Structure of Earth and magmatic processes Flashcards
What are the steps of earth formation?
Planetesimals grow from collisions
Irregular proto-shape earth develops Interior heats becomes soft
Gravity re-shapes proto to sphere
Interior separates to core and mantle
What is the LHB?
a spike in asteroid bombardment 4 billion years ago
Who was one of the first to try and age earth?
Lord kelvin 1862
How did lord Kelvin try do date earth?
based on calculation of how long earth would take to cool if it began as a molten mass
What was lord Kelvins estimate for the age of the earth?
20-400 million years old (wrong by significant margin as actual= 4.5 billion)
What is the reason for why lord Kelvin might have been wrong in his ageing of earth?
He didn’t know of the existence of radioactive elements which decay and release energy affecting internal heat
What does temperature inside planets getting above the melting point of most minerals mean?
separation into more and less dense layers
What are the layers of earth?
Crust
Upper mantle
Lower mantle
Outer core
Inner core
How will silica and iron abundance be affected by earth layering?
Silica- most abundant at crust decreasing towards core
Iron- least abundant at crust increase with depth
What is chemical differentiation?
When different minerals have different compositions and densities allowing physical partitions to occur
How can we tell the interior of earth must be much denser than the crust?
Average earth density= 5.5g/cm3
Crust range-2.5-3g/cm3
Interior must be more dense to bring up average
How can seismology and seismic waves be used to probe earths interior?
measuring the time required for seismic waves to travel through the earth by different paths
What are the 2 main types of seismic wave?
Primary = P
Secondary = S
What type of movement are P waves?
Compressional
What is the speed of P waves?
Propagate fastest
6.5km/sec
What states of matter can P waves travel through?
Solid
Liquid
What type of movement are S waves?
Shear
What state of matter can S waves travel through?
Solid
What is the speed of S waves in the crust?
4km/sec
What can a shadow zone with no S waves show us after a quake? (opposite side to quake)
earth must have a liquid core
What is the crust?
less dense rigid layer of rock forming earths exterior
What is the base of the crust defined by?
Mohorovic discontinuity (Moho)
What happens to seismic velocity and why at the moho?
distinct increase due to more dense material
Does the moho remain constant throughout crust?
No it is variable
What are the characteristics of oceanic crust?
5-10km thick
Denser so faster seismic velocity
Subducts
Basaltic rock
What are the characteristics of continental crust?
20-90 km thick (thicker under mountain, thinner under continental shelf)
Not dense enough to subduct
Granitic rock
What can the crust be split into?
Oceanic
Continental
What is the upper mantle like?
denser than the crust but still silicate rich
What will the topmost part of the upper mantle be like?
rigid
What will the topmost part of the upper mantle and the crust form?
tectonic plates
What is the top rigid upper mantle called?
The lithosphere
How thick is the ocean lithosphere?
50-140km
How thick is the continental lithosphere?
40-280km
What starts to happen when you travel below the lithosphere?
Temp and pressure increase allowing some flow
Why is drilling into earth to see the layering not possible?
Complex and costly with deepest hole being 12km which doesn’t pass into deeper layers
What is the ductile region in the upper mantle?
the asthenosphere
What is the boundary between the lithosphere and asthenosphere?
1300*c isotherm
What is the remaining upper mantle? (not lithosphere or asthenosphere)
mesosphere
What happens to material as you go deeper into earth?
more dense less viscous
What happens to the structure of rock as you travel through the mantle?
Changes twice as 410 and 660km resulting in abrupt density change
What marks the boundary between the upper and lower mantle?
the 660km disconformity
How deep does the lower mantle go down till
3000km
What is at the base of the lower mantle?
200km thick heterogenous layer of anomalously low seismicity (D layer)
What might the “D layer” at the base of the lower mantle be made of?
ancient subducted plates
What is the core-mantle boundary a major transition between?
silicate mantle changes to the denser iron rich metallic iron core
What form will the iron rich metallic alloy be in the outer core?
liquid as temperatures are 5000*c
Why is the inner core solid even though its so hot?
due to the immense pressure exerted on it
What is the composition of the crust?
Si/Al rich
Na/K/Ca rich
What is the composition of the mantle?
Fe/Mg rich
Si/Al poor
What is the composition of the core?
Nearly pure Fe/Ni
What % of earths surface is active volcanism confined to?
<0.5%
How much of active volcanism/magma is ejected at subduction zones?
20% - 6km*3/yr
How much of active volcanism/magma is ejected at intraplate?
10%- 2.5km*3/yr
How much of active volcanism/magma is ejected at mid-ocean ridges?
70%- 21km*3/yr
How much of magma is ejected as lava?
10%
Where does the other 90% of magma occur in the crust?
solidifies in crust as igneous intrusion
How does the mantle flow?
as it is not solid or liquid it flows by convection
What type of rocks is the mantle composed of?
Ultrabasic
Ultramafic
What will basic magmas solidify into?
basaltic lava
gabbroic intrusions
What happens to pressure with depth?
it increases due to the weight of the overlying material
What happens to temperature with depth? (earth structure)
Temp increases due to transfer of heat from planet interior to surface with insulating properties of overlying material
What are the 2 sources heat in earth?
Early accretion and differentiation
Heat from radioactive breakdown of unstable nuclides
What proportion of earths heat comes from breakdown unstable nuclides?
75-90%
How is heat transported through earth?
Conduction
Convection
What is conduction?
transfer of kinetic energy from hotter atoms to adjacent cooler atoms
Where does conduction occur in earths structure?
the rigid sections (lithosphere)
What is convection?
movement of ductile material due to density differences caused by thermal or compositional variation
Where does convection occur within earths internal structure?
outer core
mantle
What is the geothermal gradient?
variation of temperature with increasing temperature and pressure
How does the presence of thinner lithosphere affect convection?
allows convection heat transfer to shallower depths resulting in a higher geothermal gradient
How is the geothermal gradient useful?
it graphically shows how and where the mantle undergoes partial melting to form magma
What is solidus?
point at which rock begins to melt
What is liquidus?
where the rock becomes 100% melted
What happens when the geothermal gradient crosses solidus?
you get partial melting
How does the geotherm cross the solidus?
changing the position of the geotherm (thinning lithosphere)
Changing the position of the solidus
How does thinning lithosphere move the geotherm?
convecting asthenospheric mantle rises adiabatically to fill the gap
What does adiabatically mean?
without losing heat
How can the solidus location be moved?
addition of volatiles (water) lower solidus (and liquidus) to promote partial melting
How do volatiles like water end up in the mantle?
subducting oceanic crust which is saturated with water
