Geology Rocks! Trivia Quiz

Answer: A mountain stream

Sherlock says, "Well, Watson, the large size of the sediment, fist-sized cobbles, indicates that this was not far from the source area. After all, if the cobbles had been transported much further, say to a river delta or all the way out to sea, there would have been time to break them down further.

There must also have been some way to transport the cobbles, which, after all, are relatively large. This points to a "high-energy" environment, such as a mountain stream, where the force of gravity on steep slopes would have helped move the rocks, or a glacier.

However, we know that it came from a stream because the chips and fractures, known as 'crescentic impact fractures,' are only found in stream environments. Elementary, my dear Watson."

Answer: A desert

Cross-bedding always indicates ripples or dunes. There is a difference between the two-- a ripple is less than 6 cm tall, a dune is greater than 6 cm tall. The height of the beds indicate the height of the dunes, in this case, approximately 20 feet high. Ripples and dunes cannot be higher than the depth of the fluid they form in, so in this case, streams are out. Now, it takes energy to move sediment into ripples and dunes, and it takes more energy (and bigger grain sizes) to form bigger ripples and dunes.

It is unlikely there would have been sufficient energy in a calm lagoon. This leaves a beach and a desert. The red color in the sandstone is likely an iron oxide coating on the sand grains, which is only found in desert environments.

In fact, this was probably an "erg," an extensive desert area covered by deep sand dunes.

Answer: All of these

An asymmetric ripple (symmetric ones will be considered in a minute) has two different sides: a long side with a shallow slope, the "stoss side," and a shorter side with a steeper slope, the "lee side." The current flows from the stoss to the lee side.

Individual sand grains are pushed up the stoss slope to the crest of the ripple, and then cascade down the lee slope. At the bottom of the lee slope, they begin their journey up the stoss slope of the next ripple. Hence, you can tell the direction of the current from a ripple. You can tell the relative speed of the current from the shape of the ripple. Bigger ripples, also called dunes, indicate higher current flow (though the size of the sand grains affect this as well.) But there is also the top view of a ripple to consider: ripples with wavy crests come from higher velocity environments than ripples with straight crests. You can tell the rate of deposition of a ripple from the ratio of how much the ripple grew up to how much crest of the ripple moved over, the "rise over run" from high school algebra.

In the case of a symmetric ripple, where you can't tell the stoss slope from the lee slope, it probably formed from wave action.

Answer: Yes

This suspect obviously did not know too much about geology. The sand in a limestone area is generally light-colored, and the sand that comes from basalt is dark-colored, nearly black. He's lying! This is a simple example, and lest you think this situation is too unlikely, one of my geology professors was indeed called upon to investigate the origin of some sand found in a suspect's trouser cuff.

Answer: Olivine weathers very quickly in the presence of water

A geologist named Goldich did a study of the rates of chemical weathering in different minerals. Olivine is one of the minerals which weathers the quickest with exposure to water. It is extremely unlikely that olivine sand would have been around for millions of years in the presence of water.

Incidentally, the rate at which minerals weather is inversely proportional to the temperature and pressure at which they crystallize. So, a mineral which crystallizes at a high temperature and pressure weathers quickly, while a mineral which crystallizes at a lower temperature and pressure weathers slowly. Diamonds crystallize at very high temperatures and pressure.

A diamond is forever... oops. But if you do want to get your love a long-lasting mineral, get zircon, which is almost as sparkly as diamond, certainly less expensive, and can last up to 4 billion years. Yes, zircon crystals from Western Australia have been measured to be 4.1 to 4.4 billion years old, on par with the estimated age of the crystallization of the Earth.

But back to this question. Mars is red because the dust grains are covered with a layer of iron oxide, which is red. And, sand refers the size of a grain of sediment, not the composition, so it is quite possible for sand to be made up of minerals other than quartz. This experiment was really conducted!

Answer: This place was underwater about 300 million years ago!

Limestone is only deposited in shallow marine environments. There were no ocean-going dinosaurs: plesiosarus, pliosaurs, and ichthyosaurs were aquatic, but they weren't dinosaurs! Crinoids, also known as sea lilies, flourished maybe 350-250 million years ago, and while most of them went extinct at the end of the Paleozoic, there are still a few species alive today.

They look roughly like flowers, but are actually animals related to starfish and sea urchins. Columnals are the hard parts that make up the stem of the crinoid.

They look like beads, and are very common in some Paleozoic rocks. You also know that this area had salt water, not fresh water-- crinoids are very intolerant of salinity levels much different from oceanic salinity.

Answer: Dinosaur National Park, Vernal, UT

Once upon a time, there was a sandbar on a river. The river would periodically flood, and wash dinosaur carcasses onto the sandbar. 145 million years later, the place is a sandstone formation containing lots and lots of dinosaur bones. On August 17, 1909, a paleontologist named Earl Douglas discovered this formation, and began excavations.

In 1915, the quarry was declared a national monument, and nowadays there is a visitor center on the site. The building backs up against the quarry, and one entire wall is simply the rock with the bones still embedded in the sandstone.

It is really quite impressive.

Answer: The composition of the lava

The difference is in the composition of the magma. The "runny" volcanoes like the Hawaiian volcanoes have magma that is rich in iron and magnesium. The word to describe this composition of magma is "mafic." The crystal structure of the minerals in the lava is relatively simple, and is not able to trap much dissolved gas.

The "constipated" volcanoes like Mount Saint Helens have "felsic" magma, without the iron and magnesium, which has a very complicated molecular structure, which can trap a lot of dissolved gas.

When the pressure builds too much-- BLAM! The pressure from the gas creates a very violent explosion. The magma from mafic and felsic volcanoes produce different rock types, basalt and gabbro for mafic, rhyolite and granite for felsic. Which type of volcano would I rather live near? A mafic one. You still wouldn't find me living near any volcanoes, though...

Answer: They precipitate around a nucleus, sometimes consisting of organic matter

Sometimes concretions form by mineral precipitation around organic material. Sometimes concretions can preserve fossils in exquisite detail! The only problem is getting the fossils out, since the concretions are usually harder than the surrounding rock.

Interestingly, in Italy, concretions seem to have religious significance-- there are shrines with concretions topped by crosses, statues of the Virgin Mary, etc.

Answer: The cement holding the grains together was calcite

The grains in a sandstone can be cemented by any number of minerals, like quartz, calcite, dolomite, kaolinite, chlorite, or others. The cement mineral does not necessarily have to be the same as the mineral the grains are made out of. In this case, a neat explanation for why the sandstone was deteriorating from the acid rain, but the individual grains were still largely intact, would be that the sandstone was cemented by calcite cement, which is not resistant to sulfuric acid.

Incidentally, people drilling oil wells will sometimes dump sulfuric acid into the well to dissolve the calcite cement and improve the porosity of the rock, all the better to extract the oil. But, make sure the cement really is calcite, and not chlorite, or else you'll end up with a nasty sludge that clogs all the porosity!