Geophysics, Lecture Notes- Physics - Prof Gavin Bell 17

Geophysics, Lecture Notes- Physics - Prof Gavin Bell 17 -...

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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 treversal 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 from geology.about.com). 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 rock. Foreground: small chamber poorly supplied with molten material, leading to slow spreading. Background: fast spreading region due to large magma chamber. [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). 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 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.

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