Ch 2

Question 1

The solar system begins: rotating spherical cloud of gas, ice, dust, and debris

Answer 1

• Particles accrete under gravity into a cloud
• The cloud contracts, speeds up, and flattens into a disk

Question 2

The Sun

Answer 2

• Accumulates by accretion most of the disk’s matter (H and He)
• Central temp increases beyond 10^6 C
• Nuclear fusion: H -> He + heat

Question 3

How Planets Form

Answer 3

• Gravity sorts the cloud into rings
• Gravity sorts the rings -> planetesimals -> planets
• EM radiation scrubs gas/liquid from terrestrials (close, rocky)
• Giant planets collect and retain gas and liquid (distant, gassy)

Question 4

International Astronomical Union Definition of a Planet

Answer 4

1) Elliptical orbit
2) Large and dense enough to become spheroidal
3) No other planets or planetesimals in its orbit
(Pluto fails condition 3 -> eccentric orbit and crosses Neptune)

Question 5

The Moon

Answer 5

• Mars-sized body collides with Earth (moon is chemically similar to Earth’s crust and mantle)
• Gases and liquids scrubbed
• Less dense than Earth (impact did not disturb Fe-rich core)

Question 6

Earth History

Answer 6

• Accretion mostly ceases ~4.6bil ya
• Processes of planet formation creates tons of heat
• Impact energy
• Decay of radioactive elements
• Frictional energy from differentiation into layers under gravity

Question 7

Differentiation

Answer 7

• Fe melts at ~1000C
• Liquid Fe is denser than the surrounding rock, descends to Earth center due to gravity
• Low-density melt displaced by Fe melt and rises (forms solid crust and oceans/atmosphere)

Question 8

Major events in Ga

Answer 8

4.4 Ga: large oceans, small continents
3.5 Ga: life (photosynthetic bacteria)
2.5 Ga: supercontinent (first of four)
1.5 Ga: plate tectonics

Question 9

Earth Layers

Answer 9

• Differentiated based on increasing density
• Fe-rich core 7000km in diameter (solid inner core = 2450km in diameter, liquid outer core = 4550km thick)
• Liquid outer core viscous convention currents = magnetic field
• Mantle surrounds core = 2900km thick (83% of Earth’s vol, 67% of mass)

Question 10

Layers can be described in terms of

Answer 10

• Different density (different chemical and mineral compositions)
• Different strength (lithosphere overlies asthenosphere and is rigid, asthenosphere can flow)

Question 11

Elastic deformation

Answer 11

reversible/recoverable -> object returns to its original shape

Question 12

Ductile deformation

Answer 12

permanent -> stress applied over long time or at high temperature

Question 13

Brittle deformation

Answer 13

permanent -> stress applied v quickly to shatter or break object (i.e earthquake fault)

Question 14

Asthenosphere

Answer 14

• ~250km thick
• Daylights at MORs
• Facilitates Earth’s oblate-spheroid shape
• Continents float by isostasy (buoyancy of solids)
  > over vast periods of time
  > relatively mobile asthenosphere accommodates isostasy

Question 15

Internal Sources of Energy

Answer 15

1. Impact energy
2. Energy of differentiation under gravity
3. Radioactive isotopes

Question 16

Impact Energy

Answer 16

• Lots of smaller bodies hit Earth early after formation, converting kinetic energy into thermal energy

Question 17

Energy of differentiation under gravity

Answer 17

• As earth is pulled to smaller + denser mass, frictional energy is released as thermal energy

Question 18

Radioactive isotopes

Answer 18

• decay and release thermal energy
• Early Earth had more short-lived radioactive elements and therefore much greater thermal energy production
• Internal heat conducts to surface v slowly

Question 19

Total internal heat drives:

