Mid ocean ridges A2 Flashcards
Evidence from direct surveying
interior cannot be directly studied as molten
investigate lithosphere directly adjacent e.g. O floor + O core complexes
deep ocean subs take range of measurements e.g. temp, density, comp + care samples
No matter where O floor sampled= same structure
All the I rocks form from magma rising + cooling at MOR
Types of remote surveys at MOR
Heat flow surveys
Gravity anomalies
Conflicts in survey data
Heat flow surveys results at MOR and what it indicates
highest heat flow above axial rift
>
indicates upwelling of M (by Partial melting due to decomp)
Gravity anomalies at the MOR
indicates Low density zone in upper Mantle below MOR >
indicates deficit of mass >
Due to upwelling of less dense (due to partial melting) material
decompression > expand > less dense
negative bouguer anomaly
conflicts in survey data
sometimes multiple types of remote surveys leads to conflicts
often due to H20 present
also may do electromagnetic/ seismic survey
Ophiolite + ophiolite suit definition
ophiolite- section of earths O crust that have been tectonically moved moved/obducted onto C crust
suits- large area of ophiolites
rocks that make up ophiolite suits
basalt dolerite gabbro maybe peridotite
chefs hat diagram on its side
formation of an ophiolite suite
ocean curst subducts below C crust
subduction continues
O crust is trapped between 2 sets of C crust
C crusts collide and some of the O crust is trapped/ scaped off and forms ophiolite suite
(End of the Wilson cycle)
How to calc rate of spreading
date rocks + measure distance from ridge
distance/ time
(*2 as happening on both sides)
cmyr^-1
What determines rate of spreading
amount of upwelling + partial melting > ridge Push
amount of subduction > slab pull
C currents may play a role in plate movement
effect of MOR spreading on global sea levels
fast flowing ridges create lots of magma in ocean > decrease space/vol in ocean > sea level rise > H20 displaced > flooding
FOCUS
EXTRA- O crust move away from MOR , cools, sinks, more dense, increases vol of ocean basin + decreases sea level
Magma formation at MOR
as pressure drops> decomp melting occurs as mantle moves freely so MP falls (e.g. peridotite)
STILL ONLY PARTIAL MELTING
about 3% for ever 10km
adiabatic cooling > pressure drops > expands > vol increases > cools
amount of magma depends on latent heat of fusion > takes a lot of energy to melt silicic material > partial melting
Seismic studies show that some magma chambers have thin layer of molten rock > mush zone
melted zone only 10ms thick
magma chambers erupt continuously> some magmas close to surface cool due to water infiltration
2 types of MOR
fast flowing
slow spreading
Slow spreading ridge
(see booklet for diagram)
e.g. MAR
have large wide rift valleys (10-20km)
very rugged terrain- blocky abyssal hills
some covered by young lava flows
+some faults
DISCONTIOUS magma chamber (each eruption is distinct)>insufficient partial melting + may be mush zone
some have multiple small volc cones
Fast spreading ridge
(see booklet for diagram)
e.g. East pacific rise
high elevation
dome shape topography > layers of lava
deep hot rock rise quickly > heat cant escape > lithosphere hotter > MOR cant subside due to rising magma > no rift valley > smooth flanks
CONTINUOUS MAGMA CHAMBERS
pillow lavas + rounded abyssal hills
decompression melting diagram
SEE IN BOOKLET
Geotherm = rate of change of temp with depth (geothermal gradient)
liquidus = everything to the right is completely liquid
solidus = everything to the left is completely solid
reps MP shows increase with depth due to pressure
between= mush zone (some crystals + some liquid)
Black smoker description
formed from deposits of chalcophiles e.g. iron sulphide
often in areas of volc activity e.g. slow spreading MOR
how are black smokers formed ?
hot rock rises from mantle>
submarine hot spring>
H2O superheated>
new hydrothermal fluid>
high pressure stops boiling>
far less dense>
rises quickly>
water looks like black smoker
minerals precip (helped by chemosynthetic bacteria)
build chimneys
form rich deposits
chimneys collapse>
block vent>
percolate back through>
new chimney
Mining back smokers
rich source of minerals e.g. gold zinc mickle and copper
Japan = first to successfully mine
developing deep sea mining tech>
crush + grind rock>
suck up>
dup unwanted sed>
poses threat to ocean
often mined if found in ophiolite suits
Metasomatism + example
the hydrothermal chemical alter of rocks > by hydrothermal + other percolating fluids
by add/sub of minerals
chem in OG react with H2O > dissolve form new min > replace OG crystals
e.g. Olivine + H2O = serpentine (increases vol + reduce density, accounts for size of MOR)
exo reaction raises temp of H20
heat flux + flow definition
Flux = rate of energy transfer
Flow = total amount of energy transferred
Heat flow in Ocean-heat transfer
hot H2O from H vents spirals up by convection
(12TW worldwide, compared to 180000TW from sun)
most goes to O crust then sea water at MOR through floor/ black smokers
cool H20 percolates into crust along fractures + faults
crust cool as H2O gets warmer
White smokers
lower temp- 300 vs 400
generally further from the ridge
different minerals e.g. barium, calcium and silica
