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Unformatted text preview: PX266 Geophysics (2010/11)
Lecture 16 Handout – Mid-ocean Ridges
Dr. Gavin Bell
Age of ocean floor Colour scale represents ocean floor age based on radiometric dating. Very oldest oceanic
plate material is around 200Ma due to continual subduction.
Clear pattern in age of ocean floor: youngest material by mid-ocean spreading ridges where
magma upwells and cools to form basalt. The mid-Atlantic ridge is very obvious and was
originally responsible for the break-up of Pangea (Jurassic – north Atlantic opens up; later in
the Cretaceous, the south Atlantic opens). Paleomagnetism and sea-floor spreading
Magnetic polarity can be measured by ship-towed magnetometer. “Stripes” of width x v0
2 observed parallel to spreading ridges. The reversal chronology can be verified by radiometrically dating oceanic basalts (e.g. by potassium-argon isochron method). Topography of mid-ocean ridges Figure adapted from Fowler. Typical ridge structure: gentle rise up towards ridge axis with
deep trench at the axis.
The depth of water near ridges increases as the square root of the age of the rock (which
increases as you move away from the ridge axis) up to about 70Ma. This comes from a simple
model (“half-space cooling model”, see Q. 22) assuming the hot upwelled lithosphere cools as
it ages and moves away from the ridge, hence thermally contracting to give deeper ocean. Gravity surveys at mid-ocean ridges Figure adapted from Fowler.
Free-air and Bouguer anomalies across a typical mid-ocean ridge.
The anomalies can be computed – two density structures are shown for the Bouguer anomaly.
NOTE – the density structure is not uniquely determined by the gravity survey.
We need extra evidence to understand the real structures – primarily seismology. The free-air anomaly is positive and indicates lack of complete isostatic compensation. What is the free-air correction for these measurements? Why is it positive given a low density structure beneath the ridge?
The Bouguer anomaly becomes less positive over ridge due to the underlying low density
structure. What “known” density variation have we accounted for? Why is the Bouguer anomaly positive over the sea even though it becomes less
positive at the ridge? Models for magma upwelling
Various magma supply models describe how magma might break through to the surface along
a ridge. One possibility is a large, stable magma chamber beneath the surface whose surface
layers “peel off” to form new crust. Another possibility is discrete packets of rising magma
which arrive near the surface and nucleate a new crack in the crust through which magma
escapes and cools.
Typically, extruded magma forms a
structure such as a pillow lava (picture
These occur with slow extrusion rates
(compared to a violent volcanic eruption,
say) and rapid cooling in sea water.
Exotic life forms exist in the warm waters
around mid-ocean ridge vents. Magma chambers cause the
pronounced upward bulge of the
ridge structure and comprise
mostly molten rock on top of a
reservoir of partially melted
Foreground: small chamber
poorly supplied with molten
material, leading to slow
Background: fast spreading
region due to large magma
[From web version of Sci. Am.
paper, K. Macdonald, UCSB.] Seismic tomography at mid-ocean ridges
Recall: we discussed the overall vertical density structure of the Earth as determined by
seismology and self-compression (PREM – preliminary reference Earth model) but
mentioned that there is also lateral variation in seismic wave speeds. 3D model of wave
speeds derived from detailed travel time measurements: seismic tomography.
A large magma chamber should produce a region of lower seismic wave speeds and high
attenuation. Such gross structures can be observed. Detailed measurements can also show
complex, asymmetrical structures such as the example below.
Example: East Pacific Rise measured by MELT experiment, 1998.
Red: slower P-waves than expected (by up to a few %).
Blue: faster. NB.
Absolute wave speeds are not important, in seismic tomography one measures wave speed
“residuals”, i.e. difference from expected speed. Figs. From: MELT Seismic Team, et al. Science 280, 1215 and 1224 (1998).
Make sure you understand qualitatively the gravity anomalies, topography and density
You should be able to sketch the magnetic reversal pattern and be able to calculate “stripe
widths” based on spreading rate and reversal dates.
Plenty of supplemental reading available – Fowler goes into lots of detail (interesting, but
much more than you need for this course).
Q22 Quick question to check your understanding of magnetic inclination measurements.
About heat flow through mid-ocean ridges based on half-space cooling model. ...
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This note was uploaded on 09/12/2011 for the course ECON 102 taught by Professor Gavinbell during the Spring '11 term at LSE.
- Spring '11