THE UNIVERSITY OF WESTERN ONTARIO

LONDON CANADA

Final Examination - Summer Term 2003

Time: 3 hours Earth Sciences 240A Distance Studies

(No electronic materials permitted)

PART A

Answer ONE of the following three questions. Each question is valued at 20 marks.

Each question may be answered in as little as two pages of single spaced (essay format) writing (if you prefer double spaced answers, do the math!). Obviously, you may use as little space as you wish, but each topic must be fully discussed in whatever space you use. Please do not use point-form answers except where a listing is appropriate within an essay answer. Use illustrations to strengthen/support discussions, but not in substitution for words.

1. Meteor Crater in Arizona was discovered a long time ago, but was not acknowledged to be the product of impact until relatively recently. Based upon (a) the knowledge you now have of that crater, the area around it, and the character of the meteoroid that produced it; and (b) your current excellent knowledge of impact objects and their effects, discuss how you would identify and fully define such a feature if you stumbled across an identical one today in, say, Canada’s Northwest Territories.

Answer points:

The first thing you need to do is briefly describe the identifying characteristics of Meteor Crater (since it supposedly is an identical feature that you have found).

Approx. 1 km circular crater (‘simple’ structure; i.e. below critical 4 km diam.)

Uplifted and overturned rim

Ejecta blanket

(Flat-lying sediments on basin floor covering) at least 265 m thickness of breccia

(Of course recent seds may not be present, but breccia must)

Meteorite fragments surrounding crater (in this case, siderites)

Impact features such as coesite, stishovite, tektites and shatter cones (define each)

May or may not be preserved evidence of local fire

The second thing you must do is describe the local geology for your area; you are simply eliminating the possibility of mistaking the feature for a volcanic vent or something like the dissolved crater in a salt dome (as Meteor Crater was once thought to be).

Finally, to ‘fully define’ the feature, you need to consider an age date.

2. You have been mapping a continuous sequence of horizontally stratified marine sedimentary rocks. From the bottom of the sequence up, there has been an abundance of hard-shelled fossils, but at one very well defined horizon they nearly all disappear! The rocks immediately above that horizon have almost no fossils, but progressively higher layers in the sequence gradually contain more and more fossils. Describe in detail what you would do to determine if this is evidence of a previously undiscovered mass extinction event.

Answer points:

The first thing you should do is to look carefully at the suggested extinction horizon to make certain that it is not an unconformity (i.e. one of those horizons representing missing strata; remember Hutton’s work?); you could assume that ‘continuous’ means just that, but it’s best to check.

Whether you do the above step or not, you must collect representative samples of the questionable horizon and have them age dated (Here, you must say a few words about how dating works for this scenario). Having obtained a date, you then must go through the records to determine if rocks of that time have ever been previously determined to show mass extinction evidence.

Whether the horizon is new or not, you must define as much as possible of the character of that horizon. Suppose you find (above) that there is another extinction date very close or even the same to the number you got; if this previous one has been defined as one showing clear evidence supporting an impact event (Ir spike, global fires, tektites, lots of ‘rock ash’, etc.) and your horizon shows none of that stuff, you’ve got some explaining to do!

3. In Mid-Cretaceous times, only about 12% of the globe was dry land, thus seas were abundant. Seas were relatively warm and all life –especially marine life – was thriving. At the end of Cretaceous there was a terrific mass extinction of all life, terrestrial and marine. At just about this time, India was situated over a mantle plume (creating a dramatic hot spot) and the Deccan Traps were erupted. No matter what you (or I) might think about an impact cause for the mass extinction, a host of scientists still believe it was caused (totally) by the Deccan Trap eruptions. Your task is to critically review the hard evidence collected from the geologic record, and convince that host of scientists that they are wrong.

Answer points:

Since this is to be based on hard scientific evidence, by far the easiest thing to look at is:

Ir anomaly:

The Ir in question is found at the 66 mya horizon all over the world, thus must have an atmospheric source.

This is a sensitive indicator of extraterrestrial matter simply because there’s little Ir on Earth’s surface – nearly all is at Earth’s core. In order to test how much Ir might be brought to surface by a volcanic eruption (even flood basalt), we need to see how much comes from a very deep source. That would be via a mantle plume. The usual source to test is the mantle plume beneath Hawaii and the products from it. Recent Hawaii lavas contain Ir at 0.28 to 0.40 ppb; Deccan Trap lavas contain 0.006 to 0.026 ppb. The partition of Ir is very strongly to the liquid phase – hardly any can be volatilized. The 66 mya horizon contains, on average, 5-7 ppb. No atmosphere associated with Deccan Traps magma could account for that content.

[obviously, other arguments could be made, but they all have some speculation attached]

PART B

Fill in the missing word(s). Please put your answers, in sequential order, in the answer booklet – NOT on this examination sheet. Each question is valued at 1 mark.

