Mid Ocean Ridges Flashcards
Mid Ocean Ridge =
Long, seismically active submarine ridge system situated in the middle of an ocean basin and marking the site of the upwelling of magma associated with sea-floor spreading
Seismically and volcanically activity
Total length of MORs across the Earth
65,000km
Bathymetry
MOR itself is 2-3km higher than mature ocean floor which is at approx. 5km depth
As you travel further away from the MOR, depth increases
- older, colder, more dense
Free Air Anomaly
Matches bathymetry
Close to 0 as MOR are in isostatic equilibrium
Only small-scale features are uncompensated
- perhaps flexural support?
N.B. The FAA does not tell you what TYPE of isostasy is occurring but if isostasy is complete i.e. in equilibrium the FAA = 0
Bouger Anomaly
Large -ve bouger anomaly
Suggests the elevation of the MOR is supported by a mass deficit i.e. a low density body at depth
= PRATT
Heat flow
Elevated above MOR and decreases with increasing distance away from MOR
Evidence of hydrothermal activity in MOR regions
= hot, low density material
= PRATT
What type of partial melting?
Decompression melting
- the adiabatic cooling gradient is greater than the solidus
- upwelling of the mantle causes a large decrease in pressure, temperature remains fairly similar
= BASALTIC MELT RICH IN IRON AND DEPLETED IN MG
What are the main constituents of igneous oceanic crust?
Basalt - extrusive
- cools quickly, fine grained
Gabbro - intrusive
- cools slowly, coarse grained
Mafic igneous rocks
- mostly plagioclase and pyroxene
How can we determine oceanic crustal structure?
Seismic evidence (indirect) Ophiolites (direct) = fragments of ancient oceanic crust
0-1km EXTRUSIVES (basalt)
1-2km INTERFINGERING/UNDULATING TRANSITION AND SHEETED DIKES
2-3km INTRUSIVES (gabbro)
Characteristics of fast spreading slopes
Rapid adiabatic rise of mantle material
Continuous melt generation
Gentle slopes
Lack prominent rift valley at crest
Ridge has an axial high flanked by small scale normal faulting and volcanoes
Characteristics of slow spreading slopes
Slow adiabatic rise of mantle material
Cooling by vertical heat conduction inhibits melting
= Intermittent melt generation
Steeper slopes
- extension in part taken up by magmatism and by a greater proportion of tectonic faulting
Prominent rift valley at crest
- 2km deep, 30km wide
Flanks formed by 200m high NORMAL faults
Cooling and subsidence
Older = colder = subsides more
Direct relationship between depth and age
Active hydrothermal circulation
Circulation of well known, high temperature vent waters near ridge crests
Passive hydrothermal circulation
Low temperature diffuse flow of water through sediments and buried basalts
