RockOn #10

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 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.)

Which is accurate about the Grand Canyon, in Arizona:

A great thickness of sedimentary rocks exists in Death-Valley-type faulted basins, which can be seen deep in the canyon in many places. Well over two miles of Precambrian sedimentary rocks can be seen in the deep part of the canyon, all slanted from horizontal and preserved where they were dropped by faulting. The sedimentary rocks above are right-side up, and the Coconino Sandstone is well below the Kaibab Limestone of the rim, which 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.

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

B, F, E, D, C 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.

Which is younger:

Fault H 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:

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). 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 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 more than about one-hundred-million years old. 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 long have you personally been alive?

Much less than the thickness of a sheet of paper. If the 4.6 billion years of Earth history are 100 yards, then the few thousand years of written history are just one-millionth of that history, just over the thickness of a sheet of paper. And your small piece of written history must be only a small fraction of a sheet of paper, roughly 1/200th or so.

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. 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 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. 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. 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 younger:

The tree: 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 inches across from the Grand Canyon . The shape and polish of the rock are interesting .It is likely that the rock:

Was scratched and polished by silt-laden river water, during carving of the Canyon by the Colorado River. 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.

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. 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.