Ch 2 Flashcards
The solar system begins: rotating spherical cloud of gas, ice, dust, and debris
- Particles accrete under gravity into a cloud
- The cloud contracts, speeds up, and flattens into a disk
The Sun
- Accumulates by accretion most of the disk’s matter (H and He)
- Central temp increases beyond 10^6 C
- Nuclear fusion: H -> He + heat
How Planets Form
- 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)
International Astronomical Union Definition of a Planet
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)
The Moon
- 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)
Earth History
- 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
Differentiation
- 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)
Major events in Ga
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
Earth Layers
- 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)
Layers can be described in terms of
- Different density (different chemical and mineral compositions)
- Different strength (lithosphere overlies asthenosphere and is rigid, asthenosphere can flow)
Elastic deformation
reversible/recoverable -> object returns to its original shape
Ductile deformation
permanent -> stress applied over long time or at high temperature
Brittle deformation
permanent -> stress applied v quickly to shatter or break object (i.e earthquake fault)
Asthenosphere
- ~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
Internal Sources of Energy
- Impact energy
- Energy of differentiation under gravity
- Radioactive isotopes
Impact Energy
- Lots of smaller bodies hit Earth early after formation, converting kinetic energy into thermal energy
Energy of differentiation under gravity
- As earth is pulled to smaller + denser mass, frictional energy is released as thermal energy
Radioactive isotopes
- 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
Total internal heat drives:
- Plate tectonics
- Earthquakes
- Volcanic eruptions
Dev of Plate Tectonics Concept
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
Earth’s Magnetic Field
- 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)
Magnetization of Volcanic Rocks
- 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)
Magnetized Patterns on the Seafloors
- 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)
Deep earthquakes
- 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)
Ocean Basins
- 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
Ages from hot spot plumes
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
Ocean is ~3.7km deep with 2 areas of exception:
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
Seafloor depth
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
Fit of continents
- 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%
Changing Positions of Continents
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
The Grand Unifying Theory
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
Uniformitarianism:
Natural laws are uniform through time and space, the present is the key to the past
Currently modified actualism:
Rates of Earth processes can vary
- Study the present to understand the past and make probabilistic forecast of future
