hence, fault H is the youngest.

You are asked to assign as accurate a numerical age as possible (how many years old) to a sedimentary deposit. You would be wise to use: Uniformitarian techniques. Either counting of annual layers or radiometric techniques if the deposit is old (more than about 100,000 years), and radiometric techniques if the deposit is young (less than about 100,000 years). Counting of annual layers if the deposit is old (more than about 100,000 years), and radiometric techniques if the deposit is young (less than about 100,000 years). Uniformitarian techniques if the deposit is old, and counting of annual layers if the deposit is young. Either counting of annual layers or radiometric techniques if the deposit is young (less than about 100,000 years), and radiometric techniques if the deposit is old (more than about 100,000 years).

Either counting of annual layers or radiometric techniques if the deposit is young (less than about 100,000 years), and radiometric techniques if the deposit is old (more than about 100,000 years). Feedback: If you want an absolute date (number of years) rather than older/younger, you can count layers for young things, or use radiometric techniques for young things or for old ones. Uniformitarian calculations aren't very accurate.

Which is the youngest fault: H J I

H Feedback: I is cut by J, so I is older than J. And with reference to K, both I and J can be shown to be older than H.

Using only uniformitarian calculations from the thickness of known sedimentary rocks, likely rates at which those rocks accumulated, and features in and under those sedimentary rocks, geologists working two to three hundred years ago estimated that the Earth: Is about one-hundred-million years old. Is 4.6 billion years old. Has been here forever. Is less than about one-hundred-million years old. Is more than about one-hundred-million years old.

Is more than about one-hundred-million years old. Feedback: Radiometric techniques reveal the Earth to be about 4.6 billion years old, but early geologists did not have the sophisticated instruments to measure the trace radioactive elements and their offspring. Working from the rocks, the geologists knew that the age must be in the neighborhood of 100 million years, plus extra time in unconformities and additional extra time in the oldest, metamorphic rocks.

In the photograph above, a portion of cliff about 30 feet high is shown. From what location in the Grand Canyon did Dr. Alley take this image? About halfway between the top and the river, where a large fault has dragged the rocks and caused the fold. Near the bottom, where the river has cut through rocks that were cooked, squeezed, and partially melted deep in an old mountain range. Near the top, in sedimentary rocks that slumped downhill when they were soft, folding the rocks. Near the west end, where lava that came up pull-apart faults folded while flowing before hardening fully. In the gift shop, where artists have painted the cliff to look like real rocks.

Near the bottom, where the river has cut through rocks that were cooked, squeezed, and partially melted deep in an old mountain range. Feedback: This is the Vishnu Schist and Zoroaster Granite, rocks from the heart of a mountain range. The river is just barely out of the picture to the bottom.

The two pictures above, I and II, show fossils inrocks from the Grand Canyon. Each is "typical"; the rocks near sample Icontain fossils similar to those shown in sample I, and the rocks nearsample II contain fossils similar to those shown in sample II. It is likely that: Sample I is from the north shore of Lake Winna-Bango, and sample II is from the south shore, where there is plenty of moose moss to munch. Sample I is from near the river, and sample II is also from near the river. Sample I is from high in the cliffs of the Canyon, and sample II is also from high in the cliffs of the Canyon. Sample I is from near the river, and sample II is from high in the cliffs of the Grand Canyon. Sample I is from high in the cliffs of the Grand Canyon, and sample II is from much lower, near the river.

Sample I is from high in the cliffs of the Grand Canyon, and sample II is from much lower, near the river. Feedback: Sample I is a wonderful shell hash, or coquina, from the Supai Rocks well up the side of the Canyon, and contains shells from a great diversity of different creatures. Sample II includes algal-mat deposits (stromatolites) from the Precambrian Chuar Group of the Grand Canyon Supergroup, deep in the Canyon near the river, from a time when biology was not a whole lot more diverse than algal mats. Lake Winna-Bango featured in the gripping Dr. Suess tale of Thidwick, the Big-Hearted Moose, but is not pictured here.

What is accurate about the scientific results learned by counting tree rings? Study of tree rings and associated geology proves that the Earth is 5,000 years old, but no older. Study of tree rings by themselves shows that the Earth is 4.6 billion years old. Study of tree rings and associated geology shows that the Earth is 12,000 years old, but no older. Study of tree rings and associated geology shows that the Earth is exactly 12,429 years old. Study of tree rings and associated geology shows that the Earth is more than 12,429 years old.

Study of tree rings and associated geology shows that the Earth is more than 12,429 years old. Feedback: The longest continuous tree-ring record is 12,429 years, but that was published a few years ago, the trees grew in soil that was already there, and there is lots of older wood around. So, the tree rings show that the Earth is more than 12,429 years. But, we don't have overlapping trees back to the formation of the Earth about 4.6 billion years ago, so tree rings do not show that the Earth is 4.6 billion years old.

Which is not accurate about the Grand Canyon, in Arizona: The walls of the Canyon include rocks deposited in many different environments. A great thickness of sedimentary rocks exists in Death-Valley-type faulted basins, which can be seen deep in the canyon in many places. The oldest rocks are on top, with younger ones beneath, as shown by all of the footprints being upside-down in the rocks of the canyon walls. The youngest rock layer at the canyon slants downward to the north beneath still-younger rocks of Zion, Bryce, etc. The rock record of the canyon contains many unconformities.

The oldest rocks are on top, with younger ones beneath, as shown by all of the footprints being upside-down in the rocks of the canyon walls. Feedback: There are some folded rocks in the heart-of-a-mountain-range metamorphics at the bottom, but otherwise, everything is right-side up. All the other possible answers here are correct.

One practical radioactive system used to date lava flows involves: The gas argon-40, which decays to solid potassium-40. The solid potassium-40, which decays to the solid moosemossium-41. The solid potassium-40, which decays to solid argon-40. The solid potassium-40, which decays to the gas argon-40. The gas argon-40, which decays to the gas potassium-40.

The solid potassium-40, which decays to the gas argon-40. Feedback: Potassium-40 is common in solid minerals, and decays to produce the gas argon-40. And despite his great contributions to humanity, no one has named an isotope after moose moss (the favorite food of Thidwick, for you Dr. Suess fans).

Which is younger: Fault J. Rock layer B. Unconformity L. The tree. Rock layer A.

The tree. Feedback: The tree is growing on intrusion G, which can be shown to be younger than all of the others.

The picture above shows a region of hard rock about six inchesacross from the Grand Canyon. The shape and polish of the rock areinteresting. It is likely that the rock: Was scratched and polished by the hooves of mules carrying tourists into the Canyon along the Bright Angel Trail. Was scratched and polished by a glacier, which helped erode the Canyon during the ice age. Was scratched and polished by the wind, which howls through the Canyon carrying loads of sand eroded from sand bars. Was scratched and polished by motion along a fault, which helped open the Canyon so that weathering could lower the Canyon floor. Was scratched and polished by silt-laden river water, during carving of the Canyon by the Colorado River.

Was scratched and polished by silt-laden river water, during carving of the Canyon by the Colorado River. Feedback: The Canyon was carved by the Colorado River. Glaciers have not been there, and while wind, faults and mule hooves all can change the appearance of rocks, none makes something like this river-polished rock, as you saw in the class materials including in one of the Grand Canyon slide shows.