April 2015
Final Examination – Exercise Template
UNDERSTANDING PLANET EARTH
EPSC 201, SECTION 001
April 22, 2015; 2PM
Examiner: / Alfonso Mucci / Assoc Examiner:Student Name: / McGill ID:
INSTRUCTIONS: (Examples)
· This is a CLOSED BOOK examination.
· NO documentation allowed
· Calculators are NOT permitted.
· This examination consists of thirty questions (30) of equal value. Answer twenty (20) questions ONLY. If you answer more than twenty (20) questions, only the first twenty (20) answers will be graded.
· Answer in the official language of your choice (English or French).
· This examination is PRINTED ON BOTH SIDES of the paper, i.e., five pages counting this one.
· This examination paper MUST BE RETURNED
· Answer in Classroom (Examination) Book.
· Turn in the sheets of questions with the exam booklet
1)
Explain, with two lines of evidence, a theory for the formation of the Moon.
Constrained by the age of the oldest Moon rocks recovered by the Apollo missions (~4.47 Ga), it has been proposed that sometime soon after or perhaps during Earth’s differentiation (because the Moon is mostly felsic and has a very small core), a Mars-sized protoplanet slammed into the Earth. As a result the colliding body disintegrated, along with a large part of the Earth’s mantle. While some of the material was melted and/or vaporized, a ring of debris was formed and retained within the gravitational field of the Earth formed.
OR
The Moon is made of silica-rich rocks (ejected following the planetesimal collision with the Earth) of similar chemical and isotopic composition as the Earth’s crust and its age corresponds with the time of Earth’s differentiation.
2)
Why does the buildup of heavy elements in large burning stars (“Red Giants”) not go beyond the formation of iron. In other words, why are elements heavier than iron not produced in burning stars?
In the largest stars the cycle of nuclear fuel depletion, renewed collapse, core temperature rise, and ignition of a less flammable nuclear fuel is repeated several times, but as the mass number increases, each merger leads to a smaller loss of mass and to the corresponding production of heat. Finally, the buildup cannot go beyond the element iron, because beyond iron heat must be added to the reaction for the nuclei to merge. The binding energy of elements with an atomic number larger than 26 is smaller than for iron. The reaction is no longer self-sustained.
3)
Most meteorites are believed to originate from the asteroid belt between the orbits of Mars and Jupiter. As a result of collisions with one another, their orbits can be modified and some pieces can enter the Earth’s gravitational field. Why are some of the meteorites that collide with the Earth made of nearly pure iron?
Iron meteorites are believed to be the cores of small planets or large asteroids that have been broken apart.
4)
Why is the Earth’s climatological equator different than its geographical equator?
Because, at this time, more than 65% of the land surface is concentrated in the northern hemisphere. As land masses have a lower albedo than water, the northern hemisphere is warmer and the climatological equator is north of the geographical equator.
5)
What causes seafloor spreading and how do we know that it is occurring?
Seafloor spreading is caused by extension of the oceanic crust and upwelling of magma from the asthenosphere. The formation of new seafloor is recorded by symmetrical magnetic anomalies across the inflated mid-ocean ridges where anomalously high heat flows can be measured. Seafloor spreading rates can also be estimated by measuring (laser ranging, GPS measurements) the annual increase width of the ocean basins.
6)
What is cleavage and how does cleavage develop in a mineral?
Cleavage is the preferential fracture plane of a mineral. Cleavage forms in directions where the bonds holding the atoms together in the crystal are the weakest.
7)
a) Where do natural diamonds originate from?
Diamonds form at very high pressures and temperatures, conditions that are only found at depths around 150 km below the Earth’s surface well within the mantle.
b) Why are they preserved at the Earth’s surface where they should normally be converted to graphite?
In order to keep the diamonds from converting to graphite at the Earth’s surface, the magma is brought up and cools very rapidly at the surface. Kimberlite magmas are typically rich in dissolved gases and froth to the surface rapidly.
8)
What is the seismic evidence that subduction is occurring? Explain your answer.
Extensional faulting of the subducting oceanic slab produces earthquakes to depths of 650km under the overlying crust in the so-called Wadati-Benioff zone. The earthquakes occur because the subducting slab is cold and retains brittle properties to great depths in the mantle.