Answer 19

• Plate tectonics
• Earthquakes
• Volcanic eruptions

Question 20

Dev of Plate Tectonics Concept

Answer 20

1620: Francis Bacon - Africa and South America look like they fit together
Late 1800s: Eduard Suess - supercontinent Gondwanaland
1915: Alfred Wegener - Pangaea + continental drift
Earthly 20th century opposition: no drift mechanism (asthenosphere not yet mapped seismically)
20th century: ocean crust magnetism, oceanic crust extrudes at divergent boundaries (absorbed at convergent boundaries)
1960s: theory of plate tectonics developed and widely accepted

Question 21

Earth’s Magnetic Field

Answer 21

• Origin in Fe-rich outer core
• Well-modelled as bar magnet
• North and South poles
• Magnetic pole axis now inclined 11º from rotation axis (both axes meander, mag axis +/- 10º every century, mag axis reverses every ka to Ma taking a few ka for reversal, mag polarity stored in volcanic and sedimentary rocks)

Question 22

Magnetization of Volcanic Rocks

Answer 22

• Oceanic crust magnetic pattern: extruded asthenosphere at divergent boundaries cools (from >1200C to <550C, Fe minerals assume polarity of the ambient magnetic field solid oceanic crust stores a history of Earth’s mag polarity)

Question 23

Magnetized Patterns on the Seafloors

Answer 23

• Oceanic crust parallel bands of magnetized in mirror symmetry across divergent plate boundaries (bands parallel to ridges)
• Alternating polarity bands corresponds to elapsed time b/w mag field reversals (determine historical divergence rates by rock ages and width of bands)

Question 24

Deep earthquakes

Answer 24

• Most earthquakes have shallow hypocentres at depths <25km (v shallow at hot spots and divergent boundaries)
• Deep hypocentres along inclined planes to depths up to 600km at convergent boundaries (hypo centres give an image of subducting plates)

Question 25

Ocean Basins

Answer 25

• Oldest rocks on ocean floor are ~200mil y/o
• Are young features - continually being formed at MORs and destroyed at subduction zones
• Sediment on seafloor is v thin at MORs and thicker near trenches

Question 26

Ages from hot spot plumes

Answer 26

Hot spot: anomalous point of hotter, lower density in mantle (rises and melts near the asthenosphere)
- Rises through lithosphere as magma (volcanoes form on overlying crust)
- A moving plate gives a line of extinct volcanoes increasing in age away from hot spot

Question 27

Ocean is ~3.7km deep with 2 areas of exception:

Answer 27

Oceanic mountain ranges: volcanic mountains that form at spreading centers, where plates pull apart and magma rises to fill, sea mounts
Narrow trenches: extend to depths of >11km, tops of subducting plates turn downward to enter mantle

Question 28

Seafloor depth

Answer 28

Systematic increases in seafloor depth with seafloor age, moving away from MORs
- As oceanic crust gets older, it cools and becomes denser, therefore sinking a little lower into mantle
- Weight of sediment on plate also cause it to sink a little into the mantle

Question 29

Fit of continents

Answer 29

• At 1800km water depth, outlines of continents match up very well
• 1800km is also ~ the boundary b/w low density continental rocks and high-density oceanic rocks
• Continental masses cover ~40% of Earth’s surface and ocean basins cover ~60%

Question 30

Changing Positions of Continents

Answer 30

220mil ya: Pangaea covered 40% of Earth
180mil ya: Pangaea broke up into Laurasia and Gondwanaland
135mil ya: North Atlantic ocean opened; India heads towards Asia
65mil ya: south Atlantic Ocean opens; Africa and Europe collide
Present: India colliding with Asia, Eurasia and North America separated; Australia and Antarctica distant

Question 31

The Grand Unifying Theory

Answer 31

Plate tectonics require time perspect of mils and bils of years
- Plate movement may be 1cm/yr -> 75cm in human lifetime
- 1cm/yr is 10km in 1mil years
Plate tectonics also require size perspect of continents and plates

Question 32

Uniformitarianism:

Answer 32

Natural laws are uniform through time and space, the present is the key to the past

Question 33

Currently modified actualism:

Answer 33

Rates of Earth processes can vary
- Study the present to understand the past and make probabilistic forecast of future