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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 0
reversal
v
x t 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.
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).
Further study Make sure you understand qualitatively the gravity anomalies, topography and density structures. 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). Q21 Quick question to check your understanding of magnetic inclination measurements. Q22 About heat flow through mid-ocean ridges based on half-space cooling model.