9)
Compare and contrast stratovolcanoes and shield volcanoes. Why do they differ?
Stratovolcanoes are large cone-shaped (composite) volcanoes constructed of layers of lava flows and pyroclastic debris (tephra). In contrast, shield volcanoes are broad (low aspect ratio) and gentle domes. The difference reflects the differential viscosity (silica content) of their lava; silica-poor basaltic lavas are fluid and form thin sheets yielding shield volcanoes, whereas andesitic (more silica-rich) lavas are more viscous, they tend to break up as they flow, and often lead to explosive volcanism.
10)
Discuss three criteria to distinguish between immature and mature clastic sediments.
Immature clastic sediments have undergone little mechanical weathering (clasts with sharp edges and poorly sorted) and often rest near the area of origin. In contrast, mature clastic sediments contain well-rounded and often well-sorted clasts, reflecting long-range transport from their origin.
11)
How and why do rocks change texturally and chemically during metamorphism?
Metamorphic rocks originate from changes in the mineralogy and/or texture of a rock in response to modifications of its physical or chemical environment. These changes occur when the protolith is subjected to heat, pressure, differential stress (push, pull, or shear), and/or bathe in hydrothermal fluids. First, mineral grains in metamorphic rocks are typically large and interlock with each other rather than being cemented together like in a sedimentary rock (they undergo recrystallization with or without a phase change). Second, metamorphic rocks often display foliation, a parallel alignment of platy minerals and/or the presence of alternating light-colored and dark-colored layers. Finally, they contain metamorphic minerals, minerals that grow only under metamorphic temperatures and pressures.
12)
Explain the facies concept of metamorphism.
A metamorphic facies is a set of metamorphic mineral assemblages indicative of a certain range of pressure and temperature. Each specific assemblage in a facies reflects a specific protolith composition. A given metamorphic facies includes several different kinds of rocks that differ from each other in terms of composition (i.e., mineral content) but all rocks of a given facies formed under roughly the same temperature and pressure conditions.
13)
Name the five rock types that develop upon prograde metamorphism of a shale?
Slate, phyllite, schist, gneiss and migmatite.
14)
Under what conditions do high grade metamorphic rocks undergo retrograde metamorphism?
Retrograde metamorphism occurs upon the exhumation (uplift) of metamorphic rocks in the presence of water/fluids. Under cold and dry conditions, retrograde metamorphism cannot occur since the reactions are too slow and the metamorphic grade of the rock is preserved even upon exhumation.
15)
How do primary seismic waves (P-waves) travel through the Earth?
P-waves are compressional waves which deform the material by changes in volume as they go through by pulses of compression and expansion, much like a sound wave or the movement of a loudspeaker. Compression waves can travel through solids, liquids or gases.
16)
What does an unconformity represent in terms of amount of deposition and time of deposition?
Unconformities represent periods of non-deposition or erosion. Whereas an unconformity is a physical feature we can identify in a rock sequence, we normally refer to the lapse of time recorded by an unconformity as a hiatus. In other words, during this period, no new sediment will accumulate and some of the pre-existing sediment may be eroded away. It is impossible to tell the duration of the hiatus unless we can correlate the individual beds with others where a complete sequence of deposition is preserved or by dating the individual beds.
17)
Using the basic principles of stratigraphy (superposition, horizontality, continuity) and cross-cutting relationships, describe the chronological sequence of events in the following cross-section (It also suffices to list them in chronological order starting with the oldest).
A- Deformed sediments G- Deformed sediments M- Deformed sediments
B- Deformed sediments H- Sediments N- Dike
C- Sediments I- Deformed sediments O- Dike
D- Deformed sediments J- Unconformity 1- Fault
E- Igneous intrusion K- Deformed sediments 2- Fault
F- Sediments L- Deformed sediments 3- Fault
Answer: G-E-2-K-D-L-A-M-(unconformity)-B-I-O-1-J-H-C-F-3-N
18)
Explain the four factors that determine whether folds or faults form during deformation.
