OCEAN/ESS 410
Class 9. Mid-Ocean Ridges
Dax Soule
Mid-Ocean Ridges
Dax Soule
Modern Map
What makes these so different?
From MacDonald 1982
Learning Goals
• Be able to sketch the structure of fast and
slow spreading mid-ocean ridges and describe
how they differ
• Describe how and why the structure and
across-axis bathymetry of slow and fast ridges
differ in terms of magmatic, tectonic and
hydrothermal processes that form the crust
A favorite mid-term question
Catalyst Questions
• List one example for both a slow and fast
spreading rate mid-ocean ridge
• Indicate on which you would expect for find
high temperature hydrothermal venting
• Indicate on which you would expect to find
the larger earthquakes
Oceanic
Crust
Stratigraphy
• Deep sea drilling program
• Dredging of fracture zone scarps
• Ophiolites
From Karson 2002
What’s the difference?
Gabbro
Basalt
Upper crust
Upper basalt – morphology
primarily pillow, but
lobate and sheet also
evident; variable
thickness; high porosity
A/B/C
C/D
From Karson 2002
Lower basalt – lobate and
sheet are common;
increased fracturing
and hydrothermal
alteration
Transition zone – fractured
sheet flows cut by
dikes; gradational
Sheeted dikes –
subparallel alignment;
~1m width; dip away
from ridge
Upper crust – Seismic layer 2
Multi-channel seismic (MCS) studies are able
to identify layer boundaries based on
impedance contrasts; tomography identifies
changes relative to reference model
Nature of transition from 2A to 2B is focus of
community debate:
1) Lithologic – boundary between highporosity basalt flows to low-porosity
sheeted dikes
2) Hydrothermal – alteration front within
upper extrusive volcanic layer
Fracturing and hydrothermal alteration
contribute to seismic anisotropy
Modified from Nedimovic and Carbotte 2008
Seismic Layer 3
Compare and contrast
• Examine these schematic drawings of fast and
slow spreading mid-ocean ridges
• List the ways in which the structure and
across-axis bathymetry of slow and fast ridges
differ in terms of magmatic, tectonic and
hydrothermal processes that form the crust
Pair up
Compare your answers
• Magmatic systems:
• Tectonics:
• Hydrothermal systems:
Share what you have observed
Fast-spreading < 8 cm/yr
seismic zone
Courtesy of D. Fornari
Slow-spreading 2-5 cm/yr
Temporary magma chamber
Plate Tectonics
island arc
trench
fracture
zone
trench
MOR
earthquakes
ocean crust
melt
sediments, igneous
crust & mantle
magma
adiabatically rising
mantle material
continental crust
earthquakes
Mantle
Mantle
melt
Magmatics
Slow spreading ridges
• Mush-filled chamber with
no melt lens
• Short lived AMC feeds
localized volcanic structures
within the axial valley
• Undifferentiated lavas
Fast spreading ridges
• Thin, narrow, sill like body
of melt overlying a thicker,
wider crystal mush zone.
• Steady state AMC
• Wide range of differentiated
lavas
• Large low velocity zone
extending to the base of the
crust
Tectonics
Slow spreading ridges
• Large rift valleys (10 – 20 km
wide)
• Rugged topography with relief
up to 1000 m
• Earthquakes can occur to depths
of 8 km or more
• Maximum EQ magnitude = 5.5
Fast spreading ridges
• No major tectonic faults
bounding the axial valley
• Axial summit with trough ~100
wide and 10-20 m deep
• Small relief (smooth
topography like a dome)
• Seismic activity constrained to
depths shallower than the
AMC (max depth 2 km)
• Maximum EQ magnitude = 2
Hydrothermal Circulation
Fast spreading ridges
Slow spreading ridges
• Circulation depth controlled • Circulation controlled by
by the depth of the AMC
large regional faults
• Along axis convection of
• Across axis convection of
hydrothermal fluids?
hydrothermal fluids
What about intermediate
spreading-rate ridges
4-8 cm/yr
Are they more like slow or fast
spreading ridges
Juan de Fuca and
Gorda Ridges Both have a full
spreading rate of
6 cm/yr.
JdFR - looks like a
fast spreading
ridge but magma
chamber is
deeper.
Gorda looks like a
slow spreading.
Juan de
Fuca Ridge
Gorda
Ridge
Melt Production at ultraslow spreading ridges
Melting shuts off because uppermost
mantle cool conductively
Average Crustal thickness < 6km
Ultraslow spreading
ridges. Not enough
melting to generate
continuous crustal
coverage
Exit Slip
• Explain how slow and fast spreading centers
differ in terms of the cause and intensity of
seismic activity
• Sketch your choice of:
– A slow spreading center
– A fast spreading center
– A depth section showing seismic layers 1 – 3