Rock On #10

You start with 200 parent atoms of a particular radioactive type, which decays in a single step to give a stable offspring, and you start with none of those stable offspring. You wait just long enough for two half lives to pass. You should expect to have how many offspring atoms (on average)(remember that the number of parents and the number of offspring add up to 200, so if you have 10 parents, you have 190 offspring because 10 and 190 add up to 200, and if you have 20 parents you have 180 offspring, and so on):

150 Feedback: After one half-life, you've gone from 200 parents to 100, and 100 offspring have been made. In the second half-life, you go from 100 to 50 parents, and that makes another 50 offspring. Adding the additional 50 to the 100 from the previous half-life gives 150 offspring. (Typical studies of radioactive decay use many more atoms, to avoid statistical fluctuations, but the question says "on average", so we asked you about 200 rather than 200,000,000,000,000 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 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.

What is indicated by the yellow lines in the image above?

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.

Which is the second-oldest sedimentary rock layer?

D 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, and D is second-oldest.

Which is older:

Fault I Feedback: 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 I is the oldest on this list.

Which correctly gives the order of the faults, from youngest (first) to oldest (last):

H, J, I 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.

Early geologists did not have radiometric dating techniques, or long layer-counted histories. Instead, they followed William Smith in putting things in order, and then used uniformitarian calculations based on modern rates of processes and observed results of processes in the geologic record. These early geologists, using these techniques, found that the Earth:

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.

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 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.

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, how far on the football field would represent the time between the first appearance of abundant shelly creatures and today?

Just over 10 yards Feedback: If the 4.6 billion years of Earth history are 100 yards, then the 570 million years since the widespread appearance of shelly creatures is a bit over 10 yards. Most of our fossil record is limited to the last 10% of the planet's history. The shells appeared "suddenly"—in a few million years, or a few inches on the football field of time.

The above photograph was taken in the Grand Canyon, and shows a cliff that is approximately 30 feet high. What are the rocks in the cliff?

Precambrian metamorphic rocks with some igneous rocks intruded; the folding was caused by mountain-building processes when the rocks were very hot deep in a 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.

In age dating, geologists use:

Radiometric techniques and layer-counting for absolute dating of events that happened in the last 100,000 years, and other radiometric techniques for absolute dating of much older events. 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.

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 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. PreviousNext

In the two pictures above, I and II, show traces of former life in rocks from the Grand Canyon. Each is "typical";the rocks near sample I contain fossils similar to those shown in sample I, and the rocks near sample II contain fossils similar to those shown in sample II. It is likely that:

Sample I is from higher in the cliffs of the Grand Canyon, and sample II is from much lower, nearer to the river. Feedback: Sample 1 shows shells from complex creatures including trilobites and snails, from the Supai Group far up the side of the Canyon.

What is accurate about the scientific results learned by counting tree rings?

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 canyon is wider at the top and narrower at the bottom because the river was wider when the region was wetter, and has narrowed as deserts spread recently. Feedback: The idea of the river narrowing over time was the hypothesis that an interested tourist presented to one of the professors and a ranger at the Canyon a few years ago. When the professor asked whether the tourist would want to go out on a narrow point with a jackhammer, the tourist said no, because the rocks might fall off and slide down into the Canyon. When the professor pointed out the many places that rocks had fallen off and slid down, the quick-witted tourist figured out that the Canyon has been widened by such rockfalls as the river has cut downward. All of the rest are accurate.

One practical radioactive system used to date lava flows involves:

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:

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 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.