Mid-ocean ridges are underwater mountain systems formed at divergent tectonic plate boundaries where new oceanic crust is generated. They consist of a chain of mountains linked by a central rift valley and result from mantle upwelling and melting in response to plate spreading. As the buoyant magma rises to the seafloor at the plate boundary, it emerges as lava to form new ocean crust upon cooling.

1. Mid-Ocean Ridges

2. A mid-ocean ridge is an underwater mountain system formed
by plate tectonics. It consists of various mountains linked in
chains, typically having a valley known as a rift running along
its spine.
This type of oceanic mountain ridge is characteristic of what is
known as an oceanic spreading center, which is responsible
for seafloor spreading.
The production of new seafloor results
from mantle upwelling in response to plate spreading;
this isentropic upwelling solid mantle material eventually
exceeds the solidus and melts.
The buoyant melt rises as magma at a linear weakness in
the oceanic crust, and emerges as lava, creating new crust
upon cooling.
A mid-ocean ridge demarcates the boundary between
two tectonic plates, and consequently is termed a divergent
plate boundary.

4. Þingvellir National Park

14. World Distribution of Mid-Oceanic Ridges

15. Mid-Ocean Ridges

16. Modern Map

17. What makes these so different?
From MacDonald 1982

23. Oceanic
Crust
Stratigraphy
From Karson 2002
• Deep sea drilling program
• Dredging of fracture zone scarps
• Ophiolites

24. What’s the difference?
Gabbro Basalt

25. Upper crust
From Karson 2002
Upper basalt – morphology
primarily pillow, but
lobate and sheet also
evident; variable
thickness; high porosity
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
A/B/C
C/D

26. Modified from Nedimovic and Carbotte 2008
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 high-
porosity basalt flows to low-porosity
sheeted dikes
2) Hydrothermal – alteration front within
upper extrusive volcanic layer
Fracturing and hydrothermal alteration
contribute to seismic anisotropy

27. Seismic Layer 3

28. Compare your answers Share what you have observed

29. Fast-spreading < 8 cm/yr
seismic zone
Courtesy of D. Fornari

30. Slow-spreading 2-5 cm/yr
Temporary magma chamber

32. Mantle melt
adiabatically rising
mantle material
magma
MOR
Mantle
sediments, igneous
crust & mantle
island arc
trench
earthquakes
ocean crust earthquakes
continental crust
melt
fracture
zone
trench
Plate Tectonics

33. 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

34. 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

35. Hydrothermal Circulation
Fast spreading ridges
• Circulation depth controlled
by the depth of the AMC
• Along axis convection of
hydrothermal fluids
Slow spreading ridges
• Circulation controlled by
large regional faults
• Across axis convection of
hydrothermal fluids

36. What about intermediate
spreading-rate ridges
4-8 cm/yr
Are they more like slow or fast
spreading ridges

37. 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. Gorda
Ridge
Juan de
Fuca Ridge

38. Melt Production at ultraslow spreading ridges
Melting shuts off because uppermost
mantle cool conductively
Average Crustal thickness < 6km

39. Ultraslow spreading
ridges. Not enough
melting to generate
continuous crustal
coverage