Four factors determine whether a rock will deform plastically or break: the confining pressure, temperature, time (strain-rate) and the rock composition. The higher the temperature, the weaker and less brittle a solid becomes. An increase in confining pressure tends to keep a solid mass together in a single unit and hinders the formation of fractures. The lower the strain rate, the greater the tendency for ductile behaviour. There are two aspects to the composition, the water content and the mineral composition. The presence of water in interstitial voids of a rock (i.e., tiny spaces between mineral grains) enhances the ductile properties of a rock as it lubricates movement between mineral grains. The strength of individual minerals making up a rock also influences the overall strength of a rock.
19)
Draw a cross-section through a zone of continent-continent collision showing detachment and thrust faults. Explain your cross-section.
Compression upon continental collision leads to deformation, reverse and thrust faulting. Thrust faulting (e.g., shallow angle reverse faulting) can transport sheets of rocks hundred of kilometers on top of older rocks along a detachment fault. Movement on reverse faults pushes older rocks over younger ones, thereby shortening and thickening the crust.
20)
If a ship weighs 52,000 tons and is in isostatic equilibrium when 6,000 tons of cargo is added, what is the total mass of water that will be displaced by the ship after the cargo is added?
Answer: 58,000 tons
21)
22)
James Hutton, the “father of modern geology” put forth the principle of uniformitarianism. Explain this principle.
Processes we observe today also operated in the past and were responsible for the formation of the geological features we see in outcrops today. In other words that “The present is the key to the past”.
23)
Is there any evidence for extraterrestrial life on Earth? Explain.
Microscopic structures that look like modern bacteria filaments have been observed in meteorites. Likewise, the presence of strings of magnetite nanocrystals, observed in a meteorite believed to originate from Mars, are thought to have been formed by magnetotectic bacteria such as those that live on Earth today.
24)
25)
Describe and explain the evidence that bacteria oxygenated Earth’s atmosphere ~2.5 billion years ago.
Before 2.2 Ga, pyrite (iron sulphide) occurs as clasts in sediment, this would not be possible if the atmosphere contained a little oxygen since pyrite weathers rapidly in the presence of oxygen. The presence of red beds, clastic sedimentary rocks colored by the presence of bright red hematite (iron oxide) and the occurrence of banded-iron formations (BIF) deposits, the main source of iron ore today, attest to the presence of oxygen in the atmosphere after 2.2 Ga.
26)
Why is the absence of oxygen so critical in the accumulation and preservation of organic matter destined to be converted to oil, gas and coal? In other words, what is the usual fate of organic matter that accumulates in oxygenated environments?
To be preserved, organic matter must be deposited in an oxygen-poor environment, otherwise the organic matter would react with oxygen to produce CO2 or could simply be eaten by bottom-dwelling organisms such as worms and disappear. In the absence of oxygen, the degradation of organic matter is much slower and higher organisms cannot survive and are therefore absent.
27)
a) How is electricity generated from hydroelectric dams?
b) What two sources of energy are being exploited?
c) What are the drawbacks of hydroelectric power?
a) As water flows downslope, its potential energy converts to kinetic energy. The water flow drives turbines, which in turn drive generators that produce electricity.
b) Gravity (potential energy as water flows down a gradient) and sunlight (as heat evaporates water and rain redeposits it on the continents).
c) The construction of large dams floods large areas, disrupts the local hydrology, and destroy ecosystems. Furthermore, the degradation of organic matter in the inundated soils produces carbon dioxide and methane that contribute to the inventory of greenhouse gases.
28)
29)
Discuss black smokers in the context of metallic mineral deposits.
Seawater percolating through fractures in the mid-ocean ridge crests is heated and leaches metals from the fresh basalt. When the hydrothermal fluids come in contact with the cold seawater, the dissolved metals instantly precipitate as tiny crystals of metal sulphide minerals (e.g., Pb, Ni), creating black clouds called black smokers. The minerals in the cloud eventually sink and form a pile of nearly pure ore minerals around the vent. Black smokers are found at mid-ocean ridges where hydrothermal fluids at temperatures as high as 350ºC and carrying high concentrations of dissolved metal and sulphur debouch from chimneys.
30)
Course: EPSC-201 Page 9