1. Carbon isotope age dating is useful only for C-containing items less than 100,000____ (number) years of age.
2. Products of early supernovae are part of your personal composition. _True____(True/False)
3. _Cuvier____ is known as the ‘Father of Catastrophism’.
4. Earth should expect one Zhamanshin-size asteroid to impact every __1 million years.
5. In determining the kinetic energy of a meteoroid, the most important factor is ___velocity________.
6. The average life span of any species is about __4 million_ (number) million years.
7. The southern block of Pangaea was called _Gondwana__________.
8. When there is a slow but continuous movement of both sides (relatively) of a fault, the motion is called ___shear______.
9. A compression seismic body wave travels at roughly ___6_______ km/s.
10. In the case of volcanic activity, the heat of _radioactivity__ is overwhelmingly more important than any other source.
11. Magma classified as MORB is most likely found in _mid-ocean ridges______ (geographic location).
12. One of the two most abundant dissolved volatiles in magma is _CO₂/H₂O___.
13. The only flood basalt with which humans have any experience erupted at _Laki, Iceland_____ (location and country).
14. Stothers and Rampino postulated a periodicity of approximately _33±3 (number) million years for plateau basalt eruptions, magnetic field reversals and mass extinctions.
15. The action of water, together with _CO₂___________ from the atmosphere is largely responsible for production of caves in limestone.
16. Underground caves, stranded in the zone of aeration above a dropping water table, may lead to large circular depressions called _sinkholes_________.
17. All other things being equal, air density is lowest on a _hot__ (hot or cold) day.
18. The most mass of the atmosphere is in the _troposphere____ zone.
19. The percentage of incoming radiation that is reflected by a natural surface is called _albedo_______.
20. Mount St. Helens erupted in 1980, emitting approximately __1_____ cubic kilometers of material from its magma chamber.

PART C

Answer any 3 of the following 5 questions. Each question is valued at 10 marks.

1. (a) Earliest Earth had a greenhouse atmosphere loaded with CO₂. Explain why that changed and where the CO₂ went. [7]

Earliest atmospheric CO₂ decreased because of Earth’s cooling and development of continents, whose weathering provided Ca; calcite formed in oceans took the CO₂ out of circulation as limestone [ some reduction also because of photosynthesis]

(b) If major volcanic eruptions occurred nearly every year for a century, what might happen to global climate? [3]

If eruptions were quite basalt, only a CO₂-strengthened greenhouse would result (warmer climate). If the eruptions were explosive, initial result would be cooling from dust, then later warming from CO₂.

2. (a) Define latent heat. [4]

Amount of heat energy released or absorbed per gram of material undergoing a simply phase change.

(b) Explain the sequence of events that turns an African storm into a hurricane hitting the east coast of North America. [6]

Evapotranspiration from plants adds water to air and local thunderstorms develop. As these small storms sweep into the Atlantic near the tropics and over min. 26^oC water:

- water vapor rises, condenses, released large amount of latent heat

-Earth’s rotation ‘pushes’ storm westward

-adiabatic cooling is important to note

- water warm so even more latent heat added

- go through the tropical storm/hurricane statistics

- on either side of equator, cyclical action possible

-continued progression westward plus growth due to latent heat addition

3. Sketch a map of an idealized tectonic plate and evaluate the volcanic hazards along each type of plate edge. [10]

Map has to show spreading, subduction and transverse boundaries; best sketch would be combined plan and section views. Show volcanoes developing on continent side of subduction zone, along break of spreading center, and very minor eruption (‘leaks’) along transverse boundaries (in fact, inclusion of transverse is not mandatory to the answer). In evaluation, indicate quiet eruption along spreading centers, violent eruptions at subduction volcanoes. Something like cinder cones would be all you’d get at transverse breaks.

4. Explain how hot mantle rock can melt from (i) changes in temperature, (ii) changes in pressure, (iii) water content. [10]

Best to use a P-T sketch to show solidus-liquidus, and possibility to move across the boundary with change of P and T. For water, need to explain that water acts as solvent to lower melting points.

5. (a) Explain three ways that humans have caused or triggered earthquakes. [6]

Hydroelectric water dams, removal of groundwater (collapse), injection of water or other liquid (lubrication of fractures). I suppose an underground nuclear explosion would have to be accepted too.

(b) What type of fault motion best characterizes the “Basin and Range” area of the western US states in this century? [1]

Extension, so ‘strike-slip’.

(c) What Hawaiian volcanic processes cause earthquakes. [3]

Fracturing of rock ahead of upward-moving magma, and expansion (or just change of shape) of magma chamber.

PART D

Write complete definitions of each of the following; use point form if you wish. Each question is valued at 3 marks.

1. Supercell

Individual thunderstorm cells have coalesced into one huge storm with common rotation properties. Usually tiled forward and with associated tornadoes.

2. Kinetic energy

Quoting the formula was best: E=(1/2mass)x(velocity)².

3. Radioactivity

Breakdown of atomic nuclei by emission of particles and/or energy.

4. Strain

A change in form or size or position due to an external force or stress.

5. Farallon Plate

A previously existing oceanic plate which has now mostly subducted beneath the western edge of the North American Plate.

6. Rodinia

A supercontinent which assembled just prior to Cambrian time.

7. Aerolites

Commonly known as ‘stones’, these are the most common meteorites consisting of silicates, oxides and scattered metal phases. Usually divided into chondrites and achondrites.

8. Tektites

Rounded spherules of silicate glass formed from the melt of impacts.

9. Supernova

The eruption of a star that releases tremendous energy plus produces heavy elements.

10. Unconformity

Horizon representing a time during which no rocks were deposited (i.e. a continuous sequence was broken); may be represented by a weathered horizon, upper and lower sequences with different relationships, etc.