Mid ocean ridges A2

Question 1

Evidence from direct surveying

Answer 1

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

Question 2

Types of remote surveys

Answer 2

Heat flow surveys

Gravity anomalies

Conflicts in survey data

Question 3

Heat flow surveys

Answer 3

highest heat flow above axial rift
>
indicates upwelling of M (by Partial melting due to decomp)

Question 4

Gravity anomalies

Answer 4

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

Question 5

conflicts in survey data

Answer 5

sometimes multiple types of remote surveys leads to conflicts

often due to H20 present

also may do electromagnetic/ seismic survey

Question 6

Ophiolite + ophiolite suit definition

Answer 6

ophiolite- section of earths O crust that have been tectonically moved moved/obducted onto C crust

suits- large area of ophiolites

Question 7

rocks that make up ophiolite suits

Answer 7

basalt dolerite gabbro maybe peridotite

chefs hat diagram on its side

Question 8

formation of an ophiolite suite

Answer 8

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)

Question 9

How to calc rate of spreading

Answer 9

date rocks + measure distance from ridge

distance/ time

(*2 as happening on both sides)

cmyr^-1

Question 10

What determines rate of spreading

Answer 10

amount of upwelling + partial melting > ridge Push

amount pf subduction > slab pull

C currents may play a role in plate movement

Question 11

effect of MOR spreading on global sea levels

Answer 11

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

Question 12

Magma formation at MOR

Answer 12

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

Question 13

2 types of MOR

Answer 13

fast flowing

slow spreading

Question 14

Slow spreading ridge

(see booklet for diagram)

Answer 14

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

Question 15

Fast spreading ridge

(see booklet for diagram)

Answer 15

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

Question 16

decompression melting diagram

SEE IN BOOKLET

Answer 16

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)

Question 17

Black smoker description

Answer 17

formed from deposits of chalcophiles e.g. iron sulphide

often in areas of volc activity e.g. slow spreading MOR

Question 18

how are black smokers formed ?

Answer 18

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

Question 19

Mining back smokers

Answer 19

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

Question 20

Metasomatism + example

Answer 20

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

Question 21

heat flux + flow definition

Answer 21

Flux = rate of energy transfer

Flow = total amount of energy transferred

Question 22

Heat flow in Ocean

Answer 22

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

Question 23

White smokers

Answer 23

lower temp- 300 vs 400

generally further from the ridge

different minerals e.g. barium, calcium and silica