Earth outer shell, seismology, interior Flashcards
What is differentiation (in terms of planetary formation)?
The process by which random chunks of primordial matter were transformed into a body whose interior is divided into concentric layers that differ from one another both physically and chemically
How long ago did the Earth’s crust form?
4.4 billion years ago
How did the Earth’s crust form? maybe a bit simple
Lighter rocks floated to the surface of the magma ocean
When was the earliest evidence for water at the surface of the Earth?
3.8 billion years ago
When was the earliest evidence for primitive life?
3.5 billion years ago
What are cratons?
Pieces of old (>2.5Ga) and thick (>200km) continental lithosphere.
The stable infrastructure of the Earth’s continents is assembled around cratons
What are:?
- Shields
- Platforms
- Unsedimented cratons
2. Sedimented cratons
Why was Earth’s heat flow nearly 3x greater than it is today during the ARCHEAN?
Greater concentration of radioactive isotopes
Residual heat from Earth’s accretion
What did the high temperatures of the Archean result in (in terms of crust formation?)
- Greater degrees of melting at oceanic spreading ridges
- Formation of oceanic lithosphere with thick crust
- Highly depleted mantle
- Lithosphere would not subduct deeply because of its buoyancy
- Removal of denser melt further lowered density of residual mantle
What are cratonic mantle roots probably composed of?
Buoyantly subducted slabs of a highly depleted oceanic lithosphere
How do deep mantle roots increase the stability of cratons?
Make them less susceptible to tectonic thickening by collisions or destruction by subduction
When did the first continents form?
During the Archean
What is the Juvenile crust?
Younger Continental Crust that has been added to the cratons since the Archean
What % of the worlds present continental crust formed during the Archean?
Only 5-40%
What is the thickness of juvenile crust?
Half that of cratons
How was the Juvenile crust formed?
- At continental magmatic arcs & ocean island arcs (formed within subduction zones)
- In lesser amounts by mantle plumes (but under lower temperature conditions)
Why is it surprising that continental growth has occurred in 3 major periods of time?
Models of monotonous secular cooling of the core and mantle suggest a smooth distribution of the ages of the juvenile crust formed as a result of partial melting from the mantle
What are the 3 major explanations for the episodic age distribution of the continental crust (CC)?
- Incomplete sampling of CC
- Uneven preservation of CC
- Non-uniform crustal generation
Theory for uneven age distribution of CC
- 2 layer mantle convection
- No transfer of material across phase boundary between upper and lower mantle (670km discontinuity)
- Large volumes of subducted slabs accumulate at boundary at discrete periods
- Results in massive overturn of mantle
- Brings v. high T lower mantle to shallow depths produce voluminous melting at upper mantle depths
- Overturns became more frequent as Earth cooled but with reducing max’ temp of upper mantle each cycle
- Earth’s cooling resulted in change from layered to whole-mantle convection regime 1.2Ga
What occurred in the following time periods? >4.5Ga 4.5 - 4 Ga 4 - 2.5 Ga 2.5 - 1 Ga <1 Ga
Planetary embryo collisions Magma ocean & differentiation. Crust formation Formation of cratons Start of primitive plate tectonics Start of modern plate tectonic system
How do we determine the structure of the continental crust?
Controlled source seismology
Seismic refraction
What is the Moho discontinuity?
A strong velocity contrast at the base of the crust
From <7km/s to >8km/s
What is the most common model of the CC
Three-layer model
Divides CC into upper, middle and lower crust
What are the compositions of the upper, middle and lower crust?
Upper = felsic (granite/granodiorite) Middle = intermediate - felsic Lower = mafic (comp' close to basalts)
