Earth Sciences 240A – Answers to Final Exam

Part A: 1. Question: Circular ‘depression’ found by sensitive gravity survey at interface between folded/faulted gneisses and schists (700 million years old) and exposed Ordovician limestone (500 million years old). The interface is covered completely by 10 meters of limestone.

(a)    What to do/evidence to look for to prove this is (or is not) an impact crater.

The best first thing is to sketch a vertical section showing surface, limestone, depression and basement. That will remind you that you cannot see the interface. If you cannot see it, how are you going to collect physical evidence? It was my hope that you would remember the situations at both Chicxulub and Chesapeake Bay; both were buried but both were clearly identified as impact craters. You needed to do detailed surveys (such as seismic) to identify as much as possible of features such as central rebound regions before you go further. Having done that you had to get samples to work with. True, I said you could not excavate the limestone en-masse, but you most certainly could drill! Remember that both Chicxulub and Chesapeake Bay were drilled for samples. With physical sample, you then could consider things like tektites, shocked mineral grains, breccia, etc. In your consideration of evidence, you needed to be careful; if the interface as 200 million years old (this is the answer to part (c): the crater is formed in 700 m.y. rock and covered by 500 m.y. rock), the probability of finding and loose material on that unconformity surface is zero. Weathering and transportation or glaciation will have long since removed it. If your answer included all the features of evidence about an impact crater but did not consider how to get the samples, you got almost no marks for part (a) [worth 15].

(b)   About all you tell was that the object was roughly 1 km diameter; there would have been no asteroid samples left.

(c)    500-700 m.y. was all I needed

(d)   Remembering the size study we talked about in class, the answer is “No”.

 

2. This question had to do with answering certain statements make by ‘greenhouse skeptics’ from the point of view of a ‘greenhouse proponent’.

(a) “Rising levels of greenhouse gases have not caused Earth to warm”.

Start with statement that The Little Ice Age is generally accepted to mark the down-turn (from interglacial maximum) of global climate. Include a sketch graph showing EXACT correlation of CO2 content of greenhouse and trend in global temperatures. Of course, you could show that trend’s connection with the Industrial Revolution. You then need to state that there is no other factor that correlates with the proven temperature change.

(b) “It is arrogant of humans to think they are capable…….”

This, of course, is the typical thinking until 20th Century. Natural (non-human) factors that change climate are: Milankovitch factors, sun radiation variation, weathering, volcanism,…In the latter part of the course, we learned that any factor which had the ability to significantly change the strength (composition) of the greenhouse certainly would change climate. So all that’s left is to affirm that humans + the feedback we started in the natural systems have done just that.

(c) “Some areas have warmed….”

No one had trouble with documenting areas that are warming and the reasons why. Also everyone appreciated that we were talking averages. The trouble was in documenting any areas which were cooling. Some recognized that there’ve been reports about significant periodic cooling along the Atlantic eastern coastline and along the western coast of South America. However, very few appreciated that ocean current patterns are the primary cause. If you have cold water at surface, the air above it will be cool. If you have a feature like El Nino, changes in flow of water cause changes in flow of air. (By the way, the Pacific system is also present in the Atlantic but weaker and on a different cycle).

 

3. Vancouver earthquake/volcanic eruption probabilities.

Very, very few tried this one, and those who did got reasonable results. The thing to remember was the activity of the Jan de Fuca plate and the geography of it. The northern end is actually south of Vancouver (a sketch would have helped a great deal, but no one did one). That means that the most northern volcano to likely be affected will be Baker – which is a fair way south of Vancouver. On the other hand, everyone is waiting for a big earthquake, because of the immense stress on the whole of the subduction system, and that will most definitely do great damage to Vancouver/Victoria. As for prediction data, the lists are pretty standard and I’m not going to regurgitate them. [In my view, this was by far the easiest question of Part A].

 

Part B was just fill-in-the-blank and Part D was standard definitions of important things we covered. I’m not going to repeat those – you either knew them or you didn’t.

 

Part C:

1. Plume-fed continental volcano.

Certainly you needed to begin with an appreciation that the initial magma was basaltic thus very hot and fluid. It ‘burned through the continental crust like a torch’ and in so doing assimilated a great deal of silicic material, becoming more Si-rich, fluid-rich and viscous. Somewhere along route the magma will slow/stop and develop a ‘chamber’; there there will be further composition modification by crystal fractionation, wall assimilation, filter pressing. The end result will be to change the composition toward rhyolite – dacite. In some cases, you envisaged the process from here on as being on the Mount St. Helens-scale, but it actually would be more toward the Yellowstone-scale (i.e. we’re aimed toward the resurgent caldera sequence. Remember the diagrams that showed that sequence? The top figure showed a Si-rich, low density cap that bulged the ground upward; next came a Plinean eruption (following magma that got within 2 km of surface); following pyroclastic flows that pretty much emptied the chamber; the sides fell in, with a final explosion; a cap rebuilt and the process started over again.

 

2. (a) Much of the original CO2 of the thick atmosphere eventually went to large limestone deposits after continental weathering released the needed Ca (at this stage there were almost zero organisms to consume it).

(b) Much of the current CO2 goes to the atmosphere and biosphere; some, of course, still goes to rocks/shells.

(c) Major CO2 reservoir is rocks (by a huge factor).

 

3. Tsunami warning system.

TSW is a system of combined automation (early stages) followed by human participation. The early stages consist of automatic detectors in 26 countries around the Pacific that monitor earthquakes and water levels and transmit them via satellite to the central station in Honolulu. The computer evaluates the data and selects only those which have the potential to create tsunami. Warnings are sent to 2 NOAA stations (Alaska and Hawaii) from which messages are transmitted via satellite to all countries and all ships. The (US) Coast Guard is responsible for any needed evacuations and also for transmitting the ‘all clear’ when the danger is past.

 

4. (a) The sketch could have been the simple stress/strain graph or the one showing elastic rebound (both from the prerequisite material lecture). The words simply had to define elastic rebound as defined in the text of that prerequisite lecture (which most of you either didn’t read or very obviously forgot!).

(b) You needed to begin with definitions of what you intend to describe: creep and locked faults. Since the question dwelt upon evaluating hazards, it clearly was insufficient to say there were none associated with creep. Of course there are hazards! The offset distance of a creeping segment will be exactly the same as for a locked segment of the same fault  (once both move) over the long-term; the damage is slight per unit time with creep. It can still cause hydro dams to crumble or road structures to fail. You needed, of course, to describe, at least briefly, the hazard of damage from a highly stressed system over a very short time (like seconds).

 

5. (a) one-half point per factor:

            - mass extinction is 4x simple.

            - mass extinction must be: >30% extinction of species

            - world-wide

            -all environments

            -short duration

            -one cause or series of simply related causes

(b) Remember when we went through the 5 major mass extinctions and tried to find correlations of impacts? We later did it for flood basalts too. That’s what I wanted for an answer. There was a positive correlation of both impact and flood basalt for the 66 m.y. extinction (and you could talk about the evidence for and against each); there was positive correlation of flood basalt with the 250 m.y. extinction (but the really good evidence for impact correlation was only reported in the last month so I obviously didn’t expect that to show in any answer); there was positive correlation for impact evidence for the 239 m.y. extinction (no flood basalt). The others have no solid evidence pointing to either impact or flood basalts.