GEOSCI Unit 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):

50. 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.)

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. 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. 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 second-oldest sedimentary rock layer?

D 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.

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 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.

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

H, J, I 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. 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.

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.

What is accurate about the scientific results learned by counting tree rings and other annual layers?

Records in tree rings, lakes and ice all reach beyond 12,000 years, and some of them reach beyond 40,000 years. There is a continuous record of overlapping tree rings from north Germany with 12,429 years in trees (and that was published a few years ago). The longest lake-sediment record of annual layers is over 40,000 years, and there are over 100,000 years in the longest ice-core record that preserves annual layers. And, various lake, tree and ice records agree on the history of volcanoes, climate changes, etc.

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. 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 next four (4) questions refer to the diagram above. This diagram shows a geologic cross-section of some rocks, such as you might see in a cliff. The tree is growing on top of the modern surface. Rock layers A, B, C, D, E, and F are sedimentary; E contains mud cracks and fossil footprints as shown. G is igneous rock that hardened from hot, melted rock. H, I and J are faults, and K and L are unconformities. Sedimentary rocks are right-side-up unless there is some indication given to show something else. Remember that footprints and mudcracks tell you whether rocks are right-side up or upside-down, so look for those. Also, if a layer is upside-down, so are the layers that are in the same sedimentary pile, until you hit an unconformity. So, if you have layers Q, R, S and T in one sedimentary pile beneath an unconformity, and then layer U above the unconformity, and you learn that Q is upside-down, so are R, S, and T, but you must look for more information to tell which way is up for U. Referring to the rocks you see here ......

See next three questions

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. 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. 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. 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. 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.