Rock On #10

You are dating a lava flow by the potassium-argon system. However, the offspring in this system are leaking out of the minerals. Which is accurate?

You will think that the lava flow is younger than it really is, but you will be able to detect the error by comparing concentrations of offspring from the edges and centers of grains. Feedback: Argon-40 leakage will make the lava flow appear young even if the flow is old; however, the edges of grains will lose more argon-40 than will the centers, pointing to the source of the error.

You start with 800 parent atoms of a particular radioactive type, which decays to give stable offspring. You wait just long enough for two half lives to pass. You should expect to have how many parent atoms remaining (on average):

200 Feedback: After one half-life, you've gone from 800 parents to 400 parents; after a second half-life you go from 400 parents to 200. . (Typical studies of radioactive decay use many more atoms, to avoid statistical fluctuations, but the question says "on average", so we asked you about 800 rather than 800,000,000,000,000 to make the math easier.)

What is indicated by the yellow lines in the image above, which separate flat-lying sedimentary rocks, on top, from slanting sedimentary rocks beneath?

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. 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 correct age progression, from younger (first) to older (last)?

B, F, E, D, C Feedback: The package of sediments C, D, E, F is upside-down, as shown by the footprints and mud cracks, so C is oldest, and F the youngest of these. B is above the unconformity above all of C, D, E, and F, so is the youngest of these five.

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:

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 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 is the oldest fault:

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.

Which is accurate about the history of the Grand Canyon:

In the deepest part of the canyon, the river cuts through rocks formed by metamorphism of older sedimentary rocks in the heart of a mountain range. Feedback: The Colorado River is cutting through the metamorphic rocks from the heart of an old mountain range. The sedimentary rocks above are right-side up, and the Kaibab Limestone slants down to the north beneath the rocks of Zion, which are older than the rocks of Bryce, among others. Many unconformities exist in the walls of the Canyon, including the one below the Precambrian sediments and the one above those sediments. 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.

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.

What is accurate about the scientific results learned by counting annual layers in ice cores?

Many tests show that some ice cores have reliably preserved annual layers, and the longest record extends back more than 100,000 years. Feedback: As detailed in the text, ice-core layer counters do a huge amount of testing to be sure that the layers really represent years, and that the counting is as accurate as possible. The longest such record now extends beyond 100,000 years.

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.

The two pictures above, I and II, show fossils 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 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.

The picture above shows a very hard piece of rock about six inches across, in the Grand Canyon.The surface of the rock looks rather different from the surfaces of many other rocks.What made this odd-looking surface?

The river, which blasted the rock with sand- and silt-laden water during floods; this shows that even hard rocks can be eroded by rivers. Feedback: The Canyon was carved by the Colorado River. Glaciers have not been there, and while wind and faults can change the appearance of rocks, none makes something like this river-polished rock, as you saw in one of the Grand Canyon V-Trips.

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.